Variations in cyanobacterial blooms in the Baltic Sea
by
Johanna Borgendahl
Dept. of Geology and Geochemistry, Stockholm University, Sweden
Coauthors: Per Westman (Dept. of Physical Geography and Quaternary Geology, Stockholm University)

An increasing extent of cyanobacterial blooms has been observed in the Baltic Sea during the last decades. The reason for this has been suggested to be both hot summers and an increasing load of nutrients, mainly phosphorus (P), due to eutrophication. Studies have shown that the blooms are not man induced, but a natural phenomenon that has occurred in the Baltic Sea for the last 7000 years. They can be detected using zeaxanthin, a very stable plant pigment that can be used as a biomarker for cyanobacteria. The present blooms consist mainly of nitrogen-fixing species such as Aphanizomenon spp. and Nodularia spumigena. As they can use molecular nitrogen from air, they depend primarily on the supply of phosphorus in the water to bloom, which makes them competitive at low nitrogen/phosphorus (N/P)-ratios. When the Baltic Sea turned from a freshwater into a brackish water stage ca 8000 BP, the inflow of phosphorus-rich saline water probably lowered the N/P ratio and thus made conditions suitable for cyanobacterial blooms. The brackish water led to the development of a halocline, leading to stagnant and poorly oxygenated bottom water.

Results of the analysis of a sediment core from the Bornholm Basin in the southern Baltic Sea corroborates the earlier results that extensive cyanobacterial blooms have occurred in the area during the last 7000 years. Earlier blooms were found to have been at least as extensive as today and seem to have been triggered by a low N/P ratio due to high P concentrations. There was no evidence for release of ironbound P from the sediments as a result of the anoxic bottom conditions. The increasing P concentrations in the water were probably to a large extent derived from the inflowing saline water. An additional explanation to the low N/P ratio could be an increased input of land material due to enhanced erosion caused by the climatic amelioration beginning c. 8000 BP.

Date received: March 29, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cagc-55.