Physical Supplies of Oxygen to the Bottom Waters of Western Long Island Sound.

摘要

Western Long Island Sound (WLIS) bottom waters experience low dissolved oxygen (DO) levels in the summertime. The seasonal hypoxia in the WLIS motivated the development of a coupled biogeochemistry/hydrodynamic model named the System Wide Eutrophication Model (SWEM). A critical assessment of the SWEM model, however, reveals that vertical transport rates are grossly underestimated, indicating a need for data-based estimates of these rates.;I present a novel approach to estimate vertical mixing coefficients from time-series of measurements made at two or more depths by measuring the attenuation and phase lag of scalar signals generated periodically at the surface as they propagate downwards and estimate mean summertime depth-averaged downward fluxes of DO and heat of 14+/-4 muM day--1 and 17+/-10 Wm--3, respectively. In order to assess the importance of horizontal transport in the bottom waters, I present an analysis of time-series of moored temperature, DO, and current observations in the hypoxic area of Long Island Sound and estimate mean near-bottom along-channel flux differences of DO and heat as 4+/-6 muM day--1 and --5+/-6 Wm--3, respectively. I conclude that vertical transport forms the bulk of the physical supply of both DO and heat to the hypoxic zone.;When WLIS moored instrument records are examined, it is evident that near-bottom increases in DO and heat and a decrease in salt occur during the middle of the flood tide; an analysis of water mass signatures indicates that the transport involved is vertical and not horizontal. Temperature data from a thermistor string deployed in the WLIS for 16 days in August 2009 clearly shows internal waves and a pycnocline depression of approximately 25% of the water depth occurring at mid-flood. Near-bottom internal wave energy is correlated with near-bottom DO and temperature changes at both supertidal and subtidal scales, and I conclude that internal mechanisms are potentially important to vertical transport in the WLIS region.