Influence of freshwater inflow on the inorganic nutrient and dissolved organic matter within coastal sea ice and underlying waters in the Gulf of Finland (Baltic Sea) Mats A. Granskog a, * , Hermanni Kaartokallio b , David N. Thomas c , Harri Kuosa d a Arctic Centre, University of Lapland, P.O. Box 122, FI-96101 Rovaniemi, Finland b Finnish Institute of Marine Research, P.O. Box 33, FI-00931 Helsinki, Finland c School of Ocean Sciences, University of Wales-Bangor, Menai Bridge, Anglesey LL59 5EY, UK d Tva ¨rminne Zoological Station, FI-10900 Hanko, Finland Received 23 November 2004; accepted 10 May 2005 Available online 6 July 2005 Abstract A study was conducted to measure the biogeochemical characteristics of freshwater plumes underlying Baltic Sea land-fast ice, and the overlying sea ice. A 40-km long transect was conducted in the northern Baltic Sea in March 2003, following a freshwater plume from its source into the fully mixed open-sea area. The spreading of river outflow below the ice resulted in a well-stratified low-salinity surface layer further out than normally occurs in the open-water period. The freshwaters were high in dissolved organic matter (DOC, DON and CDOM), and inorganic nutrients (ammonium, nitrate and silicate), although the levels of phosphate were low. In general these parameters changed concurrently with salinity in such a way that mixing was conservative. The characteristics of the ice varied from the freshwater source to the open water, with increasing salinity and brine volumes (porosity) occurring in the more open-sea stations. Coinciding with the changes in ice properties there was an increase in sea-ice algal growth in the more marine stations along the transect. Biological activity in the ice was largely confined to bottom ice assemblages. In contrast to the conditions in the underlying water, no relationship between salinity, inorganic nutrients and organic matter was observed in the ice. In particular ammonium, phosphate, DOC and DON were present in excess of those levels predicted from the dilution curves, indicating the presence of considerable DOM production by ice assemblages, inorganic nutrient uptake and remineralization within the ice. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: coastal oceanography; sea ice; river plumes; estuarine chemistry; nutrients (mineral); dissolved organic matter; Baltic Sea 1. Introduction The transfer of inorganic nutrients and dissolved organic matter (DOM) from terrestrial systems to coastal waters is a fundamental issue governing marine systems and the ultimate source of productivity in these waters (Jickells, 1998). Carbon, nitrogen and phospho- rus are introduced in both inorganic and organic forms, the proportions of which are highly dependent on the nature of the terrestrial system through which the supplying watercourse flows (Kortelainen and Saukko- nen, 1998; Hansell and Carlson, 2002; Mulholland, 2003). However, it is not only the resulting nutrient status that is of consequence, since DOM is also the major light-absorbing agent in coastal waters, both of visible and ultra-violet radiation (Højerslev and Aas, * Corresponding author. Present address: Centre for Earth Obser- vation Science, Department of Environment and Geography, Univer- sity of Manitoba, 114 Isbister bldg, Winnipeg, Manitoba, Canada R3T 2N2. E-mail address: granskog@cc.umanitoba.ca (M.A. Granskog). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.05.011 Estuarine, Coastal and Shelf Science 65 (2005) 109e122 www.elsevier.com/locate/ECSS