Observations of water exchange, currents, sea levels and nutrients in the Gulf of Riga

摘要

The Gulf of Riga is one of the main sub-basins of the Baltic, coupled through one shallow and one deep strait. The average gulf salinity is close to the surface values of the Baltic proper, showing that the water exchange through the straits are dominating the buoyancy flux. As a result, two strong salinity fronts are regularly found, one is close to the river mouth in the southern gulf and the other is in the Irbe Strait area. The surface front is strongly variable in position and strength, depending on atmospheric conditions, whereas the deep layer front is almost a stationary phenomenon. Generally, the current structure are organized in two layers and is in geostrophic balance, but superimposed by oscillatory currents with typical diurnal and inertial periods. In the central part of the gulf - the Ruhnu Deep - southward currents in the near bottom layer prevailed during July and September 1993. Nutrient concentrations in the gulf and in the Irbe strait showed higher values than those in the Baltic proper and inorganic nutrients correlated well with the thermohaline structures, which was not the case with total nitrate and phosphate. It is believed that the gulf is phosphorus limited whereas the Baltic proper is nitrogen limited, regarding phytoplankton growth. 112 figs, 4 tabs