Towards modeling growth rates of cyanobacteria in the Baltic Sea

Abstract Potential influences of various environmental factors (light conditions, mixed layer depth, water temperature) on growth rates of different cyanobacteria in the Baltic Sea (BS) are discussed. Growth rates were modeled as a function of quanta absorbed by phytoplankton. This in turn depends on phytoplankton exposition to light, which we simulated to represent realistic conditions encountered in the Baltic Sea in summer. We considered eight realistic scenarios (two cases of water clarity and four depths of the mixed layer). According to our calculations, exposition of phytoplankton to photosynthetically available radiation (PAR) in the BS can change about five times in case of the same solar surface insolation and water turbidity, solely due to changes in the mixed layer depth from 2 to 20 m. When we additionally accounted for differences in water turbidity, phytoplankton PAR exposition changed by one order of magnitude and this resulted in one order of magnitude differences in absorbed quanta for the same species of phytoplankton. Knowledge of single-species absorption characteristics allowed us to show that under the same water column conditions, the number of absorbed quanta varies significantly between different species of cyanobacteria, because of differences in their absorptive properties. The effectiveness of light absorption translated to different growth rates estimated for each phytoplankton species. Our research shows that further progress in phytoplankton modeling requires incorporating in the models improved information about optical properties, growth rates, and temperature and nutrient dependencies for different phytoplankton types.