Warmer Baltic Sea May Promote Harmful Algal Blooms

Global warming can signal bad news for the Baltic ecosystem. If the waters of the Baltic get warmer, it may instigate low oxygen conditions and massive blooms of cyanobacteria ("blue-green algae").

Global warming affects ecosystems in complex ways. Now, a group of scientists have shown that there is an increasing danger of algal blooms and low oxygen levels in the Baltic when temperatures rise. Algal blooms already are a major problem in large parts of the Baltic, concomitant with spread of deoxygenated bottom conditions, without life, over large areas.
Massive blooms of so-called cyanobacteria, also known as blue-green algae, have been observed in recent years. Increased supply of nutrients from intensive agriculture, primarily phosphorus and nitrogen is considered a likely cause, but temperature changes in the surface waters of the Baltic Sea have also been implicated.
A group of scientists led by Karoline Kabel and Matthias Moros at the Baltic Sea Institute in Warnemunde in Germany have now found that temperature is a major player in addition to nutrients, and that rising temperatures have been part of the increasing problem. This implies that continued warming is expected to exacerbate the problems when global warming continues as expected. Their work is now published in the leading journal Nature Climate Change.
"To isolate the effect of temperature, the scientists had to take their investigations back to times prior to large-scale industrialized agriculture, before increased nutrient supply became a major influence" says professor Eystein Jansen of the Bjerknes Centre for Climate Change in Bergen, Norway, a co-author of the study.
Medieval warm period
By using sediment cores covering the last 1000 years of sedimentation in the Baltic, scientists unraveled warm periods in the past that were also characterized by algal blooms and low oxygen content. The study goes back to the Medieval warm period 1000 to 800 years ago. In the following period, often referred to as "The Little Ice Age," temperatures in the Baltic dropped 3-4 degrees. In this cold period, conditions in the Baltic were healthier, until blooms and de-oxygenated conditions emerged again in the 20th century.
New method

The scientists used a new method, TEX-86, to estimate past temperatures. Analyses of temperature sensitive biological compounds found in the sediments could thus be used to quantify past temperature changes, at times before thermometers were available. The interpretation of the sedimentary information was additionally supported by the application of ecosystem models which were used to calculate the ecosystem sensitivity to a combination of temperature and nutrient concentrations in the Baltic.
Jansen says the novelty of this study lies in the combination of ecosystem model simulations with studies of past climates.
"This is an exciting, pioneering work that shows the usefulness of combining studies of past natural climate variations with those occurring today and those that may unfold in the future. Much of the waters that flow along the coast of southern Norway originates in the Baltic, so the results may have implications also in a broader setting," says Jansen.