High-frequency Dissolved Oxygen Dynamics in an Urban Estuary, the Long Island Sound

The seasonal occurrence of deep-water hypoxia in western Long Island Sound (LIS) has been documented for decades by water quality cruise surveys and fixed mooring buoys. While previous studies have focused on factors modulating bottom dissolved oxygen (DO) at subtidal timescales, here we analyze continuous timeseries data from a moored buoy during summers 2021 and 2022 to examine factors controlling high-frequency fluctuations in surface and bottom DO at diurnal and semidiurnal timescales. Fluctuations in surface DO at diurnal timescales are associated with biological production, while fluctuations in bottom DO near semidiurnal timescales are associated with horizontal advection of DO by tides from the upper East River tidal strait into western LIS. Results from timeseries analysis are supported by weekly cruise surveys that resolve horizontal and vertical DO gradients in the western narrows. However, inferences regarding the duration of hypoxia during a given summer vary across datasets in part because weekly survey data do not resolve dominant timescales of variability within a particular summer. While prior studies have illustrated the importance of nutrient loading, stratification, and wind in controlling the development of hypoxia, the results presented here demonstrate the role of tidal advection in modulating hypoxia in far western LIS. Despite stronger stratification in 2021, the duration of hypoxia was 11.1 days shorter compared to 2022 in part due to greater advection of DO by tidal currents that intermittently increased bottom DO near the buoy. Furthermore, five-year averaged hypoxic area in the western narrows has increased since 2017, which highlights the spatially variable response of DO to nutrient load reductions. Future analysis of hypoxia in LIS should focus on leveraging high-frequency information contained in continuous datasets to improve estimates of hypoxia based on less temporally resolved water quality surveys.