Drivers of hypoxia variability in a shallow and eutrophicated semi-enclosed fjord

Seasonal deoxygenation of coastal waters has been observed with increasing frequency around the world, with consequences for ecosystem functioning and continued benthic capacity to buffer hypoxia. Here, we present a hydrodynamical-ecological model study of the Limfjord in Denmark, an example of a semi-enclosed water body affected by recurring seasonal deoxygenation. Applying observations and model results, we show that water temperature, combined with wind strength and direction are the most important controllers of short-term interannual variability of bottom oxygen, while ventilation through episodic water inflow from the North Sea and local stratification create a spatial decoupling of deoxygenation. Nutrient load to the fjord drives sustained high biological productivity, but does not affect the interannual variability to the same degree. However, high biological turnover rates likely push the system closer towards a deoxygenated state, making the fjord more sensitive to future changes in temperature, wind and ventilation by reducing the buffer capacity of the sediments.

Automated summary

This study presents a hydrodynamical-ecological model analysis of the Limfjord in Denmark, revealing the important influence of water temperature, wind strength, and wind direction on short-term interannual variability of bottom oxygen. Ventilation through episodic water inflow and local stratification cause spatial decoupling of deoxygenation. Nutrient load affects sustained high biological productivity but has a lesser impact on interannual variability. The high biological turnover rates make the fjord more susceptible to future changes in temperature, wind, and ventilation.