Demonstration of a transnational cooperation for harmonized chlorophyll a monitoring in the North East Atlantic Ocean

Abstract

The concentration of chlorophyll a (Chla), a proxy for phytoplankton biomass, is used as indicator for several criteria of three Marine Strategy Framework Directive (MSFD) descriptors (D1C6, the biodiversity of pelagic habitats; D4, food webs; and D5, eutrophication). Satellite Earth observation utilises algorithms that link the satellite observations of waterleaving radiance and the in-water Chla. Among the main sources of variability around this regression to define algorithms are the uncertainties in the in situ measurements due to the lack of consistency in the approaches employed in monitoring programs and research cruises. For example, global analyses based on measurements of Chla by high-performance liquid chromatography (HPLC), considered the reference technique for Chla, are usually derived from studies of independent investigators, so methodological differences between laboratories can introduce significant uncertainties. In addition, since HPLC is a relatively expensive and expertise-demanding technique, Chla concentration have been customarily determined in long-term oceanographic time-series programs by alternative techniques, such as spectrofluorometry (e.g., in RADIALES (Spain)) and fluorometry (e.g., in Plymouth Station L4, Western Channel Observatory (UK)). However, the agreement in the results obtained with these techniques has only been compared in a few ancient studies. The cooperation among Member States required by the MSFD for methodological harmonization has triggered a transnational collaboration involving some partners of the Interreg Atlantic Area project iFADO (Innovation in the framework of the Atlantic deep ocean) for a joint monitorization of Chla in the North East Atlantic Ocean (NEA) region. In situ data have been obtained in 21 research cruises and sampling sites, from coastal to offshore environments, by using standardized sampling and analytical methods. We will report on the results obtained from this operational demonstration and how this collaborative transnational initiative allowed us: i) to intercalibrate the methods currently used for the analysis of discrete samples (HPLC, spectrofluorometry, fluorometry) and assess them in terms of accuracy, costs and effectiveness; ii) to calibrate continuous measurements obtained with optical sensors and remote sensing results with HPLC data; iv) to extend in situ observations temporally and spatially through remote sensing for MSFD assessments; iii) to contribute to the integration of data of different accuracy, spatial scale and resolution in databases and to their dissemination in data hubs according to FAIR principles. This work will provide detailed guidelines for in situ sampling, analysis, and data quality control for Chla monitoring and will contribute harmonized data for the next MSFD assessment cycles for the target descriptors.