Analyses of sea surface Chlorophyll-a trends and variability in a period of rapid Climate change, German Bight, North Sea
Abstract. Satellite remote sensing of ocean colour properties allows observation of the ocean with high temporal and spatial coverage, facilitating the better assessment of changes in marine primary production. Ocean productivity is often assessed using satellite derived chlorophyll-a concentrations, a commonly used proxy for phytoplankton concentration. We used the Copernicus GlobColour remote sensing Chl-a surface concentration, comparing with the Helgoland Roads Chl-a in situ data, to investigate seasonal and non-seasonal variability, temporal trends, changes in spring bloom Chl-a magnitude and the relationship with sea surface temperature (SST) and mixed layer depth (MLD) in the German Bight (GB) from 1998 to 2020. Empirical Orthogonal Functions and Multi Covariance Analysis were employed, in order to investigate dominant spatial and temporal patterns (modes) related to the main processes of Chl-a variability and to extract the dominant structures that maximize the covariance between Chl-a and SST|MLD fields. High levels of Chl-a were found near the coast, showing a decreasing gradient towards offshore waters. A significant long-term positive trend was observed close to the Elbe estuary and adjacent area, while 30 % of the GB was characterized by a significant negative trend. No significant trends were observed during spring blooms, but the distribution of Chl-a anomalies changed when periods from 1998 to 2009 and 2010 to 2020 were compared, mostly showing a shift towards negative anomalies and decrease in variability. The Chl-a non-seasonal variability showed that the first four modes explained around 45 % with the first and second modes related to inter and intra-annual variability, respectively, observed in the temporal principal components spectral analyses. Linear trends between Chl-a anomalies and SST and MLD anomalies were weak and described by opposite signal in offshore and coastal areas, with negative correlation between Chl-a and temperature in offshore areas and positive in mostly coastal areas, while for MLD was the contrary. The monthly chlorophyll-a concentration anomalies covaried 45 % with sea surface temperature anomalies and 23 % with mixed layer depth anomalies. This study demonstrated that the Copernicus Global Ocean Colour chlorophyll-a concentration product can assess mostly of the known processes connected to chlorophyll-a surface variability in the German Bight The monthly averages were suitable to investigate long-term trends and variability for SST, but for MLD, higher frequency should be used. The causes for the significant negative trends in most of the central German Bight cannot be solely explained by the direct effect of warming. However, the rising water temperature, combined with its indirect effects on other variables, can partially explain these observed trends.
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