1. Introduction

Accurate knowledge of chlorophyll-a (chl-a) concentrations is essential for understanding primary production and evaluating eutrophication status in marine ecosystems. Within the OSPAR framework, chl-a serves as a key indicator for assessing eutrophication under the Common Procedure Eutrophication Assessment Tool (COMPEAT). In the previous Statistics Netherlands (CBS) report, the COMPEAT methodology used to assess chl-a concentrations in the OSPAR maritime area was evaluated, specifically focusing on OSPAR assessment areas such as the Southern North Sea (SNS), Meuse plume (MPM), and Rhine plume (RHPM)¹⁾ that (partly) overlap with Dutch waters. A newly developed method was proposed (henceforth referred to as the ‘proposed method’) to calculate growing season means and for determining confidence ratings, by first aggregating both in-situ and Earth Observation (EO) data on a grid basis, before assigning confidence ratings based on the sample size per grid. Under the COMPEAT approach currently applied by OSPAR (during the COMP4 assessment; henceforth referred to as the ‘applied method’), EO and in-situ data are combined with equal weighting (50:50), provided that in-situ data show a high confidence as otherwise they will be down-weighted, when calculating the growing season mean chl-a concentration per assessment area.

This report builds upon this proposed method and explores its implications in more detail by 1) revising a figure from the previous report to more accurately represent the temporal coverage of in-situ measurements and providing trends for the coastal OSPAR assessment areas that overlap with Dutch waters, 2) examining the method’s impact on the chl-a assessment for the three previously examined OSPAR assessment areas, 3) focusing on the smaller river plume OSPAR assessment areas to identify an optimal grid size that balances the reliability of growing season chl-a means with a high confidence rating, and 4) applying the method to additional OSPAR assessment areas that were not part of its initial development, to test its broader applicability and evaluate its influence on chl-a assessments.