The North Sea Light Climate: Analysis of Observations and Numerical Simulations.

Recent studies have indicated changes in the light climate of the North Sea. An overall reduction of water clarity over the twentieth century could be observed in measurements, and more recent analysis suggests that these trends continue. Inorganic sediment is often named one of the driving factors in these changes and it has been shown to locally increase. With 20 years' worth of satellite‐derived sediment data, we were able to conduct basin wide investigations of the temporal dynamics of the biogeochemical state of the North Sea. To identify the impact of inter‐annual and seasonal changes in sediment, we fed from two different remote sensing sources (GlobColour & IFREMER) into a 3D coupled hydrodynamic and biological model. The light scheme in the Carbon Silicon Nitrogen Ecosystem model (CoSiNE) was modified to account for sediment specific attenuation. We performed a total of five numerical experiments for the period of 2000–2017. The main two experiments were conducted using monthly averaged data. Additionally, as controls, one experiment with annually averaged and one with a 20 years average of sediment, as well as a fifth one without sediment were performed. Our model showed a clear relation between changes in sediment and water clarity. Phytoplankton biomass was reduced only in areas with high nutrient availability. Plain Language Summary: Studies using satellite or field data have shown that the North Sea has undergone decreases in water clarity and increases in sediment content, while simultaneously showing decreases in phytoplankton biomass over the past century. Declining water clarity would imply inhibited photosynthesis and phytoplankton growth. Phytoplankton itself reduces water clarity. Therefore, the coupling between the three quantities is complex. In this study, we used satellite‐derived sediment data of the years 2000–2017 from two different sources in a three‐dimensional model to account for sediment specific effects on underwater light. This way, we were able to determine how changes in sediment content affect water clarity and phytoplankton biomass. The two data sets show different long‐term behavior, with one showing increases and the other decreases. Changes in sediment directly corresponded to trends in water clarity. Long‐term changes in the light climate led to changes in biomass in areas of high nutrient availability. Key Points: Satellite‐derived sediment concentration data is fed into a model to study long‐term trends in water clarity over the period of 2000–2017Changes in sediment content correspond to changes in water clarity in the modelPhytoplankton biomass changes do not directly correspond to changes in water clarity in the model [ABSTRACT FROM AUTHOR]