Probable causes for cyanobacterial expansion in the Baltic Sea: Role of anoxia and phosphorus retention

Abstract

This study corroborates the hypothesis that nitrogen-fixing cyanobacteria have probably occurred as an important component of the phytoplankton community in the Baltic Sea at least since brackish water conditions were initiated 8,50014C yr BP. Pigment analyses indicate that extensive occurrences started prior to a sharp increase in nutrient levels dated to 7,10014C yr BP. The cyanobacteria could have functioned as a natural trigger for eutrophication in the Baltic Sea by importing nitrogen. This is also verified by a contemporaneous drop in the δ15N values from 4‰ to around 2‰. We further conclude that the spreading of cyanobacteria was probably caused by a decrease in nitrogen∶phosphorus (N∶P) in the water mass that resulted from the intrusion of oceanic water with high P levels. The fractionation of P in sediments indicated that iron-bound P was efficiently sequestered under anoxic conditions that occurred as a consequence of the establishment of a stable stratification caused by the marine intrusion. This pool only showed minor variations around 3 μmol g−1 at the freshwater-brackish water transition. All P pools except the CaCO3 fraction showed a distinct increase around 9,30014C yr BP prior to the transition. We interpreted this increase as a change in preservation of organic matter or in the source of the sediment. Slightly after 4,00014C yr BP there was a dramatic drop in all P pools without any corresponding decreases in total N and carbon. Total P decreased from around 75 to 25–30 μmol g−1. The most dramatic drop occurred in the organic bound and the detrital apatite fractions, which decreased by a factor of 3–4. We explain this as a preferential regeneration of P, especially organic P, compared to other nutrients due to more prevalent anoxic conditions.