Seasonal depletion of oxygen from bottom waters of Chesapeake Bay : roles of benthic and planktonic respiration and physical exchange processes '

Seasonal O2 budgets were developed for the mesohaline region of Chesapeake Bay (USA), which experiences bottom water O2 depletion in summer. Rates of O2 production and consumption by the planktonic community and O2 consumption by the benthos were measured at 1 to 4 wk intervals from March to October at 2 stations. Under summer anoxic conditions, rates of sulfide diffusion from sediments were also measured directly with in situ chambers. Weekly observations of water column temperature, salinity and 02, combined with wind data and regression models, allowed calculation of air-sea gas exchange. Using these rates in mass-balance analyses for the upper and lower water column layers, we were able to compute net physical O2 transport across the pycnocline and longitudinally through the bottom layer. Mean monthly estimates of these net physical O2 transports were highly correlated to their respective O2 gradients. Slopes of these correlations provided estimates of the average spring-summer vertical dispersion coefficient (0.2 cm2 S-') and net gravitational water velocity (5 cm S-'), both of which correspond to previous reports. Vertically integrated planktonic respiration rates in the lower water column layer were compared to benthic O2 consumption from April to August. In general, planktonic processes dominated 0 2 consun~ption, comprising almost two-thirds of the total. Oxygen consumption associated with benthic processes, however, exceeded planktonic rates in early spring prior to vernal warming and in late August when large S2fluxes resulted from release of accumulated pore water pools. By combining our respiration data with values from other coastal environments and plotting rates versus water-column depth, we find a general relation in which planktonic respiration exceeds benthic respiration for systems deeper than 5 m. Hence, for stratifled estuaries with bottom layers thicker than 5 m, seasonal O2 depletion is dnven primarily by planktonic respiration rather than benthic consumption of accumulated organic pools. A comparison of mean monthly rates for bottom respiration (plankton plus benthos) and net physical O2 replenishment here revealed that the 2 processes were highly correlated between March and October; both rates increased through July and declined thereafter. This strong correlation underscores a fundamental interdependence of biological O2 consumption and net physical transport, wh~ch is based on the O2 gradient by which the 2 processes are coupled. Consequently, relatively large reductions In respiratory O2 consumption (e.g. with decreased organic inputs) would lead to substantially smaller decreases in the extent of bottom water O2 depletion because of an inherent adjustment between the coupled biological and physical processes. INTRODUCTION ( 0 2 < 2 mg 1-l) bottom waters have been reported for a variety of estuarine ecosystems (May 1973, J~rgensen A common feature of many estuarine and coastal 1980, van Es & Ruardij 1982, Caumette et al. 1983, systems is the summer depletion of dissolved oxygen Jersefson & Widbom 1988). Episodic anoxic events ( 0 2 ) from deep waters. Seasonally anoxic or hypoxic have been observed also in shallow continental shelf 0 Inter-Research 1992 0171-8630/92/0085/0137/$03.00 Mar. Ecol. Prog. Ser. 85: 137-152, 1992 regions (Falkowslu et al. 1980, Leming & Stuntz 1984, Faganeli et al. 1985). In most partially-stratified temperate coastal systems, O2 depletion events are limited to the warm summer months when stratification strengthens following the spring freshet of river flow (Turner et al. 1987, Seliger & Boggs 1988). The reduced vertical exchange of water and solutes associated with stratified conditions limits the rate at which 0, can be replenished to bottom waters. Organic matter derived from spring and summer phytoplankton production provides substrate for planktonic and benthic respiratory processes (Malone et al. 1988), which tend to increase with warming waters. Thus, it is the combined effects of vernal increases in both biological (respiration) and physical (stratification) processes which lead to summer depletion of bottom water 02. Chesapeake Bay (USA) is a large temperate estuary which has experienced summertime occurrences of anoxia/hypoxia in its bottom waters since at least the mid-1930's (Newcornbe & Horne 1938). The temporal and spatial dimensions of O2 depletion, however, appear to have increased in recent decades for the main bay (Officer et al. 1984), as well as for certain tributaries (Heinle et al. 1980). Within this decade-scale trend, major meteorological and climatic events and attendant variations in runoff and wind conditions may have contributed to fluctuations in anoxia for a given year (Seliger et al. 1985). Furthermore, in response to local and far-field winds, the depth to the O2 depleted layer at any position in the Bay may vary greatly between days due to oscillations in the cross-Bay orientation of the pycnocline (Malone et al. 1986). This can result in hypoxic water welling up into shallow (< 3 m) regions of the estuary (Breitburg 1990). For many coastal regions, documented historical increases in the frequency and magnitude of anoxic events have been attributed to anthropogenic inputs of nutrients (Jargensen 1980, Rosenberg 1985, Andersen & Rydberg 1988). It has similarly been suggested that the recent increase in volume of Chesapeake Bay's hypoxic waters has resulted from a general eutrophication trend (Officer et al. 1984). Indeed, the mounting evidence for a widespread pattern of coastal eutroplcation throughout the world (Walsh et al. 1981, Nixon et al. 1986) suggests such anoxic events in marine waters may occur with increasing frequency in the future. Nutrients entering coastal marine systems such as Chesapeake Bay are delivered chiefly in terrestrial runoff (Nixon 1987). It appears that phytoplankton growth and biomass accumulation are directly related to these riverborne nutrient inputs (Boynton et al. 1982, Malone et al. 1988). Thus, years with higher riverflow are marked by greater algal biomass which, in turn, can support elevated respiration and more rapid depletion of bottom water Oz. This relationship is confounded, however, by the fact that interannual variations in salinity stratification are also directly related to riverflow (Seliger & Boggs 1988), so that distinguishing between effects of physical and biological process on interannual variations in anoxia becomes problematic. Respiratory processes consuming O2 from coastal waters can be functionally separated into the metabolism of planktonic and benthic communities. This is a useful distinction in the analysis of O2 depletion events because of the inherently different time-scales associated with processes occurring in the water column compared to the sediments. Strong positive correlations between planktonic respiration and primary production on scales of hours to days (Jensen et al. 1990) suggest a close coupling between the 2 processes. Benthic metabolism in some estuaries may respond rapidly (days to weeks) to variations in particulate organic inputs (e.g. Graf et al. 1987). The absence of correlations between primary production and benthic O2 consumption in Chesapeake Bay (Kemp & Boynton 1981), however, suggests that the 2 processes were weakly coupled on these time-scales. The quantitative contributions of planktonic and benthic respiration to O2 consumption varies with season (Hopkinson 1985) and total water column depth (Hargrave 1973) in coastal environments. Two previous reports considering respiration effects on O2 depletion in the mesohaline region of Chesapeake Bay attributed most of the total 0, consumption either to planktonic respiration (Taft et al. 1980) or to benthic processes (Officer et al. 1984). We describe herein direct measurements of benthic and planktonic O2 consumption and planktonic O2 production at selected mesohaline Chesapeake Bay sites that experience seasonal hypoxia. We compare these measurements, which focus on the spring-summer period, to observed rates of change in oxygen pools in upper and lower layers of the estuarine water column at time-scales of weeks to months. Using these data, we have developed O2 budgets to compare: planktonic O2 consumption; ben t l c O2 consumption; and physical transport (advective plus diffusive) of 0,.