Chl a trends in European seas estimated using ocean-colour products

Abstract. Ocean-colour remote-sensing products have been used to estimate Chl a trends in European seas. This work aims to develop a new indicator based on ocean-colour data for the European Environment Agency (EEA). The new indicator, called CSI023(+), produced from satellite ocean-colour products from the MyOcean Marine Core Service (www.myocean.eu) has been defined and calculated. CSI023 (+) is intended to complement the EEA CSI023 (Core Set of Indicators n.23) indicator for eutrophication, which is based on chlorophyll a (Chl a) in-situ observations. Validation of ocean-colour products has been carried out through comparison with observations of the Eionet EEA database, and we believe that such validation should continue in the future, perhaps with a dedicated data-collection exercise. Ocean colour has a much higher spatial and temporal resolution than the in-situ observations. The ocean-colour observations, however, are based on indirect measurements of the optical properties of the ocean, which are transformed to Chl a using an appropriate algorithm. This algorithm can either be a global algorithm that reproduces the average global Chl a concentrations well or one that is adjusted to specific regional conditions. In our analysis we have used both global and regional (adjusted to specific regional Mediterranean conditions) ocean-colour products, but the results highlight the fact that regional products produced with regional algorithms are recommended for the future.

This work proposes a methodology for analysing trends comparable to the method EEA uses for its CSI23 indicator. Analysis has revealed the potential of ocean colour as a CSI023(+) indicator: large-scale, and in some cases even local-scale, changes appear to be captured by the satellite images even though in general the ocean-colour products underestimate the Chl a values. CSI023(+) shows, in the period 1998–2009, decreasing Chl a concentrations throughout the Black Sea, in the Eastern Mediterranean, in the southern part of the Western Mediterranean, in the English Channel and in the north part of the North Sea, whereas large areas with increasing trends are observed in the Bay of Biscay, in the North-East Atlantic regions of Ireland and the UK, in the northern part of the North Sea, in the Kattegat and in the Baltic. Specific analysis has been performed in the Mediterranean and the Black Sea: we first defined Chl a areas and then calculated the CSI023(+) for each of the Chl a areas. This last analysis reveals that about 80 % of Chl a areas do not show significant trends; increasing significant Chl a trends were detected in 3 Chl a areas in the Black Sea and 32 Chl a areas in the Mediterranean. Decreasing significant trends were detected in 6 Chl a areas in the Mediterranean and 2 Chl a areas in the Black Sea.