Cyanobacterial blooms in the Baltic Sea: Natural or human‐induced?

Abstract

Massive summer blooms of nitrogen‐fixing cyanobacteria have been documented in the Baltic Sea since the 19th century, but are reported to have increased in frequency, biomass, and duration in recent decades—presumably in response to the well‐documented anthropogenic eutrophication of the Baltic. Here, we present an 8,000‐yr record of fossil cyanobacterial pigments, diatom microfossil assemblages, and d15N variations in sediment cores from the Baltic proper. This record indicates that nitrogen‐fixing cyanobacterial blooms are nearly as old as the present brackish water phase of the Baltic Sea, starting as far back as ca. 7000 B.P.—soon after the former freshwater Ancylus Lake turned into the brackish Litorina Sea. Demonstration of cyanobacterial blooms in the Baltic prior to the greatly increased anthropogenic nutrient inputs of the 20th century is important for setting realistic goals when trying to reduce the magnitude of present blooms. Our results suggest that the presently predominating nitrogen (N) limitation of phytoplankton in the Baltic Sea proper is not man‐induced, but a natural phenomenon, which has endured for some 7,000 yr. These cyanobacterial blooms were possibly initiated by increased availability of phosphorus (P)—from inflow of P‐rich seawater and increased P release from sediments—during periods of deep‐water anoxia, caused by the establishment of salinity stratification. Efforts to restore the Baltic proper to a more oligotrophic and natural condition should take into account that nitrogen‐fixing cyanobacterial blooms are a characteristic, natural feature of this sea.