Characterizing ventilation events in an anoxic coastal basin: Observed dynamics and the role of climatic drivers

Restricted horizontal exchange and vertical stratification restrict ventilation of bottom waters in many coastal systems, leading to stagnant basin water conditions. Dense inflows from oceanic sources can lead to dramatic changes in basin-scale properties over very short timescales. Here, we used a semi-enclosed coastal basin with deep-anoxia (Lough Furnace) as a test bed, in order to discern estuary-ocean exchange dynamics that lead to irregular and ephemeral deep-water ventilations. Measured densities and current profiler-based estimates of volume fluxes were used to compute typical renewal times of individual water masses in the stratified inner basin. Renewal timescales were primarily determined by freshwater flux and the spring-neap tidal cycle. The deployment period culminated with observations of a ventilation of the basin water which had been stagnant for over 2.5 yr. Entrainment of ambient resident water doubled the volume of the dense plume and diluted its oxygen content. Following ventilation, the volumetric extent of anoxia expanded by 20% within several months, owing to uplifting of old basin water and oxygen consumption in the new basin water. Founded on these observations, a predictive model was constructed relying only on freshwater, atmospheric pressure, and wind data and successfully recreated ventilation events over the past decade. Hindcasting the model back multiple decades implied contrasting periods of higher and lower frequency variability in ventilation occurrence. This model offers a diagnostic tool for assessing how climatic change may influence the oxygen climate in systems that experience contrasting regimes of suboxia and ventilation.