United States
Environmental Protection
Agency
Wasnmnton, D.C. CO-560
Septemccr 1982
CHESAPEAKE BAY
PROGRAM TECHNICAL
STUDIES: A SYNTHESIS
SUMMARY
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CONTENTS
Foreword 1
Nutrient Enrichment 2
Toxic Substances 7
Submerged Aquatic Vegetation 12
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FOREWORD
As part of the five year study plan for the EPA Chesapeake
Bay Program (CBP), EPA staff, officials from Maryland and
Virginia, and citizens identified 10 areas as foremost water
quality problems of the Bay, and agreed upon three as most
critical for intensive investigation: Nutrient Enrichment,
Toxic Substances, and the Decline of Submerged Aquatic
Vegetation. The EPA then initiated research to study
intensively these three problem areas.
This summary describes a 600 page synthesis of the findings
from research projects funded by the Chesapeake Bay Program in
those three technical areas. The first section, Nutrient
Enrichment, highlights findings from research on the enrichment
problem in the Bay, processes that interact to sustain the
problem, and what sources contribute nutrients to the estuary.
The second section, Toxic Substances, covers major results from
studies on sources, distribution, and concentrations of metals
and organic compounds in the Bay's waters and sediments. The
third part, Submerged Aquatic Vegetation (SAV) describes
results of investigations into the distribution and abundance
of SAV, the value of SAV to the Bay ecosystem, and reasons for
its decline.
For more information on the complete report, Chesapeake Bay
Program Technical Studies: A Synthesis, and its availability,
contact the Chesapeake Bay Program, 2083 West St, Suite 5G,
Annapolis, Maryland 21401, (301) 266-0077, FTS 922-3912.
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NUTRIENT ENRICHMENT
Nutrients, both phosphorous (P) and nitrogen (N), are
crucial to Bay life. Nutrient enrichment occurs when excessive
additions of nitrogen and phosphorous compounds enter the
water. Enrichment can lead to undesirable consequences such as
phytoplankton blooms, depletion of oxygen, and changes in kinds
of fish present. When an estuary, such as Chesapeake Bay,
becomes nutrient—enriched, algae can thrive and accumulate in
the water column. Their presence decreases light transparency,
and, when they degrade, they use up dissolved oxygen that other
plants and animals need.
Nutrient enrichment in Chesapeake Bay is evaluated by
measuring a number of related factors including nutrient
concentration and oxygen levels in the water, amounts of
chlorophyll a, (a green pigment found in most algae), and
transparency of the water (Secchi depth). Historical records
of these measurements were gathered and analyzed during the Bay
Program to look at trends in nutrients over the past 20 years.
During this time, nutrient concentrations have increased,
causing enrichment in some areas. Figure 1 shows areas of the
Bay that are enriched. These include: most of the western
tributaries such as the Patuxent, Potomac, and James; the
northern and central main Bay; and some Eastern shore
tributaries including the Chester and Choptank. These areas
show high levels of nutrients and chlorophyll a, and reduced
light transparency. The lower Bay, however, has remained
relatively unaffected. An analysis that relates these trends
to the health of fisheries in the Bay will be presented in the
CBP report entitled “Characterization of Chesapeake Bay.”
SOURCES OF NUTRIENTS
Phosphorus (P) and nitrogen (N) enter the Bay from several
major sources or pathways: atmosphere, rivers, point sources,
and sediments. The estimated percentage that each of the
sources contributes to the Bay during a year is shown in Table 1.
TABLE 1. PERCENTAGE OF ANNUAL NUTRIENT LOADINGS FROM VARIOUS
SOURCES ( 1)
Constituent
Atmospheric
Sources
Riverine
Sources
Point
Sources
Sediment
Sources
Total
nitrogen
13
56
22
9
Total
phosphorus
5
35
35
25
2
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Figure 1. Map showing portions of Chesapeake Bay that are moderately
or heavily enriched accordim to the criteria of Heinle et al. (1980).
Chesapeake Bay
Region
—
/
U
TI ’
Moderot&y Enriched
Heovi y Enriched
J
3
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(l)Definition of Terms
Atmosphere: aerial input that directly lands on
fluvial or tidal waters.
Riverine: mass loadings of nutrients to Bay from
above the head of tide.
Point sources: nutrient loads from industry and
municipalites below the head of tide.
Sediment sources: nutrient releases or loads from the
bottom sediment of Chesapeake Bay.
Riverine Sources
Riverine sources are a major contributor of N and P to the
Bay; approximately 56 percent of the total nitrogen loading
comes from these sources. This loading ranges from 39 percent
in summer to 64 percent in spring when river flows are
highest. Riverine source loads for P are about 35 percent of
the total annual input and range from 12 percent in summer to
57 percent in spring.
Of all the river sources, the Susquehanna River is the
major contributor of P and N, as shown in Table 2. The
Susquehanna River has by far the largest drainage area and
annual flow discharge among the river sources. This at least
partly accounts for the relatively higher contribution of N and
P from the Susquehanna. This river carries about 70 percent of
the total nitrogen and 56 percent of the total phosphorus
delivered to the Bay each year from riverine sources. Most of
these loads enter during the winter and spring.
The Susquehanna produces only about 40 percent of annual
sediment load, because the particulate matter is trapped in
reservoirs located on the lower 60 miles of the main stem of
the river. Only a large flow, above 400,000 cubic feet per
second (cfs), will transport sediment through the reservoir and
deliver them to the Bay. Such flows occur only one percent of
the time.
TABLE 2. ESTIMATED PERCENTAGE OF TOTAL ANNUAL RIVERINE NUTRIENT
AND SEDIMENT LOADS FROM CHESAPEAKE BAY TRIBUTARIES
Other
Constituent
Susquehanna
Potomac
James
Tributaries
Total nitrogen
70
19
6
5
Total phosphorus
56
22
16
6
Sediment
40
33
16
11
Major land uses in the Chesapeake Bay basin and their
estimated contribution to riverine nutrient loads are shown in
Table 3.
4
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TABLE 3. MAJOR LAND—USES ABOVE THE FALL LINE AND THEIR
ESTIMATED CONTRIBUTION TO RIVERINE NUTRIENT LOADS
Land Use
Percent In Basin Percent of Riverine Nutrient Loads
i
Cropland
Pasture
Forest
Urban
15—20
8—12
60—65
3— 5
45—70
4—13
9—30
2—12
60—85
3— 8
4 8
4—12
Riverine loadings can vary considerably among land uses.
The highest riverine loading rates come from cropland, and
lowest from forest sites. Agricultural land appears to produce
the largest fraction of the riverine loads by at least a factor
of three for both nitrogen and phosphorus, due to the high
unit—area loadings and large percentages of land used for
agriculture in this area. The CBP’s Bay—wide watershed model
has estimated the relative contributions of nutrients from all
nonpoint sources. These results will be presented in the CBP
report “Management Strategies for Chesapeake Bay.”
Point Sources
Most of the remaining nutrients in the Bay are contributed
from point sources, such as sewage treatment plants and
industries lying below the head of tide (see Table 1). These
point sources account for about 22 percent of total nitrogen
load and some 35 percent of total phosphorus input. The
percentage of nutrient load from point sources ranges from 15
in spring to 29 in fall, while phosphorus percentages range
from 59 percent in fall to 21 percent in summer.
Other sources include the atmosphere and bottom sediments.
Atmospheric contribution constitutes about 13 percent of the
total nitrogen and five percent of the annual phosphorus input,
while bottom sediments make up about 10 percent of the annual
nitrogen and 25 percent of the annual phosphorous load.
SEASONAL NATURE OF NUTRIENT LOADS
The largest portion of the annual nitrogen load enters the
Bay during the winter and spring, while the highest portion of
the annual phosphorus load enters during the spring and
summer. These nutrient inputs support increases in algal
standing crop. Since the relative abundance of nitrogen and
phosphorus changes from spring to summer, so the potential
limiting nutrient for the algal standing crop may change.
5
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The limiting nutrient changes during the year in Chesapeake
Bay as a result of three prominent events. The first is the
substantial nitrate input with a spring runoff from the
Susquehanna River. The second event occurs during mid—summer
when very low oxygen concentrations in deeper Bay water permit
release of phosphate from Bay sediments and accumulation of
both phosphate and ammonium in the deep water. The third event
is the fall nitrite maximum observed in both mid—Bay and in the
lower Potomac River estuary. Thus, peak nitrogen availability
occurs in spring, while peak phosphorus availability occurs in
Summer.
Consequently, phosphorus concentration is generally higher
in deep water during summer. Addition of phosphorus during the
other seasons could cause the standing crop of phytoplankton to
increase, if nitrogen is available. Thus, phosphorous appears
to be the biomass limiting, or regulating, nutrient for spring,
fall, and winter. Nitrogen, however, is at its lowest levels
and could be limiting in summer; additions at this time may
cause phytoplankton to grow if phosphorous is available from
the deep water due to recycling processes. An awareness of the
response of phytoplankton to available nutrients is important
when considering effects on Bay resources and how to control
input. Because phytoplankton form the base of the Bay’s food
web, increases in their populations will create more food for
other Bay inhabitants, to a point. Beyond this point (we feel
that Figure 1 indicates what areas of the Bay are at this
point) growth of phytoplankton can be detrimental to the Bay’s
water quality and its resources.
MANAGEMENT IMPLICATIONS
Management strategies to address the problem areas must
take into account the seasonal patterns of nitrogen and
phosphorous we have described and the degree to which each
contributing source may be controlled, its relative costs to
achieve this control, and trade—offs between point and nonpoint
sources. The possible management strategies will be shown in
the CEP report “Management Strategies for Chesapeake Bay”.
6
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TOXIC SUBSTANCES
Toxic substances constitute the second of three critical
areas studied under the CBP. The research focused on
determining the status of both metals and organic compounds in
Chesapeake Bay, including their concentration in the water
column, bed sediments, suspended sediments, and in some
bivalves. Sources of metals and organic compounds were also
investigated. A limited amount of research was performed on
assessing the toxicity of point source effluents and Bay
sediments.
Toxic substances are usually defined as chemicals or
chemical compounds that can harm living plants and animals,
including humans, or impair physical or chemical processes.
The two general classes of toxic substances studied were
inorganic and organic compounds. Inorganic materials are
metals such as arsenic (As), cadmium (Cd), chromium (Cr),
copper (Cu), and zinc (Zn). Many of the organic compounds are
products of human activities and include pesticides, phthalate
esters, polynuclear aromatic hydrocarbons (PNA’s), and other
chlorinated hydrocarbon compounds (PCBs, etc.).
CURRENT STATUS
The highest concentrations of metals in Bay sediment occur
in Baltimore Harbor and the Elizabeth River. In the main Bay,
the highest metals concentrations in sediment occur in the
northern Bay and particularly near the western shore where
cadmium, cobalt, copper, manganese, nickel, lead, and zinc are
enriched (elevated relative to natural levels) two to eight
times above natural levels from the Susquehanna Flats to
Baltimore Harbor region. At least half of the metal loads for
chromium, cadmium, copper, and lead orginate from human sources.
Metals tend to partition with fine particulate matter such
as detritus and silt. Consequently, highest concentrations of
metals in suspended material (ug of metal per gram suspended
material) occur in near—surface water in the central Bay where
organic matter tends to be high. Cadmium, lead, copper, and
zinc display the highest concentrations. Because this enriched
zone is an area of high organic activity where organisms
respire, reproduce, and grow, metals are available for uptake
by phytoplankton and marine organisms. Once in the plankton,
the metals can be passed through the food chain.
Like metals, organic compounds tend to cling to fine
material that is suspended in the water. When this material
settles, organic compounds will accumulate on the Bay floor.
Concentrations of organic compounds in bottom sediments are
highest in the northern Bay. They exhibit similar trends to
metal enrichment, with highest concentrations occurring in the
vicinity of Baltimore Harbor. Concentrations tend to increase
up the Bay from the Potomac River mouth toward the Patapsco
River. North of the Patapsco River, elevated concentrations
7
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are found to exist to the Susquehanna River mouth. It appears
that many of these organic compounds may have entered from the
Susquahanna River. In the southern Bay, the highest
concentrations of organic compounds are found where the river
estuaries enter the main Bay.
The sediments of the Patapsco River estuary show the
highest concentrations of organic compounds. Highest levels
occur near source locations. These sediments appear to be
largely trapped within Baltimore Harbor.
Oysters collected from around the Bay and oyster—tissue
extracts were examined for organic compound concentrations.
These bivalves did accumulate some toxic compounds. There were
42 compounds detected whose individual concentrations exceeded
50 parts per billion. The mouth of the James River had 29
percent, and Baltimore Harbor 24 percent of these 42 compounds.
SOURCES
Riverine sources above the fall line, point sources below
fall line, and atmospheric sources, contribute most of the
metals to Chesapeake Bay as shown in Table 4. Of the three
major rivers in which metal concentrations were measured
(Susquahanna, Potomac, and James), the Susquahanna contributes
the greatest amount of metals. These river loads include
municipal, nonpoint, and industrial sources above the fall
lines. The annual loadings of various metals of the three
rivers are compared in Table 5. The concentration levels of
metals in the three rivers are similar, however, the
Susquehanna has greater loadings because of its higher flow.
The Susquehanna River is also very significant to quality of
water in the Bay proper, because the loads it delivers enter
the Bay directly and are not trapped in the sub—estuaries like
those from the James and Potomac.
Industrial and municipal input below the fall line are a
major contributor of metals to the Bay (Table 4). For example,
industrial loads account for 66 percent of total cadmium load.
Municipal POTWs account for 19 percent of total chromium load.
The distribution of these loadings for POTWs and industries
below the fall line (Pennsylvania counties, thus, not included)
by counties is shown in Table 6. The inputs of Cd, Cr, Cu, Fe,
and Zn in Baltimore County and Baltimore City far exceed those
from other counties. Substantial inputs from POTWs are also
noted for Cr, Fe, and Zn in Richmond City; for Cr, Fe, and Zn
from Norfolk City; and for Cr, Fe, and Zn at Hopewell City.
The industrial load exceeds POTW loadings by two times.
Loadings from urban runoff and atmospheric sources are also
significant for several metals as shown in Table 4.
Results from the CBP show that sources of organic compounds
to the Bay are human—related. In particular, organic compounds
in northern—Bay sediments are probably from the Susquehanna
River, and possibly some from the Patapsco. Concentrations of
organic compounds in the Bay should be highest in areas of
8
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TABLE 4. LOADINGS OF METALS FROM THE MAJOR SOURCES AND PATHWAYS TO
CHESAPEAKE BAY (VALUES IN METRIC TONS/YEAR)
Source
Cr
Cd
Pb
Cu
Zn
Fe
Industry
1
200 (19) 178
(66)
155
(22)
190
(22)
167
( 6)
2,006
( .1)
Municipal
Wastewater
200 (19)
6
( 2)
68
(10)
99
(12)
284
(10)
625
( 1)
Atmospheric
———
3
( 1)
34
C 5)
28
( 3)
825
(29)
87
C 1)
Urban Runoff
10 ( 1)
7
C 2)
111
(16)
9
( 1)
63
C 2)
977
C 1)
Rivers
551 (53)
75
(28)
307
(43)
517
(59)
1444
(50)
199,682
(77)
Shore Erosion
83 ( 8)
1
C 1)
28
C 4)
29
( 3)
96
( 3)
57,200
(22)
1 Values in parenthesis
represent
percent
of
total
MAJOR TRIBUTARIES
(VALUES IN METRIC
OF
TON
THE CHESAPEAKE
S/YEAR) (FROM
BAY
LANG
FOR 1979—1980 PERIOD*
AND GRASON 1980)
Parameter
Susquehanna
Potomac
James
@
Conowingo Dam
%
@
Chain Bridge
@
Cartersville, Va.
z
Totals
Al—T
As—T
161,618 69
82 71
37,626 16
13 12
33,884 15
20 17
233,128
115
Cd—T
65 87
4 5
6 8
75
Co—T
59 40
39 27
48 33
146
Cr—T
383 70
105 19
63 11
551
Cu—T
390 75
86 17
41 8
517
Fe—D
1,844 57
839 26
567 17
3,250
Fe—S
192,422 65
76,227 26
27,783 9
296,432
Mn—T
14,469 77
1,933 10
2,327 13
18,729
Ni—T
229 57
109 27
64 16
402
Pb—T
174 57
102 33
31 10
307
Zn—T
837 58
322 22
285 20
*Values listed represent the mean of 1979 and 1980 calender year loadings.
(Note: Percentages above are approximate numbers)
0 — Dissolved
S — Suspended
T — Total
9
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TAIILE 6. POINT SOURCE LOADINGS OF METALS FROM INDUSTRIES 2 AND PUBLICLY OWNED TREATMENT WORKS (P01 14’S) 1 IN COUNTIES
BELOW Till FAIL LINE FOR CR, CD, PB, CU, ZN, FE, IN METRIC PER YEAR
Metal
POW I POTW 1 P0 1W 1
Cr Cd Pb Cu Zn Fe
P01W P01W I P01W I
Arundel
Baltimore
7.3 0.7
59.5
0.2 0.2
24.1
2.4
3.1
17.5
3.4 2.4
88.5
9.9
1.3
59.1
21.8
9.4
Baltimore City
78.9 47.2
1.8 142.3
25.5
9.9
37.1 20.7
106.8
45.5
234.4
Culvert
3.8
—
8.9
1.9
Caroline
0.0
0.0
0.0
0.0
0.0
—
Cecil
0.6 0.0
0.0 0.0
0.2
0.0
0.3 0.0
0.9
0.0
Charles
0.0
0.0
0.0
0.0
0.0
Dorchester
4.9 0.6
0.1 0.1
1.6
0.2
2.3 0.4
6.6
0.4
0.0
Harford
2.2 0.8
0.1 0.2
0.7
0.1
1.0 0.6
2.9
—
Howard
0.0
0.0
0.0
0.0
6.5
—
Kent
Prince Georges
0.2 0.0
12.8 1.8
0.0 0.0
0.3 0.0
0.1
4.1
0.0
4.1
0.1 0.0
6.0 0.9
0.3
17.3
0.0
0.0
0.7
37.9
0.0
—
Saint Mary’s
0.0 0.0
0.9 0.0
0.3
0.0
0.4 0.0
1.2
0.0
Wicomico
1.8 0.1
0.0 0.0
0.6
0.0
0.9 0.0
2.5
0.1
—
Alexandria City
10.2 2.2
0.2 —
3.3
5.1
4.8 1.1
13.7
—
30.2
0.1
Chesterfield
2.5 9.6
0.1 0.1
0.8
17.5
1.2 4.0
3.3
2.4
Nenrjco
1.1
0.0
0.4
0.2
1.9
Hopewell City
14.6 7.6
0.3 0.3
4.7
2.9
6.9 1.4
19.7
13.9
43.3
0.0
Louisa
22.7
—
53.0
11.4
—
Newport News city
Norfolk Cttj
12.8 9.6
16.7 2.0
0.3 3.9
0.4 0.9
4.2
5.4
2.7
0.4
6.0 13.9
7.8 3.0
17.4
22.6
9.3
2.0
38.1
49.5
—
36.7
5.9
Northampton
0.0
0.0
0.2
0.0
0.0
Portsmouth City
5.2 14.4
0.1 5.9
1.7
3.1
2.5 ,20.8
7.0
14.9
15.5
—
Prince William
13.4 2.0
0.3 —
4.3
4.7
6.3 1.0
18.1
—
Richmond City
22.6 1.1
0.5 0.6
7.3
2.5
10.6 12.8
30.5
6.7
67.1
—
—
Spotsylvania
0.2 0.0
0.0 0.0
0.1
0.0
0.1 0.0
0.3
0.0
0.6
Westmore land
0.3 0.1
0.0 0.0
0.1
0.0
0.1 0.1
0.4
0.1
Williamsburg City
2.3 0.1
0.1 0.0
0.7
0.0
1.1 0.0
3.1
0.1
6.8
0.2
—
York
12.1
0.2
18.7
4.5
8.9
—
TOTAL
1 POTW loadings were calculated for facilities where flows were 0.5 M CD.
2 Loadings computed from approximately 122 industrial disehargern.
— Industry
199.5 199.1 5.7 178.8 68.1 155.0 98.9 189.6 284.4 167.3 624.6 2008.2
10
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sedimentation near industrial regions and high population
areas. The CBP is further investigating sources of toxic
substances and will present the results in CBP report
“Management Strategies for Chesapeake Bay”.
In certain areas, present levels of toxic substances could
threaten the health of organisms. Bioassay tests on bottom
sediments from the Bay show that sediments from the Patapsco
and Elizabeth Rivers and northern Bay are potentially more
toxic than elsewhere. This toxicity is probably produced by a
combination of high metal content and large loads of organic
compounds. These tests on bottom sediments found
concentrations that cause mortality. The highest mortalities
occurred on samples from the upper reach of the Patapsco and
Elizabeth Rivers, and the northern Bay. Tests performed on
effluent from industrial plants around the Bay area revealed
that up to half of effluents sampled killed test fish and
invertebrates. The significance of these results and their
relationship to Bay resources will be discussed in CBP report
“Characterization of Chesapeake Bay”.
MANAGEMENT IMPLICATIONS
Managing toxic substances requires a priority, or ranking,
framework that evaluates toxic material for its greatest
potential to affect human and environmental health. As with
nutrients, areas where environmental quality is severely
degraded should be established, based on all available
environmental quality data (sediment, biota, and water) and
should be top priority for cleanup. The priority areas will be
examined in the CBP report “Characterization of Chesapeake Bay”.
11
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SUBMERGED AQUATIC VEGETATION
PATTERN OF DECLINE
Submerged aquatic vegetation (SAy) has, in the past, been
very abundant throughout Chesapeake Bay. Our current evidence
indicates a pattern of SAV decline that includes all species in
all sections of the Bay. A marked decline has occurred
throughout the estuary since the mid—1960’s. Present abundance
of Bay grasses is at its lowest level in recorded history.
Historical analysis of sediments on Bay—grass seeds and
pollen indicates a continuous presence of Bay grasses from the
17th century. In the last 50 years, there have been several
distinct periods and patterns where Bay grasses have undergone
major changes. An outbreak of eelgrass wasting disease
occurred in 1930’s and reduced SAV populations, as did a
watermilfoil outbreak in the late 1950’s and early 1960’s.
However, a far more dramatic and Bay—wide decrease in SAV
populations occurred in the 1960’s and 1970’s where, unlike the
eelgrass and milfoil events, all species in almost all areas of
the Bay were affected. The change is not attributable to
disease.
Because there has not been a significant change in SAV
distribution along the east coast of the United States
comparable to the Chesapeake Bay decline, it is most likely
that water quality problems affecting the distribution of
grasses in Bay are regional and specific to the Bay, its
tributaries, and their drainage basin. Recent international
studies have found that SAV declines in other countries are
highly correlated with changing water—quality conditions, such
as decreasing water clarity resulting from increased
eutrophication, as sewage, agricultural runoff, and suspended
sediment inputs increase. CBP work suggests that sediment
composition and light availability are the most important
factors controlling the distribution of SAV within regions of
the Bay. In addition, SAV decline parallels historical
increases in nutrients and chlorophyll a concentrations in the
upper Bay and major tributaries that occurred first in
freshwater parts and have now moved “down—river”.
VALUE
The severity of the decline is heightened by the importance
of SAV to the vitality of the Bay. The Bay grasses are vitally
important to the Bay because of their value as large primary
producers, food sources for waterfowl, habitat and nursery
areas for many commercially important fish, controls for
shoreline erosion, and mechanisms to buffer negative effects of
excessive nutrients.
Numerous studies have shown that the primary productivity
of SAV communities is among the highest recorded for any
aquatic systems. However, trends in SAV biomass production
follows those of its distribution and abundance. The average
12
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Figure 2. Pattern of recent changes in the distribution of SAV in
Chesapeake Bay. Arrows indicate former to present limits.
Solid arrows indicate areas where ecigrass ( Zostera marin i )
dominated. Open arrows indicate other SM species.
(Orili Ct al. 1982)
13
w
0
0
w
-J
0
LU
0
-J
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biomass estimates for SAV in the Bay are low relative to other
communities. For example, we have estimated that some 40
percent of primary production in Bay was attributable to SAV in
1963 while only six percent is attributable to SAV in 1975.
These trends along with other results are indicative of
stressed plants, particularly in the upper Bay.
SAV provides food and habitat for many species of birds and
animals. The most definitive linkage is between SAV and
waterfowl. Some types of SAV are excellent food for
waterfowl. In recent years, the most important waterfowl
wintering areas have also been the most abundantly vegetated
areas. Waterfowl have adapted to the SAV decline primarily by
wintering elsewhere in the Atlantic Flyway.
SAV beds in Chesapeake Bay support larger populations of
most animals than nearby unvegetated bottoms, and provide
significant protection from predators. Fish abundance in SAV
communities in the upper Bay are among the highest ever
recorded, indicating that SAV are sources of food either
directly, or indirectly, to important Bay species. Few
commercially—important finfish use SAV beds as significant
nursery habitats. However, lower Bay beds do serve as a
primary blue crab nursery, supporting a very large number of
juvenile blue crabs throughout the year.
Work in the upper Chesapeake Bay has shown that SAV is
important in stablizing suspended sediments. As turbid water
enters SAV beds on rising tides, sediments are effectively
removed, and light transparency increases. Sediment
resuspension is reduced in proportion to SAV biomass.
SAV also reduces nutrient levels in the water. Our studies
show that, at moderate loading rates, nutrient concentrations
are consistently lower in SAV communities than in unvegetated
sites. Ainmonium concentrations were one to 10 times lower,
nitrate two to 10 times lower, and orthophosphate generally two
to four times lower in the SAV community than in deeper,
offshore waters. When loading rates and nutrient
concentrations reached high levels, SAV was no longer effective
in reducing nutrient levels.
CAUSE OF THE DECLINE
During the Bay program, investigators looked at light
reduction as a major cause of SAV decline. Overall, factors
governing light energy availability to submerged aquatic
vegetation are the principal control for growth and survival.
Bay grasses are currently living in a marginal light
environment, and water quality problems, such as increases in
nutrients and chlorophyll a concentrations in major tributaries
and the main stem of the Chesapeake Bay over the past several
decades, are seriously affecting the distribution and abundance
of grasses in the Bay region. Epiphyte communities, those
organisms that directly attach to submerged aquatic plant
blades, can also limit light availability.
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Another important factor contributing to the stress of SAV
in the Bay is the input of herbicides to the ecosystem. Our
laboratory and field experiments indicate that herbicides are
not generally available to SAV in toxic levels, and their
presence alone probably did not cause the SAV decline.
However, herbicide—induced impacts could, in concert with the
other major stresses (such as those from light limitation),
create intolerable conditions for SAV existence.
In summary, the SAV decline parallels a general increase in
nutrients, chlorophyll a concentrations, and turbidity in the
upper Bay and major tributaries. This decline first ocurred in
freshwater portions, and has moved down—river. The upper—Bay,
western—shore, and lower—Bay communities have been the most
severely impacted. Light, restricted by organic and inorganic
suspended particles from runoff and nutrient loads, and by
changes in physical—chemical regimes (salinity and
temperature), is the principal factor controlling Bay—grass
growth and survival. Bay grasses are now living in a marginal
light environment and will be adversely stressed if water
quality in the Bay declines further. Management programs that
minimize sediment and nutrient loads will have to be improved
and expanded if SAV is to flourish again throughout the Bay.
The “Characterization” report will address relationships
between SAy, other natural resources, and water quality
trends; the “Management Strategies” report will suggest ways to
protect and/or enhance these resources.
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