5. Discussion

The assessment of chl-a levels is essential for understanding eutrophication in the OSPAR maritime area, and the current method of combining in-situ and Earth Observation (EO) data provides a valuable, albeit imperfect, approach to estimating chl-a concentrations. One key issue identified in this analysis is the disparity between the temporal and spatial distributions of the in-situ and EO datasets, with in-situ samples being far less representative for the entire assessment area. Despite this, the current methodology often assigns significant weight to in-situ data, even when its confidence rating is low. This overemphasis may lead to inflated growing season means in years where in-situ measurements are coincidentally biased towards high chl a values, such as algal blooms.

Our findings suggest that the current weighting approach, especially the 50:50 weighting used in most years, can introduce bias into the overall chl-a means. In years with high in-situ chl-a concentrations, particularly during the early monitoring period, this bias leads to a moderate downward trend in chl-a levels from 1998 to 2020. However, when applying the alternative method presented in this report—where in-situ and EO data are aggregated on a year–month–grid basis—the trend is stable over time. This indicates that too much weight is given to the sporadic high in-situ values in the current method, and that the alternative method may provide a more accurate reflection of overall eutrophication trends. It is noteworthy that this new approach significantly diminishes the influence of in-situ measurements in monitoring eutrophication, bringing it close to negligible levels.

Furthermore, the introduction of confidence ratings based on the relative margin of error (MOE) offers a robust framework for assessing the reliability of chl-a estimates. By linking confidence ratings to sample sizes, we have provided a method for determining ‘low,’ ‘moderate,’ and ‘high’ confidence levels across different OSPAR assessment areas. The confidence rating thresholds for these classes can be determined per OSPAR area. For instance, the coastal the Meuse and Rhine plumes have higher sample size thresholds than the Southern North Sea due to increased variability in chl-a concentrations closer to shore. However, differences in confidence rating thresholds between these assessment areas were relatively small, meaning these thresholds may be determined more generally for the entire OSPAR maritime area. They could be established conservatively, including more grids with higher chl-a variation, but this would require data from more assessment areas. As an example the MOE threshold for a high rating was set to 0.05, and the threshold for a moderate rating to 0.1 in this report, however these thresholds can be adjusted to the acceptable error range. Moreover, data was aggregated on a 10x10 km scale, as it provided the sufficient sample size for reliable chl-a estimates, while minimizing edge effects and resolution losses. Optimal grid size should also be determined for other assessment areas.

In conclusion, the revised approach reduces the bias introduced by low resolution in-situ measurements and provides a clearer picture of eutrophication trends, supporting more informed decision-making for environmental management in the OSPAR maritime area.