Limited capacity to retain phosphorus in the Baltic proper offshore sediments

Based on calculations using a phosphorus (P) budget model, Stigebrandt (2018) concludes that the P-concentration in the water column of the Baltic proper (BP) could be reduced by about 70% within 10–15 years if the deep bottoms were oxygenated. The calculations assume an annual influx from the sediment to the water column of almost 100 000 tonnes, which could be switched off if the sediments were oxidised.

However, the P storage capacity is strictly limited to the uppermost oxidised layers of the sediment cores. Below the sediment redox cline, the total P-concentration levels out around 1000 mg kg-1 dw regardless of the oxygen conditions in the upper sediment layer. The maximum capacity of BP sediments to retain P from recent settling matter would be around 100 000 tonnes in total, assuming oxygenation of 50 000 km2 currently anoxic bottom.