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

A 3 yr record (1992 to 1994) of dissolved oxygen (DO) concentrations from a n intensive grid of 21 sampling stations in a similar to 100 km(2) study ar ea of western Massachusetts Bay (similar to 25 to 50 m water depth) showed a regular seasonal decline in bottom waters during stratification, but cons iderable spatial and temporal variability within and across years. Mean nea r-bottom, subpycnocline DO concentrations for the area reached 7.57, 7.85, and 6.2 mg l(-1) in mid October 1992, late September 1993, and late Septemb er 1994, respectively; individual station readings were as low as 4.8 mg l( -1) in 1994. Overall stratified-season rates of subpycnocline DO decline we re similar to 0.025 to 0.031 mg l(-1) d(-1), but rates increased late in th e season as the bottom layer sharply warmed to its annual temperature maxim um. Concurrent with relatively lower DO concentrations in 1994, field measu rements indicated high bottom-water temperatures >12 degrees C (greater tha n or equal to 4 degrees C above 1992 to 1993) and a deepened pycnocline jus t prior to overturn. To address how factors like temperature and vertical s tructure of the water column interact with metabolic processes to shape obs erved trends in DO decline and spatio-temporal variability, we used a simpl e model with physical and biological measurements from field monitoring as inputs. From field and model sensitivity results, we conclude that temperat ure and stratification strongly influence DO minima and rates of decline, a nd these factors interact with the bathymetric slope, as well as the topogr aphic and depositional heterogeneity of the study area, to create subpycnoc line variability in DO. With respect to lower DO in 1994, temperature contr ibuted by accelerating both water and sediment metabolism, but a major effe ct was the late-season deepening of the pycnocline that enhanced the contri bution of sediment respiration to DO decline by isolating a thin near-botto m water layer. In addition, dynamics of seasonal pycnocline deepening are a principal influence on interannual variability in bottom-water DO because, in contrast to the late-season effect, early in the stratified season a sh allow pycnocline depth may moderate DO decline by allowing mid-water primar y production to add DO to subpycnocline water.