Statistical combination of different types of chlorofyll-a measurements in the Dutch North Sea
1. Introduction
One of OSPAR's key objectives in the OSPAR maritime area is to address eutrophication. Chlorophyll-a (chl-a) levels within OSPAR assessment areas are one of four primary indicators used to assess the state of eutrophication. Therefore, it is crucial that measurements provide a comprehensive and reliable representation of chl-a concentrations. The current method combines chl-a in-situ measurements with Earth Observation (EO) data using a weighted average to calculate chl-a mean values during the growing season. These means are then compared against assessment area specific reference values to assess eutrophication levels. The weighting applied in this method is based on the confidence ratings of both in-situ and EO data, tailored to each OSPAR area.
This report evaluates the current method of combining chl-a data by: 1) exploring the in-situ and EO datasets in both temporal and spatial contexts, 2) assessing how the weighting process influences growing season mean values, and 3) reviewing the criteria for determining the confidence rating. Thereafter, alternative methods for calculating chl-a growing season means and confidence ratings will be explored. The primary focus will be on the Dutch OSPAR assessment areas, mainly the Southern North Sea (SNS), but two smaller coastal areas, the Meuse plume (MPM) and the Rhine plume (RHPM), will also be discussed.