Nutrient enrichment effects of atmospheric N deposition on biology in oligotrophic surface waters - a review (ICP Waters report 101/2010)

Abstract

Atmospheric deposition of nitrogen (N) leads to enhanced leaching of N species to surface waters in natural and semi-natural ecosystems. The reigning paradigm of freshwater primary productivity is limitation by phosphorus (P), which suggests that additional N does not affect growth of algae and higher aquatic plants. This literature review shows that increased availability of N, related to atmospheric N deposition, in nutrient-poor temperate, boreal and arctic lakes affect freshwater biology. Lake sediment studies show shifts in algal communities and increases in algal growth related to higher N concentrations. Regional surveys in boreal lakes show higher chlorophyll concentrations per unit P in areas with higher N deposition, indicative of higher primary production. Experimental nutrient additions in lakes (mesocosm studies and bioassays) support the finding of the regional surveys by showing that N limitation of algal growth is common, especially under conditions of low N availability. Increased N availability can stimulate productivity of sediment- and rock-dwelling algae (benthic algae) but here data are scarce. Water plant dynamics from oligotrophic lakes relate significant plant community shifts (loss of key species, dominance of new species) to increased ammonium deposition and increased availability of N species, but other factors (CO2, liming) may also explain some of the observed changes. The findings in this report are used to evaluate critical loads for atmospheric N in freshwater habitats. New empirical loads for nutrient-N are proposed, while an approach for modelling of critical loads of nutrient N with a mass balance model is suggested.