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

Abstract

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.

Physical mixing in coastal waters controls and decouples nitrification via biomass dilution

“…If water supplied by mixing or advection is relatively poor in viable nitrifier biomass but contains ammonium or organic N, ammonium and nitrite may consequently accumulate due to delayed nitrification and associated temporal decoupling of nitrification. This could apply to subsurface waters of seasonally mixed lakes where climatic variations can affect the extent of spring mixing (53) as well as fjord-like basins and estuaries similar to BB with seasonal stratification-mixing cycles and/or irregular intrusion events (22,35,54). The mechanism might even operate in shelf ecosystems subject to seasonal blooms and sinking of organic matter, periodic upwelling, or seasonal changes in shelfslope exchange (15,55,56).…”

“…Moreover, all these effects are linked to weather and climate, since atmospheric forcing plays an important role for stratificationmixing patterns of ocean waters (61). For example, infrequent deep water renewal events in a coastal basin have been linked to variations in freshwater run-off and other atmospherically controlled variables (54). In BB, winter mixing is largely driven by heat loss to the atmosphere (32,39).…”

The Role of Boundary Mixing for Diapycnal Oxygen Fluxes in a Stratified Marine System

“…Studies in lakes, coastal seas, and the ocean have shown that vertical mixing is often confined to the turbulent bottom boundary layers (Ledwell & Hickey, 1995; P. L. Holtermann et al., 2012) and may exhibit a strong seasonal variability (Axell, 1998; P. Holtermann & Umlauf, 2012; Williams et al., 2022). Often both large‐scale advective (e.g., Kelly et al., 2020; P. Holtermann et al., 2020) and small‐scale turbulent vertical transport (Rovelli et al., 2016) contribute to the total oxygen transport.…”