Internal Nutrient Fluxes Counteract Decreases in External Load: The Case of the Estuarial Eastern Gulf of Finland, Baltic Sea

Abstract

Despite a decrease of about 30% in the external nutrient inputs, no corresponding improvement has been observed in the state of the open Gulf of Finland. At the end of the 1990s the external nutrient load to the Gulf totalled 120,000 tonnes (t) yr-1 of nitrogen (N) and 7000 t yr-1 of phosphorus (P). Relative to its surface area, the nutrient load of the Gulf is 2 to 3 times the average of the Baltic Sea. Despite the decrease in loading, an increase in the phosphate-P concentration was observed both in the surface and near-bottom layers around the mid-1990s. The reason for this development was most probably the acceleration of internal loading, triggered by poor oxygen conditions at the sediment-water interface of the eastern Gulf, where the oxygen conditions weakened during the 1990s, after being relatively good in the 1980s and the early 1990s. On the basis of experimental data from the coastal Gulf of Finland, the phosphate-P flux from the reduced surface sediment to water averaged 13 kg km-2 d-1. This corresponds to total amounts which can explain the observed trends of P in the open Gulf. The low N:P ratio of the sediment efflux can partly explain the N limitation of primary production in the Gulf.