Seasonal deepening of the pycnocline in a shallow shelf ecosystem and its influence on near-bottom dissolved oxygen

Abstract

A 3 yr record (1992 to 1994) of dissolved oxygen (DO) concentrations from a n intensive g n d of 21 sampling stations m a -100 km2 study area of western Massachusetts Bay (-25 to 50 m water depth) showed a regular seasonal decline in bottom waters during stratification, but considerable spatial and ten~poral variability within and across years. Mean near-bottom, subpycnocline DO concentrations for the area reached 7.57, 7.85, and 6.2 mg 1-' in mid October 1992, late September 1993, and late September 1994, respectively; individual stahon readings were as low as 4.8 mg 1-' in 1994. Overall stratified-season rates of subpycnocline DO decline were -0.025 to 0.031 mg 1-' d-', but rates increased late in the season as the bottom layer sharply warmed to its annual temperature maxlrnum. Concurrent with relatively lower DO concentrations in 1994, field measurements indicated high bottom-water temperatures >12"C (24C above 1992 to 1993) and a deepened pycnocline just prior to overturn. To address how factors like temperature and vertical structure of the water column interact with metabolic processes to shape observed trends in DO decline and spatio-temporal variability, we used a simple model with physical and biological measurements from field monitoring as inputs. From field and model sensitivity results, we conclude that temperature and stratification strongly influence DO minima and rates of decline, and these factors interact with the bathymetric slope, as well as the topographic and deposihonal heterogeneity of the study area, to create subpycnoclme variability in DO. With respect to lower DO in 1994, temperature contributed by accelerating both water and sediment metabolism, but a major effect was the late-season deepening of the pycnocline that enhanced the contribution of sediment respiration to DO declme by isolating a thin near-bottom water layer. In addition, dynamics of seasonal pycnocline deepening are a pnncipal influence on interannual variabihty in bottom-water DO because, in contrast to the late-season effect, early in the stratified season a shallow pycnocllne depth may moderate DO decline by allowing mid-water prinlary production to add DO to subpycnocline water.