Impact of increased river discharge on the phytoplankton community in the outer Oslofjord,Norway

An extraordinary flood event occurred in SouthernNorway in May–June 1995. The impact of fresh waterdischarge from the river Glomma on phytoplanktonspecies composition and on primary production duringthe flood was studied. The phytoplankton community wasdominated by species usually found in the fjord, thediatom Skeletonema costatum dominated thecommunity together with cyanobacteria. Potentiallyharmful species (e.g. Chrysochromulinapolylepis) were found in low cell densities. No largeincrease in nutrient concentrations were found.Primary production and uptake rates of nitrate andammonium were very low in low-salinity water near theriver outlet. Nutrients from the river water wereefficiently utilized in the open part of the fjord.The flood trigged a phytoplankton bloom dominated byharmless species commonly observed in the fjordplankton. This revised version was published online in July 2006 with corrections to the Cover Date.