Transition from P- to N-limited phytoplankton growth in an artificial lake on flooded cutaway peatland in Ireland

Question: Which nutrient limits primary production in a lake created by flooding industrial cutaway peatland? Location: Clongawny Lake (53 degrees 10' N, 07 degrees 53' W), County Offaly, Ireland Methods: Nutrient concentrations in lake water and the dynamics of phytoplankton populations were monitored over a 38-month period. The ratio of dissolved inorganic nitrogen to total phosphorus (DIN:TP) and nutrient enrichment bio-assays were used to investigate temporal changes in nutrient limitation. Results: Primary production in the new lake was phytoplankton-driven due to the scarcity of recolonizing macrophytes. Phytoplankton growth was initially phosphorus-limited. The runoff of phosphate fertilizer from an adjacent coniferous forestry plantation raised the TP concentration of lake water 5.5-fold. Consequently, the biovolume of phytoplankton increased 30-fold, and chlorophyll-a concentrations increased eightfold, reaching hyper-eutrophic levels. A concurrent depletion of nitrogen in lake water reduced the DIN:TP ratio from 17.8 to 0.6, and phytoplankton growth rapidly became nitrogen-limited. Phytoplankton composition shifted from dinoflagellates to minute, unicellular chlorophytes, with a coincident decline in species diversity. Cyanobacteria did not proliferate, most likely due to the acidic nature of the lake. Conclusions: Results illustrated the vulnerability of newly created cutaway peatland lakes to developing severe phytoplankton blooms and coincident secondary nitrogen limitation in the presence of moderate external phosphorus inputs.