Does DOM composition help explain bioavailable macronutrient concentrations in organic matter-rich freshwaters?

Dissolved organic matter (DOM) is a major source of macronutrients in freshwaters, yet has variable and poorly understood bioavailability to bacteria and other organisms. Because intrinsic variation in bioavailability is caused by chemical structures of organic nutrients, DOM composition data should improve predictions of bioavailable resource pool sizes. We hypothesized that bioavailable organic carbon (C) and nitrogen (N) fractions are made up of freshly produced humic- and protein-like DOM, respectively, whereas bioavailable phosphorus (P) is linked to microbially-derived DOM with potential organophosphate content and/or to chemical structures associated with DOM-Fe-phosphate complexes. These ideas were tested in eight, unproductive and organic matter-rich stream and lake sites, where we performed C, N and P bioassays with bacteria in combination with analyses of DOM composition using fluorescence excitation-emission matrix (EEM) analysis. Bioavailable C followed the predicted patterns, with strong links to fluorescent features indicating recently produced DOM. Surprisingly, bioavailable N was poorly related to DOM composition, including protein-like fluorescence, and was instead driven mainly by the amount of inorganic N. Bioavailable P was best linked to microbially-derived organic components. The standard nutrient variables explaining most of the bioavailable total dissolved C, N and P, respectively, were dissolved organic carbon, dissolved inorganic nitrogen and total phosphorus. In addition, DOM composition variables made significant unique contributions to explaining the variance in bioavailable C (19%), N (13%) and P (18%). Overall, DOM composition analysis is a promising tool to improve prediction and develop our understanding of bioavailable macronutrients in organic matter-rich freshwaters.