Chesapeake Bay Monitoring
"Monitoring for Management Actions"

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.

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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