Nutrient Status of Phytoplankton Blooms in Norwegian Waters and Algal Strategies for Nutrient Competition

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Abstract
During blooms in Norwegian waters phytoplankton communities are phosphorus limited in fresh and brackish waters, and balanced or even nitrogen limited in high-salinity marine waters. This results from the high N/P ratio for available nutrients before onset of blooms in freshwater relative to seawater (N/P (atoms) >100 and 12–16, respectively). Algal N/P ratios at nutrient saturation vary between 8 and 27 among species, and average 16. A low ratio at nutrient saturation may imply P limitation even in high-salinity waters (e.g. Skeletonema costatum). In general, nutrient deficiency becomes more pronounced as the biomass increases. Addition of sewage shifts natural systems toward N limitation, which may therefore be a secondary effect of accelerated eutrophication. interspecific competition in nutrient-limited communities depends on the nutrient requirement of individual species and the mode of nutrient uptake. In our example the desmid Staurastrum luetkemuellerii outcompetes the cyanobacterium Microcystis aeruginosa in a chemostat when the nutrient supply is continuous; the opposite happens when nutrient supply is pulsed.