Panayiota Siskos ’23
The Baltic Sea has a history of having anthropogenic river nutrients from agriculture and wastewater discharges as well as phytoplankton biomass. A combination of physical, chemical, and biological factors forms the marine ecosystem, making it difficult to identify which impacts are climate change and human caused concentration of nutrients, and studies identifying them are necessary for a healthy marine ecosystem. Seasonal ice acts as an indicator of winter severity and limits light transmission through its surface, affecting underwater light, water mixing, and nutrient circulation. Changing environmental conditions lowered sea ice coverage and thickness and water temperature increases. There have been reports that warming of the Baltic Sea led to earlier onset and longer duration in phytoplankton distribution during the spring as well as a shift in their biomass composition. Diatoms are a major component of spring phytoplankton in coastal ecosystems while vernal phytoplankton tend to have cod-water dinoflagellates. The factors which promote dinoflagellates are seen as inferior to diatoms due to low growth rate and capacity for nutrient uptake. Dinoflagellate phytoplankton has increased over time, and this study aims to examine the effects of sea ice and wind speed on spring bloom of phytoplankton and describe influencing mechanisms. The hypothesis was that thinner sea ice or low wind speed leads to early seeding and dinoflagellate accumulation, outcompeting diatoms.
To test this, ice-free conditions were simulated with wind-induced turbulence being the deciding factor, and the mechanism effect on diatoms and dinoflagellates was tested with the use of observational data. Results of models with and without sea ice were compared, and all runs had the same initial and boundary conditions while sea ice conditions had differences.
Diatoms dominated when sea ice was absent, while dinoflagellates dominated when sea ice was present. It was also found that dinoflagellate growth was dependent on temperature of the water; however, after examining dinoflagellate concentrations in the simulations, it was found that temperature difference was not large enough to cause a difference in phytoplankton growth rates. This indicated that ice prevented diatom dominance. In areas partially covered with ice, water density of surface layers was lower than that of deeper layers, allowing the lighter dinoflagellates to be on the surface while heavier diatoms sank. When diatom concentration increased in upper layers during no ice runs due to turbulent mixing of water, because of fast growth rate, they absorbed light, restricting access to dinoflagellates in lower layers. It was also found that low wind speeds cause conditions favoring dinoflagellates while higher speeds cause greater turbulent mixing. These findings are significant, as they show that due to climate change, as sea ice disappears diatoms will dominate in the future in the Baltic Sea due to dinoflagellates dependence on ice conditions. Due to simple parameters of the phytoplankton groups being limiting and preventing more comprehensive answers, future directions for this study include updating understanding of Baltic Sea phytoplankton ecology, focusing on resting cysts, and utilizing more diverse phytoplankton functional groups.
 O. Pärn , G. Lessin, and A. Stips, Effects of sea ice and wind speed on phytoplankton spring bloom in central and southern Baltic Sea. PLoS ONE 16, 1-1 (2021). doi: /10.1371/journal.pone.0242637.