Long-term land-based and internal forcing of the nutrient state of the Gulf of Riga (Baltic Sea)

Abstract

The long-term nutrient trends (N, P, Si) in the Gulf of Riga and their driving factors were studied based on monitoring data from 1974 to 2000. Each nutrient showed individual temporal behaviour. The nitrate-N pool peaked in 1989–1990, but after that, the increasing trend switched to a decrease. The P pool, in turn, constantly increased till the mid-1990s; afterwards, a tendency was unclear. The obvious depletion of the silicate-Si pool reversed after 1995. Algal growth limitation responded to the changes in the nutrient pool. P deficiency during the vernal phytoplankton bloom in 1979–1990 switched to further alternate P or N limitation or co-limitation. Since 1991, Si shortage set in for diatom growth in spring. Both land-based loading and internal processes have contributed to the long-term change of the nutrient pools. The dynamics of nitrate-N was mainly determined by riverine N loading, i.e., N leaching losses from watersheds. On the contrary, the P variation did not correlate with riverine inputs because the external P loading relative to its pool in the water layer and to its sediment–water exchange was less significant. Weakening in agricultural soil leaching that diminished riverine N export to the Gulf occurred 6–7 years after reduction of fertiliser application at the end of the 1980s. Internal processes—P outflux from the sediments and N losses via denitrification—varied in the long term and affected the magnitude of the P and N pools significantly. P bottom supply sustained the upward P trend during the 1990s. The silicate-Si dynamics reflected inverse changes in P concentrations. Current changes in the nutrient state of the Gulf and still declining anthropogenic loading present an optimistic perspective on further water quality improvement.