Earlier spring phytoplankton blooms disrupt the balance in the Baltic Sea

Zooplankton are captured using a large net. Photo: Michaela Lundell

Plankton or microscopic organisms floating in the water form blooms at different times over the year when conditions of temperature, light and nutrients are ideal, turning the water brown in spring and green in summer. But how these multiple blooms are changing with climate warming and how the diversity of zooplankton, their main grazers, are responding has been difficult to measure. A new study utilizing continuous data collected by the Swedish National Pelagic Monitoring from the Baltic Sea shows that spring blooms are occurring earlier in the year but the timing of zooplankton peaks, the main prey for commercial fish, are not changing, causing a decline of a preferred fish prey.

"We find that the earlier occurring spring blooms are related to a decline of a key copepod species that is an important prey for fish" says Kinlan Jan, a PhD student at Stockholm University and first author of this article. "The reason for this is because spring blooms are starting about one week earlier today as compared to ten years ago and blooms peak in April when water temperature is below five degrees. Copepods have not had the time yet to grow and reproduce at this cold temperature and appear too late when the spring bloom is gone."

The dominating zooplankton species instead peak during the summer month and co-occur with cyanobacteria blooms that are also starting earlier and have become more intense in some areas. But the timing of copepod and cladoceran zooplankton was remarkable stable, and did not change with increasing temperature. This highlights that plankton species are responding differently to climate change, which largely depends on the cues that trigger growth. For some species the onset of growth is related to temperature but for others to light or driven by an internal clock, which is not changing with climate warming.

The study also found newly arriving autumn blooms in the northern central Baltic Sea and that these blooms have moved northwards. How this is affecting fish reproduction will be questions of coming projects.   

Studies of marine systems predicting how climate is affecting plankton blooms typically assume one single zooplankton group that follows the spring bloom. However, this study shows that spring, summer and even autumn phytoplankton blooms support fish and bird production. It also shows that the diversity of zooplankton response to temperature need to be considered to understand how climate affects marine ecosystems.

Analyzing count data of multiple phytoplankton taxa and zooplankton species, the researchers analyzed how the timing and magnitude of blooms has changed at three stations of the central Baltic Sea over the last 15 years. If the timing of plankton blooms is not shifting synchronously among all species, can disrupt the delicate balance between species and particularly between predators and their prey availability, and ultimately harming fish production. 

"The exceptional high quality of the Swedish National Monitoring datasets made it possible for the first time to study changes of spring, summer and autumn seasons of multiple species in marine systems. This gives valuable insights in how climate and environmental change is affecting the timing of these key biological events", says Prof Monika Winder and leader of the research project that financed the study.

More information

The article "Plankton blooms over the annual cycle shape trophic interactions under climate change" is published in the scientific journal Limnology and Oceanography Letters.

The research was funded by the Swedish Research Council for Sustainable Development (Formas).