The key role of nitrogen in boosting algal growth in Arctic rivers

Increasing nutrient loads are impairing water quality and ecological status of aquatic ecosystems globally, but their effects on Arctic freshwaters remain less studied. Nutrients in Arctic freshwaters are increasing due to expanding anthropogenic land use and climate change induced alterations in nutrient leaching and transport. Also increased occurrences of non-native semelparous Pacific pink salmon ( Oncorhynchus gorbuscha) in the northern European rivers can enhance nutrient availability. These additional nutrients may be readily available for aquatic primary producers in often inherently (ultra-)oligotrophic Arctic freshwaters. We used an experimental field set-up with nutrient diffusing substrates to study temporal and spatial differences in the response of primary producers to increased phosphorus (P) and nitrogen (N) levels in Arctic rivers. Additional NO 3 -N enhanced algal growth (chlorophyll -a (Chl- a) amount) of epilithic biofilm. PO 4 -P alone had no effect, indicating N as the primary limiting factor for algal growth. This was particularly evident in autumn while early summer showed evidence for secondary P limitation. The response to nutrient additions was mostly dictated by the background availability of inorganic N. Our results suggest that Arctic rivers are highly sensitive to nutrient additions, especially N. Thus, any increase in nutrients can boost algal growth causing cascading effects throughout Arctic ecosystems.