Chlorophyll-a Trends in OSPAR Assessment Areas: Applying and Evaluating a New Statistical Approach

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This report expands on a newly proposed method for assessing chlorophyll-a (chl-a) concentrations and eutrophication status within the OSPAR framework. The method aggregates in-situ and Earth Observation (EO) data on a grid-month-year basis before calculating the growing season mean, with a confidence rating derived from the sample size for each grid. This approach aims to reduce biases introduced by limited and sporadic in-situ sampling, which often disproportionately influences chl-a concentrations. Compared to the applied method on COMPEAT, which equally weights in-situ and EO data, the proposed method leads to lower mean growing season chl-a concentrations and a higher Ecological Quality Ratio Scaled (EQRS), often resulting in improved eutrophication status. This difference is particularly evident in areas with limited in-situ data, such as small river plume assessment areas.
The report also examines the influence of grid resolution, used in the proposed method, on assessment outcomes, recommending a 5×5 km grid for areas smaller than 205 km2 and a 10×10 km grid for areas larger than 580 km2. A confidence rating system based on sample size further enhances the robustness of the assessment, providing a clearer understanding of the uncertainty involved.
The proposed method offers a transparent, objective, and spatiotemporally representative approach to eutrophication assessments, aligning with OSPAR’s goals for harmonized data aggregation. Its broader applicability across OSPAR assessment areas demonstrates its potential for improving future assessments and policy decisions related to marine eutrophication.