| Summary: | The influence of sea ice seeding on the northern Baltic Sea´s pelagic phytoplankton spring bloom was studied in a laboratory experiment in which microcosms mimicked sea conditions. On March 26th, 2018, samples (ice cores and seawater) were taken from land-fast ice at a coastal station in the Gulf of Bothnia. The seeding experiment lasted for 9 days, during which a 12:12 hours light:dark incubation took place. Four different treatments (two with ice and two without it) were set up in twelve incubated microcosms. Samples for analyses were taken on days 0, 3, 6 and 9. On day 0, measurements were carried out on four melted ice cores and on seawater. On the remaining days analyses were performed on the incubated microcosms. The measured variables were: chlorophyll a, phytoplankton abundance, bacterial abundance, conductivity and nutrients (TDN and TDP). The most abundant algal species were identified in a qualitative analysis. The obtained data were processed to calculate the average and standard deviations and to assess the existence of statistically significant differences among the treatments. A significant increase in chlorophyll a, phytoplankton and heterotrophic bacteria abundances was detected. A parallel decline in the nutrient concentrations was observed. A relationship between phytoplankton´s degree of influence and cell-size is suggested: cells > 3µm were more abundant in ice than in seawater, and the opposite tendency was appreciated for cells < 3 µm. My study shows that sea ice seeding can have a marked seeding effect on the size structure of the spring phytoplankton bloom.
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