SEDIMENT ACCUMULATION OF NUTRIENTS (N, P) IN THE EASTERN GULF OF FINLAND (BALTIC SEA) J. LEHTORANTA I, H. PITKANEN I and O. SANDMAN 2 I Finnish Environment Institute, Box. 140, FIN-O0251 Helsinki, Finland "South Savo Regional Envirotmwnt Center, J,Y,~,qrinkatu 14, FIN-50100 Mikkeli Finland Abstract. Dry weight (DW), ignition loss (IL) and concentrations of total nitrogen (TN) and total phosphorus (TP) of the sediment surface layer (0 to 10 cm, 1 cm slices) were analyzed from 20 sites in the eastern Gulf of Fin- land. The distance of the sampling sites from the mouth of the River Neva explained the nutrient concentrations of Ihc sediments well, while the effect of water depth was negligible. The increase of TN and the decrease of TP along the transect from the river mouth towwds the open Gulf were caused by the diminishing share of allocht- honous material supplied from the River Neva. The mean TN concentration of the different accumulation areas w~Ls about 40 % higher in the sediment surlhce than in the deeper layer (9 to I0 cm). The corresponding difference for TP varied fi-om 53 to 56 e/,. The results suggest considerable netflux of nutrients from sediment to v,,ater. The net sediment accumulation of nutrients were estimated as 6.0 g m2 at of N and 1.7 g m'~ a t of P corresponding 22 000 t a~ of N and 6 100 t al of P for the whole eastern Gulf. Key words: sediment, nitrogen, phosphorus, org:mic matter, cluster analysis, Gulf of Finland, estuaries 1. Introduction The general trophic status (chlorophyll-a concentration and biomasses of auto- and heterotrophic organisms) is far higher in the eastern Gulf of Finland than in the western Gulf or in the other parts of the northern Baltic. This is principally due to the heavy nutrient loading lrom the St. Petersburg area and the River Neva, coupled to the hydrodynamics of tile area (Pitk~inen et al., 1993: Kauppila et al., 1994). According to mass balance calculations the eastern Gulf of Finland retains amounts corresponding 69 % of tile land-derived and atmospheric N and 100 % of the corresponding P inputs (Pitkfinen, 1994). The suspended particulate matter (SPM) is sedimented primarily in the Neva Estuary. Biological processes can explain a part of the losses of P from the euphoric surface layer (Pitkfinen and Tamminen, 1995). However, it is also possible that a part of the P co-precipitates with iron (Fe) and manganese (Mn) oxides already in the water column (Golterman, 1980). The general features of Fe and P removal, involving rapid aggregation of riverborne colloids, have been reproduced in laboratory simulations of estuarine mixing (Bale and Morris, 1981 ). The objective of this study is to classify different types of sedimentation areas of the eastern Gulf of Finland and to find out the /actors causing the observed differences in concentrations of organic matter and nutrients between various areas. With a multivariate method (cluster analysis) it is possible to classify the sampling sites to different groups according to the measured variables. This information is simplified and used for new hypotheses. Water, Air and Soil Pollution 99: 477-486, 1997. 9 1997 Kluwer Academic Publishers. Printed in the Netherlands.