Hidden Hypoxia in Coastal Waters

Coastal benthic hypoxia and anoxia develop in thermally stratified coastal waters during warm summer months. They alter the chemical composition, biogeochemical cycling, and ecosystem functioning at the seafloor and can render the benthic habitat uninhabitable for higher life forms. With more and longer heatwaves expected due to global warming, the strength and persistence of stratification is expected to increase leading to longer and more extensive bottom water hypoxia in the coastal ocean. However, on short timescales benthic oxygen availability can be dominated by highly dynamic lateral transport and transient vertical mixing events that can compensate for the sediment oxygen demand through short-term ventilation events. The occurrence, temporal dynamics, and quantitative impacts of these ventilation events have so far been poorly understood.

We present results of a two-week summer field campaign at a 38 m deep thermally stratified  coastal site in the western Baltic Sea. An autonomously operating benthic lander system equipped with stationary oxygen optodes at fixed depths, a continuously profiling multiparameter probe, a high-frequency downward-looking ADCP was deployed together with an eddy correlation system, within 50 meters distance. The setup enabled the study of the vertically resolved temporal evolution of oxygen in relation to hydrodynamic parameters in the bottom waters at a second- and centimetre-scale resolution for a 280-hour long deployment period together with continuous measurements of the benthic oxygen consumption. At the beginning of the deployment bottom-water free-flow velocities were on average 1.6 cm/s consisting of a translatory and a rotating diurnal oscillatory component. Weakening of the translatory current component gradually turned the system into an almost pure oscillatory state with free-flow velocities of about 0.8 cm/s. Bottom-water oxygen concentrations were constant down to 5 cm above the sediment at an initial normoxic concentration of 170 μmol l^-1 that decreased with decreasing flow velocity to hypoxia below 63 µmol l^-1 by the end of the measurement series. During purely oscillatory flow the balance between sediment oxygen uptake and vertical transport resulted in a net bottom water oxygen loss of 6.4 μmol l^-1d^-1 increasing to -14.5 μmol l^-1d^-1 following a resuspension event. Even at low-flow velocities the bottom water remained well mixed. Bottom water oxygen loss was not continuous and instead varied between +43.5 and -45.2 μmol l^-1d^-1 corresponding to changes in lateral transport. Temporary changes (<2 hours) up to 30 μmol l^-1 were found due to convergence/divergence events of the bottom water during flow reversals.

These dynamic bottom water changes would have been undetectable using conventional shipboard tools due to their close proximity to the sea floor. We suggest that areas undergoing frequent hidden hypoxia and re-ventilation are more common than previously thought and have so far unexplored effects for benthic ecosystem functioning.