Bacteria, dissolved organics and oxygen consumption in salinity stratified Chesapeake Bay, an anoxia paradigm

ROBERT B. JONAS2

Department of Biology George Mason University, 4400 University Drive, Fairfax, Virginia, VA 22030

SYNOPSIs. Chesapeake Bay is a bacterially dominated ecosystem driven, at least under summer conditions, by high levels of labile dissolved organics. Bacterioplankton are exceptionally abundant (20 x 10 ^ sup 9^ cells liter 1) and productive (7 x 109 cells liter-' d-l), and their biomass can equal or exceed 60% of phytoplankton biomass. In the salinity stratified Chesapeake Bay bacterioplankton account for 60-100% of planktonic oxygen consumption, potentially driving the Bay to anoxia in days to weeks. Sulfide, released from sediments by sulfate reducing bacteria, chemically consumes oxygen at rates up to 9 mg O2 liter -1 d-1 maintaining the oxygen deficit. The organic matter driving this oxygen demand in the summer season is functionally dissolved. Dissolved organics, measured as biochemical oxygen demand, account for about 60% of microbially labile organics throughout the water column and 80% (sometimes 100%) in the subpycnoclinal water. Field studies suggest that reduced oyster stocks in Chesapeake Bay may be a major factor in the shift to this bacterially dominated trophic structure.

INTRODUCTION

Although the importance of bacterial communities in ecosystem function is not in dispute, bacterial distributions and metabolism of organic matter are rarely considered directly in zoological investigations. The reason for considering these topics here is that there is now a large body of evidence indicating that, in some aquatic ecosystems, bacterial processes have changed, and now control important aspects of the physicochemical environment in which the macrobiota live. Therefore, investigating the distributions and adaptations of the macrobiota must begin with an understanding of bacterial dynamics in these systems.

The goal of this paper is to present evidence that dissolved organic matter and bacterial metabolism of that organic matter are, quantitatively, major components of aquatic ecosystems, including salinity stratified Chesapeake Bay, the nation's largest estuary. It is a mesotrophic ecosystem which suffers from seasonal hypoxia and anoxia, produced directly, and maintained directly and indirectly, by bacterial activity (Jonas and Tuttle, 1990; Jonas, 1992). Consideration of data from Chesapeake Bay indicate that a modified trophic paradigm is needed. Rather than the classical trophic pyramid, the high concentrations of dissolved organics and extreme abundance of metabolically active bacteria imply that a "teardrop" shape might better represent this portion of the trophic model or paradigm (Fig. 1). In grammar the term "paradigm" is an example of a conjugation giving ALL the inflections of a word. In the Chesapeake Bay and, perhaps other estuaries, the critical role of bacteria should be recognized as one important "inflection" of ecosystem structure and function.

Low oxygen conditions in Chesapeake Bay are not new. Sale and Skinner (1917) (Fig. 2A) found deep water oxygen concentrations in the lower Potomac River to be only 20% of saturation in the summer of 1912. Newcomb and Home (1938) reported "true" anoxia in the Bay's mainstem during 1936. Salinity stratification, high salt concentrations in the deep water, provides the physical environment which allows oxygen depletion in the Bay (Boicourt, 1992), but anoxia events were quite rare and transient until the 1970s, even in years when the water column was highly stratified (EPA, 1982). During the past two decades summer-long anoxia has occurred almost yearly, affecting the mainstem, from north of Annapolis, Maryland to an area south of the Potomac River, and the lower reaches of the subestuaries. The damage which can be done by this situation is evident from the seicching event documented in 1984 (Fig. 3) (Malone et al., 1986). Anoxic water, containing toxic hydrogen sulfide (H2S), "sloshed" to the western side of the Bay mainstem reaching depths as shallow as 2 meters and covering nearly all of the benthos.

It is generally understood that heterotrophic microbial processes led to the oxygen depletion. However, very little was known about the nature of the microbial community involved, its rate of metabolism or the organic matter fuelling the oxygen consumption. Therefore, extensive investigations were conducted to probe the relationships among nutrient enrichment, phytoplankton production and bacterioplankton distributions and metabolism (Tuttle et al., 1987b Smith et al., 1992).

EXPERIMENTAL APPROACH

It is beyond the scope of this summary to describe these studies in detail. The basic approach was to conduct repeated seasonal cruises on the Chesapeake Bay occupying multiple stations along cross-bay transects located within the region affected by anoxia. At each station, vertical profiles of nutrient conditions, phytoplankton and bacterial distributions and activities, and organic carbon distributions were determined. In the case of the bacteria and organics, measurements were made of bacterial abundance, production, and metabolism of selected organic carbon compounds. Biochemical oxygen demand (BOD) (measured by oxygen concentration changes during 5day incubations in dark bottles) and "dissolved" BOD (filterable organics passing Gelman Type A/E glass fiber filters under 5 in. Hg vacuum) were measured to estimate the amount of bacterially available organic matter. Concentrations of reducing sugars (reported as glucose) and amino acids were assayed chemically (Bell et al., 1988; Jonas et al., 1988a; Jonas and Tuttle, 1990; Jonas, 1992).