Discharges from hydroelectric power schemes as a trigger for marine algal blooms

It has been suggested that the occurrence of marine algal blooms world-wide, has increased over the last few decades due in large part to nutrient eutrophication (Smayda, 1990). Specific reference was made to the Skagerrak and Kattegat where reports of unusual blooms (North Sea Task Force, 1993) contributed to their classification as eutrophication ‘Problem Areas’ by the Paris Commission (Oslo and Paris Commissions, 1992). Part of the supporting evidence was based on two exceptional blooms, with associated deoxygenation and fish kills in 1980 and 1981 (Lindahl and Hernroth, 1983; Dahl et al., 1982; Edler, 1984). This paper shows that these blooms coincided with unusually large run-off from rivers regulated by hydroelectric power schemes. Such large freshwater discharge to the sea causes shallow and stable stratification; ideal conditions for rapid phytoplankton growth (Cloern, 1984; Therriault and Levasseur, 1985). These events, as does a similar example in 1982 in France (Rossignol-Strick, 1985), throw in doubt any direct causative relationship with eutrophication, and highlights the need for a careful evaluation of all parameters that are used as assessment criteria for eutrophication problem areas.