Statistical combination of different types of chlorofyll-a measurements in the Dutch North Sea

Summary

This report evaluates the current methodology used to assess chlorophyll-a (chl-a) concentrations in the OSPAR maritime area, specifically focusing on the Dutch OSPAR subareas such as the Southern North Sea (SNS), Meuse plume (MPM), and Rhine plume (RHPM). Chl-a levels are a key indicator of eutrophication, and the method combines in-situ measurements with Earth Observation (EO) data to calculate chl-a growing season means from March to September. These values are then compared to subarea-specific reference levels to determine eutrophication status.
The current approach uses weighted averages of in-situ and EO data based on confidence ratings, where the weight given to in-situ data can range from 10% to 50%, depending on the confidence in the data. However, significant discrepancies exist between the datasets, with in-situ measurements being sparse in both time and space. This can result in overrepresentation of in-situ data and leads to potential biases in the calculated growing season means.

The report proposes an alternative method that aggregates in-situ and EO data on a grid basis and imputes missing values before calculating growing season means. This approach aims to reduce the bias introduced by the low resolution of in-situ measurements. The report also introduces a new method for determining confidence ratings using the relative margin of error (MOE) to account for sample size variations across year–month–grid combinations. By setting thresholds for MOE, the report defines confidence ratings as ‘low,’ ‘moderate,’ or ‘high,’ providing a more systematic approach to assess the data quality.

Ultimately, too much weight may be given to the in-situ data in the current weighted approach, as this shows a moderate decrease in the chl-a trend between 1998 and 2020, while the alternative method produces a stable chl-a trend. The proposed changes offer a more accurate and balanced representation of chl-a levels, helping to improve the assessment of eutrophication in the OSPAR area.