Dissolved organic N in shallow tropical lakes and reservoirs: Contribution to total dissolved N and relationships with eutrophication

Dissolved organic nitrogen (DON) plays important biogeochemical and ecological roles in aquatic systems. However, little is known about its importance in small tropical lakes and reservoirs. We hypothesized that (a) DON is higher in these systems than dissolved inorganic N (DIN), and that (b) relative contributions of DON and DIN to the total dissolved nitrogen (TDN) pool decrease and increase, respectively, with trophic status. To test these hypotheses, we measured DON and DIN in six lakes and two reservoirs at the Atlantic forest-Cerrado savanna transition in Southeast Brazil, in the rainy and the dry season. With one exception in each season, all oligo- and mesotrophic systems had DON:DIN ratios >1. DON:DIN ratios were negatively correlated with total phosphorus (TP). This pattern was due to high DIN, mainly ammonium, in eutrophic and hypertrophic systems. DON:DIN ratios <1 occurred at TP higher than 103 and 104 µg L⁻¹ in the rainy and dry season, respectively. We found a general dominance of DON in near-natural lentic systems at the Atlantic forest-Cerrado savanna transition, which was more pronounced than that reported for other regions. Human alterations to the TDN pool may affect biological communities and ecosystem functioning of these systems.