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.

Popular Abstract

Phytoplankton blooms play a crucial role in marine ecosystems and the composition of Earth’s atmosphere. The blooms provide food to higher trophic levels and are known to regulate the climate by removing CO2 from the atmosphere. Climate change affects the bloom patterns which in turn can impact the whole of marine life. Additionally, harmful algal blooms may start to be a more common occurrence with the rising temperatures of the ocean. In the North Sea, that has rich fishing grounds, such changes can have an influence on the region's economy; therefore, monitoring and understanding what drivers influence the bloom pattern is critical for better planning to counter any potential harmful changes to the environment.
This study used remote sensed chlorophyll-a (chl-a) data and TIMESAT software as a modelling tool to better understand the North Sea bloom phenomenon. Investigation of interannual fluctuations and long-term trends in phytoplankton blooms timing and intensity in the North Sea (between the years of 2002 and 2020, except 2017) was performed. Additionally, it was investigated if changes in Sea Surface Temperature (SST) and Sea Surface Salinity (SSS), as a proxy for nutrients, can explain the fluctuations in phytoplankton blooms. The results show that the timing of the spring blooms in the North Sea remained unchanged, and the chl-a abundance generally decreased; however, there were significant differences at the regional level. Concentration of chl-a was decreasing in areas of the Scottish coast and the region between Denmark, Germany and the Netherlands whilst it increased in the south. Warmer temperatures and higher salinity led to earlier and larger blooms in some regions. The remaining regions could not be defined solely by temperature and salinity, which indicate that other local environmental factors must influence the blooms in those areas. This study brings further insight into understanding the timing and intensity of phytoplankton spring bloom in the North Sea.

• author: Muszel, Krystyna
• supervisor: Marko Scholze
• organization: Dept of Physical Geography and Ecosystem Science
• course: GISM01 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Geography, GIS, North Sea, phytoplankton, bloom, phenology, remote sensing, TIMESAT software
• publication/series: Master Thesis in Geographical Information Science
• report number: 156
• language: English
• id: 9111016
• date added to LUP: 2023-02-16 16:53:17
• date last changed: 2023-02-16 16:53:17