A recently published study by researchers at the Swedish Board of Fisheries among others, suggests a link between the reduced size of the cod population and the increase in algal blooms in the Baltic Sea.
Using field data collected during a 33-year period, the researchers conclude that the dramatic reduction of the cod population has consequences for the entire ecosystem of the Baltic Sea, all the way down to the level of the zooplankton.
The research by Casini et al indicates that there is a cascading effect in the ecosystem. The decrease in the main top predator, the psicivorous cod, cascaded down the food web, directly affecting its main prey, the zooplanktivorous sprat, and indirectly the summer biomass of zooplankton and phytoplankton. The cascading effects are strongest in ecosystems with a low biodiversity, like the Baltic Sea.
The cod drop is most likely due to high fishing pressure, but was also facilitated by recruitment failure caused by the lack of salt- and oxygen-rich water inflows from the North Sea, which reduced the water volume suitable for cod reproduction. The sudden increase of sprat after the cod collapse occurred probably because cod is the most important piscivore in the Baltic ecosystem and sprat its main prey.
The summer phytoplankton community is largely constituted of cyanobacteria, whose intense blooms have before been attributed to eutrophication and high temperature. However, research shows that cyanobacterial blooms can be suppressed before the build up process if there is sufficient amount of grazing zooplankton in the system. In the Baltic Sea, zooplankton feed also on the toxic species of cyanobacteria.
These findings highlight the fact that, in order to dampen the summer blooms of phytoplankton, effort should be made not only to control anthropogenic nutrient inputs but also to prevent large changes at the higher levels of the food web.