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

Increasing nutrient loads are impairing water quality and the 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 NO3-N enhanced algal growth (chlorophyll-a (Chl-a) amount) of epilithic biofilm. PO4-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.     

Methods
The study was conducted within the Teno River catchment in northern Finland. In total, 18 spatially independent study sites were selected from the catchment area based on their location and size (1st, 3rd and 5-6th order). We used Nutrient Diffusing Substrates (NDS) to test the reactivity of epilithic algal communities to additional nutrients and, consequently, to identify the resources limiting algal productivity. The experiment was conducted in 2022 and replicated at the same study sites twice: June-July and August-September. NDS were prepared following the instructions by Tank et al. (2006; Methods in Stream Ecology. Elsevier, pp. 213-238) with minor adjustments. Sets of NDS cups (each treatment replicated four times) were incubated in the rivers for 23 – 26 and 25 – 29 days in early summer (13.6.-13.7.2022) and autumn (15.8.-16.9.2022), respectively. After each incubation period tiles were removed from the NDS cups and stored in a dark cold box until freezing to -20°C within the same day. To obtain chl-a from the tiles we used acetone extraction following closely the protocol presented in Steinman et al. (2017; Methods in Stream Ecology. Elsevier, pp. 223–241). Extraction was done separately for each tile resulting in four replicates per treatment per site and per time.