A case study of seasonal variation in the chemical composition of accumulating suspended sediments in the central Gulf of Finland

A study was made of suspended sediments from a basin in the eastern part of the Baltic Sea, in the central Gulf of Finland. In this investigation, moored sediment traps were used to collect suspended particulate material during the half-year period from April to October 1996. An equivalent to the primary net sedimentation was estimated through studies of the variations in the content of dry matter, carbon, nitrogen, phosphorus, arsenic, cadmium, cobalt, chromiun, copper, nickel, lead, titanium, vanadium, iron, manganese, aluminium and zinc in accumulated particulate matter through the period. Two algal blooms, the spring bloom and the summer bloom, were included in the period. The accumulation rate of total particulate matter (TPM) increased during the blooms by a factor of 2.5 – 3.0. In autumn there was clearly an additional period of increase in the TPM accumulation, not connected with an actual algal bloom. Metal accumulation was lowest during the spring bloom but started rising immediately after that, being highest in the autumn. Calculations show that the metal concentrations accumulated in the trap moored 20 m above the sea bottom correspond to 20–37 % of the concentrations that are deposited in the bottom sediments. According to the TPM accumulation, resuspension seems to correspond to at least 10 – 35 % of the matter in the trap moored 11 m above the sea floor.