Bianchi, T. S. Tulane University,
Westman, P. S. Stockholm University,
Rolff, C. Stockholm University,
Engelhaupt, E. Tulane University,
Andren, T. Stockholm University,

The massive cyanobacterial blooms regularly recorded in the Baltic Sea in summer are among the most impressive bacterial manifestations anywhere, and are easily observable from space. Such blooms have been documented 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 (HELCOM 1996). Here we present an 8000 year record of fossil cyanobacterial pigments, diatom assemblages and 15N variations in sediment cores from the Baltic proper. This record indicates that nitrogen-fixing cyanobacterial blooms are as old as the present brackish water phase of the Baltic Sea, starting as far back as c. 7000 years BP, soon after the former freshwater Ancylus Lake turned into the brackish Litorina Sea. The blooms were likely initiated by increased availability of phosphorus (P), from inflow of P-rich sea water and increased P release from sediments, due both to higher salinity and to periods of deep water anoxia, caused by the establishment of salinity stratification. Our results further suggest that the presently predominating nitrogen (N) limitation of Baltic Sea phytoplankton is not man-induced, but a natural phenomenon, which has endured for some 7000 years.
Day: Friday, Feb. 5
Time: 11:30 - 11:45am
Location: Eldorado Hotel
Code: SS42FR1130E