Bioavailability of wetland-derived DON to freshwater and marine bacterioplankton

Access to bioavailable nitrogen often limits primary production in marine a nd freshwater ecosystems. Around 70% of nitrogen transported by rivers worl dwide 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 bacter ial utilization of the high-molecular-weight fraction of DON isolated by ul trafiltration from three wetlands in South Sweden throughout the year. The bioavailability of low-molecular-weight and bulk DON was also analyzed in o ne 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 impo rtant nitrogen source for the biota of coastal waters. DON may be the domin ant input of bioavailable nitrogen during summer, when nitrate concentratio ns in rivers decrease and DON bioavailability increases. Marine bacteriopla nkton assimilated a substantially larger fraction of DON than did freshwate r bacterioplankton, on average by a factor of 2.4. This finding indicates t hat the susceptibility of DON to bacterial mineralization increases as it i s transported from freshwaters into saline environments.