Chesapeake Bay Monitoring
"Monitoring for Management Actions"

7. ecosystem processes

The Ecosystems Processes Component (EPC) of the water quality monitoring program focuses on the exchange of organic matter, oxygen, and nutrients between Bay waters and sediments. The importance of these exchanges in regulating the productivity and water quality of estuaries has become increasingly clear over the last decade. For example, we know that many bottom sediment (benthic) communities, including commercially important shellfish and finfish, are nourished by organic matter produced in the overlying water. At the same time, the phytoplankton production of many estuaries, including portions of the Chesapeake Bay, depends on the release of fertilizing nutrients - dissolved inorganic forms of nitrogen (N), phosphorus (P), and silicon (Si) - from bay sediments. In addition, oxygen consumption by organisms in sediments is an important factor in the depletion of oxygen from bottom waters.

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Interaction between bottom sediments and the water column in the Chesapeake Bay ecosystem involves exchanges of oxygen, nutrients and organic matter. Dissolved N, P, and Si are introduced into Bay waters from surface and groundwater runoff, sewage and industrial effluents, and rainfall. As discussed in previous chapters, these nutrients may then be taken out of solution and incorporated into growing phytoplankton. A significant portion of this phytoplankton biomass sinks, either as intact cells or in various stages of decay, and eventually reaches the Bay bottom. On the Bay bottom, much of this rich organic matter is eaten by benthic organisms ranging from microscopic bacteria to large shellfish. This feeding is accompanied by the consumption of oxygen and the formation of "remineralized" inorganic nutrients. The remineralized nutrients in Bay sediments are released back to the overlying water where they may then mixed upwards into the sunlit portions of the water column to support additional phytoplankton production.

Thus, many of the linkages between benthic and water column systems can be characterized as having "positive feed-backs". For example, enhanced phytoplankton production in the upper water column leads to even greater deposition of organic matter to deeper waters and sediments. This, in turn, can fuel greater oxygen-consuming and nutrient-releasing activities by organisms living in the sediments. Unchecked, the cycle of production, deposition, consumption, and remineralization contributes to the lack of oxygen in both sediments and bottom waters. This eventually leads to the deterioration of aquatic habitats for important living resources, a symptom which is characteristic of overly fertilized, or eutrophic, estuarine systems.

The conceptual model of sediment-water column coupling outlined above predicts the following relationships. If total loading of nutrients and organic matter to Bay waters decrease, then deposition of organic matter to Bay sediments, sediment oxygen demand, and the return flux of remineralized nutrients from sediments will also decrease. Sediment processes, therefore, not only contribute to changes in water quality, they also serve as important indicators of these changes. In practical terms, the effectiveness of controls on nutrient loading will be reflected by changes in the rate of deposition of organic matter to Bay sediments plus changes in the rates of metabolic activities in sediment communities. It is because of these links between nutrient loading, sediment nutrient exchange dynamics, and water quality that long-term trends of deposition of organic matter and sediment-water exchanges of oxygen and nutrients need to be monitored.

DESIGN CONSIDERATIONS

The primary objective of the Ecosystem Processes Component of the OEP Chesapeake Bay Water Quality Monitoring Program is to determine current conditions in the exchange of organic matter, oxygen, and dissolved inorganic nutrients between waters and sediments of Chesapeake Bay and to provide data needed to identify long-term trends in these same exchanges. Additionally, the data collected in this component are to be integrated with those from other monitoring components to produce information needed to develop and evaluate water quality management programs.

To meet these objectives, the EPC project is divided into two complementary parts: (1) determinations of exchanges of oxygen and nutrients across the sediment-water boundary (Sediment Oxygen and Nutrient Exchange - SONE) and (2) measurements of the rate of downward transport of particulate organic matter through the Bay's water column (Vertical Flux - VFX).

Sediment Oxygen and Nutrient Exchange (SONE): Previous studies in Chesapeake Bay have shown that sediment communities undergo seasonal cycles, reflected in the predictable seasonal variations in sediment oxygen demand (SOD) and sediment-water nutrient exchanges. Earlier work has also shown that short-term (daily to monthly) variability in these exchanges at any one location is small compared with seasonal variability at a particular location and variability between stations.

This information indicated that a regional view of these processes could be achieved with quarterly measurements at ten stations located within the Maryland portion of the Bay (see Map 1, Chapter 3). Four stations were identified to cover the salinity gradient along the Bay's mainstem. Within the major tributaries themselves, spatial patterns are examined with an upriver and a downriver SONE station in the Potomac, Patuxent and Choptank rivers.

The net exchanges of oxygen and nutrients across the sediment-water boundary are determined on board ship using intact sediment samples. Triplicate cores from each station are placed in a darkened water bath to maintain normal water temperature. The cores are sealed from the atmosphere and held in the dark for two to five hours. Concentrations of dissolved oxygen and nutrients are measured over time in the water overlying each core and in a control core without sediment. Using these data, exchanges of oxygen and various nutrients can then be calculated to provide a direct indication of the influence of Bay sediments on water quality.

Vertical Flux (VFX): The design of VFX monitoring is governed by constraints somewhat different from those applying to SONE monitoring. Unlike sediment-water exchanges, daily to monthly variability in vertical flux was expected to be large, particularly during the summer. This variability is attributable to the unpredictable and patchy distribution of plankton blooms, variations in zooplankton grazing rates, freshwater flow, and zooplankton grazing rates, freshwater flow, and other factors. Obtaining an accurate estimate of annual and seasonal organic matter deposition therefore requires intense sampling during the spring, summer, and fall, with less frequent sampling during the colder months. A station was established at a deep-water, mid-bay location that is representative of areas in the Bay that suffer from low dissolved oxygen in bottom waters (see Map 1, Chapter 3).

The downward flux of particulate material is determined using cup-like traps fixed at 3 distinct depths. The uppermost trap estimates the vertical flux of particles from surface waters to the level of the pycnocline, the middle trap allows estimates of particle fluxes across the pycnocline to deeper waters, and the near-bottom trap collects "new" material reaching the Bay bottom as well as "old" sediment resuspended by tidal currents and waves. Traps are routinely deployed for periods of one to two weeks. Analyses of collected material, which are used to calculate rates of deposition, include particulate N, P, and C, total dry weight, organic fraction and phytoplankton species composition.

Supporting Data: Additional water column and sediment analyses are routinely carried out as part of the EPC effort. These supporting data are used to assist in the interpretation of ecological events at each station and add to the temporal coverage of monitoring data being collected in other components of the program. For example, profiles of temperature, salinity, and dissolved oxygen conditions are determined through the water column whenever a SONE or VFX station is occupied. Particulate matter concentrations in the water and sediments are also routinely collected. Most importantly, EPC monitoring locations and sampling schedules are coordinated with those of other program elements. Because of this, there is a rich source of additional complementary data with which to interpret EPC trends as well as to enhance the information base developed from other monitoring components.

RESULTS

Sediment Oxygen Exchange

Sediment oxygen demand rates determined during the first two years of SONE monitoring ranged between 0.1 - 3.9 grams of oxygen consumed per square meter of Bay bottom per day (g 02/m2/d). These values are comparable to rates found in other productive estuarine systems. Seasonal measurements of SOD (Map 11) revealed that oxygen consumption in tributaries was generally greater than along the mainstem and that sediments from the upper Bay had greater SOD rates than those in mid-bay regions. Some of the highest rates were found in the lower Patuxent and lower Choptank Rivers.


Map 11

SOD rates were often highest in the warmer sampling periods. This is due to the higher rates of oxygen demanding processes, such as biological metabolism and chemical reactions, caused by higher temperatures. In addition to temperature, the spatial and seasonal patterns in measured SOD rates can be attributed to factors such as the deposition of organic material to the sediments and the availability of oxygen in the water overlying sediments. For example, SOD was low - less than
0.7 g 02/m2/d - even in warmer months when oxygen concentrations in the overlying water were less than 2.5 mg/l. The low bottom-water oxygen levels help explain why the two stations in the lower mainstem exhibited low SOD rates in August despite the warm temperatures. It is now clear that SOD rates are ultimately regulated by a combination of interacting environmental factors.

It has recently been learned that certain meteorological conditions can lead to the mixing of surface-water oxygen into oxygen-depleted bottom waters. These events appear to occur repeatedly during the summer and have considerable impact on water quality conditions. One such impact appears to be on SOD rates. While SOD rates are known to be small when oxygen concentrations are very low, the response to suddenly increasing oxygen concentrations due to a mixing event was not known. Would the SOD rate remain low because most of the organisms living in sediments had already died from lack of oxygen? Alternatively, would SOD rates increase greatly due to the accumulation of compounds which quickly react and combine with oxygen. Recent measurements made during the OEP program strongly suggest that the initial response to increased oxygen availability is a large, approximately ten fold, increase in SOD rates. This response is likely due to the chemical reaction of oxygen-consuming sulfur compounds which are generated by bacteria and accumulate during anoxic conditions. The rapid growth of oxygen-consuming bacteria is also a probable factor contributing to the increased oxygen demand following the introduction of oxygen. The important point here is that SOD may be capable of rapidly returning bottom waters to hypoxic or anoxic conditions even following summer mixing events.

SOD at the station in the mainstem off the mouth of the Choptank River (see Map 1, Chapter 3) exhibits a typical seasonal cycle for this area, marked by lowest oxygen uptake rates in late summer when oxygen concentrations are low. Assuming that the yearly average SOD at this station was about 0.8  g 02/m2/d and that yearly phytoplankton production in that region (see Map 7, Chapter 5) was about 450 g C/m2, carbon consumption by mid-bay sediment communities in the presence of oxygen accounts for the utilization of about 25% of primary production of the overlying waters. Recent research suggests that significant additional amounts of carbon consumption by the sediments are occurring when the overlying water is devoid of oxygen, although this is not measured routinely in the monitoring program. This additional amount of carbon consumption would increase the percentage of primary production being utilized by the sediment community. Similar values have been found from a variety of other coastal marine systems and serve to illustrate once again that the bottom community can be a significant consumer of oxygen and organic carbon in the Bay ecosystem.

Sediment Nutrient Exchange

Ammonium, perhaps the most important form of nitrogen for phytoplankton growth, is the dominant form of nitrogen released from estuarine sediments. Consistent with the findings from other estuaries, the fluxes of ammonium found in the OEP program (Map 12) were always directed from the sediments into the overlying water and were as high as 6.5 milligrams of nitrogen released per square meter of Bay bottom per hour (mg N/m2/h. The spatial patterns of ammonium release differed from that found for SOD measurements. Ammonium release was highest in the lower reaches of the mainstem and the lower Potomac and Choptank Rivers and generally declined toward the upper Bay and the upper tributaries. On a seasonal basis, ammonium flux rates were generally highest in summer and lowest in spring or fall.
Map 12

Three other important nutrient fluxes were measured in the SONE program - nitrate, silica, and dissolved inorganic phosphate. Although the results for these nutrients are not presented in detail here, some general findings are summarized below. Nitrate fluxes ranged from -1.4 to +2.1 mg N/m2/h, the negative and positive signs indicating that nutrient were entering or leaving the sediments, respectively. Fluxes were always positive in upper mainstem and Patuxent sediments and, in October, nearly all stations exhibited positive nitrate fluxes. These observations offer evidence that the early fall, when large amounts of oxygen are introduced into summer-depleted bottom waters, may be notable for active sediment nitrification. Nitrification is a bacterially-mediated chemical reaction between certain nitrogen compounds and oxygen which yields nitrate.

Dissolved inorganic phosphate fluxes ranged from 0.15 - 0.77 mg P/m2/h and tended to be highest in the upper portion of tributary rivers and at stations in the Bay experiencing summer anoxia. Again, the magnitude of the dissolved inorganic phosphate fluxes were such that they could have considerable influence on water quality conditions. This effect can clearly be seen in mainstem bottom waters during summer when dissolved inorganic phosphate builds to high concentrations (see Figure 11, Chapter 4).

Silica, an essential nutrient for diatom growth, was always released from sediments to overlying waters. Diatoms are important species of phytoplankton in Chesapeake Bay, especially prominent in the fall through spring period. Average fluxes ranged from about 12 - 36 mg Si/m2/h and tended to be somewhat higher at the down-Bay mainstem stations than in the upper mainstem and tributary rivers. The observed fluxes appeared sufficient to have substantial impacts on water column concentrations.

Vertical Flux

Vertical flux rates from surface waters to bottom waters of the Bay are expressed as the mass of organic matter, presented here as carbon, passing through an imaginary plane of given area during a day. Average monthly values for two full years at the central bay station ranged from a low of about 0.5 grams of carbon deposited per square meter per day (g C/m2/d) to a high of 1.3 g C/m2/d (Figure 19). These are substantial deposition rates which would be expected to have a considerable impact on water quality conditions, especially the dissolved oxygen regime in bottom waters. The general seasonal cycle indicated that deposition of organic material was sustained at a relatively constant level of 0.5 - 0.6 g C/m2/d in the cold months of January, February and March and then rose sharply to the yearly peak of around 1.3 g C/m2/d in April. In the May through October period, the monthly-averaged rates varied between about 0.6 and 0.9 g C/m2/d. It is interesting to note that organic matter deposition rates in the colder months are only slightly lower than in summer. The result of this sustained winter and early spring deposition of phytoplankton is a growing accumulation of phytoplankton in bottom waters through the winter with a peak in the spring (see Figure 13, Chapter 5).

The relationship between seasonal phytoplankton productivity in the upper water column, organic matter deposition and bottom water dissolved oxygen is displayed in Figure 19 for the mainstem station located off the mouth of the Choptank River. This relationship is a critical one to understand for assessing management actions aimed at reducing phytoplankton productivity to improve bottom water oxygen problems. Several patterns are evident in this relationship which suggest seasonally varying levels of coupling between surface and bottom waters. In the period from winter through the middle of the spring phytoplankton bloom in April, most of the phytoplankton production appears to enter bottom waters. The period from May through August is characterized by a much lower fraction, generally about 1/3 to 1/2 of phytoplankton production, reaching bottom waters. In September, when the Bay destratifies and phytoplankton productivity declines from its summer peaks, most of the productivity again reaches bottom waters. An October peak in phytoplankton production is not reflected in the deposition rate although the lack of a correlation may be due to the fact that deposition was only monitored during the first half of October.

There are some probable explanations for the observed relationships between phytoplankton productivity and organic matter deposition. From January through April and again in September, the plankton component of OEP's monitoring program has found that much of the phytoplankton community is composed of species, such as diatoms, that tend to sink at relatively rapid rates. This is a likely cause for the apparently high fraction of phytoplankton productivity that is being deposited to bottom waters. During the summer, more buoyant phytoplankton cells and a strong density barrier between surface and bottom layers may serve to inhibit the downward transport of organic material.

Another interesting aspect of these data concerns some of the weekly variability in carbon deposition that was observed during summer months. For example, rates varied between 0.25 and 0.8 g C/m2/d over a two week period during the summer of 1984. The reason for this variability appears to be periodic, short-term blooming and subsequent death and sinking of phytoplankton communities.


Figure 19.  Relationships between seasonal phytoplankton productivity, organic matter deposition and bottom water dissolved oxygen for the Chesapeake Bay mainstem station off the mouth of the Choptank River.

The seasonal cycle of oxygen in bottom waters at the mid-Bay station is typical of this region (Figure 19). During spring, as strong density stratification develops, levels of oxygen decline rapidly and then remain low during summer. In the fall there is a rapid return to the higher oxygen levels observed during colder months. The organic matter that deposits gradually over the colder months and in a large spring pulse is probably responsible for much of the oxygen consumption in bottom waters and sediments that occurs between March and June. This spring period encompasses a decline in average monthly oxygen concentrations near the bottom of about 9 mg/l. During summer, continued substantial deposition of organic matter, coupled with limited reaeration, appears to be sufficient to maintain average dissolved oxygen levels below 1 mg/l. In fall, the enhanced reaeration of Bay waters due to lower density stratification overwhelms oxygen-consuming processes which are declining, in part because of falling temperatures. This leads to a rapid increase in bottom-water oxygen concentrations. The physical ability of water to hold more oxygen at colder temperatures reinforces and partially contributes to the seasonal patterns just described.

CONCLUSIONS

  • The rates of oxygen consumption by Bay bottom sediments contribute significantly to bottom-water oxygen depletion which is a major problem in Chesapeake Bay.
  • The release of recycled nitrogen, phosphorus and silicon by Bay bottom sediments exert a substantial impact on water column levels of nutrients, phytoplankton production and other water quality indicators.
  • The deposition of organic matter, produced by phytoplankton in surface waters, is substantial year-round, ranging in the mid-Bay from 0.5 to 1.3 grams of carbon deposited on a square meter of Bay bottom each day. Highest rates occur during the Bay's annual spring bloom. These high deposition rates are a major factor causing low dissolved oxygen levels to occur in bottom waters.
  • The relationship of the Bay's phytoplankton production with organic matter deposition is now becoming clear with data collected under the OEP program. This relationship varies seasonally, with most of the water column production being deposited to Bay bottom waters in colder months and about 1/3 to 1/2 being deposited during summer. An understanding of this relationship between production and deposition is important to formulating management actions to improve bottom-water oxygen levels.

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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