Chesapeake Bay Monitoring
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.
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.
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.
Sediment Oxygen Exchange
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
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.
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.
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