Again and again there is mass multiplication of cyanobacteria in the Baltic Sea. To find out which factors favor this, researchers have now looked far back into the past – with sediment drill cores that are up to 7,000 years old. Biomarkers in these drill cores and climatic data reveal that blue-green algae blooms are less dependent on nutrients than expected. Instead, the water temperature plays the decisive role.
Blue-green algae, the so-called cyanobacteria, spread rapidly in water, especially in the summer months. The inflow of fertilizers from agriculture increases the nutrient concentration in the water and the warm temperatures also favor the development of algal blooms. Not only does it make swimming in affected waters less attractive, it can also threaten ecosystems such as the Baltic Sea. Because when the algae die, they sink to the sea floor. When they decompose, oxygen is consumed and low-oxygen “death zones” are created.
Searching for traces at the bottom of the Baltic Sea
Researchers have been observing extensive algal blooms in the Baltic Sea time and again since the 1980s. They are clearly visible in satellite images. But the reasons for these explosive growth phases of cyanobacteria remained unclear. Because so far the observations did not go back far enough to show connections with climate fluctuations or the nutrient input into the Baltic Sea. That is why researchers led by Jérôme Kaiser from the Leibniz Institute for Baltic Sea Research in Warnemünde have used two biomarkers to look further back into the past. These markers are produced by the cyanobacteria found in the Baltic Sea. These are two hydrocarbons that bacteria produce from fatty acids. They have the advantageous property of not decomposing even within millennia and of remaining in the sediment for years.
To find out how the amount of these biomarkers in the sediment is related to the intensity of the algal bloom, the scientists carried out experiments with sediment traps. These are funnel-like vessels that are anchored to the seabed and collect everything that falls into them in collecting containers, separated by weeks. The evaluations showed that the biomarkers not only indicate the presence of blue-green algae, but also allow conclusions to be drawn about their density. On the basis of these findings, Kaiser and his team then examined a sediment drill core from the Baltic Sea dating back 160 years.
Hardly any influence of over-fertilization
The researchers found only low densities of cyanobacteria in the sediment core until 1920. After that, periods alternated with high and low frequency. “Interestingly, there does not seem to be a significant increase in cyanobacteria incidence in the 1950s,” report Kaiser and his team. “During this time the over-fertilization and the oxygen consumption of the Baltic Sea increased considerably.” Accordingly, the increased supply of nutrients seems to play only a subordinate role for the cyanobacteria blooms.
The scientists found a positive correlation between cyanobacteria and sea temperature: in the years when the surface temperature of the Baltic Sea was higher in summer, there was also an increase in blue-green algae. As the researchers explain, this is related to two effects: On the one hand, a hot summer also increases solar radiation, which increases the growth of algae. On the other hand, a higher surface temperature stabilizes the stratification of the Baltic Sea water and also the growth of cyanobacteria. However, the drill core data also showed that these temperature fluctuations over the past 160 years are mainly due to natural influences such as cyclically fluctuating ocean currents.
Long-term effect of warming
To find out whether long-term warming, such as that caused by climate change, also had an influence, Kaiser and his colleagues examined another 7,000-year-old sediment core from the Bottensee, a basin in the north of the Baltic Sea. This time comprises a warm period that began with the end of the last ice age. It is particularly interesting for today’s climate research because the average temperatures in the northern hemisphere were 1 to 1.5 degrees Celsius higher than today.
The result: In the section of the sediment core from this time, the content of biomarkers was up to 100 times higher than in today’s central Baltic Sea. This suggests frequent and strong cyanobacterial blooms in this warm climatic period. The biomarkers not only enable a glimpse into the past: “They showed us that cyanobacteria can react drastically to climate anomalies. In view of the ongoing global warming, we should keep that in mind, ”says Kaiser.
Source: Leibniz Institute for Baltic Sea Research Warnemünde, Article: Biogeosciences, doi: 10.5194 / bg-17-2579-2020