Effects of sea ice on Baltic Sea eutrophication

This study investigates the influence of sea ice on eutrophication in the Baltic Sea ecosystem by comparing simulations from 1953 to 2017, with ice and without ice cover. We assessed the impact from ice cover by using eutrophication indicators defined by the Marine Strategy Framework Directive (MSFD), the Dia/Dino index and the newly proposed Trophic Transfer Index (TTI). Five out of six indicators suggest a negative impact of sea ice on the eutrophication status of the Baltic Sea, with a marked increase in ice impact observed in the early 1970s, followed by a decline in the late 1980s. The linear correlation between ice impact on MSFD indicators and nutrient loads suggests that the influence of ice becomes more pronounced under conditions of elevated nutrient loads. Around 1988, both the TTI and Dia/Dino index indicate eutrophication amplification, with ice cover significantly impacting both indicators (approximately 30 %), leading to a shift towards dinoflagellate dominance. While ice influences plankton timing and ecosystem structure, nutrient loads remain the primary driver of the timing of spring and summer blooms. According to our study the reduction in sea ice cover due to climate change, could contribute to the faster achievement of the Baltic Sea's Good Environmental Status.