Seasonal variations in phosphorus species in the surface sediments of the Gulf of Riga, Baltic Sea

The redox-dependent variations in concentrations of phosphorus at two diffe rent accumulation bottom areas were investigated in the Gulf of Riga (Balti c Sea) between December 1993 and January 1995. The sediment samples from ni ne sampling occasions were analyzed for phosphorus forms and redox potentia l. The average concentrations of total phosphorus measured in 0-1 cm (65 an d 89 mu mol P g(-1) for sites G5 and T3, respectively) were among the highe st reported from the entire Baltic Sea. Redox-dependent "mobile" phosphorus (MP) contributed more than 50% of total in the uppermost-oxidized centimet er, whereas in reduced layers it was 16-18% throughout the year. The signif icant differences (ANOVA, P <0.01) among months of inorganic phosphorus (IP ) concentration at 0-1 cm were observed at site G5 due to temporary accumul ation of mobile phosphorus mediated by redox-dependent bacteria activity du ring summer. On the contrary no accumulation was observed at T3 probably as a result of low redox potential caused by high accumulation rates and low bioturbation. Although the water column above sediments remained oxic throu ghout the investigation period, the redox potential at site T3 was close to the redoxcline (i.e., +230 mV) during summer. Further increase of eutrophi cation might lead to development of anoxic conditions at sediment-water int erface and that in turn will result in rapid release of redox-dependent pho sphorus stored in surface sediments. The availability of excess phosphorus will further enhance eutrophication in partly phosphorus-limited Gulf of Ri ga. (C) 2001 Elsevier Science Ltd. All rights reserved.