Effects of reduced nutrient inputs, climate change and heat waves on microalgae (phytoplankton) in our coastal waters

Author: Gerald Schernewski (IOW)

Our coastal waters are benefiting from a sustained reduction in external nutrient loading. However, this development is being overshadowed by climate change, which is leading, among other things, to rising temperatures and more frequent heat waves. These changes can significantly alter our coastal water ecosystems and affect water quality and phytoplankton. There are concerns that potentially toxic cyanobacteria could proliferate in the summer.

Summer cyanobacteria blooms in the Oder Lagoon 2001

In cooperation with the State Agency for the Environment, Nature Conservation and Geology (LUNG), the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) has combined long-term observation data from the last 40 years with a spatially and temporally high-resolution ecosystem model to investigate these changes and future prospects in detail.

Despite the significant reduction in external nutrient loading, the average annual phytoplankton biomass and the long-term species composition in the Szczecin Lagoon have not yet changed significantly, although extreme summer blooms have tended to decline. In summer, cyanobacteria, predominantly the potentially toxic Microcystis species, can reach up to 90 percent of the total algal biovolume. The variability of phytoplankton biomass and composition between years is very high. However, heat waves and cold spells lasting a few days or weeks do not show any immediate or lasting effects. Nevertheless, the results suggest that cyanobacteria, particularly Microcystis, benefit from hot summers. Model simulations indicate that an increase in air temperature of 1.5 °C would increase the average summer algal biomass by 4–9%, thus promoting cyanobacterial growth.

Summer cyanobacteria blooms in the Oder Lagoon 2001

These results are the product of a collaboration between the DAM projects PrimePrevention and Coastal Futures II.

Publication:

Schernewski, G., Schneider, M., Neumann, T., von Weber, M. (2025): Phytoplankton Dynamics in a Large Lagoon: Nutrient Load Reductions, Climate Change, and Cold- and Heatwaves. Environments 12, 370. https://doi.org/10.3390/environments12100370