The biogeochemistry of the river-sea interface was studied in the Kem' River (the largest river flowing to the White Sea from Karelian coast) estuary and adjacent area of the White Sea onboard the RV "Ekolog" in summer 2001, 2002 and 2003. The study area can be divided into 3 zones: I - the estuary itself, with water depth from 1 to 5 m and low salinity in the surface layer (salinity is lower than 0.2 psu in the Kem' River and varies from 15 to 20 psu in outer part of this zone); II - the intermediate zone with depths from 5 to 10 m and salinity at the surface from 16 to 22 psu; III - the marine zone with depths from 10 to 29 m and salinity 21-24.5 psu. Highest concentrations of the suspended particulate matter (SPM) were registered in the Kem' mouth (5-7 mg/1). They sharply decreased to values <1 mg/1 towards the sea. At beginning of July 2001, particulate organic carbon (POC) concentration in the river mouth was 404 μg/1 and POC content in total SPM was 5.64%. In the marine part of the studied area the POC concentration varied from 132 to 274 μg/l and the POC contents in suspended matter increased to 19-52.6%. These studies show, that the majority of river-borne suspended matter in the Kem' estuary deposits near the river mouth within the 20 psu isohaline, where sedimentation of the suspended matter takes place. The role of freshwater phytoplankton species decreases and the role of marine species increases from the river to sea and the percentage of green algae decreases and the role of diatoms increases. The organic carbon (Corg) to nitrogen (N) ratio (Corg/N) in both suspended matter and bottom sediments decreases from the river to the marine part of the mixing zone (from 8.5 to 6.1 in the suspended matter and from 14.6 to 7.5 in the bottom sediments), demonstrating that content of terrestrial-derived organic matter decreases and content of marine organic matter increases from the river mouth to the sea. The Kem' estuary exhibits a similar character of biogeochemial processes as in the large Arctic estuaries, but the scale of these processes (amount of river input of SPM, POC, area of estuaries) is different. © 2005 Author(s). This work is licensed under a Creative Commons License.