Chesapeake Bay Monitoring
"Monitoring for Management Actions"

5. plankton

Plankton, the microscopic plants and animals suspended in the water column, are the foundation of the food web in aquatic ecosystems such as Chesapeake Bay. The plankton also represent one of the most direct and profound responses to pollution entering the Bay. In fact, the degree of eutrophication or nutrient enrichment is often gauged by the amount of plankton growth in an aquatic environment. Because of their critical position at the foundation of the food web, the plankton response to pollution has many ramifications. For example, the increased growth of plankton in response to excessive nutrient additions initiates a chain of events that leads to the adverse symptoms of eutrophication, such as low dissolved oxygen concentrations and consequently the loss of habitats for living resources.

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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
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.
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

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