6. Concluding remarks
The present study expands on the proposed methodology developed in the previous CBS report for assessing chl-a and eutrophication status within the OSPAR framework. By first aggregating in-situ and EO data on a year-month-grid basis and deriving confidence ratings from concomitant sample sizes, the method provides a more objective and spatiotemporal representative approach to eutrophication assessment.
Across assessment areas, results consistently showed that mean growing season chl-a concentrations derived with the proposed method were slightly lower and less variable than those currently published on COMPEAT with equal weighting of EO and in-situ data. This reduction reflects the mitigation of biases caused by uneven or limited in-situ sampling, particularly in coastal waters where a few high-value observations can disproportionately influence the mean. Consequently, the proposed method often produced higher EQRS values and, in several cases, a more favorable eutrophication status, while simultaneously providing a clearer indication of the underlying uncertainty through the confidence rating.
The proposed method aligns closely with the direction outlined in OSPAR’s Annex 14: Improving and harmonizing methods for data aggregation in space, time and between data types7), which emphasizes the need for transparent, harmonized procedures to integrate diverse data types and avoid spatial or temporal bias. Similar to examples one and three described there, the method first aggregates EO and in-situ data on a common spatial grid and at a monthly temporal scale before deriving growing-season means. This first aggregation step prevents bias toward high-frequency EO data and ensures that each grid cell and period is represented by a single, balanced statistic with an associated confidence rating. In doing so, it operationalizes the principles OSPAR identified as essential for future eutrophication assessments, combining methodological consistency and transparency, while demonstrating their feasibility in practice. However, unlike the examples in Annex 14, we did not apply separate weighting to combine the different datatypes, because the pronounced spatiotemporal imbalances between EO and in-situ data would have introduced bias.
Analyses of grid resolution further showed that assessment outcomes were largely stable across scales, although spatial detail does decrease at coarser resolutions, particularly in small river plume areas such as MPM and SCHPM2. Using a 5×5 km grid in the first aggregation step might strike a practical balance in these smaller areas: it captures enough spatial representativeness while maintaining sufficient statistical confidence. Based on the results, we recommend that a 5×5 km grid should be applied to areas smaller than 205 km2, and that a 10×10 km grid can be applied to areas larger than 580 km2. Further analysis is needed to determine the exact surface area threshold between resolutions. These insights support a flexible, area-specific approach rather than a one-size-fits-all resolution.
Applying the proposed method beyond the OSPAR areas that overlap with Dutch waters, including CFR, KC, and OC, demonstrated its broader applicability. In these regions, the method again produced more stable and representative chl-a means and eutrophication classifications, reducing the disproportionate influence of limited in-situ measurements.
Overall, the proposed method strengthens the scientific basis of eutrophication assessments under OSPAR by (1) integrating EO and in-situ data in a statistically consistent, spatiotemporally explicit framework, (2) quantifying uncertainty through objective confidence ratings, and (3) providing a transparent and adaptable approach for regions of varying size and data availability. Its implementation would contribute to more consistent and policy-relevant eutrophication assessments across the OSPAR maritime area. In this context, it is important that the right management signals are sent, and therefore the relation of the assessment results to a verified and, if necessary, adjusted eutrophication threshold value must also be correctly established.