Signature patterns of chlorophyll a variability in the Greater North Sea

The greater North Sea comprises a high diversity of marine systems within a relatively small geographical area: shallow and well-mixed systems, stratified systems, continental shelf, margin and deep ocean. Across these systems, different seasonal and interannual patterns of chlorophyll a (Chl) variability may be observed. Characteristic features of the Chl dynamics are used to identify typical areas across the studied domain. Remote sensing observation of Chl (ENVISAT-MERIS) offers a comprehensive picture of the spatial dynamic of surface phytoplankton biomass over almost a decade (2003-2011). The Cloern and Jassby (2010) method of Chl signal decomposition was applied to every pixel separating the Chl signal into four components: 1) the grand mean, 2) the interannual, 3) the seasonal and 4) the residual components for the considered 9-years period. The method assumes no trend a priori over the considered period and the standard deviations for each component are compared in a relative fashion to describe the local dynamics of Chl. The pixel-wise calculated components are represented in individual maps to depict the spatial patterns of chlorophyll a variability and outline areas of high or low seasonal and interannual variability. Subsequently the decomposed Chl signal is used in a K-means cluster analysis to classify areas in the greater North Sea. These areas correspond to different categories of Chl dynamics for the considered period 2003-2011, providing a governance tool for policy makers. It is also shown how these areas directly reflect physical features of the sea, suggesting invariant structure of phytoplankton dynamics.

In: 4th International Symposium on Research and Management of Eutrophication in Coastal Ecosystems (EUTRO 2018), 18-20 June 2018, Nyborg, Denmark.