Allochthonous dissolved organic matter (DOM) plays an important role in the food web of boreal lakes in the temperate zone by providing energy and nutrients (Tranvik, 1992). It also governs environmental conditions like light attenuation of the water mass (Schindler et al., 1996). Allochthonous DOM is together with primary production the primary carbon source of the food web in these lakes. In some lakes rich in humic substances (HS), DOM is more important than the primary production for the microbial production (e.g. Jansson et al., 1999).
However, our knowledge about fate and kinetics of the terrestrial DOM and thus TOC (total organic carbon) is poor. In spite of this, it has important implications for the terrestrial and marine ecosystems as well as the global carbon cycle. The role of rivers and lakes in this degradation process has been studied in the past and one result is that only those systems with long hydraulic residence time may significantly alter the TOC composition and concentration (Curtis, 1998; Pers et al., 2000). In fact, peat and sediments in lakes and coastal waters represent the only significant long-term carbon sinks in the ecosystem of northern Scandinavia (Erikssson, 1991 ). The riverine organic matter that escapes these systems will be most likely degraded in the marine environment. In fact, of the annual sedimentation of roughly 20 g C m·' yr·' in the Bothnian Bay, up to 90% may be remineralised, the carbon is probably released as COz, and only about 2 g C m·' yr'1 is assumed to be sequestered (Eimgren, 1984). These figures support a rapid decomposition of terrestrial humic substances in the marine environment. Also, Carlsson and Graneli (1993) have shown that organic bound nitrogen in humic substances may enhance marine phytoplanktongrowth. Thus the humic substances support the marine production in this Bay.