Phytoplankton responses to nitrogen and phosphorus enrichment in unproductive Swedish lakes along a gradient of atmospheric nitrogen deposition

Lake sampling and in situ nutrient enrichment enclosure experiments with nitrogen (N) and phosphorus (P) were conducted in unproductive Swedish lakes along a gradient of increasing atmospheric N-deposition. The regional and seasonal patterns of nutrient limitation of phytoplankton were clearly related to the amounts of N-deposition and N-inputs the lakes received. In areas of low N-deposition in northern Sweden, N-limitation of phytoplankton was evident throughout the summer season due to high catchment N-retention and very low dissolved inorganic N (DIN) inputs during the early summer. High N-deposition in the south was accompanied by high lake DIN-concentrations during the early summer and subsequent P-limitation of phytoplankton. However, P-limitation did not persist over the summer and, as a consequence of a declining DIN-pool, the lakes switched to dual- and co-limitation by N and P, and then to N-limitation. Generally, the lakes were N-limited rather than P-limited during the summer. We conclude that N-limitation is probably a natural state of the unproductive lakes studied, but P-limitation of variable intensity and duration has been induced by elevated atmospheric N-deposition.