A Diagnostic Study of the Nutrient Loading at Swan Lake, Montana
Swan Lake Clean Lakes Project, Phase One
FLBS Open File Report 138-95
by
Nancy M. Butler, James A. Craft, and Jack A. Stanford
Funding Provided by
Water Quality Bureau
Montana Department Of Health and Environmental Sciences
Room A-206 Cogswell Building
Helena, Montana 59620
Loren L. Bahls, Project Officer
in cooperation with
Clean Lakes Program
U. S. Environmental Protection Agency
EPA Region 8
999 18th Street, Suite 500
Denver, Colorado 80202
David Rathke, Project Officer
November 1995
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TABLE OF CONTENTS
Section
Title Page
Table of Contents
List of Figures
List of Tables
Introduction and Overview
Study Area Description
Methods
Results
Physical Limnology
Chemical Limnology
Primary Production
Weather Station Data
Biological Oxygen Demand
Isotopic Analyses
Mass Balance Calculations
Analytical Accuracy and Quality Control
Conclusions
Recommendations
Summary
Acknowledgments
Literature Cited
Appendix
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LIST OF FIGURES
p. 2 Figure 1. Map showing location of Swan Lake and the Swan River sub-basin
in the Flathead River catchment basin in Montana, U.S.A. and British Columbia,
Canada.
p. 6 Figure 2. Map of Swan Lake showing location of sampling sites and weather
station.
p. 14 Figure 3. Location of 1994 sampling sites for collecting whole water
samples and autochthonous and allochthonous organic matter for stable isotope
p. 16 Figure 4. Discharge calculations for the tributaries, including data logger
discharge data for Swan River.
p. 17 Figure 5. Temperature (° C) and dissolved oxygen (percent saturation)
profiles for the north basin of Swan Lake during the period of July 1992 to
November 1993.
p. 18 Figure 6. Temperature (° C) and dissolved oxygen (percent saturation)
profiles for the south basin of Swan Lake during the period of July 1992 to
November 1993.
p. 20 Figure 7. Temperature (° C) and dissolved oxygen (percent saturation) data
for Swan River, Spring Creek, and Sixmile Creek during the period of July
1992 to November 1993.
p. 21 Figure 8. Transmittance and photosynthetically active radiation (PAR)
profiles and Secchi depth readings for the south basin of Swan Lake during the
period of July 1992 to November 1993.
p. 23 Figure 9. Turbidity readings for the three tributaries (A) and the south
basin of Swan Lake (B).
p. 24 Figure 10. Total suspended solids for the three tributaries (A) and the south
basin of Swan Lake (B).
p. 25 Figure 11. Ammonium nitrogen concentration in the three tributaries (A)
and the south basin of Swan Lake (B).
p. 27 Figure 12. Nitrate/nitrite nitrogen concentrations (/jg/l N) in the three
tributaries (A) and the south basin of Swan Lake (B) during the period of July
1992 to November 1993.
p. 28 Figure 13. Total persulfate nitrogen concentrations (/vg/l N) in the three
tributaries (A) and the south basin of Swan Lake (B) during the period of July
1992 to November 1993.
p. 30 Figure 14. Total soluble reactive phosphorous concentrations (/vg/l P) in
the three tributaries (A) and the south basin of Swan Lake (B) during the
period of July 1992 to November 1993.
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p. 32 Figure 15. Total phosphorous concentrations (jyg/l P) in the three
tributaries (A) and the south basin of Swan Lake (B) during the period of July
1992 to November 1993.
p. 33 Figure 1 6. Dissolved inorganic carbon (DIC) concentrations (mg/l C) in the
three tributaries (A) and the south basin of Swan Lake (B) during the period of
July 1992 to November 1993.
p. 34 Figure 17. pH (A) and alkalinity (B) measurements for the south basin of
Swan Lake during the period of July 1992 to November 1993.
p. 36 Figure 18. Dissolved organic carbon (DOC) concentrations (mg/l C) in the
three tributaries (A) and the south basin of Swan Lake (B) during the period of
July 1992 to November 1993.
p. 37 Figure 19. Non-dissolved organic carbon (NDOC) concentrations (mg/l C)
in the three tributaries (A) and the south basin of Swan Lake (B) during the
period of July 1992 to November 1993.
p. 39 Figure 20. Soluble silica concentrations (mg/l SiO^ ) in the three
tributaries (A) and the south basin of Swan Lake (B) during the period of July
1992 to November 1993.
p. 40 Figure 21. Sulfate concentrations (mg/l S04 ) in the three tributaries (A)
and the south basin of Swan Lake (B) during the period of July 1992 to
November 1993.
p. 41 Figure 22. Primary production rates (mg Cm2 -hr) in the south basin of
Swan Lake during the period of July 1992 to November 1993.
p. 42 Figure 23. Fluorescence profiles in the south basin of Swan Lake during the
period of July 1992 to November 1993.
p. 44 Figure 24. Phaeopigment and chlorophyll a concentrations in the south basin
of Swan Lake during the period of July 1992 to November 1993.
p. 46 Figure 25. Results of chemical analysis of precipitation collected at the
weather station at the south end of Swan Lake during the period of July 1992 to
November 1993.
p. 47 Figure 26. Air temperature data collected at the weather station located at
the south end of Swan Lake during the period of July 1992 to November 1993.
p. 48 Figure 27. Wind vectors (upper graph, graphed as km/h, all dates
combined) and wind speed data (lower graph) collected at the weather station
located at the south end of Swan Lake during the period of July 1992 to
November 1993.
p. 49 Figure 28. Monthly wind vector data collected at the weather station located
at the south end of Swan Lake during the period of July 1992 to November
1993.
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p. 50 Figure 29. Post-incubation oxygen levels in BOD bottles suspended at 5 m
(squares), 10 m (triangles), and 35 m (circles) in the south basin of Swan
Lake during the period of May 1993 to September 1993.
p. 53 Figure 30. Stable isotope signatures of organic material in whole water
samples collected during the period of May to October 1993 from Swan Lake
south basin and the tributary sites identified in Figure 2.
p. 55 Figure 31. Stable isotope signatures of organic material in whole water
samples collected during the period of May to September 1994 from the Goat
Creek and Lost Creek drainages at the sites identified in Figure 3.
p. 56 Figure 32. Mean isotopic signatures (± 1 S.D.) of stream produced
(autochthonous) and terrestrially derived (allochthonous) organic matter in
the North and South Forks Lost Creek.
p. 56 Figure 33. Stable isotope signatures of organic material in whole water
samples collected during the period of May to September 1994 from GC (Goat
Creek), LC (Lost Creek), and SR (Swan River) sampling sites identified in
Figure 3.
p. 59 Figure 34. Mean daily load of total phosphorous, nitrate/nitrite, and total
persulfate nitrogen entering Swan Lake from each of the four sources studied
and the relative contribution of each source expressed as percent of total daily
load.
p. 60 Figure 35. Average daily transport of total phosphorous (kg/d P),
nitrate/nitrite (kg/d N), and total persulfate nitrogen (kg/d N) leaving Swan
Lake via the Swan River (upper graph) and entering Swan Lake (lower graph)
via the four sources studied.
p. 61 Figure 36. Average daily balance of total phosphorous (kg/d P),
nitrate/nitrite (kg/d N), and total persulfate nitrogen (kg/d N) entering and
leaving Swan Lake during the period of October 1992 to November 1993..
p. 62 Figure 37. Particulate carbon (NDOC) load (kg/d C) entering Swan Lake via
the Sixmile and Spring Creek (upper graph) and Swan River (lower graph)
during the period of August 1992 to November 1993..
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vi
LIST OF TABLES
p. 4 Table 1: Mean conductivity (^mhos/cm), nutrient concentration (jvg/l) and
ratio of total phosphorous to total nitrogen for selected lakes, of the Flathead
River Basin.
p. 10 Table 2. Sampling protocol and schedule for Swan Lake and tributary sites
identified in Figure 2.
p. 12 Table 3. Biophysical variables measured in time-series at Swan Lake and
tributary sites identified in Figure 2 and analytical protocol.
p. 50 Table 4. ANOVA table for change in oxygen level in BOD bottles during the six
hour incubation period.
p. 57 Table 5. ANOVA table for effects of sampling date (effect 1), drainage basin
(Lost vs. Goat - effect 2), and logged vs. unlogged stream (effect 3) on isotopic
signature of stream transported organic matter.
p. 57 Table 6. ANOVA table for effects of sampling date (effect 1) and stream (GC,
SC, and SR, as identified in Fig. 3) on isotopic signature of stream transported
organic matter.
p. 76 Table A-1. Summary of water chemistry data for all stations.
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INTRODUCTION AND OVERVIEW
The glacial lakes of western Montana are among the most pristine in the world and their
water quality is generally not measurably impacted by human activities. Concentrations
of plant growth nutrients are naturally low, with concentrations of soluble reactive
phosphorous and the soluble forms of nitrogen usually at or below detection limits (<0.5
ljg/\ and <20.0 ^g/l, respectively). Consequently, production of algae is limited and, in
lakes that have deep basins, Secchi depths are usually > 10m, even though lake surface
areas are often less than 5% of the total catchment and spate-induced sedimentation is
often high (Ellis and Stanford 1986; Stanford and Ellis 1988; Ellis and Stanford
1988a).
Despite their reputation as pristine lakes, however, a few of the larger, lowland lakes in
western Montana have been showing signs of eutrophication. Flathead Lake, the largest
(480 km2), has been a focal point of concern for the last two decades, owing to its
centerpiece setting in the Flathead River Catchment (Figure 1) and the extreme value of
property on the lake shore. Studies of Flathead Lake have documented urban sewage as a
primary source of labile nutrients that have stimulated abnormal algae production and
otherwise reduced water quality (Stanford and Ellis 1988; Stanford et al. 1990). As a
result of these studies, strategies to limit nutrient loading from these point sources have
been implemented and potential nonpoint problems are being evaluated in an ongoing
study (Flathead Basin Commission 1991).
With its primary goal to protect water quality, the Flathead Basin Commission developed
and initiated a basin-wide monitoring program and sponsored cooperative research
initiatives (Flathead Basin Commission 1986, 1988, 1991). Results from the
monitoring program and from site-specific studies by The University of Montana's
Flathead Lake Biological Station have increased concern for two other large lowland
lakes, Swan and Whitefish (Figure 1). These lakes, which are located in rapidly
developing economic centers within the Flathead Catchment, have been showing clear
signs of chronic cultural eutrophication within the last two decades in the form of a
hypolimnetic oxygen deficit.
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CLARK FORK
Figure 1. Map showing location of Swan Lake and the Swan River sub-basin in the
Flathead River catchment basin in Montana, U.S.A. and British Columbia, Canada.
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This study focuses on water quality problems in Swan Lake. In spite of the large size of
the lake, its socio-economic importance as a major home and recreation site, and its
location in a catchment that contains some of the most concentrated logging activity in
North America (Potts 1991b), very little monitoring or historical data are available
for Swan Lake. Knowledge of the limnology of Swan Lake is limited to informal data
collection (e.g., summer profiles of temperature and dissolved oxygen collected by
students at The University of Montana Flathead Lake Biological Station) and a few
preliminary measures of nutrient concentrations (Table 1). There are no water quality
monitoring sites located within the Swan River Catchment, with the exception of a site
located below Swan Lake, where the Swan River enters Flathead Lake. Data have been
gathered at that site continuously since 1977, during which time nutrient levels have
been consistently low and comparable to Flathead Lake values. However, the recent
appearance of a hypolimnetic oxygen deficit (Spencer 1991a) suggests a potential threat
to the lake's status as a pristine, oligotrophic lake. Because it is a phenomenon generally
associated with eutrophic lakes, the oxygen deficit in Swan Lake may be a warning sign
for impending decline in water quality.
Here we present data collected during the period of July 1992 through September 1994.
This study utilizes a variety of techniques to monitor seasonal changes in the biological,
chemical, and physical parameters of Swan Lake and its tributaries and elucidate the
influences of external factors, such as land use patterns, on these parameters.
Limnological parameters were quantified in time series measurements during the period
of July 199Z through November 1993 at two lake stations and three tributary stations.
During the same period, information was collected from a weather station and
precipitation gauge located on the south shore of the lake. These data were used to
describe seasonal trends in water quality in the lake and tributaries and seasonal changes
in the relative influence of major nutrient sources (i.e. tributaries, precipitation) on
the nutrient balance of Swan Lake. During the period of May to September 1994, select
tributaries located up valley from Swan Lake were sampled at regular intervals to
provide information on the source and transport of organic carbon from the watershed
into Swan River and ultimately into Swan Lake.
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Site Cond SRP SP N03-N NH3-N P:N n (yr)
Gyrfalcon
30.3
<1
1.6
23.4
<5
1:28
14 (6)
(19-42)
(<1-<1)
(<1-2.7)
(<10-85)
(<5-13)
McDonald
95
<1
<1
152.0
<5
1:103
8 (4)
(74-117)
(<1-<1)
(<1-<1)
(128-164)
(<5-<5)
Whitefish
151
<1
3.7
20.8
5.1
1:17
60 (2)
(138-164)
(<1-<1)
(2.6-4.2)
(20-60)
(<5-6)
Swan
152
<1
N D
40.0
6.2
1:32
4 (1)
(130-191)
(<1-1.3)
(35-54)
(<5-8)
Rathead
166
<1
4.5
38.4
5.6
1:17
564 (10)
(156-177) (0.1-1) (2-20) (5-136) (<5-12)
TABLE 1: Mean conductivity (^/mhos/cm), nutrient concentration (/vg/l) and ratio of
total phosphorous to total nitrogen for selected lakes, of the Flathead River Basin shown
in Figure 1. Ranges are given in parentheses.
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THE STUDY AREA
Swan Lake
The study was conducted in Swan River catchment, located in the northwest corner of
Montana (Fig. 2). Of glacial origin, Swan Lake is long and narrow and oriented on a
SE-NW axis. Although the morphometry of the lake had not been accurately mapped prior
to this study, the surface area was estimated at 2688 acres (10.8 km^)and there are
two deep water basins, one located just north of the Swan River inlet (South Basin) and
the other at the northwest end of the lake (North Basin). Swan River, Sixmile Creek,
and Spring Creek are the three major, year-round tributaries, but the lake has a
number of seasonal streams along its periphery. Swan River, by far the largest
tributary, enters at the south end of the lake and leaves the lake at the northwest end.
Historically, the majority of development along the lake has occurred along the east
shore, with intensive development in the vicinity of the Swan Lake town site on the south
east shore. More recently there has been an increase in development along the northwest
shoreline and at the north end of the lake. Public access to the lake is provided by a road
network around the lake, a State Park on the southeast shore (administered by the
Montana Department of Fish, Wildlife, and Parks) a wildlife refuge on the south shore
(administered by the US Fish and Wildlife Service), and federal lands on about 50% of
the shoreline (administered by the US Forest Service). The lake is used for a variety of
water sports, waterfowl hunting, fishing, and for domestic water supply. The lake is
classified in the Montana Surface Water Quality Standards as A-1.
As mentioned previously, a hypolimnetic oxygen deficit has been observed in both basins
of Swan Lake, with the problem especially severe in the south basin (Figure 2).
Depletion of oxygen from the deep water layers could change the redox gradient at the
water sediment interface, resulting in the release of large amounts of soluble reactive
phosphorous (SRP) and ammonia into the water column of the lake. Generally lakes
accumulate high concentrations of SRP at the water-sediment interface as a function of
anoxic bacterial metabolism in the sediments, coupled with upward migration of water
upon compaction. If oxygen concentrations are high, the SRP is held in the sediments at
the interface by the positive (oxidizing) redox gradient. However, anoxic conditions at
the lake bottom would be expected to result in a sudden release of SRP from the
sediments into the water column. Since autotrophic production by most lakes in the area
(e.g. Flathead, McDonald) is P limited or co-limited by P and N, such a release could
result in a sudden and drastic change in the trophic state of the lake.
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5 m Contour Interval
Maximum Depth: 43 m
Lake Volume: 2.2 x 108 m3
Surface Area: 1,327 ha
Figure 2. Bathymetric map of Swan Lake, with inset map indicating location of
sampling sites and weather station. SB: south basin; NB: north basin; Sixmile:
Sixmile Creek; Spring: Spring Creek; Swan: Swan River; WS: weather station and
precipitation collector.
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Other than the presence of an oxygen deficit, Swan Lake appears to have the
characteristics of an oligotrophy lake. The lake contains a popular cold water fishery
and shoreline residents routinely use lake water as a potable supply with little or no
treatment. On the basis of data in Table 1, Swan would be included among the nation's
cleanest lakes. Estimates of nutrient concentrations indicate that the lake is chemically
similar to Flathead Lake and other lakes of similar size, morphometry, and geology.
These lakes all have low levels of primary production, as would be expected under such
low nutrient conditions, and none experience a hypolimnetic oxygen deficit (Stanford and
Ellis 1988; Ellis and Stanford 1988a). Thus the oxygen deficit is Swan Lake is highly
unusual and perplexing as to its cause.
Pollution sources to Swan Lake have not been quantified in any detail. Although domiciles
and businesses along Swan Lake dispose of sewage in leach fields within the glacial tills
that characterize the shoreline, development along the shoreline seems too limited for
septic contamination to cause such an oxygen deficit. Even if all the leach fields were
faulty and polluting the lake, it is unlikely they would be causing the deficit given the
dilution effect of the lake and the relatively short water retention time (estimated at less
than 2 years). A more probable explanation may lay with land use practices within the
catchment which accelerate nutrient loading by increasing water yield and erosion.
Research by Spencer (1991) suggests that sedimentation rates in Swan Lake since the
late 1800's are closely correlated with logging activities within the catchment.
Increased sedimentation may accelerate microbial activity either by increasing
bioavailable nutrient loads in the photic zone (resulting in increased photosynthesis and
associated microbial respiration) or by entraining sediments and organic matter within
the water column (resulting in increased bacterial bioproduction and respiration).
Watershed Description
The surface area of Swan Lake (10.8 km2; 2668 acres) represents only about 0.67% of
the total catchment area upstream of the lake (1,738 km^. Therefore, the river has
the potential for a profound advective effect on the limnology of the lake, particularly
during snowmelt (May - June) when river discharge may increase as much as 3 to 4
orders of magnitude over base flows. The catchment is entirely within the Precambrian
formations of the Belt Series, which are characterized by minimal dissolution and
release of ions due to weathering processes. Given this low rate of release into solution
and the overall low nutrient levels within the lake, input of nutrients associated with
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suspended sediments could have a marked effect on the lake's bioavailable nutrient budget
and associated biological responses, particularly during high flow periods.
With the exception of a few isolated ranches and about 25% of the lake shore, nearly all
land within the catchment is in federal and state ownership or in large tracts of
timberland owned by Plum Creek Timber Company. It is uncertain how much of the
catchment has been directly influenced by logging or road building. Records prior to
1 950 are incomplete and a great deal of logging occurred between 1920 - 1950. We do
know that at least 15% of the entire catchment was clearcut at least once since 1950 and
a majority of the total harvest activity occurred within the last two decades (Hauer
1 991; Spencer 1991). In addition, nearly 70% of the catchment has had road building
activity.
METHODS
The primary purpose of this diagnostic study, as presented in the original proposal, is to
determine if there is a relationship between the observed oxygen deficit and nutrient
loads derived from shoreline tributaries, including Swan River. Input of materials into
the lake from the catchment (via Swan River and large shoreline creeks) and the airshed
will be quantified in time series. The intent of the study is not quantify individual
sources of nutrients, but to evaluate lake and watershed characteristics.
The specific tasks proposed were:
1. Accurately map the morphometry of the lake so that area/volume relations
can be established.
2. Conduct a lake sampling program to provide data relating the timing and
extent of nutrient loading to lake trophic status, as indicated by temporal
dynamics of ambient nutrient concentrations, distribution and abundance of
plankton, seasonal rates of phytoplankton production, and temporal dynamics
of hypolimnetic stagnation.
3. Perform analyses to estimate mass transfer coefficient for water, sediments
and nutrients, using data obtained under Task 1, and relate the limnological
and loading variable in time series to observed patterns of oxygen depletion in
the lake.
4. Report research results from Tasks 1 -3 in relation to general measures or
observations of land use in the Swan River catchment.
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Lake Mapping
Lake mapping was conducted during the summer of 1995 and the information obtained is
presented in Figure 2. The lake volume determined by this mapping did not differ
substantially from the volume estimated previously and used in this report in
calculating nitrogen and phosphorous mass balance; therefore, those values were not
adjusted.
Limnological Analysis
Two sites on Swan Lake, one on Swan River, and two tributary sites (Fig. 2) were
sampled at monthly intervals during the period of July 1992 to November 1993.
Originally three tributaries in addition to Swan River were to be sampled, but only two
tributaries (Sixmile and Spring Creeks) consistently had sufficient flow to permit
collecting water chemistry and flow data. Rather than include a site that would permit
only seasonal sampling, it was decided to use the two streams with year round flow and
increase the variety of analyses run. In addition, it was originally proposed to collect
samples for chemical analysis at the north and south basins. However, funding
limitations would permit collection of only one sample from each site on each date. By
collecting multiple samples (surface, bottom, and integrated) at the south basin site we
would gain more information on the nutrient dynamics as pertained to the seasonal
development of the oxygen deficit. The sampling protocol is summarized in Table 2.
Stream and river discharge data were determined from cross-sectional flow/depth
analysis. In addition, flow data were collected with a Druck© pressure transducer
located on the Swan River site and stored by an Omnidata data logger. These data were
collected at 5 minute intervals and the stored data were recovered monthly.
A weather station and precipitation collector were located on the south shore (Fig 2).
Weather data (wind direction, wind speed, and air temperature) were collected at 5
minute intervals with an Omnidata data logger during the period of October 1992 to
August 1994 . The stored weather data were downloaded monthly. Precipitation
samples were collected monthly and after major precipitation events during the period
of October 1992 to November 1993. The collector consisted of two 50 cm diameter
funnels open to the atmosphere and connected such that water, melted snow, and
particulate matter would flow into collection bottles. After removing the collection
bottles with the collected precipitation, the funnels were rinsed with 10% HCI and then
thoroughly rinsed with distilled water. The collection bottles were replaced with clean,
acid washed bottles and the samples transported on ice to the laboratory for analysis.
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North & South Basin
Hydrolab (temp, ph, DO, Cond, ReDox, %Sat)
Surface, 1 m intervals to 20m, 2m intervals to bottom
SQMth Bqsin
Fluorometer
Surface, 1m, 2m, 2m intervals to bottom
Primary Production: 14C Light/Dark Bottles
1m, 5m, 10m, 20m, 30m
Water Chemistry
(D0C,DIC,ND0C,TP,SRP,TPN,N02/3,NH3 N,Si03 ,S04,TSS,TURB,CHLa,ALK)
5m, 30m, 0-30m (integrated)
Chlorophyll
0-30m (integrated), depth with Fluorometer max.
Phytoplankton (for reference)
5m, bottom, 0-30m (integrated)
Zooplankton (for reference)
0-30m (integrated), 0-thermocline (integrated)
Light Attenuation
Surface, 1 m intervals to bottom
Light Meter
Hourly (Sunrise to sunset)
Transmittance
Surface, 1m intervals to 20m, 2m intervals to bottom
Secchi Reading
Tributaries
Hydrolab (temp, ph, DO, Cond, ReDox, %Sat)
Water Chemistry (as for lake)
grab sample
Flow rate and stream depth
Sampling Dates (1992-1993): 13 Jul, 20 Sep, 21 Oct, 16 Nov, 22 Jan, 11 Mar, 20
Apr, 13 May, 27 May, 10 Jun, 26 Jun, 1 Aug, 6 Sep, 26 Sep, 9 Oct, 6 Nov
Table 2.. Sampling protocol and schedule for Swan Lake and tributary sites identified in
Figure 2.
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During the summer months, the benthic zone along the developed areas of the shoreline
was visually surveyed for signs of septic contaminated groundwater seepage, which
would appear in the form of localized patches of algal blooms. If such signs were found,
the specific sites of seepage would be located via in situ conductivity measurements and
water samples for chemical analysis collected at those sites. At no time during this study
was there any sign of such seepage; therefore, no samples were collected.
The water chemistry analytical protocol is summarized in Table 3. In addition to
analyzing field collected samples, analytical performance was monitored for precision
(defined as < ± 1 s. d. among replicated samples) and accuracy (defined as 90-110%
recovery of a standard solution added to a sample). Analytic performance was evaluated
through analysis of quality control samples, supplied by the U. S. Environmental
Protection Agency. Analytical and quality control data were archived in the Flathead Lake
Biological Station data storage and retrieval system.
Suspended sediment samples were collected from the south basin site in Swan Lake and
from the three tributary sites (Swan River, Spring Creek, and Sixmile Creek) and total
suspended sediments (TSS) determined gravimetrically (Table 3). The bioavailability
of nutrients associated with the sediments was calculated based upon the work of Ellis
and Stanford (1986 & 1988) in Flathead Lake. Because the Swan and Flathead basins
are situated in geologically similar bedrock and the river-transported sediments are
derived from geologically similar strata, it is reasonable to expect the sediments carried
by the Swan River to interact with phosphorous in a manner similar to that observed in
Flathead Lake. Therefore, we extrapolated from their work in calculating bioavailability
in the present study.
Biological Oxygen Demand
Biological oxygen demand (BOD) was measured at monthly intervals from May through
September 1993 to determine the relationship between microbial activity, organic
carbon volume and sources, and changes in dissolved oxygen concentrations in the water
column. Respiration bottles (2 light and 2 dark for each depth) were suspended at 5 m,
10 m, and 35 m depth in the south basin of Swan Lake. The bottles were filled with
water collected from the test depth and incubated in situ for a period of 4 to 6 hours.
Oxygen concentrations were measured at the beginning and end of each experimental run
using a YSI model 5739 probe. Data were analyzed using multiple analysis of variance
(MANOVA) to test for the effects and interactions of depth and sampling date.
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Variable Method (reference) Detection Limit Sites
Grab Samples
Phosphorous
total
persulfate digestion; automated
0.5 fjg/\-P
SB, TRIBS, PREC
ascorbic adic (1)
soluble reactive
filtration; auto, ascorbic adic (1)
0.4 fjg/\-P
SB, TRIBS, PREC
Nitrogen
total persulfate
persulfate digestion (2); auto.
20 //g/l-N
SB, TRIBS, PREC
cadmium reduction (1)
nitrate/nitrite
filtration; auto, cadmium red. (1)
2.0 j/g/l-N
SB, TRIBS, PREC
ammonia
filtration; auto, phenate (1)
2.0 ^g/l-N
SB, TRIBS, PREC
Soluble silica
filtration; auto, molybdate-
0.2 mg/l-Si02
SB, TRIBS
reactive silica (1)
Sulfate
filtration; ion chromatography (1)
0.05 mg/l-S04
SB, TRIBS, PREC
Carbon
non-dissolved organic
filtration; persulfate oxidation;
0.1 mg/l-C
SB, TRIBS
infrared C02 detection (3)
dissolved organic
filtration; persulfate oxidation;
0.1 mg/l-C
SB, TRIBS
infrared CO2 detection (3)
dissolved inorganic
filtration; acid liberation;
0.1 mg/l-C
SB, TRIBS
infrared CO2 detection (3)
Alkalinity
titration (1)
0.5 mg/l-CaC03
SB
Total suspended solids
filtration; gravimetric (1)
0.5 mg/l
SB, TRIBS
Turbidity
nephelometry (1)
0.10 NTU
SB, TRIBS
Bioloaical SamDles
Chlorophyll a
acetone extractions (1,4)
1.00 mg/m3
SB
Phytoplankton
Utermohl counts
NA
SB
Phytoplankton production
incubation of14 C02-label in light
NA
SB
and dark bottles (5)
In Situ Analyses
Temperature
thermistor (6)
.15 °C
NB, SB, TRIBS
Dissolved Oxygen
electrode (6)
0.20 ppm
NB, SB, TRIBS
pH
low-ion electrode (6)
0.1 units
NB, SB, TRIBS
Conductivity
electrode (6)
1.5 ^mhos/cm
NB, SB, TRIBS
Secchi depth
secchi disc
NA
SB
Water clarity
submarine transmissometer
% transmission
SB
1 APHA 1 985 and as modified for analysis of ultralow concentrations of nutrients
Technicon Auto Analyser
2D'Elia eta/. 1977
^Oceanography International TOC system, Menzel and Vaccaro 1964
^Marker et al. 1980
^Wetzel and Likens 1991
^measure in situ using Hydrolab Surveyor III
Table 3. Biophysical variables measured in time-series at Swan Lake and tributary
sites identified in Figure 2 and analytical protocol. NB = north basin site,SB = south
basin site, TRIBS = tributary sites, PREC = precipitation samples.
-------
13
Carbon Source Pools
Whole water samples for isotopic analysis were collected in conjunction with BOD
measurements to determine the source of the organic matter in the water column. Four
liter whole water samples were collected on each sampling date during the period of May
to September 1993 from each of the three tributary sites and from 5, 10, and 35 m
depth (corresponding to the BOD test depths) in the south basin of Swan Lake.
To determine the source pools of organic carbon entering the Swan River system and the
influence of land use practices on those sources pools, thirteen tributary sites (Fig. 3)
were sampled monthly during the period of May through September 1994. Squeezer
Creek (stations SQ1 and SQ2) and South Fork Lost Creek (stations SF1 and SF2) are
located in areas that have not been logged in over 20 years. Goat Creek (stations G1, G2,
and G3) and North Fork Lost Creek (stations NF1, NF2, and NF3) flow through areas
that have been recently logged, with logging sites located in some instances immediately
adjacent to the stream bed. Whole water samples ( 4L grab samples) were collected at
each stream site. Terrestrial and stream plant material samples also were collected for
isotopic identification of allochthonous and autochthonous (respectively) organic matter.
Whole water samples were filtered through 4.7 cm Whatman glass microfiber filters
(0.7 jjm pore size) to isolate stream-transported particulate matter. The filters were
dried and then transported to the laboratory of Dr. Joseph Montoya at Harvard
University for isotopic analysis via mass spectrophotometry. The data were analyzed
using multiple analysis of variance (MANOVA).
Statistical Analyses
All statistical analyses were conducted using the statistical package Statistica V3.0b (©
StatSoft Inc.).
-------
Figure 3. Location of 1994 sampling sites for collecting whole water samples and
autochthonous and allochthonous organic matter for stable isotope analysis. SB: south
basin of Swan Lake; SR: Swan River; LC; Lost Creek; NF: North Fork Lost Creek; SF:
South Fork Lost Creek; GC: Goat Creek; SC: Squeezer Creek. Shaded areas represent
sites of recent logging in close proximity to the stream drainage.
-------
15
RESULTS
PHYSICAL LIMNOLOGY
Tributary Discharge Rates
All three tributaries reached maximum flow rates during the late spring and early
summer (Fig. 4), reflecting the influence of snow melt runoff. Although spring fed.
creeks are usually noted for fairly consistent flows, Spring Creek (so named because
it is spring fed) had extremely variable flow during the period of April through
July, experiencing as much as a 60% reduction or 150% increase in flow over
average values. While it is worth noting, this variation in flow rate is most likely a
consequence of seasonal variations in surface water runoff and interstitial flow
masking the more consistent input from subterrain springs and is probably not a
cause for concern within the context of this study. The other two tributaries
maintained fairly consistent flow during the periods outside spring runoff and had
far less erratic variation in flow among sampling dates between April through July.
As expected, given its large size and its role as a conduit of flow from the valley south
of Swan Lake, Swan River flow volume was far greater than what was recorded for
the other two tributaries.
Temperature and Dissolved Oxygen Profiles
In the north basin, seasonal changes in temperature and dissolved oxygen followed the
pattern which is typical for temperate, dimictic lakes (Fig. 5). During the summer
months, the water was stratified, with warmer waters overlying colder. In
November, the lake turned over, as evidenced by a unithermal water column. During
the winter months, the lake became reverse-stratified, with colder water overlaying
warm, until spring turnover once again produced a unithermal water column.
Associated with the onset of summer stratification was a slight decline in oxygen
concentrations in the deeper waters, reaching a minimum of 20% saturation by fall
turnover.
In the south basin, the pattern of temperature stratification was similar to that
observed in the north basin. The thermal profiles of the basin indicate turnover in
the fall, with reverse stratification during the winter and subsequent spring
turnover. However, oxygen depletion in the deep water zone was more pronounced
than was observed in the north basin, with oxygen concentrations falling as low
0.1% saturation in October 1992 (Fig. 6).
-------
1992 1993
Figure 4. Discharge calculations for the tributaries, including data logger discharge
data for Swan River.
-------
17
Temperature (°C)
0 5 10 15 20
Dissolved Oxygen (% Sat)
Figure 5. Temperature (° C)1 and dissolved oxygen (percent saturation) profiles for
the north basin of Swan Lake during the period of July 1992 to November 1993. Fall
turnover (November 1992 and 1993) and spring turnover (April 1993) are evidenced
by unithermal temperature profiles. Shaded diamonds = temperature data; open squares
= dissolved oxygen data.
-------
18
Temperature (°C)
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Figure 6. Temperature (c C) and dissolved oxygen (percent saturation) profiles for
the south basin of Swan Lake during the period of July 1992 to November 1993. As in
the north basin, fall turnover (November 1992 and 1993) and spring turnover (April
1993) are evidenced by unithermal temperature profiles. Shaded diamonds =
temperature data; open squares = dissolved oxygen data.
-------
19
Water temperature in the tributaries varied in a pattern consistent with seasonal
changes in climatic conditions (Fig. 7). Water temperatures approached 0° C during
January and reached maximum values (« 17° C) in late summer. Dissolved oxygen
followed a pattern similar to that observed for temperature, reflecting a combination
of aeration effects during periods of high flow during the spring, oxygen production
via high primary productivity during late summer, and temperature dependent
saturation effects. The latter two effects were visibly evident in Spring Creek in
August 1992 (Fig. 6B), when oxygen concentration exceeded 130% and submerged,
rooted macrophytes could be observed to produce bubbles of oxygen in the water
column.
Lake Water Clarity
The field measures of water clarity (Secchi depth, photosynthetically active
radiation (PAR), and transmittance) were taken at the South Basin sampling site in
Swan Lake on each sampling date and are presented in Figure 8. Secchi depth is a
measure of visibility and water clarity. The lowest reading, indicating low
visibility, was recorded in May 1993, with a Secchi depth measurement of 2.1 m.
Maximum values approached 10 m and were recorded during late summer and mid
spring. PAR records the rate of light attenuation with water depth and is a measure
of the amount of light available for photosynthesis at each depth interval.
Transmittance is a measure of horizontal light attenuation (i.e. the ability of a light
beam to penetrate the water column at a specific depth). Observed values generally
ranged between 45% and 80% (compared to 100% transmittance in air) at all
depths, with two exceptions. During late summer and early fall, transmittance in
the deep water declined sharply. Possible implications of this decline with respect to
organic carbon transport will be discussed later in this paper. In the late spring and
early summer, transmittance in the upper 10 m of the water column was sharply
reduced, coinciding with the influx of the spring sediment plume and the decline in
Secchi depth values.
-------
20
t 105
5 --
JUL SB3 NOV JAN MAR MAY JUL SB3 NOV
Figure 7. Temperature (° C) and dissolved oxygen (percent saturation) data for Swan
River, Spring Creek, and Sixmile Creek during the period of July 1992 to November
1993. Shaded diamonds = temperature data; open squares = dissolved oxygen data.
-------
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-------
22
Turbidity and Total Suspended Solids (TSS) are measurements of water clarity and
particulate matter concentration, respectively. At the three stream sites and the
lake site (Fig. 9), turbidity was low for most of the year, remaining below 4 NTU .
However, during periods of peak flow, turbidity levels in two of the stream sites
(Swan River and Sixmile Creek) and in the lake increased markedly. There was an
associated increase in TSS levels (Fig. 10), especially in the tributaries and in the
lake at 5 meters depth. The latter was most likely a reflection of the sediment
plume spreading laterally across the thermocline, rather than mixing vertically
throughout the water column, as suggested by the sharp drop in transmittance above
the thermocline during spring runoff (Fig. 8). There was no evidence of a change in
turbidity or TSS concentration associated with spring runoff at the Spring Creek
sampling site. Because the stream meanders through a marsh before entering Swan
Lake, presumably the majority of particulates acquired from overland rijnoff were
lost before reaching the lake and the sampling site.
CHEMICAL LIMNOLOGY
Nitrogen
Ammonia nitrogen (NH3-N) data are presented in Fig. 11. Because it is readily
utilized by primary producers and oxidized to nitrate/nitrite by bacteria, ammonia
nitrogen is usually present only in very low concentrations in aerobic waters.
Accumulations of ammonia nitrogen are generally associated with increased organic
matter sedimentation. Tributary values were generally below 10 pg/l N, with two
notable exceptions. In October 1992, NH^-N concentrations in Spring Creek reached
nearly 20 £/g/l N, with a less pronounced increase observed in October 1993. These
increases may reflect degradation of debris deposited during spring runoff and the
decline and degradation of the macrophyte populations. A similar peak was recorded
in the Swan River in June 1993 and was presumably the result of degradation of
organic matter deposited during spring runoff. Concentrations recorded for the south
basin of Swan Lake also tended to be less than 10 A/g/1, with the exception of the 35
m sample collected 10 June 1993, which had a NH3-N concentration of nearly 30
A/g/l N, again presumably reflecting increased deposition and degradation of organic
matter.
-------
23
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to November 1993.
-------
24
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1993.
-------
26
The most common form of inorganic nitrogen entering freshwaters is nitrate
nitrogen (N03-N). High levels of nitrate can indicate input of biological wastes or
runoff from heavily fertilized fields. Because it is readily oxidized to nitrate,
nitrite nitrogen (N02 -N) rarely accumulates in measurable quantities; therefore
the data are presented as nitrate/nitrite concentrations. During the study period,
nitrate/nitrite nitrogen (NO2/3-N) concentrations varied seasonally within the
stream sites (Fig. 1 2A). Higher concentrations were recorded during the spring and
lower concentrations during the summer months, most likely a reflection of
increased input from meltwaters in the former case and a combination of uptake by
primary producers and decreased run-off in the latter case.. In addition, a strong
isolated pulse in NO2/3-N concentration was observed during spring runoff in Swan
River. In the lake (Fig. 12B), NO2/3-N concentrations in the surface waters (5 m)
were highest during the late spring and lowest during the late fall with the reverse
pattern observed in the deeper waters (35 m). A similar pattern has been observed
for Whitefish Lake, although the concentrations are much lower (0.5 to 0.1) than in
Swan Lake (Butler 1994). NO2/3-N concentrations in the integrated samples were
fairly constant year round, with values similar to those reported for integrated
samples from Flathead Lake during the same period (Stanford et al. 1994).
The pattern of seasonal change in total persulfate nitrogen (TPN) concentration was
similar to that observed for nitrate/nitrite nitrogen concentration, with the
following exceptions. In Swan River and Sixmile Creek, peak TPN concentrations
occurred in May 1993, with the peak particularly pronounced in Sixmile Creek
(Fig. 13A). In the south basin of Swan Lake (Fig. 1 3B), there was less variation in
the range of TPN concentrations than occurred for nitrate/nitrite concentration.
Nitrogen concentrations (NH3, NO2/3. and TPN) in Swan Lake and in Swan River
during the period of June through September for both sampling years (1992 and
1993) were not substantially different from those reported previously at those sites
and for Flathead Lake and Flathead River during the same time of year (Spencer
1991 a). The overall range of values recorded during this study were similar to
what has been recorded for Flathead Lake during the period of 1977-1992 (Stanford
et al. 1992) and for integrated samples collected from Flathead Lake during the same
time period (Stanford eta/. 1994).
-------
27
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tributaries (A) and the south basin of Swan Lake (B) during the period of July 1992
to November 1993.
-------
Six Mile Creek
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Figure 1 3. Total persulfate nitrogen concentrations (/vg/1 N) in the three
tributaries (A) and the south basin of Swan Lake (B) during the period of July 1992
to November 1993.
-------
29
Phosphorous
Soluble reactive phosphorous (SRP) data are presented in Fig. 14. In Swan River
and Spring Creek, SRP concentrations were generally below 3 \ig/\rV throughout the
year. However, Sixmile Creek consistently had concentrations above 3 /jg/l P,
reaching a peak value of nearly 1 5 jvg/l P in October 1993. During casual
conversation with a local land owner, we discovered that a privately maintained trout
pond is located upstream from the sampling site on Sixmile Creek and it is seasonally
flushed and refilled. The presence of the fish in the pond and the periodic flushings
most likely account for the higher SRP concentrations at the Sixmile Creek samping
site.
In the south basin of Swan Lake (Fig. 14 B), SRP concentrations remained below 2
/vg/l P throughout most of the year. There was a pronounced peak in SRP
concentration in October 1993 for all three depths, but there was no obvious
seasonal pattern in SRP concentrations. A similar late summer increase in SRP
concentration was also observed in Whitefish Lake (Butler 1994) at concentrations
similar to those recorded in Swan Lake in this study.
Total phosphorous (TP) data are presented in Figure 15. The results of Ellis and
Stanford (1986 & 1988) indicate that less than 10% of the TP load associated with
the sediment plume is actually available for bio-conversion during high flow events
(defined as when TSS exceeds 10 mg/l), with the majority being trapped on the
sinking sediments and lost from the open water phosphorous pool. Accordingly, three
data points (Swan River and Sixmile Creek on 13 May and Swan River on 26 May
1993) were adjusted to reflect this difference. For the majority of the year,
however, concentrations in the tributaries remained consistently low (Fig. 15A),
with values generally below 20 /vg/l P.
There was a slight seasonal pattern to TP values in the south basin of Swan Lake (Fig.
15B), with minimum values recorded during the summer months and maximum
values during the winter for all depths, but values were generally low, ranging
between 5 and 10 /vg/l P. Concentrations of TP in the integrated samples were
similar to those reported for Flathead Lake during the same time period (Stanford et
al. 1994) and are well within the range of values recorded during the period of
1977 - 1992 (Stanford eta/. 1992).
-------
30
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Figure 14. Total soluble reactive phosphorous concentrations (//g/l P) in the
three tributaries (A) and the south basin of Swan Lake (B) during the period of July
1992 to November 1993.
-------
31
Phosphorous concentrations (SRP and TP) in Swan Lake and in Swan River during
the period of June through September of 1992 and 1993 were not substantially
different from those reported for the same time of year by Spencer (1991a) at those
sites and in the Flathead River and Flathead Lake. The range of values observed
during this study were similar to what has been reported for Flathead Lake (Stanford
et a/. 1992).
Carbon
Dissolved inorganic carbon (DIC) data are presented in Fig. 16. These data represent
the amount of inorganic carbon available for photosynthesis. In addition,
accumulations of DIC in the hypolimnion generally result from decomposition of
organic matter. In the tributaries, there was no seasonal pattern in DIC
concentration, although a consistent difference in DIC concentrations was observed
among the tributaries. Sixmile Creek consistently had the lowest and Spring Creek
the highest DIG concentrations.
DIC concentrations in lake samples collected at 5 m varied markedly over the year,
with minimum concentrations occurring during the early summer and uniformly
higher concentrations recorded for the remainder of the year (Fig. 16B).
Concentrations in the 35 m samples were fairly uniform throughout the year and
consistently higher than concentrations in 5 meter samples during the period of
April to September 1993. Oligotrophic lakes typically have an orthograde DIC
curve; that is, there is little change in DIC with depth. A clinograde DIC curve, in
which DIC increases sharply with depth, is more typical of eutrophic lakes. This
increase reflects the increase in decomposition in the tropholytic zone and is
typically associated with decreasing pH and increasing alkalinity. While DIC
concentration and alkalinity were not measured at depth intervals sufficient to graph
changes with depth, the higher DIC values recorded for 35 m samples were
associated with a decrease in pH with depth (Figure 17A), as predicted. However,
alkalinity at 35 m (fig. 1 7B) did not vary with DIC and showed little variation
among sample dates. It is possible that the hypolimnetic waters were not sufficiently
acidic to cause dissolution of CaC03 sinking from the surface waters. Also, sorbed
coatings of dissolved organic matter can also reduce CaC03 dissolution. Either of
these could have prevented the expected increase in alkalinity typically associated
with increasing DIC concentrations and decreasing pH.
-------
30
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Figure 1 5. Total phosphorous concentrations (j/g/l P) in the three tributaries
(A) and the south basin of Swan Lake (B) during the period of July 1992 to
November 1993.
-------
33
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to November 1993.
-------
A PH
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Figure 17. pH (A) and alkalinity (B) measurements for the south basin of Swan
Lake during the period of July 1992 to November 1993.
-------
35
Dissolved organic carbon (DOC) varied seasonally within the tributary sites (Fig.
18A), with maximum values during the late spring runoff period. There was no
obvious seasonal trend in the lake (Fig. 18B). Although a large proportion of the
DOC pool is secreted by phytoplankton and littoral flora, these compounds tend to be
quite labile and quickly decomposed. Because DOC secretion and decomposition occur
so rapidly (i.e. < 48 hr), it is difficult to delineate the dynamics of these events
within the sampling time frame of this study. However, major changes in DOC could
indicate a change in source pools (e.g. increased allochthonous DOC input or increased
bacterial chemosynthesis of organic matter, with release of DOC). The sharp
increase in DOC concentration in the 35 m sample recorded in 26 September 1993
was not associated with an increase in allochthonous DOC concentrations (i.e. there
was no change in the tributary DOC levels associated with this increase). It would
appear that this increase in DOC concentration observed at 35 m was associated with
microbial decomposition of organic matter in the hypolimnion.
Nondissolved organic carbon (NDOC) concentrations in the tributaries remained low
throughout the year (Fig. 19A), except for a peak during spring run off. The most
obvious change in NDOC concentration in the lake samples (Fig. 19B) occurred
during the period of November 1992 to September 1993. During this period, NDOC
concentrations for all depth strata were at their lowest immediately before and after
spring runoff, increasing by a factor of 2 to 3 during spring runoff.
Silica
There was no seasonal trend in dissolved silica (Si02) concentrations within the
tributaries other than a slight reduction in concentration during the early summer
(Fig. 20A). For the remainder of the year, Si02 concentration was fairly constant.
Swan River concentrations were consistently lower than those for the other two
tributaries, while Sixmile Creek values were higher. Within the lake depths
sampled (Fig. 20B), there was again a slight seasonal trend in variation, with
minimum concentrations recorded during early summer. Among the depths sampled,
Si02 concentrations were highest in the 35 m samples and lowest in the surface (5
m) samples. Throughout this study, Si02 concentrations were high compared to what
would be expected for high productivity eutrophic lakes (which generally have
epilimnetic concentrations of less than 1 mg/l), and were more similar to
concentrations typical for oligotrophic lakes (which typically have epilimnetic
concentrations between 5 and 10 mg/l SiO}.
-------
10.0
36
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30 meters
Integrated (0-30 m)
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0.0
Figure 1 8. Dissolved organic carbon (DOC) concentrations (mg/l C) in the three
tributaries (A) and the south basin of Swan Lake (B) during the period of July 1992
to November 1993.
-------
37
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Figure 1 9. Non-dissolved organic carbon (ND0C) concentrations (mg/l C) in the
three tributaries (A) and the south basin of Swan Lake (B) during the period of July
1992 to November 1993.
-------
38
Sulfate
There was no distinct seasonal pattern in variation of sulfate concentration for either
the tributaries or the lake (Fig. 21). Among the tributaries, there were distinct
differences in sulfate concentration, with Spring Creek having the highest
concentrations and Sixmile Creek the lowest concentrations. There was no consistent
difference among the depths sampled in the south basin of Swan Lake.
PRIMARY PRODUCTIVITY
Net primary productivity data from the south basin of Swan Lake are presented in Fig.
22. In general, rate of carbon fixation (reported as mg C-m^-hr1) was maximum
during the summer months (June through November, 1992 and 1993), while rates fell
to near zero in mid winter when the lake was ice covered. The seasonal range of values
were similar to those characteristic of oligotrophic lakes.
Fluorescence (Fig. 23) provides a field estimate of the distribution of primary
producers in the water column, with the depth of maximum fluorescence generally
associated with the depth of maximum primary production. Maximum values were
observed during the late summer between 5 and 10m depth, with substantially reduced
values observed above 5 m during the same period. This reduction in surface levels was
most likely a consequence of light saturation of primary production and decreased light
utilization efficiency in the high light environment of the surface waters. A similar
reduction in fluorescence in the surface waters was observed during mid-winter,
probably reflecting a combination of temperature and light inhibition effects at the
water/ice interface. The depth of maximum fluorescence, presented in Fig. 24 with the
pigment data, varied seasonally, with minimum depth of the maxima observed in mid
winter (January 1993) and in early summer (May/June 1993). At these times the
maxima was recorded at approximately 5m. The remainder of the year the maxima
occurred at around 1 5m.
-------
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Figure 21. Sulfate concentrations (mg/l S04) in the three tributaries (A) and the
south basin of Swan Lake (B) during the period of July 1992 to November 1993.
-------
41
f
c5
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o
a
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A S 0 N D
1992
M J J A S 0 N
1993
Figure 22. Primary production rates (mg C-m2-hr) in the south basin of Swan
Lake during the period of July 1992 to November 1993.
-------
Fluorometer Reading
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. 13-Jul
22-Jan
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Figure 23. Fluorescence (fluorometry readings) profiles in the south basin of Swan
Lake during the period of July 1992 to November 1993.
-------
Laboratory measurement of photosynthetic pigments (e.g. chlorophyll) can provide an
estimate of phytoplankton biomass. Because chlorophyllous pigments degrade to the
more stable phaeophytin products, the latter provides a measure of non-biologically
active chlorophyll and can be used to indicate "crashes" or sudden declines in the
phytoplankton community. There was no consistent seasonal trend in phaeopigment
concentration in either the integrated samples or the samples collected at the depth of
maximum fluorescence (Fig. 24A). Nearly all declines in phaeopigments were
associated with declines in chlorophyll concentrations (Fig. 24B). A marked exception
to this pattern was the sharp reduction in phaeopigment concentration during peak
chlorophyll concentrations in late summer 1993. This deviation probably reflects a
situation of prime growth conditions coupled with low cell mortality. Chlorophyll a
concentrations at the depth of maximum fluorescence and in integrated water samples
(Fig. 24B) varied in a pattern consistent with that observed for net primary
production, with the highest concentrations recorded during the periods of June through
November, 1992 and 1993. Chlorophyll a concentrations in the integrated samples
were similar in value to those reported for Flathead Lake during the same time period
(Stanford eta/. 1994).
Examination of phytoplankton samples collected during the course of this study did not
indicate the presence of blue green algae or other bio-indicators of eutrophication in
Swan Lake. There was also no evidence of algal species of stream origin in the lake
phytoplankton samples except during the period of spring runoff (May 1993), as would
be expected under conditions where periphyton is scoured from the stream bed.
-------
44
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Figure 24. Phaeopigment and chlorophyll a concentrations Qvg/I) in the south basin
of Swan Lake during the period of July 1992 to November 1993. Solid bars =
concentration in integrated samples; hatched bars = concentration in samples
collected at the depth of maximum fluorescence; solid line = depth of maximum
fluorescence.
-------
WEATHER STATION DATA
Precipitation Chemistry
Precipitation chemistry data are presented in Figure 25. The variables monitored
showed distinct seasonal patterns and generally maximum concentrations occurred
during the period of July and August 1993, with the following two exceptions.
Nitrate/nitrite concentrations appeared to vary randomly during the sampling period
nor was there a distinct seasonal pattern to pH, which averaged 4.67 (± 0.607 SD) for
the sampling period. In general, concentrations of TP, SRP, N02/3, NH3, and TPN were
substantially lower than what has been recorded for bulk precipitation on Flathead Lake
(Stanford et al. 1992). Maximum concentrations were typically 0.5 to 0.1 times the
maximum values recorded for Flathead Lake.
Weather Data
During the period of October 1992 through October 1994, average seasonal
temperatures (Fig. 26) ranged from mid-winter lows of -25 °C to mid-summer highs
of 25 °C. Wind vector and wind speed data for the same sampling period are presented in
Figure 27. No predominant wind vector pattern (i.e. speed and direction) was revealed
when all data were combined, nor was there a distinct pattern in the monthly wind
vector data (Fig. 28). Wind speed data (Fig. 27B) indicated maximum winds generally
occurred during mid-winter and this pattern was echoed in the monthly wind vector data
(Fig. 28).
BIOLOGICAL OXYGEN DEMAND (BOD)
The results of the BOD study are presented in Figure 29 and in Table 4. There was no
readily apparent pattern nor was there a statistically detectable difference in oxygen
consumption between the light and dark bottles. Rather than indicating a lack of
microbial oxygen consumption, this most likely reflects the limitations in sensitivity of
the oxygen probe, with the changes in oxygen level falling below detection limits. There
was a statistically significant effect of sampling date and sampling depth and a significant
interaction effect between those two variables, but this is more a function of seasonal
and depth changes in the water column and, therefore, the water used to fill the bottles.
-------
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Figure 25. Results of chemical analysis of precipitation collected at the weather
station located at the south end of Swan Lake during the period of July 1 992 to
November 1993.
-------
35
Oct Dec Feb Apr Jun Aug Oct
1992 1993
Dec Feb Apr Jun Aug
1994
Oct
Figure 26. Air temperature data collected at the weather station located at the south
end of Swan Lake during the period of July 1992 to November 1993. Upper line =
daily maxima; lower line = daily minima; open symbols = daily average.
-------
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1994
Figure 27. Wind vectors (upper graph, graphed as km/h, all dates combined) and
wind speed data (lower graph) collected at the weather station located at the south
end of Swan Lake during the period of July 1992 to November 1993.
-------
OCTOBER 1992
20 •
NOVEMBER 1992
20
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Figure 28. Monthly wind vector data collected at the weather station located at the
south end of Swan Lake during the period of July 1992 to November 1993.
-------
51
ISOTOPIC ANALYSES
In addition to their unstable (radioactive) forms, most elements occur in the form of two
or more stable isotopes. The stable forms are generally not in equal abundance (i.e. one
form far exceeds the others in abundance) and their relative abundance (the isotopic
ratio) can serve as an isotopic "signature" for the material analyzed. Stable isotope
ratios are measured by means of isotope ratio mass spectrometry and the data generated,
the natural abundance isotopic signature, can be used to study patterns and process at the
single organism level, within food webs, or through entire ecosystems.
Isotope ratio mass spectrometry measures the ratio of the heavy and light isotopes
within a sample. These ratios are usually quite small and, therefore, their measurement
is prone to error due to such problems as variations in sample preparation and mass
spectrometer fluctuations (Hayes 1982). To offset these problems, the isotopic ratio of
a sample (/?sam) is compared to that for a standard (Rst^ so that variations due to
sample preparation and machine fluctuation will be reflected equally in both the
standard and the sample. For carbon, the stable isotopes of interest are 13C and 12C, with
13C being the heavier isotope, and the reference standard is a marine limestone fossil,
Pee Dee Belemnite (PDB) (Craig 1953). The isotopic ratio (/?) is expressed as the
ratio of the heavier isotope to the lighter. The differences are expressed as "del"((5)
values with units of per mil (%,) and are calculated as follows:
. Rsam - Rstd „
S(%o) = x 1000.
Rstd
Samples that contain more of the heavier isotope have a greater Svalue and are referred
to as being "enriched" and "heavier" than the other samples, which are considered to be
"depleted" and "lighter". Fractionation refers to the enrichment or depletion of the
heavy isotope during chemical or physical processes. For example, reactions of uptake
and loss discriminate against the heavier isotope, resulting in changes in the isotopic
signature of an element with each chemical transformation as it moves through trophic
levels. These changes are useful to ecologists because they occur by way of predictable
physical and chemical discriminations and result in different and specific isotopic
compositions (Ehleringer et a!., 1 986). For more detailed information on stable isotope
theory and application in ecological research, the reader is referred to Lajtha and
Michener (1994) and Rundel eta/. (1989).
-------
This study focused on the carbon isotopic signature of NDOC. Because the transfer of
carbon isotope ratios is conservative between trophic levels, it is very easy to
distinguish between allocation of carbon sources when there are only two major inputs
(in this case, terrestrial and stream comparisons and logged versus unlogged
comparisons) and those sources have distinct ratios (Michener and Schell 1994). If
only one carbon source is important, the isotopic signature of the consumer will be quite
similar to that of the source. If two sources with distinct ratios are equally important,
the isotopic signature of the consumer will be midway between the signatures for the two
sources. In addition, the isotopic composition of organic matter (NDOC) is not readily
altered as it is consumed and decomposed and the isotopic signature persists for as long
as the non-dissolved status persists.
Stable isotope data (<513C) collected during 1993 from Swan Lake and the three main
tributaries are presented in Figure 30. There was a statistically significant difference
among the tributary sites (ANOVA p < 0.001), with Spring Creek <513C values
substantially lower than those recorded for the other two tributaries (Fig. 30A). There
was a significant trend toward decreasing <513C values over the course of the year for all
three lake depths (r2 = 0.427; p < 0.001) but there was no statistically detectable
pattern of difference among depths (ANOVA p > 0.1).
-------
-29-
-30-
-31 -
-32-
1 1 1
July August September October
1993
May
June
-28
-30'
-32'
-34'
-36'
-38 -i 1 ( 1 1 r
May June July August September October
1993
Figure 30. Stable isotope signatures of organic material in whole water samples
collected during the period of May to October 1993 from Swan Lake south basin and the
tributary sites identified in Figure 2.
-------
Figure 31 presents <513C data collected in 1994 from the Goat Creek and Lost Creek
drainages. In both drainages, particulate organic matter <513C values for the unlogged
streams were higher (i.e. isotopically heavier) than for the logged areas. Analysis of
variance (Table 5) revealed statistically significant effects of date, drainage basin, arid
logging (p < 0.001 for each) on <513C. There was a significant interaction between
sampling date and logging effects on 513C (p <0.05) and between sampling date and
drainage basin effects on<513C (p <0.05). The terrestrial (allochthonous) matter was
significantly heavier (isotopically) than stream produced (autochthonous) organic
matter in the North Fork Lost Creek valley (the logged drainage) (Fig. 32).
Autochthonous organic matter produced in the unlogged South Fork Lost Creek drainage
was isotopically heavier than autochthonous organic matter from the logged drainage.
The isotopic signatures of particulate organic matter entering Swan River from the Goat
Creek and Lost Creek drainages and entering Swan Lake from Swan River are shown in
Fig. 33. Analysis of variance indicates a significant effect of date and drainage on the
isotopic signature and significant interaction between those variables (Table 6).
-------
55
O
m
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co
0
"O
lH autochthonous
F2 allochthonous
logged
unlogged
Figure 32. Mean isotopic signatures (± 1 S.D.) of stream produced (autochthonous)
and terrestrially derived (allochthonous) organic matter in the North and South Forks
Lost Creek.
O
CO
-------
STAT 1 ST 1Cfl
summary
of all effects; design:
GENERAL
1-DATE,
2-U3. 3-LOG
MANOUA
df
MS
df
MS
Effect
Effect
Effect
Error
Error
F
p-1 eve 1
1
4
19.50727
50
.6591315
29.59540
.0000000
2
1
42.01595
50
.6591315
63.74440
.0000000
3
1
5.34745
50
.6591315
8.11287
.0063636
12
4
2.29554
50
.6591315
3.48267
.0138329
13
4
1.71742
50
.6591315
2.60557
.0467305
23
1
.27586
50
.6591315
.41853
.5206320
123
4
.68422
50
.6591315
1.03807
.3969732
Table 5. ANOVA table for.effects of sampling date (effect 1), drainage basin (Lost vs.
Goat - effect 2), and logged vs. unlogged stream (effect 3) on isotopic signature of
stream transported organic matter, as per Figure 31.
STflT1 ST 1Cfl
summary
of all effects; design:
GENERAL
1-DATE,
2-SITE
MANOUA
df
MS
df
MS
Effect
Effect
Effect
Error
Error
F
p-1 ewe 1
1
4
8.171671
12
.0344738
237.0398
.0000000
2
2
4.457952
12
.0344738
129.3141
.0000000
12
8
.484075
12
.0344738
14.0418
.0000524
Table 6. ANOVA table for effects of sampling date (effect 1) and stream (GC, SC, and
SR, as identified in Fig. 3) on isotopic signature of stream transported organic matter,
as per Figure 32.
-------
MASS BALANCE CALCULATIONS
Phosphorous and nitrogen loads from the four sources studied (three tributaries and
precipitation) are presented in Figure 34. Daily loading estimates were made by
interpolating between known concentrations of nutrients in the tributaries and
precipitation and multiplying those values by daily tributary flow and precipitation
values. Maximum daily loading occurred during periods of peak flow, reflecting the
large influence Swan River has on lake nutrient balance. Swan River consistently
supplied the majority of the nutrients entering Swan Lake. Daily loading values for
nitrogen and phosphorous were substantially less than what is observed in Flathead Lake
(Stanford et al. 1995). The average daily loss via the Swan River outlet and average
daily input (tributaries and precipitation combined) are presented in Figure 35 and the
daily net change (input - loss) is presented in Figure 36. Total phosphorous input
approximately equaled output throughout the study, with no net change. Nitrogen loss
(TPN and NO2/3) at the Swan River outlet exceeded input during the periods immediately
prior and subsequent to peak flow, while input exceeded loss during peak flow. There
was no net change for the remainder of the sampling period.
Mass balance calculations could not be made for carbon because carbon is not analyzed in
samples collected at the Bigfork site for the Flathead Lake Monitoring Program.
However, particulate carbon (NDOC) loading from the three tributaries could be
calculated and these data are presented in Figure 37. Spring Creek showed little seasonal
variation in NDOC load, although there was a slight increase in loading during the period
of peak runoff. However, the NDOC loads in both Swan River and Sixmile Creek
increased over 14 fold during periods of peak runoff.
-------
50
40
30
20
1 0
90(9
800
700
600
500
400
300
200
1 00
0
500
000
500
000
500
0
Total Phosphorous
Total Nitrogen
I | WW |
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Figure 34. Mean daily load of total phosphorous (kg/d P), nitrate/nitrite (kg/d N),
and total persulfate nitrogen (kg/d N) entering Swan Lake from each of the four
sources studied (three tributaries and precipitation) and the relative contribution of
each source expressed as percent of total daily load.
-------
1600 T
Figure 35. Average daily transport of total phosphorous (kg/d P), nitrate/nitrite
(kg/d N), and total persulfate nitrogen (kg/d N) leaving Swan Lake via the Swan
River (upper graph) and entering Swan Lake (lower graph) via the four sources
studied.
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61
Figure 3G. Average daily balance of total phosphorous (kg/d P), nitrate/nitrite
(kg/d N), and total persulfate nitrogen (kg/d N) entering and leaving Swan Lake
during the period of October 1992 to November 1993..
-------
25 T
05
i 20 --
Q
<
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z
o
CO
QC
<
o
111
D
O
H
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15 --
10
5 --
° Sixmile Creek
— Spring Creek
ASONDJ FMAMJ JASON
1992
DATE
1993
O
<
o
8 o
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< 2
s-
§ 1)
3 &
O
H
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<
Q.
1992
DATE
1993
Figure 37. Particulate carbon (NDOC) load (kg/d C) entering Swan Lake via the
Sixmile and Spring Creek (upper graph) and Swan River (lower graph) during the
period of August 1992 to November 1993..
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63
ANALYTICAL ACCURACY AND QUALITY CONTROL
Quality control tests were conducted on approximately 1 out of every 1 5 samples
analyzed in the Freshwater Research Laboratory (FRL) at the Biological Station.
Analytical precision was determined by ± 1 sd of replicated analyses of individual
samples. Analytical accuracy was determined by 110% > X > 90% recovery of a known
standard solution (X) added to selected samples. Performance of laboratory personnel is
evaluated twice yearly by analysis of quality control samples provided by the U.S.
Environmental Protection Agency. Correct analysis exceeds 90% for the samples
provided by the EPA despite the fact that the concentrations of the unknowns provided by
the EPA were typically more than 10 times more concentrated than the solute-poor lake
samples routinely analyzed by the FRL. Most of the error in analyzing the unknowns can
be attributed to inaccurate dilution of the samples into the FRL's normal working range,
which is near the detection limits of the analytical methods. Results of analytical
accuracy and quality control tests (i.e. laboratory standard curves, quality control
information, and performance evaluations) are on file at the Biological Station and are
archived in the Biological Station's data storage and retrieval system.
CONCLUSIONS
1. Water clarity in Swan Lake appears to be influenced primarily by the influx of
sediment laden river water during spring runoff. Whereas decreases in water
clarity in lakes characterized by high nutrient loads and high productivity tend to
coincide with seasonal increases in phytoplankton density (and primary .
productivity), the decreases in water clarity observed in Swan Lake coincided with
increased suspended sediment input from Swan River and not maximum
productivity. Periods of maximum water clarity in Swan Lake (e.g. Secchi depth
maximum, minimum light attention) occurred during periods of maximum
productivity and chlorophyll a concentration. In addition, Secchi depths recorded in
Swan Lake were consistently within the range characteristic of oligotrophic lakes
(Wetzel 1983). According to Carlson's Trophic State Index (Carlson 1977), Swan
Lake has an index ranking of between 20 and 30. Using Secchi depth, total
phosphorous, and chlorophyll measures, this index ranks lakes on a scale of 0 to
100, with values between 0 and 30 encompassing the trophic range of ultra-
oligotrohic (0) to oligo-mesotrophic (30). Thus, Swan Lake falls within the range
typical for oligotrohic lakes. It should be noted, however, that Carlson (1977)
cautions against using the index as a measure of water quality and critics of the
-------
index (e.g., Lorenzen 1980; Megard et al., 1980; Edmonson 1980) argue that it is
ambiguous and should not be used in making management strategies.
In addition to possibly compromising the aesthetic appeal of the lake as a
recreational resource, low water clarity may impact lake food web dynamics on two
levels, inhibiting primary production by limiting light availability in the water
column and by interfering with feeding by crustacean zooplankton (Butler, 1995).
Because seasonal variation in water clarity in Swan Lake is an externally driven,
rather than internally produced, phenomenon, it is important that water resource
managers be sensitive to upstream land use practices that will affect sediment loads
to the source tributaries. Sedimentation records for Whitefish Lake, Swan Lake,
and Lake McDonald have suggested increased sediment accumulation rates associated
with periods of road building and logging activity (Spencer 1991 b). This
relationship is particularly evident in Swan Lake, where the highest sedimentation
rates were associated with a doubling of logging activities in the basin during the
1980's. Similarly, Hauer and Blum (1991) have reported increases in stream
nutrient and sediment loads that were correlated with increased logging activity.
Primary productivity levels in Swan Lake approached maximum rates towards the
end of spring runoff. These increases in productivity were most likely driven by a
combination of events. During peak runoff, the concentrations of biologically
available nutrients in the water column were at their seasonal maxima while water
clarity was at minimum values. Under these conditions, primary production was
most likely limited more by light availability than by nutrient availability. Thus,
while there was a substantial increase in input of biologically available nutrients
during the periods of peak flow, phytoplankton production did not appear to be
immediately responsive to this input.
The daily rates of primary production and chlorophyll a concentrations observed
during this study concur with values characteristic of oligotrophy lakes (Wetzel
1983). In addition, the concentrations of nutrients were consistently within the
range associated with oligotrophic lakes (Wetzel 1983). Furthermore,
examination of phytoplankton samples indicated bluegreen algae (which are
typically associated with eutrophication) were either absent or present in very low
numbers throughout the year. Thus, by these measures, Swan Lake appears to
typify an oligotrophic lake.
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65
3. Dissolved inorganic carbon (DIC) is the primary carbon source for photosynthetic
algae and aquatic macrophytes. Natural sources include respiratory production,
atmospheric transfer, and incoming water. In the 5 m samples, DIC concentrations
were low during periods of peak primary production while the reverse was true
during periods of reduced primary production (e.g. September 1992 and winter
1992/93). Given that there was no concurrent change in input from source pools,
these changes most likely reflected greater in situ respiratory production of CO2
by the lake biota rather than increased input from external sources. This point will
become more relevant in the discussion of the stable isotope data later in this
report.
Nearly all organic carbon in aquatic ecosystems is present either as dissolved
compounds (DOC) or as particulate carbon (NDOC). Whereas little dissolved
organic material is utilized directly, particulate organic matter is often a major
food source to the aquatic fauna, eventually being converted to soluble organic
compounds by specialized microflora. The peak in NDOC input from the tributaries
during spring runoff most likely reflects a combination of eroded terrestrial
organic matter and scoured stream produced material. The abundance of stream-
originating algae (periphyton) in whole water samples collected from Swan Lake
during peak runoff is indicative of stream scouring, whereas periphyton were
absent from samples collected in June (post runoff) and September.
Two weeks after the peak in NDOC (May 26) there was an increase in dissolved
organic carbon concentration, most likely reflecting in situ decomposition of
particulate carbon into dissolved carbon compounds. The peak in DOC observed in
the hypolimnetic samples (26 September 1 993) coincided with the onset of the
oxygen deficit, which reaches its maximum by the subsequent sampling date (9
October). The coincident timing of the DOC pulse and the oxygen decline soon after
the peak in NDOC concentrations in the hypolimnion suggests the NDOC deposited in
the hypolimnion was the primary source of organic particulate matter, with the
peak in DOC and subsequent oxygen depletion reflecting microbial breakdown of that
organic matter.
4. The trend for <513C values to become more negative over the course of the summer in
samples taken from 5 and 1 Om in Swan Lake suggests phytoplankton were utilizing
-------
13C-deplete CO2 produced by respiration of metabolized isotopically light organic
matter. These data are consistent with observations made in 1 993 and 1994, which
indicate that the isotopic signature of particulate organic carbon carried into Swan
Lake from Swan River and the Goat Creek and Lost Creek drainages became more
negative over the course of the summer.
Because organic matter from logged areas was isotopically lighter (i.e. more
negative) than that from unlogged streams, the trend of decreasing513C signatures
observed over the summer both in the streams as well as in Swan Lake suggests
temporal variation in the relative input of organic material derived from logged and
unlogged drainages. Neither the allochthonous nor the autochthonous organic
material varied seasonally in <513C signature. In addition, decomposition does not
substantially alter the <513C signature of the remaining (i.e. non-decomposed)
organic matter (Fry & Sheer, 1984), further reducing the likelihood that the
decrease in <513C signatures was a consequence of signature variation of the organic
material. It is possible that seasonal variation in groundwater input could alter the
signature within the stream biota (if groundwater-originating CO2 had an
isotopically distinct signature), but again, there was no indication that the isotopic
signature of the autochthonous material changed over the course of the study. The
similarities in isotopic signature between allochthonous and autochthonous material
produced within a drainage possibly indicates a close link in carbon cycling. In situ
decomposition of terrestrial material would produce 1 3C deplete CO2 which in turn
could be utilized by primary producers in the stream. Thus, the stream produced
(autochthonous) organic material would have lower <513C signatures as a
consequence of using the ^C deplete CO2 as a carbon source.
There is no evidence to support the remote possibility that the oxygen deficit in
Swan Lake is caused by entrainment of groundwaters within the deep basins. The
HydroLab© profiles of temperature do not suggest entrainment of cold groundwater
and the oxygen profiles provide no evidence of oxygen deplete groundwater entering
the basin. The seasonal temperature profiles of the lake indicate that both basins
are dimictic, undergoing complete turnover and mixing each fall and spring. In
addition, there is no suggestion of a groundwater source of nutrients, including DOC,
stimulating microbial production and respiration, based upon the depth profiles of
nutrient concentrations. Furthermore, there was no evidence of septic contaminated
-------
67
groundwater entering the lake during the course of this study, based upon shoreline
surveys.
6. The oxygen deficit in the south basin of Swan Lake appears to be driven by a
combination of events, but ultimately from allochthonous carbon inputs. Spencer
(1991a) concluded in his study of Swan Lake that allochthonous input of organic
matter could be a contributing factor to the observed oxygen deficit. Isotopic
evidence suggests that the majority of the organic matter decomposed in the lake
originates from outside the lake. In addition, streams in areas recently logged,
which are characterized by isotopically light particulate organic matter, may
provide a substantial proportion of the organic carbon that is decomposed in the
lake. The morphology of the lake may exacerbate the situation, trapping water in the
deep basins and isolating from oxygen replenishing overlying waters during periods
of stratification.
7. If the hypolimnion of Swan Lake goes to complete anoxia, the result will be sudden
release of nutrients from the sediments into the water column. This is a scenario
that has been well documented in Lake Erie (Braidech et al. 1971; Charlton and
Lean 1 987; Mortimer 1971) and could very well occur in Swan Lake. The
potential impact of this release can occur on many levels. First, the nutrients will
in effect fertilize the lake, resulting in changes in productivity, algal community
structure, and higher trophic levels. In a worst case scenario, total collapse of the
resident fish communities could result. In addition, the enhancement of nutrients in
the lake will result in increased outflow of nutrients into the Swan River, nutrients
which will alter the river community in a manner similar to that predicted for the
lake. Nutrients which enter Flathead Lake via the Swan River could result in algal
blooms and oxygen depletion problems similar to what has been reported for Big
Arm Bay (Stanford et al. 1994). Thus, the potential ramifications of a decline in
water quality in Swan Lake extend far beyond the perimeter of the lake itself and the
effects could well be manifest in Flathead Lake as well.
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68
RECOMMENDATIONS
Based upon the results of this study, the following recommendations are made with
respect to monitoring for changes in water quality in the tributaries of the Swan River
catchment and for preventing further degradation of water quality in Swan Lake.
1. A specific objective should be to encourage community involvement through a
volunteer stream monitoring program. At present, the local group "Friends of the
Wild Swan" is the most active water quality "watchdog" in the Swan basin. The
"Adopt A Stream" program in the Montana River Action Network would also be a
suitable mechanism for coordinating community members with interest in specific
drainages throughout the Swan River catchment. The value of such citizen groups is
reflected in the success of the Volunteer Lake Monitoring Program, which is a
tribute to the dedication of volunteers with an interest in water quality issues.
2. One area that has been overlooked in research on Swan Lake is the bay at the south
end of the lake. This bay, which is subject to intense development as well as
recreational use, is essentially a back water zone with minimal flushing influence
from Swan River flow and may function as a nutrient and organic matter sink for
material entering from the south shore wetland via Spring Creek. This scenario is
similar to what has been observed in Big Arm Bay, in Flathead Lake, where high
productivity, high nutrient concentrations, and a deep water oxygen deficit appear
to be related to the physical characteristics of the bay (Stanford et al. 1994). A
major concern is that, while the influences on water quality will initially be
concentrated in this bay area, they may ultimately have a substantial impact on the
main body of the lake with a more catastrophic, rather than gradual, effect. At the
minimum, there should be preliminary studies of the nutrient dynamics in this bay
and the degree to which the water in the bay interacts with the main lake.
3. A more detailed investigation of source pools of detrital material is warranted. The
results presented here indicate decomposition of organic carbon from external
sources affects deep water oxygen levels more than organic matter produced in the
lake. Because of the influence of these external organic carbon sources, it is
important to identify the extent to which specific upstream land use practices affect
the instream organic carbon load. Lake phytoplankton samples and isotopic evidence
indicate that two processes act together to influence organic loading of the
tributaries and the lake. During spring runoff, there is a sufficient amount of
-------
69
scouring of the stream and river bed to permit detection of stream-originating
diatoms in the lake water column. Although there is a higher periphyton standing
crop in streams flowing through logged watersheds (Hauer & Blum 1991), isotopic
analysis does not provide evidence that autochthonous material comprises the
majority of stream transported organic matter in logged watersheds. On the other
hand, isotopic analysis does indicate that organic matter derived from logged areas
comprises an ever increasing proportion of the organic matter in the streams and
river over the course of the summer, perhaps reflecting increased input due to
surface runoff and transport effects as well as loss of senescent stream periphyton.
Thus, these results suggest areas impacted by logging may supply a substantial
portion of the organic carbon load in Swan River and the origin of these materials
and the factors affecting their abundance may vary seasonally.
4. One of the most technically and politically difficult decisions resource managers face
is assessing the sensitivity threshold of an ecosystem (Potts 1 991 a). However,
given that decomposition of organic matter from external sources is the primary
cause of the oxygen deficit in Swan Lake, it is necessary to determine the sensitivity
of the lake to organic carbon loading. Presently the south basin reaches oxygen
levels that hover on the brink of anoxia. It is not known how great an increase in
the organic carbon load would result in decomposition leading to total oxygen
depletion, but once total depletion occurs, release of trapped nutrients from the
sediments will most surely result. This is an area that deserves more detailed
research if we are to make informed decisions about future land use practices in the
Swan valley and their potential impact on our water resources.
5. Carbon loading rates from selected logged and unlogged watersheds need to be
measured and extrapolated throughout the Swan Catchment (on a per-acre basis) to
estimate the overall impact of logging. The lack of accurate information on the
nature and extent of logging activities in the Swan valley and the lack of a historical
limnological data base make it difficult to assess the present day and long term
effects of logging activities on water quality in Swan Lake. It is important to
emphasize that degradation of water quality has already occurred on Swan Lake, as
indicated by the dissolved oxygen deficit. The issue is not to prevent a decrease in
water quality, but rather to prevent any further degradation of water quality and, if
possible, to reverse the degradation that has already occurred.
-------
Assuming that logging activities in the Swan valley are conducted according to BMP
guidelines, those practices may be insufficient to reverse downstream degradation of
water quality. While on site monitoring of logging activity may not indicate any
problems, the cumulative downstream effects are potentially devastating for water
quality in Swan Lake. If the majority of organic material entering Swan Lake and
causing the oxygen deficit is originating from logged drainages, as this study would
suggest, then an obvious goal is to take steps to reduce transport of organic material.
Halting all logging activity is not compatible with the multiple uses of forest lands
in the Swan Lake catchment, therefore we must search for alternative ways to
modify and minimize the cumulative impact of logging and other activities associated
with timber harvest (such as road building). One possible suggestion would be to
increase the width of the unlogged buffer zone that parallels the streams beyond
current BMP widths. This would have two effects: one would be decreased light
penetration and the other would be a greater zone of stable vegetation along the
stream bed. The former effect would result in decreased instream organic
production and, therefore, decreased organic material to be scoured and transported
during peak runoff. The second effect would result in greater stream bank stability
and therefore, a decreased likelihood of stream bed erosion in areas where this is a
problem. In addition, there would be increased surface area for trapping organic
matter transported via surface run-off before it enters the streams.
While it would be advantageous to gather data at each logging site, it would be
difficult to implement an onsite monitoring program to follow individual logging
activities. There are economic as well as technical problems associated with
implementing such a program. It would also be difficult to devise a mechanism by
which transport of terrestrial organic matter could be tracked downstream from the
disturbance site to the lake. Downstream transport of organic matter of terrestrial
origin most likely occurs over time as a series of deposition and resuspension
events, rather than in a continuous flow to Swan Lake. While the surface runoff
effect of a terrestrial disturbance event might dissipate after one or two years, the
instream transport of organic material may occur over a number of years, as
material gets trapped in sand/gravel bars to be resuspended and transported in
subsequent high flow events. Thus, it is quite possible that the full impact of an
individual logging event may not reach the lake until considerable time has passed.
However, it may be feasible to implement a monitoring program on a larger scale,
-------
with a design that permits comparison of logged subbasins with unlogged subbasins.
On such a large scale, downstream transport of materials could be tracked using
labeling techniques to identify the relative input of potential source pools. In
addition, the effects of logging on such processes as nutrient cycling between
terrestrial and stream ecosystems could be determined.
SUMMARY
While the majority of the limnological parameters studied on Swan Lake during this
study correspond with values typical in oligotrophic lakes, the seasonal oxygen deficit in
the south basin remains a primary concern and should not be ignored. The oxygen
deficit is evidence that degradation of water quality has already occurred on Swan Lake.
At issue is not only a continuation of best management practices and other efforts to
prevent decreased water quality, but also prevention of further degradation of water
quality and, if possible, reversal of the degradation that has already occurred. Given that
this study identifies upstream influences as having a primary influence on the lake in
terms of water clarity and production of the hypolimnetic oxygen deficit, the impact of
upstream land use should remain a major concern in efforts to maintain water quality in
Swan Lake.
ACKNOWLEDGEMENTS
The authors would like to thank the following people for their assistance in various
phases of this project: Kristin Olson (laboratory and field assistance and chemical
analyses), Geoffrey C. Poole (data management), and Dale Chess (field assistance). The
authors would also like to thank Loren Bahls, Randall Apfelbeck, Christian Levine, and
David Rathke for their many helpful comments on earlier drafts of this document.
-------
LITERATURE CITED
Braidech, T. E., P. E. Gehring, and C. 0. Kleveno. 1971. Biological studies of oxygen
depletion and nutrient regeneration processes in the Lake Erie central basin. Proc.
14th Conf. Great Lakes Res. 1971: 805-817.
Butler, N. M. 1994. A diagnostic investigation of nutrient loading in Whitefish Lake,
Montana. Summary Report, Montana Water Resources Center.
Butler, N. M. 1995. Effects of sediment loading on food perception and ingestion by
freshwater zooplankton. Mar. Freshw. Behav. Physiol. (In Press)
Carlson, R. E. 1977. A trophic state index for lakes. Limnol. Oceanogr. 22: 361-369.
Charlton, M. N. and D. R. S. Lean. 1987. Sedimentation, resuspension, and oxygen
depletion in Lake Erie (1979). J. Great Lakes Res. 13: 709-723.
Craig, H. 1953. The geochemistry of the stable carbon isotopes. Geochem. Cosm. Acta 3:
53-92.
Edmondson, W. T. 1 980. Secchi disk and chlorophyll. Limnol. Oceanogr. 25: 378-379.
Ehleringer, J.R., P. W. Rundel, and K. A. Nagy. 1986. Stable isotopes in physiological
ecology and food web research. Trends Evol. Ecol. 1: 42-45.
Ellis, B. K. and J. A. Stanford. 1986. Bioavailability of phosphorous fractions in
Flathead Lake and its tributary waters. Open File Report #091-86. Flathead Lake
Biological Station, The University of Montana, Poison, Montana.
Ellis, B. K. and J. A. Stanford. 1988a. Nutrient subsidy in montane lakes: fluvial
sediments versus volcanic ash. Verh. Inter. Verein. Theor. Ang. Limnol. 23: 327-
340.
Ellis, B. K. and J. A. Stanford. 1988b. Phosphorous bioavailability of fluvial sediments
determined by algal assays. Hydrobiologia 160: 9-18.
Flathead Basin Commission, "1986. Biennial Report. State of Montana, Governor's
Office. Helena, Montana.
Flathead Basin Commission, 1988. Biennial Report. State of Montana, Governor's
Office. Helena, Montana.
Flathead Basin Commission, 1 991. Biennial Report. State of Montana, Governor's
Office. Helena, Montana.
Hauer, F.R. 1991. An analysis of the effect of timber harvest on streamflow quantity
and regime: an examination of historical records. Flathead Basin Forest Practices
Water Quality and Fisheries Cooperative Programs, Flathead Basin Commission and
Flathead Lake Biological Station Open File Report Number 112-90.
Hauer, F. R. and C. 0. Blum. 1991. The effect of timber management on stream water
quality. Open File Report #121-91. Flathead Lake Biological Station, The
University of Montana, Poison, Montana.
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73
Hayes, J. M. 1982. Fractional et at an introduction to isotopic measurements and
terminology. Spectra 8: 3-8.
Lajtha, K. and R. H. Michener(eds) 1994. Methods in Ecology: Stable Isotopes in
Ecology and Environmental Science. Blackwell Scientific Publications, Oxford. 316
pp.
Lorenzen, M. W. 1980. Use of chlorophyll-Secchi disk relationships. Limnol.
Oceanogr. 25: 371-372.
Megard, R. 0., J. C. Settles, H. A. Boyer, and W. S. Combs, Jr. 1980. Light, Secchi
disks, and trophic states. Limnol. Oceanogr. 25: 373-377.
Michener, R. H. and D. M. Schell. 1994. Stable isotope ratios as tracers in marine
aquatic food webs. pp. 1 38-1 57 In: (K. Lajtha and R. H. Michener, eds) Stable
Isotopes in Ecology and Environmental Science. Blackwell Scientific Publications,
Oxford.
Mortimer, C. H. 1971. Chemical exchanges between sediments and water in the Great
Lakes - Speculations on probable regulatory mechanisms. Limnol. Oceanogr. 16:
387-404.
Potts, D. 1991a. A forest management nonpoint source risk assessment geographic
information systems application. Flathead Basin Cooperative Program Final Report
1991: 1 15-124.
Potts, D. 1 991 b. Application of the Sequoia method for determining cumulative
watershed effects in the Flathead Basin. Flathead Basin Cooperative Program Final
Report 1991: 105-111.
Rundel, P. W., J. R. Ehleringer, and K. A. Nagy (eds) 1989. Stable Isotopes in
Ecological Research. Ecological Studies Vol. 68. Springer-Verlag, New York. 525
PP-
Spencer, C. 1991 a. Comparative limnology of Swan Lake and Flathead Lake,
northwestern Montana. Open File Report #126-91. Flathead Lake Biological
Station, The University of Montana, Poison, Montana.
Spencer, C. 1991 b. Evaluation of historical sediment deposition related to land use
through analysis of lake sediments. Open File Report #123-91. Flathead Lake
Biological Station, The University of Montana, Poison, Montana.
Stanford, J. A., B. K. Ellis, D. W. Chess, J. A. Craft, and G. C. Poole. 1992. Monitoring
water quality in Flathead Lake, Montana. Open File Report #128-92. Flathead Lake
Biological Station, The University of Montana, Poison, Montana.
Stanford, J. A., B. K. Ellis, D. G. Carr, G. C. Poole, J. A. Craft, and D. W. Chess. 1992.
Diagnostic analysis of annual phosphorous loading and pelagic primary production in
Flathead Lake, Montana. Open File Report #132-94. Flathead Lake Biological
Station, The University of Montana, Poison, Montana.
Stanford, J. A., B. K. Ellis, and G. C. Poole. 1 995. Influences of nitrogen and
phosphorous loading on water quality in Flathead Lake, Montana. Open File Report
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#1 34-95. Flathead Lake Biological Station, The University of Montana, Poison,
Montana.
Wetzel, R. G. 1983. Limnology. Saunders. 767pp.
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75
APPENDIX
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DIC
DOC
NDOC
NH3-N
N02/3
SI02
S04
SRP
TP
TPN
TSS
TURB
Sixmile Creek
Mean
9.93
2.09
0.43
8.15
14.54
9.27
0.92
7.1 7
15.94
87.96
9.60
1 .86
(sd)
(2.60)
(1.66)
(0.62)
(15.87)
(12.36)
(0.82)
(0.29)
(2.51)
(12.36)
(1 17.08)
(19.44)
(3.84)
min
1 .83
0.70
0.10
1.00
2.00
8.16
0.51
3.76
7.91
26.53
1 .49
0.19
max
13.11
7.51
2.68
69.48
51.21
1 1 .85
1.87
14.49
60.76
562.07
53.67
13.00
Spring Creek
Mean
43.94
2.65
0.26
7.1 7
27.10
8.38
2.13
1.16
5.17
127.15
1 .52
0.74
(sd)
(5.16)
(1.48)
(0.08)
(3.73)
L(26.04)
(0.95)
(0.40)
(0.70)
(2.22)
(48.15)
(2.62)
(0.40)
min
35.47
0.63
0.16
4.77
2.00
7.01
1 .48
0.62
1 .72
65.13
0.50
0.32
max
52.99
7.1 1
0.46
19.33
73.98
10.62
2.84
3.51
9.94
225.58
7.45
1 .60
Swan River
Mean
24.05
1.63
0.30
5.65
40.64
6.26
1.33
0.97
7.81
90.63
17.69
4.22
(sd)
(3.26)
(0.80)
(0.49)
(3.96)
(28.83)
(0.95)
(0.19)
(0.46)
(10.91)
(68.72)
(30.77)
(9.92)
min
18.25
0.89
0.09
1.67
13.02
4.50
1 .03
0.42
1 .65
46.62
1 .60
0.38
max
28.89
3.47
2.17
19.50
126.21
7.45
1.65
2.38
48.98
355.19
86.91
36.50
South Basin
Mean
24.18
2.1 1
0.23
25.95
45.34
6.53
1.40
0.94
6.17
1 16.86
1 .26
2.13
(0-30 m)
(sd)
(1.63)
(0.82)
(0.05)
(82.50)
(11.95)
(0.62)
(0.19)
(0.59)
(1.63)
(35.28)
(0.72)
(5.65)
min
22.27
0.98
0.12
1.67
23.26
5.67
0.88
0.40
4.10
74.48
0.50
0.36
max
26.82
4.71
0.35
345.95
58.18
7.50
1 .75
2.75
9.46
227.84
2.18
24.00
South Basin
Mean
24.88
1.59
0.26
4.95
25.04
6.20
1.26
0.94
6.16
95.75
2.00
2.25
(surface)
(sd)
(3.04)
(0.56)
(0.07)
(2.26)
(22.05)
(0.91)
(0.27)
(1.30)
(2.02)
(33.61)
(1-28)
(5.61)
min
20.01
0.79
0.1 1
1.34
2.00
4.87
0.37
0.40
3.18
51 .79
0.90
0.32
max
31.50
2.85
0.38
10.70
61.51
7.54
1.53
5.78
10.48
165.76
4.16
23.00
South Basin
Mean
25.48
2.27
0.18
7.12
90.19
7.65
1.50
1.09
6.95
145.58
0.99
1 .49
(bottom)
(sd)
(2.16)
(1.96)
(0.04)
(5.65)
(36.72)
(0.73)
(0.17)
(0.86)
(1.48)
(36.03)
(0.50)
(2.98)
min
20.59
1.05
0.10
1.23
30.19
6.61
1.03
0.40
4.33
98.54
0.50
0.47
max
27.89
9.68
0.24
25.71
149.65
9.00
1.80
4.20
8.99
204.50
1.64
13.00
Precipitation
Mean
403.06
137.36
0.67
46.24
111.09
676.54
(sd)
(530.74)
(50.39)
(0.33)
(86.27)
(181.88)
(698.99)
min
41.39
55.67
0.25
0.40
5.21
1 89.70
max
1849.73
235.23
1 .33
311 .12
587.46
2941.81
Le A-l: Summary statistics for water chemistry analyses for fcaries. Swan Lake south basin, and precipitation.
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