Utilization of terrestrially derived dissolved organic nitrogen by aquatic bacteria

I investigated the bacterial utilization of terrestrially derived dissolved organic nitrogen (DON) in aquatic ecosystems, and assessed the potential role of river-born DON in marine eutrophication. To determine the bioavailability of DON, I developed a bioassay employing N-limited bacterial regrowth cultures, which were calibrated with additions of inorganic nitrogen. I found that between 2 and 72% of DON from various rivers and wetlands was available for microbial uptake. On average, a two-fold larger fraction of terrigenous DON was available to marine bacterioplankton compared to freshwater bacterioplankton. Saline cultures also exhibited higher cell-specific aminopeptidase activity than freshwater cultures. Thus, enhanced utilization of organically bound riverine nutrients in estuarine mixing zones may increase productivity in coastal marine waters. In streams, bioavailability of soil-derived DON increased during spring floods. In medium-size and large rivers, DON bioavailability was high during summer, plausibly due to algal production of labile organic material. In a wide spectrum of waters, I did not observe UV-induced changes in the bioavailability of bulk DON. Thus, the earlier reported photochemical stimulation of DON bioavailability does not seem to be ubiquitous in aquatic ecosystems. In most analyzed samples, concentrations of bioavailable DON exceeded concentrations of urea and amino acids, suggesting that bacteria utilized other DON compounds. Spectral properties of dissolved organic matter, especially the DOC-specific absorbance, were good indicators of DON bioavailability, implying that nitrogen bound into high molecular weight organic compounds, rich in aromatic groups, was more resistant to microbial degradation than algal-derived DON. According to my rough estimate, riverine input of bioavailable DON to the Baltic Sea during summer stratification is four-fold lower than the input of inorganic nitrogen. Thus, riverine DON does not seem to play a major role in the eutrophication of the Baltic Sea, mainly due to high loads of inorganic nitrogen from the densely populated southern part of the watershed. However, riverine DON may significantly enhance primary production in N-limited pristine coastal waters. Assuming DON bioavailability to be 30% (average for the rivers from the Baltic Sea watershed), and DON to constitute 70% of the total N in world rivers (literature value), around 40% of the bioavailable nitrogen loading globally to coastal areas is DON.