Eutrophication, acidification and abundance of Baltic herring larvae in the Vaasa archipelago, Gulf of Bothnia in 1983–1991

Surface water nitrogen and phosphorus concentration, pH and chlorophyll-a content were monitored at 28 stations in the Vaasa archipelago (63°05´N, 21°30´E) in June–August 1983–1991, and pH additionally during the spring runoff maximum. Herring larvae were sampled weekly or biweekly in May–August during the period. High levels of nutrients and chlorophyll-a were found in the innermost archipelago receiving non-point runoff load and municipal sewage load. Elevated nutrient and chlorophyll-a levels extend to the outer border of the archipelago. River water entering the area is acidified by acid sulphate soils, and pH values of 5 or lower were measured in spring each year in runoff and in the innermost bays. The areas affected by acid runoff have low levels of phosphorous and chlorophyll-a. Herring larvae densities were highest in the outer part of the study area, and there was a negative correlation between larval density and the chlorophyll-a content. It was concluded that the entire study area is to some extent eutrophicated by runoff and sewage nutrient load. Recurring acidity limits phytoplankton production in the innermost bays. Acidity or excessive eutrophication may preclude the utilization of the innermost bays for feeding by herring larvae.