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

The Gulf of Riga is one of the main subbasins of the Baltic, coupled through one shallow and one deep (the sill depth in Virtsu is 5 and in Irbe 35 meter) strait. The fresh water flux to the gulf from river Daugava is one of the highest in the Baltic. Nevertheless, 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 second one in the Irbe Strait area. The frontal zone in Irbe is often S-shaped with inflow along the southern part and a outflow along the northern side. The surface front is strongly variable in position and strength, depending on atmospheric (winds and summer heating) 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. Two meters above bottom high speed bursts were recorded intermittently with velocities up to 25 cm/s. It was found that these bursts were coincident with passing atmospheric low pressure systems, which caused strong fluctuations in the sea level with intensive barotropic currents as a result. It is expected that the sea level variability also influence the water exchange in the two straits. Long time series at several stations around the gulf show large variability with many fluctuation periods. The most frequent period is the annual cycle with low levels in May to June and high levels in November and December. In addition periods close to 70 days, diurnal and inertial were recorded. Nutrient concentrations in the gulf and in the Irbe strait showed higher values than those in the Baltic proper and in organic 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 where as the Baltic proper is nitrogen limited, regarding phytoplankton growth. In the Irbe frontal zone, mixing might create a water mass including both nutrients and hence a strong plankton growth potential. However, the observations did not favor this hypothesis