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Because of plankton's fundamental importance to the eutrophication process,
limitation of their growth, or production, is often one of the direct
targets of management actions. These actions are typically directed at
reducing nutrient inputs as a means of limiting plankton growth. The
limitation of plankton growth is in turn expected to improve some of the
impacts that result from excessive growth. Thus, an assessment of water
quality to guide and evaluate management actions logically includes the
measurement of plankton communities and their growth rates.
Plankton assessments in the water quality monitoring program are targeted
at three functional groups - phytoplankton, microzooplankton and
mesozooplankton. All of these groups are microscopic in size. Phytoplankton
represent a diverse group of organisms that, as plants, capture the sun's
energy, convert it into living tissue and thus support all life in
Chesapeake Bay. A wide variety of phytoplankton species' exists, some
living as single cells and some as colonies. A knowledge of the types of
species occurring in the Bay is important to an understanding of how these
organisms are utilized in the system. For example, certain species can be
easily consumed by zooplankton grazers to support higher life forms. Other
species, often indicative of eutrophic conditions, are resistant to normal
phytoplankton predators and my therefore be more prone to enter the
decomposition pathways which contribute to low dissolved oxygen problems.
Zooplankton are the primary consumers of phytoplankton and bacteria,
funneling food energy from phytoplankton production and bacterial
decomposition up to higher organisms such as fish. Larval fish survival in
spawning areas is dependant upon sufficient densities of appropriate
zooplankton species to feed upon. The zooplankton food supply in spawning
grounds during spring is one of the critical factors currently being
examined in relation to the success or failure of striped bass
reproduction. Certain fish such as bay anchovy and silversides remain
zooplankton feeders throughout their lives. Still other species, such as
menhaden, consume zooplankton as larvae and juveniles and then switch to
feeding exclusively on phytoplankton as adults. Thus, a knowledge of the
species composition and abundance of zooplankton communities is required to
assess the food supply for the Bay's fisheries resources.
Zooplankton consumption of phytoplankton and bacteria can be a regulating
force over these communities; in turn, excretion by zooplankton is one of
the most significant recycling mechanisms that supplies phytoplankton with
nitrogen and phosphorus for growth. Therefore, an evaluation of the
zooplankton community is critical to understanding both the fate of
phytoplankton production and nutrient recycling.
Microzooplankton are organisms less than about 2 hundredths of an inch and
are comprised for the most part by protozoans such as tintinnids, rotifers
and juvenile stages of mesozooplankton. Mesozooplankton are defined in this
program as those animals larger than about 1 hundredth of an inch and
include organisms such as tiny crustaceans and larvae of bottom-dwelling
organisms such as oysters, crabs and worms. Large, visible plankton
organisms such as sea nettles and comb jellies, that are important in
Chesapeake Bay, are enumerated in samples collected for the mesozooplankton
program; results for these organisms, however, are not presented in this
report.
DESIGN CONSIDERATIONS
Plankton measurements are conducted at a subset of stations sampled for
chemical and physical conditions (see Map 1)
that are representative of the tidal-fresh, salinity-transition and
lower-estuarine zones in both the mainstem and major tributaries. Samples
are taken at the same time as physical and chemical measurements to allow
for direct linkage between these two programs.
 Phytoplankton:
The Bay's phytoplankton biomass, species composition and productivity is
assessed 18 times per year. Phytoplankton biomass is determined indirectly
by measuring chlorophyll concentrations in water samples as part of the
chemical/physical program. Chlorophylls are complex molecules contained
within plant cells that help to capture the sun's light energy. Because
chlorophyll molecules are only found in photosynthetic organisms such as
phytoplankton, their concentration in water samples can be used as a
reliable indicator of phytoplankton biomass. Because the Bay's
phytoplankton are not distributed evenly in the water column, either
vertically or horizontally, special techniques are required to provide
reliable estimates of average chlorophyll concentrations. At all plankton
stations, all 22 mainstem stations and 10 stations in the Patuxent estuary,
a surface-to-bottom depth profile of chlorophyll is determined to provide
better vertical resolution. As the sampling vessel cruises between stations
on the Patuxent River and the mainstem, additional horizontal resolution of
chlorophyll is obtained by continuous monitoring of surface water
concentrations. At all plankton stations, phytoplankton species composition
and abundance are determined in waters both above and below the pycnocline
(the density gradient separating surface and bottom waters). Samples for
this enumeration are composited from several depths to produce a large,
well-mixed sample.
A final, but critical, estimate of the phytoplankton community is the
measurement of phytoplankton primary production, or growth rate. This
measurement complements the "standing stock" assessment of biomass and
species composition described above and may be a more sensitive indicator
of trends. It is quite conceivable that management actions may severely
limit algal growth rates while at the same time standing stocks may not
change appreciably. This would be a significant achievement that could go
unnoticed without primary productivity estimates. The primary production
estimates also provide valuable information for interpreting and
 synthesizing
information from other monitoring components such as vertical flux rates of
organic matter, nutrient fluxes between the sediments and water column,
dissolved oxygen deficits, zooplankton communities, benthic communities and
nutrient inputs from point and nonpoint sources.
Zooplankton: The zooplankton community is sampled at the same set of
stations and at the same times as those used for the phytoplankton
community. Because zooplankton usually have longer generation times than
phytoplankton, the zooplankton community generally changes at a slower rate
than the phytoplankton community. Thus, the once-monthly sampling frequency
for zooplankton provides temporal resolution comparable to the
phytoplankton sampling program. Large volume samples are composited and
concentrated separately from above and below the pycnocline for
microzooplankton and from the entire water column for the larger
mesozooplankton. Zooplankton biomass as well as species composition and
abundance are determined. Separate biomass estimates are made for the
contribution from large jellyfishes, such as sea nettles, and comb jellies.
RESULTS
Phytoplankton
Results from the OEP phytoplankton component in 1984 - 1985 indicate that
the waters of Chesapeake Bay sustain some of the highest phytoplankton
biomasses and growth rates found among estuaries world-wide. Average
surface-water chlorophyll concentrations in most seasons exceeded 20 mg/1
(mg = micrograms of mass) in large areas of the Patuxent, Potomac, Patapsco
and Choptank Rivers and in the mainstem region centered around the
Annapolis Bay Bridge (Map 6). Peaks of over 50 mg/l, although hidden
in the seasonal averages, were common in these same areas. Average
chlorophyll levels in the upper Patuxent River were particularly high,
exceeding 35 mg/l in all seasons except winter, and several peaks of over
90 mg/l were observed in all 3 salinity zones sampled (see Figure 15
for tidal-fresh zone). The summers of 1984 and 1985 in the upper Potomac
River were characterized by average surface chlorophyll concentrations of
35-40 mg/l. In mid-August, 1985, nuisance species of blue-green algae, a
recurring problem in the upper Potomac, reached bloom levels of around 100
mg/l.
Map 6
One of the most significant findings from the phytoplankton program was the
relatively high biomass observed during winter and spring in bottom as well
as surface waters in lower estuarine regions of the mainstem and
tributaries. This contrasts with summer months when bottom water
chlorophyll concentrations are quite low in these areas. An example of
these seasonally changing surface and bottom-water concentrations is shown
for the mainstem in Figure 13 (below). The high bottom concentrations result
from the settling of cells growing in surface waters and from transport
up-Bay in bottom waters. During these colder winter and spring months,
unlike at other times of year, phytoplankton are apparently able to survive
longer in bottom waters that do not have sufficient light for growth. This
is probably
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due to their
lower metabolic rates at low temperatures and the fact that oxygen
concentrations are relatively high in bottom waters at this time.
This large pool of living phytoplankton that builds through winter and
spring disappears rapidly with the onset of increasing temperatures in late
spring, presumably as a result of rapid decomposition. The decomposition of
this phytoplankton biomass is probably a major factor contributing to the
development of low oxygen conditions in the stratified portions of
tributaries and the mainstem. The management implication is
that we cannot ignore the growth of phytoplankton that occurs during colder
months since it may result in water quality impacts during warmer seasons. |
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Figure 13. Active chlorophyll a
concentration in surface and bottom waters of the Chesapeake Bay
mainstem from June, 1984 through December, 1985. |
Primary productivity, or the growth of phytoplankton, is measured as the
photosynthesis of new cellular material and can be measured in terms of
carbon. Rates are then expressed as the amount of carbon produced per day
within a surface to bottom column of Bay water with a cross-section of 1
square meter. Primary productivity in the Bay is highest in spring, summer
and fall when temperatures are warmer and days are longer (Map 7).
Thus, winter is consistently the time of lowest productivity, even though
the biomass of phytoplankton (see Map 6) may not decline as
precipitously during the colder months. This point is well illustrated by
seasonal patterns in the mainstem data as shown on the maps. The winter
decline in productivity is less pronounced in high salinity areas where
water temperatures remain warmer than in freshwater areas. Again, the
mainstem transect from the freshwater head of the Bay to the State line,
where higher salinities occur, illustrates this point.
Map 7
Two of the highest seasonal phytoplankton productivity rates were recorded
during summer in the tidal fresh areas of the Patuxent and Potomac Rivers
with values of 2 grams of carbon produced per square meter per day; the
rates in these areas during other seasons, however, dropped to less than
half of the summer values. Summer productivity in the lower Patuxent,
Potomac, Choptank and Patapsco Rivers, and in the mainstem, was less than
in the upper Patuxent and Potomac, but rates in these higher salinity zones
generally remained substantial during spring and fall as well. Overall, the
mainstem productivity rates were comparable to those found in most areas of
the tributaries. This contrasts with the finding of substantially higher
phytoplankton biomass concentrations in most of the upper tributaries
relative to the mainstem. This apparent contradiction leads to the
conclusion that phytoplankton growth rates on a per unit biomass basis are
lower in the upper tributaries than in the mainstem. Light limitation of
phytoplankton growth in the tributaries is a likely explanation for these
lower growth rates since nutrients, another potentially limiting factor,
are actually more abundant in the upper tributary regions. This explanation
is supported by high inorganic sediment concentrations and low water
transparencies that are typically found in the upper tributaries to
Chesapeake Bay.
Figure 14. Historical record of
phytoplankton primary productivity (grams of carbon/meter squared/day) at
the Chesapeake Bay mainstem station off the mouth of the Choptank River.
(Data from 1972 through 1977 is from Mihursky,
J.A., D.R. Heinle and W.R. Boynton. 1977. Ecological Effects of
Nuclear Steam Electric Station Operations on Estuarine Systems. Univ.
of Md. Chesapeake Biol. Lab. Ref. No. 77-22-CBL).
Productivity rates measured in the mid-portion of Chesapeake Bay, off the
mouth of the Choptank River, during 1984-1985 are similar to rates measured
in the same region between 1974-1977 (Fig. 14) suggesting that annual
phytoplankton production is relatively constant over the last decade for
this region of the Bay. The high summer peaks in 1972 and 1973 are thought
to be the result of Hurricane Agnes which brought tremendous loads of
nutrients into the Chesapeake Bay system. This decade of information is a
model for what we expect the present program to yield in future years at
many other representative sites within the Bay system.
Zooplankton
Zooplankton biomass was found to be highest in the tidal freshwater areas
of the Patuxent, Potomac and Choptank Rivers (Map 8). Biomass was
also relatively high in downstream areas of the Patuxent and Choptank
Rivers. By contrast, in the tidal fresh portion of the mainstem at the
mouth of the Susquehanna, zooplankton biomass was low except in the fall.
Other mainstem areas also had relatively low biomass levels when compared
to the tributaries.
Seasonally, the greatest variations in zooplankton biomass occurred in the
upstream regions of tributaries, although the response differed between
rivers. In the upper Patuxent and Potomac Rivers, peaks occurred in the
warmer months and winter levels were low. In the upper Choptank, however,
peaks occurred in both summer and winter. During spring, the spawning
period for striped bass and other fishes in the upper tributaries,
zooplankton food resources for the developing larvae were abundant in the
upper Patuxent, Potomac and Choptank. In the historically important striped
bass spawning grounds at the head of the Bay, however, zooplankton biomass
was relatively low. This may be an important factor to examine as an
explanation for declines in striped bass recruitment observed for this
region in recent years.
Map 8
Phytoplankton and Zooplankton Relationships
Many of the patterns observed for zooplankton (Map 8) can be
explained by looking at their food supply, the phytoplankton (Map
6). The high levels of zooplankton in the upper Patuxent, Potomac and
Choptank Rivers correspond to high levels of phytoplankton in these
regions. The seasonal patterns also show close correspondence between these
two monitoring components. Low phytoplankton concentrations during winter
in the upper Patuxent and Potomac Rivers correspond to low zooplankton
concentrations at this time of year. The upper Choptank station, when
contrasted with the other two tributaries, exhibited relatively high
zooplankton densities during winter. It also maintained much higher
phytoplankton biomass levels than the Patuxent and Potomac during winter.
The station at the head of the Bay also demonstrated a correspondence
between relatively low phytoplankton levels and relatively low zooplankton
densities.
A more detailed example of the relationship between phytoplankton and
zooplankton communities is presented in a time series for the upper
Patuxent River station (Figure 15). This station is located in the
lower tidal fresh region and is in a striped bass spawning area. The
spawning period, during which striped bass eggs and larvae were actually
collected at this station by the zooplankton sampling program, is denoted
by a shaded area. The figure clearly depicts late summer to fall peaks of
phytoplankton in both years followed by increases in the zooplankton
community. In spring through mid summer, which encompasses the spawning
period, both phytoplankton and zooplankton are maintained at moderately
high levels. This suggests that zooplankton food resources were adequate
for the survival and growth of larval fish in this area. In winter, both
phytoplankton and zooplankton biomass is low. The strong correlations in
seasonal cycles between the two communities at this station, as well as the
spatial correlation discussed above, are evidence for the link between
these two communities and the ability of the present program to define
those linkages. With this ability to define relationships, the plankton
program should add considerably to our understanding of water quality
problems such as phytoplankton blooms and low dissolved oxygen waters. In
addition, it will be a valuable water quality indicator for evaluating the
response of the Bay to management actions and assessing the linkage between
water quality and living resources.
Figure 15. Seasonal
phytoplankton, zooplankton and fisheries relationships in the upper
Patuxent River at Nottingham between July, 1984 and December, 1985.
CONCLUSIONS
- Phytoplankton biomass is high throughout the Bay system but
especially so in the tributaries where seasonal chlorophyll averages
typically exceed 20 μg/l. Blooms of over 50 μg/1 are common throughout
the Bay and its tributaries.
- Phytoplankton biomass builds to considerable levels in both surface
and bottom waters of high salinity areas during winter and spring. This
is probably an important factor in the development of hypoxic waters
during warmer months.
- Phytoplankton productivity is more evenly balanced spatially than
biomass throughout different regions of the mainstem and tributaries;
winter levels are low, with the seasonal patterns most pronounced in
lower salinity areas.
- A comparison of spatial patterns in phytoplankton productivity with
phytoplankton biomass and nutrient concentrations suggests that light may
be an important factor limiting phytoplankton growth in the upper
tributary regions.
- Zooplankton biomass shows strong correlations in space and time with
phytoplankton biomass. The demonstration of relationships between these
two communities with the current monitoring program indicates that
important program objectives, including the evaluation of water quality
processes and the impact of water quality conditions on living resources
are achievable.
Contents
a.
Preface
b.
Acknowledgements
1. Introduction
2. Understanding The Bay's Problems
3. Program Description
4. Chemical and Physical Properties
5. Plankton
6. Benthic Organisms
7. Ecosystem Processes
8. Pollutant Inputs
9. Management Strategies and the Role
of Monitoring
10. Glossary |
