Phytoplankton in boreal SubArctic lakes following enhanced phosphorus loading from forest fire: Impacts on species richness, nitrogen and light limitation

Forest fire in peatland environments can cause enhanced loading of coloured compounds and of phosphorus relative to nitrogen resulting in reduced light penetration and nitrogen to phosphorus ratios in lake water. To determine the potential impacts of forest fire in peatland dominated catchments, we tested nitrogen (N), phosphorus (P) and light limitation of pelagic phytoplankton with in situ microcosms in three lakes from a Boreal SubArctic ecozone. To assess if phytoplankton assemblages were influenced by water chemistry changes following fire, phytoplankton species were identified from 10 lakes in unburnt and 10 lakes in burnt catchments. In the microcosm study, P limitation and concurrent N + P limitation of phytoplankton biomass were apparent (P<<0.01) in the two lake waters representing the range of N and P concentrations for lakes in unburnt catchments. In the lake with water representative of takes in burnt catchments, nitrogen limitation was observed (P<<0.01). Light limitation of phytoplankton biomass was observed in microcosms from one lake in a burnt and one lake from an unburnt catchment likely due to high water colour in both take waters (> 200 mg/L [Pt]). For the 20 surveyed lakes, phytoplankton species richness was 36% lower (P<<0.01) in lakes from burnt compared to unburnt catchments. Phytoplankton communities in all lakes in this study were dominated by cyanobacteria. Phytoplankton communities in boreal forest lakes may be particularly sensitive to catchment disturbances such as fire because changes in phosphorus and carbon loading from peatlands enhance nitrogen and light limitation.