In the open sea, the winter concentration of inorganic nitrogen, which algae can use, varies by sea area: in the Bay of Bothnia and the Bothnian Sea respectively, it is 80–140 and 40–70 mg/m3, while in the Archipelago Sea and the Gulf of Finland levels are 70–100 and 70–150 mg/m3, respectively. In coastal waters, and especially in estuaries, the nitrogen concentration is markedly higher than in the open sea.

Thirty years ago, as wastewater treatment became more efficient, the nitrogen content of water began to decrease markedly off the coastal cities of Oulu and Helsinki. By contrast, nitrogen concentrations in the open sea areas of the Gulf of Finland increased until the 1990s. Since then, they have fluctuated markedly without a clear trend.

Learn more about the long-term trends and seasonal fluctuations of nitrogen concentrations in the Baltic Sea!