Ecological consequences of internal seiches in a semi-enclosed, anoxic coastal basin

Climate-related increase of near-surface water temperatures in aquatic environments both decreases oxygen solubility and intensifies vertical stratification of the water column, prompting concern over the depletion of dissolved oxygen (DO) in deep interior waters. Understanding bio-physical interactions in affected ecosystems is therefore paramount, as physical processes often determine the spatial extent of deoxygenated zones. One pertinent example, often linked to mass-mortality events of marine organisms, is wind-driven upwelling of low-DO deep waters into shallower, nearshore oxygenated areas. In this note, mortalities recorded during routine fishery surveys in Lough Furnace, a semi-enclosed estuarine basin with deep anoxia, are shown to result from significant wind events. Advection of deoxygenated deep water into nearshore areas occupied by fishing nets occurred following strong winds. Relaxation of the initial wind stress generated a baroclinic standing wave (internal seiche) and a succession of upwelling and downwelling events which caused drastic fluctuations in nearshore DO concentrations that persisted for several hours. The internal seiche dynamics were confirmed following analysis of water density oscillations at a mid-basin and boundary location. These results highlight the importance of bio-physical interactions in aquatic ecosystems with DO-depleted deep waters, as hydrodynamic processes can mediate the exposure of biota to deoxygenated water conditions.