Effects of increased concentrations of inorganic nitrogen and dissolved organic matter on phytoplankton in boreal lakes with differing nutrient limitation patterns

Global changes are currently contributing to increasing concentrations of nitrogen (N) and dissolved organic matter (DOM) in boreal lakes. While these changes can affect phytoplankton, the relative importance of each of these subsidies and how their effects vary with nutrient limitation patterns remain unclear. We investigated the effects of increasing N and DOM on phytoplankton in two relatively remote boreal regions of the northern US with lakes that differ in N and DOM concentrations: Acadia National Park (ACAD) in the northeastern US, and Isle Royale National Park (ISRO) in the north-central US. Nutrient enrichment assays were conducted in two lakes from each area first to assess nutrient limitation patterns. To determine the effects of increasing N and DOM, changes in phytoplankton biomass and community structure were examined in bioassay experiments using a N gradient combined with either DOM enrichment or DOM shading treatments. Algal biomass response indicated N limitation in ISRO and N&P co-limitation in ACAD. In the N gradient experiments, algal biomass in the N-limited lake increased along the N gradient, while it was unaffected in the N&P co-limited lake. Regardless of nutrient limitation pattern, algal biomass increased with DOM enrichment, with DOM additions primarily stimulating chlorophytes, diatoms, and chrysophytes. In contrast, shading with DOM had no effect on any phytoplankton response metrics. In boreal lakes, the effects of N enrichment were dependent on nutrient limitation status of the lake, whereas the effects of DOM enrichment were observed across lakes regardless of nutrient limitation status, suggesting that increasing DOM may have widespread effects on aquatic systems.