Limnol. Oceanogr., 44(6), 1999, 1477–1485

Access to bioavailable nitrogen often limits primary production in marine and freshwater ecosystems. Around 70% of nitrogen transported by rivers worldwide consists of dissolved organic nitrogen (DON), but its bioavailability has been poorly investigated. To assess the potential bacterial growth on DON, we developed a bioassay employing natural DON and bacterial inocula in medium manipulated to make N the limiting nutrient. We analyzed the bacterial utilization of the high-molecular-weight fraction of DON isolated by ultrafiltration from three wetlands in South Sweden throughout the year. The bioavailability of low-molecular-weight and bulk DON was also analyzed in one of the wetlands, where inorganic nitrogen concentration was sufficiently low and did not interfere with bioassays. The bioavailability of bulk DON in the latter wetland varied from 2% to 16%, suggesting that DON is an important nitrogen source for the biota of coastal waters. DON may be the dominant input of bioavailable nitrogen during summer, when nitrate concentrations in rivers decrease and DON bioavailability increases. Marine bacterioplankton assimilated a substantially larger fraction of DON than did freshwater bacterioplankton, on average by a factor of 2.4. This finding indicates that the susceptibility of DON to bacterial mineralization increases as it is transported from freshwaters into saline environments. Access to bioavailable nitrogen often limits primary production in marine environments (Ryther and Dunstan 1971; Granéli et al. 1990) and in humic (Jansson et al. 1996), ultraoligotrophic (Axler et al. 1982), and phosphorus-sufficient (Schindler 1977) lakes. Enhanced eutrophication of aquatic ecosystems due to nitrogen loading is a worldwide problem (Meybeck 1982). Research on the role of nitrogen in aquatic eutrophication is almost exclusively focused on dissolved inorganic nitrogen (DIN) forms. However, about 70% of the dissolved nitrogen transported by rivers worldwide (1012 g yr21) is dissolved organic nitrogen (DON; Meybeck 1982). DON comprises 35–50% of the total nitrogen reaching coastal waters from Sweden (Fleischer and Stibe 1989). In temperate regions, the share of DON in total nitrogen increases during summer, when nitrate concentrations decrease in many freshwater (Pardo et al. 1995) and marine (Carlsson and Granéli 1998) environments. Usually, only a small fraction of DON is in a form readily available for algal uptake, e.g., urea or free amino acids (Antia et al. 1991). Dissolved organic matter (DOM) is mainly composed of high molecular weight (HMW) polymeric compounds, particularly fulvic and humic acids (Thurman 1985). Some of the HMW DON can be assimilated by bacterioplankton, be mineralized in the pelagic food web, and become available for phytoplankton. Several researchers (Tranvik 1988, 1990; 1 To whom correspondence should be addressed.