Eutrophication assessment of the Southern North Sea using multi-satellite datasets

Abstract

The North Sea (NS) is a highly productive semi-enclosed shelf sea of the Atlantic Ocean located in the northern Europe. The southern North Sea (SNS), which is shallow and very well mixed, has long suffered from eutrophication problems. As a result, various policy measures have been taken by the NS surrounding countries (OSPAR Convention) with the aim of achieving good environmental status (GES) of the NS by 2020. The use of satellite remote sensing is a coherent method of data acquisition and provides information with generally much greater spatial and temporal coverage than in-situ data, which provide very localized information in space and time. Satellite remote sensing therefore offers an effective method to address long-term changes at the scale of an entire basin, such as the SNS. The main aim of this master thesis was to use high-level satellite gap-free products to assess the evolution of the eutrophication status and the phytoplankton dynamics of SNS over a period from 1998 to 2017. Our analyses showed a strong gradient of chlorophyll (CHL, a proxy for phytoplankton biomass) from coastal areas (higher CHL) to offshore areas (lower CHL). At the scale of the SNS domain, CHL increased between 1998 - 2004, stagnated thereafter, until 2014, when it started to decrease, probably as a consequence of the reduction of river-bone nutrient inputs. Our analyses also showed that the suspended particular matter (SPM) increased over the period by 0.042 mg/L.year. In addition, sea surface temperature (SST) also increased by 0.021°C/year in the SNS, which is positively correlated to the North Atlantic Oscillation over our period. Another convincing result is that we observed a phenological shift of about 1 month in the onset date of phytoplankton blooms. While it is difficult to identify a factor responsible for the observed phenological shift, it has been observed that the climate regime of the SNS changed during late 1990s. We therefore assume that this phenological shift is due to a change in planktonic communities associated with the rise in temperature as well as the de-eutrophication trend occurring in the SNS, which could have favored the emergence of winter diatom blooms.