Impact of Sea Surface Temperature and Salinity on Phytoplankton blooms phenology in the North Sea

Abstract: Various environmental forcings can affect phytoplankton blooms, resulting in changes in bloom timing and abundance, which can have many far-reaching consequences. Understanding bloom phenology is crucial to predict and counter any potential harmful changes. The North Sea is an important fishing ground, and any changes in phytoplankton bloom patterns can have a significant impact on the region's economy. The aim of this thesis was to assess interannual and long-term changes in the timing and abundance of phytoplankton blooms in the North Sea, and to determine if Sea Surface Temperature (SST) and Sea Surface Salinity (SSS), as a proxy for nutrients, can explain the variability of those parameters. Merged satellite data from 4 sensors: SeaWiFS, MERIS, MODIS AQUA and VIIRS were used to evaluate bloom patterns, using the chlorophyll-a concentration between 2002 and 2020 (except 2017) inside TIMESAT software. Due to satellite data limitations, it was possible to survey only spring bloom, as the data is not available year-round. There was no evident change in the spring bloom timing over the studied period. The chl-a maximum concentration values decreased over time, with statistically significant results for the region A, along the Scottish coast (Pearson r=-0.75, p<0.001) and region F, between the coast of Denmark, Germany, and the Netherlands (r=-0.68, p<0.05). The southern region (region E) was an exception from the general pattern and showed an increase of the chlorophyll-a concentration during the investigated years (r=0.47, p<0.05). SST showed a medium correlation with chl-a data (r=0.54, p<0.001), but it did not explain the fluctuations of the peak values of chl-a in specific regions, proving the need to take into consideration local environmental processes. SSS showed only a negative correlation with bloom parameters in the Dogger Bank region (region D) explaining the bloom timing fluctuations. Higher salinity correlated with a faster start of bloom (r=-0.60, r=-049 with a p<0.05 for salinity in summer and in winter respectively) and faster peak time (r=-055, p<0.05 for salinity in summer). In conclusion, this study presents a further insight to spring phytoplankton bloom timing and bloom intensity in the North Sea. The phytoplankton bloom trends differed across the study area and could not be explained only by the SST and SSS. This variability in bloom parameters proves that other local environmental factors need to be considered to understand better which processes influence the bloom phenology.