LAS VEGAS BAY STUDY
1973
Report to the Enforcement Division
of the U.S. EPA, Region IX
Prepared by Milton G. Tunzi
Laboratory Support Branch
Surveillance and Analysis Division
EPA, Region IX, San Francisco
Acknowledgement: The assistance of the Monitoring Operations
Laboratory of the National Environmental Research Center of
Las Vegas and of the U.S. National Park Service of Lake Mead
is gratefully appreciated.
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17486
LAS VEGAS BAY - FIELD STUDY
Introduction
A one-day survey of Las Vegas Bay and of the inflow into
the- Bay (called the Las Vegas Wash) was made on August 30, 1973.
The purpose of the survey was as follows:
1. To determine if algal bloom problems presently exist in
Las Vegas Bay and to quantify the extent of the bloom.
2. To gather nutrient data that would show whether or not
nutrients are in excess for algal growth.
3. To collect data needed for establishing water quality
standards.
Rationale for Sampling Site and Parameter Selections
Five sampling sites were chosen in Lake Mead which would
show the magnitude of any algal blooms relative to the Las Vegas
Wash inflow. Thus/ the first station was in Las Vegas Bay
about one mile from the Wash, and the last near Saddle Island
outside of Las Vegas Bay proper. Samples were also taken in
the Wash near a highway crossing.
The sampling parameters in the Wash flow were those which
would be measures of its sewage and industrial effluents. Those
in Lake Mead were nutrients important to algal growth, physical
and chemical measurements affecting or related to algal concen-
tration, and biological measurements indicative of algal mass.
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2.
Field and Laboratory Procedures
The sampling in Las Vegas Bay was done on August 30, 1973
from a boat furnished by the U.S. National Park Service. An
aerial photograph was taken by EPA NERC personnel during the
same time period the boat sampling was being carried out. In
addition, photographs of the water were taken from the boat
at each station.
The first measurements of the water made at each station
in Las Vegas Bay were temperature and Secchi disc depth. Then
three samples were taken from each sampling depth, composited,
and mixed well. Separate portions were filtered for chlorophyll
extraction, preserved with 4% formalin for algal count, pre-
served with HgCl2 for N and P analyses and put into an ice
chest for subsequent bioassay. The filtering for chlorophyll
extraction was done on the boat at each station, with the
filters stored in dessicant jars which were then put into dry
ice. The extraction of the filters was carried out at the
Alameda, California EPA Laboratory, with optical density readings
taken with a Beckman DU and the formulas of Strickland and
Parsons (1970) utilized for the calculation of chlorophyll a
concentrations. In vivo readings were taken with a Turner
Model III Fluorometer. Bioassay procedures are described in
Region IX, Biology Laboratory Methods (1974) .. The samples
were incubated under constant light (400 foot-candles) and
temperature conditions (20°C) and algal concentration determined
when growth had reached a plateau. The samples were assayed
with and without nutrient additions. The nutrients were added
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3.
in separate series of 1.0 mg NOs-N/1 and 0.2 mg P04-P/1.
These concentrations would make these limiting nutrients in
excess of needs for algal growth.
Samples and field measurements in the Las Vegas Wash were
taken twice: on the evening of 8/29/73 and on the morning of
8/30/73. Oxygen and temperature were measured with an IBC
oxygen meter. All other parameters were determined after the
samples had been brought to the Alameda Laboratory. These
were done following EPA procedures (1971). Personnel at the.
marinas and the rangers who patrol the lake were questioned
regarding that summer's algal bloom.
Results
Significant features of the data in Table 1 are the higher
surface water temperatures in the protected part of Las Vegas
Bay near Las Vegas Wash (29°C), compared to those at the other
stations (25-27°C), nearly equal and low concentrations of
ortho-phosphate at all stations (-<0.01-0.02 mg P/l) , contrasting
with declining values for total phosphorus from the inflow with
declining values for total phosphorus from the inflow station
(0.06 mg P/l) to that furthest from the Wash (0.02 mg P/l).
Total Kjeldahl nitrogen values were between 0.29-0.60 mg N/l
for all stations with considerable variations in the values from
the samples taken at the two depths. Nitrate values were highest
at the Saddle Island Station (0.05 and 0.06 mg-N/l).
The results of the chemical, physical and biological
determinations are shown in Tables 1, 2, 3 and 4.
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4.
Table 1 gives the analyses for the five (5) lake stations;
Table 2 lists the water quality measurements made on the lake
inflow; Table 3 shows algal species and counts at the lake
station; and Table 4 gives the bioassay responses.
The sampling locations are shown and identified in Figure 1.
A large aerial photograph taken by the'Las Vegas EPA Western
Environmental Research Laboratory has the stations numbered
corresponding to those shown in Figure 1. This as shown reduced
in Figure 2. (This photograph is not an integral part of
each copy of the report. It will accompany the copy of the
report sent to Region IX's Enforcement Division). An example
of the photographs taken from the boat of the water at each
station is attached in Figure 3.
The inflow into Las Vegas Bay (Las Vegas Wash) had expected
high values of orthophosphate (*3.5 mg P/l) and nitrate («8.7
mg N/l) CTable 2) . There was very little of either nitrogen
or phosphorus in an organic form. The water was well-oxygenated.
It was visually turbid, with samples having the indicated
Jackson Turbidity Units (25 and 27).
The algal counts were between approximately 3000-8000
cells/ml and roughly approximated the chlorophyll concentration
(Table 3). The algal species were mostly blue-greens or diatoms.
Dominant species were Oscillatoria, Cyclotella, and Stephanodiscus,
There was a noticeable increase in the presence of Navicula in
stations furthest from the Wash.
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5.
The bioassay responses given in Table 4 are the initial
sample chlorophyll a_ concentrations and the terminal
chlorophyll a_ concentrations of samples without added nutrients
and those with 0.10 mg N03-N/1 and 0.2 mg P04-P/1 additions.
The stations closest to the Wash ( No. 1 and 2) clearly showed
responses to the additions of NO3-N. There were no differences
in the other samples at the 95% confidence level between the
unspiked ones and those with either NO3-N or P04-P additions.
The photographs of the water taken from the boat gave a
quasi-uniform view of the algal population so that the photo-
graphs at the five stations appear similar. The aerial photo-
graph shows changes in water color which are entirely missed
by the ground photographs. In addition, the high in vivo
fluorescence near the Black Island station (No. 4) probably
results from material seen in the aerial photograph to be in
drifts at that station.
The persons questioned about the algal bloom differed
in their responses. Those near the Boulder Marina (near
Saddle Island) said they experienced no algal problems whereas
those at the Las Vegas Marina said the algal blooms were the
worst they had ever experienced.
Discussion
The algal population was easy to quantify by chlorophyll
extraction, however, the algal species composition should not
be overlooked. Thus, blue-greens were very much in evidence
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6.
at most of the stations, diminishing in concentration near
Saddle Island. In 1967, algal counts were less than 2000/ml
at Saddle Island and in the unpolluted parts of Lake Mead
(Anon, 1967). Counts made in 1970 of water from the Boulder
Canyon area of Lake Mead were approximately the same (Anon, 1970)
Counts made during this survey yielded higher cell concentra-
tions of about 3000 cells/ml. According to the 1967 FWPCA
report, Saddle Island was free of Wash influence, but judging
«
from the algal counts, does not appear to be so now. However,
the proportion or concentration of blue-greens was not given
in the previous studies. They were very much in evidence at
all stations during this survey and it would have been useful
to have them counted in previous surveys.
Las Vegas Bay nutrient concentrations were not easy to
relate to the algal concentrations. It would be expected
that algae biomass would have a relation to organic phosphorus
and organic nitrogen concentrations. The organic fraction of
total phosphorus was greatest near the Wash and decreased in
magnitude at the stations farthest from its influence. Although
most phosphorus in the Wash was in an oxidized form, the
concentration of organic phosphorus in the stream was still •"
five-fold higher than in the station closest to the Wash
inflow, 0.2 mg P/l and 0.04 mg P/l respectively. In contrast,
total Kjeldahl nitrogen values were approximately equal in the
Wash and in Las Vegas Bay.
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7.
Inorganic nutrients most readily used by the algae,
N03-N and PO^-P, were in approximately a 1:1 ratio at the
two stations closest to the Wash. At the other stations N and
P were at a 4-6:1 ratio. This would not include NHs-N which was
not measured. Based on algal content of N and P (Mackenthun
and Ingram, 1967) , nitrogen should be limiting at least at
Stations 1 and 2. This is verified by the bioassays in which
excess nitrate additions gave positive growth over that of the
controls at those stations only. These results should not be
construed to imply that nitrogen is limiting in all parts of
Lake Mead, but only in the area of Lake Mead nearest the Wash
where there is an excess of inflow phosphorus with an N to P
ratio of 3 to 1. Previous extensive data shows N and P in
most of Lake Mead to be in a 20-100 to 1 ratio with phosphorus
definitely limiting (Anon, 1970).
In Las Vegas Wash, N03-N and PO/j-P were at concentrations
200 times those found in Las Vegas Bay. Undoubtedly this
nutrient input would contribute to or be almost entirely
responsible for the algal;,blooms found in the Bay. Furthermore,
these nutrients will eventually lead to an extension of the
problem to other parts of Lake Mead and to reservoirs on the
Colorado River below Lake Mead.
There was unexpected high in vivo fluorescence in samples
from Station 4 near Black Island. The in vivo fluorescence
is usually proportioned to the chlorophyll a_ concentration,
since filters are selected to detect chlorophyll a fluorescence.
However, other materials both dissolved and particulate can
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8.
fluoresce within the chlorophyll a limits (Tunzi, 1974).
Fluorescence at Station 4 was clarified by the aerial photo**
graph which showed material in drifts at that station. This
material could be bottom silt or perhaps Wash inflow coming
to the surface. Wash inflow has been shown to flow in a
definite density layer, either along the bottom of Las Vegas
Bay or above the thermocline (Deacon and Tew, 1973).
Conclusion
1. Algal populations were highest near the marina and diminished
in magnitude (as measured by chlorophyll concentration)
in samples taken at stations furthest from the Wash. The
algal, populations on the sampling date could be termed
undesireable at the Marina and acceptable at Saddle Island.
2. P04-P was found close to its lower limits of detectibility;
however, concentrations appeared to be ample for further
algal growth.
3. N03-N concentration was low when evaluated both in relation
to concentrations needed for algal growth and to its rato
to PO4-P judged by algal uptake of N to P in a 10:1 ratio.
4. The presence of chlorophyll in the phaeophytin form does
indicate that a larger algal population had been present
previous to the sampling.
5. Complaints about algae were related to distance from the
Wash, with persons at the Las Vegas Marina experiencing
the worst blooms.
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9.
6. In vivo fluorescence would not always be a valid approach
in measuring algal chlorophyll as other non-chlorophyllous
substances in the water can contribute to the fluorescence.
7. Interpretation of some of the field data was only possible
because an aerial photograph had been taken.
Recommendations
1. Algal populations should continue to be monitored in Lake
Mead. Chlorophyll a determination are the simplest approach.
2. The monitoring should be more frequent, once every 2-3
weeks between March and August appears to be the best.
3. Monitoring should continue even if discharge from the Wash
has lower nutrient because of better treatment. Verifica-
tion of the role of Wash input upon algal population would
be extemely important.
4. -The EPA Laboratory at Las Vegas, U.S.B.R., or University
of Nevada could be contacted to do the sampling.
5. It appears futile to continue measurements other than
chlorophyll a. Inflow source and nutrient concentration
are well-established. Those in the lake would be in a
state of flux depending on algal uptake, mineralization
rates, and hydrolic dispersion.
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10.
REFERENCES
Anon. 1967. Report on Pollution in, Las Vegas Wash and
Las Vegas Bay. FWPCA. Dept. of Interior, Cincinnati/ Ohio.
24 pp.
Anon. 1970. Analysis of water quality and proposed standards
for Lake Mead and the Lower Colorado River, FWQA. Pacific
Southwest Region, San Francisco. 63 pp.
Anon. 1971. Methods for Chemical Analysis of Water and Wastes.
Environmental Protection Agency. NERC. Cincinnati, Ohio.
312 pp.
Deacon, J. E. and R. W. Tew. 1973. Interrelationships between
chemical, physical and biological conditions of the waters of
Las Vegas Bay of Lake Mead. The University of Nevada,
Las Vegas. 186 pp.
Strickland, J.D.H. and T. R. Parsons, 1968. A Practical Hand-
\
book of Seawater Analysis. Bull. 167. Fish Res. Bd. of Can.
Ottawa. 311 pp.
Tunzi, M. G. 1974. Limitations of in vivo fluorescence as
a measure of chlorophyll a. Proceedings of the Workshop on
Algae-Nutrient Relationships in the San Francisco Bay-Delta.
July 1974.
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TABLE 1. Chemical, biological, and physical measurements from the Las Vegas Bay section of Lake Mead (8/30/73).
Parameter
Temperature (°C)
mg/1
Total P
Ortho-P
Tot. KJeld. N
NOa - N
Algal Cells/ml
Secchl Disc Depthl
Total chl a/12
phaeophytin-free
chl a/1
Fluorescence
(10 x scale)
Las Vegas Bay
Near Mouth of Wash
1m 2m
29.0 27^0
.06 .05
.02 .01
.49 .36
.02 .04
7940 7450
1.98 m
9.75
7.40
19.0
" Near
Las Vegas Marina
1m 2m
26.0 26.0
.05 .05
.02 .02
.45 .40
.02 .01
4120 7250
1.98 m
15.4
12.5
20.0
Near
Sand Island
1m 2.5 m
26.0 26.0
.03 .03
<.01 .01
.38 .37
.04 .04
5240 4510
3.05 m
8.49
7.48
11.0
Near
Black Island
1m 3m
27.0 26.5
.02 .02
.01 .01
.50 .29
.03 .04
4210 5240
3.66 m
5.76
4.72
19.0
Near
Saddle Island
1m 3m
25.0 24.0
.02 .02
.01 .01
.52 .60
.05 .06
3330 2990
3.66 m
3.89
3.47
8.5
1. Measured In feet and converted to meters.
2. Composite value of three equal-volumed samples from each depth.
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TABLE 2
Chemical and Physical Measurements of Las Vegas Wash taken near the North Shore Road Bridge
Date and Spec. Cond. Tot.
Time JTU °C O2 pH jumhos/cm TDS Tot. P. Ortho-P Kjeld. N
8/29/73 25 23.0 9.0 8.0 5,000 3,960 3.4 3.2 .50 8.9
7:15 pm
8/30/73 27 17.5 9.7 8.0 4,800 3,830 3.6 3.4 .53 . 8.4
6:45 am
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I
TABLE 3. Plankton species and counts (no/ml) In samples taken from Las Vegas Bay (8/30/73)
GREEN ALGAE
Acanthosphaera
Carterla
Chlamydomonas
Chlorella
Chodatella
Scenedesmus
Tetradesmus
BLUE GREEN ALGAE
Anabaenopsls
Anacystls type
Calothrix
Oscillatoria
Phormidium
Spirullna
DIATOMS
Centric
Cyclotella & Stephanodiscus
Pennate
Fragillaria
Navicula
Nitzschla
Synedra
DINOFLAGE'LLATES
Glenodinlum
Peridinlum
EUGLENOIDS
Trachelomonas
Saddle Island Black Island Sand
1 meter 3 meters 1 meter 3 meters 1 meter
49
49
49
49 147
49 196 49 ^
637 588 1,470 2,009 1,568
196 245 294 294
98 294
931 1,029 1,225 1,274 1,519
49
1,470 1,274 1,176 1,372 392
49 98
98
'
Island
2-1/2 meters
49
49
196
1,372
98
294
1,176
49
686
147
Near Las Vegas Marina
1 meter 2 meters
49
980
49
1,225
294
392
4,067
49
49
98
98
49
49
833
1,225
784
686
4,165
98
147
49
98
147
Mouth of Las
1 meter
49
49
392
98
98
1,568
1,078
490
784
2,940
98
147
49
49
49
Vegas Wash
2 meters
'
•
49
98
980 -
784
294
392
4,606
49
49
98
49
•-.
U)
TOTAL/ml 3,332 2,989 4,214 5,243 4,508 4,116 7,252 8,428 7,938 7,448
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14.
Table 4
Bioassay responses of Lake Mead samples
with and without nutrient additions
Amp ling
sites
§s Vegas Bay
ar Wash (1)
tar Las
gas Marina (2)
4 Near
nd Island (3) .
_ Near
vack Island (4)
•
Near
*ddle Island (5)
Initial
9 . 8 :
15.4
8.5
5.8
3.9
oig chl a/1
Terminal
Unspiked
13.5
29.3
13.0
5.4
2.4
1 mg N03-N/1
99.4
73.4
16.7
5.8
2.8
0.2 mg POa-P
13.8
27.3
14.9
7.8
3.2
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15.
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•I TO JUNCTION V I. • ). •• ANO
fflNOMAN r»
1. Las Vegas .pay. near mouth of- Wash
2. Near Las Vegas Marina
3. Near Sand Island
4. Near Black Island
5. Near Saddle Island
Figure 1. Sampling stations in the Las Vegas survey of
August 30, 1973.
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