Consequences of coastal upwelling events on physical and chemical patterns in the central Gulf of Finland (Baltic Sea)

Abstract

The consequences of a coastal upwelling event on physical and chemical patterns were studied in the central Gulf of Finland. Weekly mapping of hydrographical and -chemical fields were carried out across the Gulf between Tallinn and Helsinki in July–August 2006. In each survey, vertical profiles of temperature and salinity were recorded at 27 stations and water samples for chemical analyses (PO 4 3−, NO 2 −+NO 3 −) were collected at 14 stations along the transect. An ordinary distribution of hydrophysical and -chemical variables with the seasonal thermocline at the depths of 10–20 m was observed in the beginning of the measurements in July. Nutrient concentrations in the upper mixed layer were below the detection limit and nutriclines were located just below or in the lower part of the thermocline. In the first half of August, a very intense upwelling event occurred near the southern coast of the Gulf when waters with low temperature and high salinity from the intermediate layer surfaced. High nutrient concentrations were measured in the upwelled water – 0.4 μmol l −1 of phosphates and 0.6 μmol l −1 of nitrates+nitrites. We estimated the amount of nutrients transported into the surface layer as 238–290 tons of phosphorus (P)-PO 4 3− and 175–255 tons of N-NO x for a 12 m thick, 20 km wide and 100 km long coastal stretch. Taking into account a characteristic along-shore extension of the upwelling of 200 km, the phosphate-phosphorus amount is approximately equal to the average total monthly riverine load of phosphorus to the Gulf of Finland. It is shown that TS-characteristics of water masses and vertical distribution of nutrients along the study transect experienced drastic changes caused by the upwelling event in the entire studied water column. TS-analysis of profiles obtained before and during the upwelling event suggests that while welled up, the cold intermediate layer water was mixed with the water from the upper mixed layer with a share of 85% and 15%. We suggest that the coastal upwelling events contribute remarkably to the vertical mixing of waters in the Gulf of Finland. Intrusions of nutrient-rich waters along the inclined isopycnal surfaces in the vicinity of upwelling front were revealed. The upwelling event widened the separation of phosphocline and nitracline which in turn prevented surfacing of nitrate+nitrite-nitrogen during the next upwelling event observed a week after the upwelling relaxation. A suggestion is made that such widening of nutricline separation caused by similar upwelling events in early summer could create favourable conditions for late summer cyanobacterial blooms.