3. Current method

3.1 Confidence rating

In the current approach for combining in-situ and EO data to determine chl-a growing season means, the means of both datasets are first calculated separately. A weighted average is then computed based on the confidence rating of each dataset, following criteria that consider both spatial and temporal confidence aspects. These criteria are detailed in Annex 13 of the OSPAR eutrophication status assessment procedure . If the in-situ data has high confidence, a weighting of 50:50 (in-situ/EO) is applied; for moderate confidence, the ratio is 30:70 (in-situ/EO), and for low confidence, it shifts to 10:90 (in-situ/EO).

Even when the confidence of the in-situ data is low, assigning it a 10% weight remains significant, especially considering the large discrepancy in the representativeness of in-situ versus EO data across the growing season (see chapter 2), with the in-situ data representing at best 0,0039% of all combined measurements taken in 1999.

To demonstrate the impact of different weightings on growing season means, Figure 5 illustrates all weighting scenarios considered in the present study, including the individual means of in-situ and EO data, as well as the weighting used in OSPAR. With the exception of 2002, a 50:50 weighting was applied in all years. However, these weighting factors were determined somewhat subjectively. There is no statistical backing for boundaries at which the confidence classes and the corresponding weights are set⁵⁾, but as a result, high in-situ values can disproportionately affect the calculated growing season means.

Since in-situ data is collected sporadically compared to EO data, there is a possibility that extremely high chl-a concentrations may be captured due to the timing of in-situ sampling during periods of high phytoplankton density, such as algal blooms, and/or the higher variation in sample sites. For instance, in 1999, the area around the British shore was extensively sampled in contrast to other years (Figure 1), and samples were only taken during four months of the growing season (Table 1).

An argument often cited to support the use of in-situ data is the claimed higher precision of chl-a measurements compared to EO data. However, comparisons between in-situ and EO chl-a measurements in the Baltic Sea reveal that the uncertainties associated with both methods fall within the same range⁶⁾. Furthermore, these comparisons were made using chl-a products derived from MERIS and OLCI satellites. In contrast, the Sentinel satellites, which have been in use since 2016 and incorporate the JMP EUNOSAT quality control, offer even greater precision.

3.2 Conclusion current method

The current method of integrating in-situ and EO data is highly sensitive to temporal and spatial biases in the in-situ measurements. As a result, the elevated chl-a concentrations observed at the start of the time series are likely to be artifacts of the methodology. Investigating alternative approaches for data integration could provide more robust results and is recommended.