Modelled long-term development of hypoxic area and nutrient pools in the Baltic Proper

The development of nutrient pools and sediment areas covered by hypoxic water in the Baltic Proper was modelled for the 1900–2009 period. Reconstructed physical forcing and estimated past nutrient loads were used when direct observations were unavailable. The modelled pool of dissolved inorganic phosphorus (DIP) more than doubled in size over the last century — partly a direct effect of an increased external phosphorus load, and partly an indirect effect of less effective phosphorus retention in sediments associated with deteriorating oxygen conditions. As opposed to the DIP pool, the modelled pool of dissolved inorganic nitrogen (DIN) decreased over the last ~ 50 years due to more effective denitrification associated with spreading hypoxia. The results of a hindcast scenario with low external nutrient loads indicate that long-term climate change had no significant effects on hypoxic conditions or nutrient pools during the study period. The deteriorating oxygen conditions since the 1950s were mainly related to increased external nutrient loads in combination with internal nutrient feedback processes. Furthermore, simultaneously reducing both nitrogen and phosphorus loads would have a larger positive effect on the oxygen conditions than would phosphorus reduction alone. Reducing only nitrogen loads would eventually be compensated for by nitrogen-fixing cyanobacteria.

► A coupled physical–biogeochemical model was used for the Baltic Sea.
► Nutrient pools and hypoxic areas were modelled for the 1900–2009 period.
► The influence of external nutrient loads was compared to that of physical processes.
► Increasing hypoxia due to external nutrient loads and internal feedback processes.
► No significant effect related to physical processes on longer time-scales.