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

Abstract

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 S2-fluxes 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, w h ~c h 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.