Abstract
Abstract. A 15-year (1997–2012) time series of chlorophyll a (Chl a) in the Baltic Sea, based on merged multi-sensor satellite data was analysed. Several available Chl a algorithms were sea-truthed against the largest in situ publicly available Chl a data set ever used for calibration and validation over the Baltic region. To account for the known biogeochemical heterogeneity of the Baltic, matchups were calculated for three separate areas: (1) the Skagerrak and Kattegat, (2) the central Baltic, including the Baltic Proper and the gulfs of Riga and Finland, and (3) the Gulf of Bothnia. Similarly, within the operational context of the Copernicus Marine Environment Monitoring Service (CMEMS) the three areas were also considered as a whole in the analysis. In general, statistics showed low linearity. However, a bootstrapping-like assessment did provide the means for removing the bias from the satellite observations, which were then used to compute basin average time series. Resulting climatologies confirmed that the three regions display completely different Chl a seasonal dynamics. The Gulf of Bothnia displays a single Chl a peak during spring, whereas in the Skagerrak and Kattegat the dynamics are less regular and composed of highs and lows during winter, progressing towards a small bloom in spring and a minimum in summer. In the central Baltic, Chl a follows a dynamics of a mild spring bloom followed by a much stronger bloom in summer. Surface temperature data are able to explain a variable fraction of the intensity of the summer bloom in the central Baltic.
Highlights
Global to regional monitoring of the surface ocean is believed to be an essential element for the sustainability of the ocean resources
CHL1 parameter is meant to provide the best performances over Case I waters and is not recommended for use over optically complex waters, but no alternative is given for the Baltic Sea
A 15-year merged multi-sensor daily data set of satellitederived chlorophyll a (Chl a) contains very valuable information for ecological studies, if information is properly processed
Summary
Global to regional monitoring of the surface ocean is believed to be an essential element for the sustainability of the ocean resources. The present work aims at assessing the performance of existing Chl a algorithms for operational applications over the Baltic Sea. Chl a is routinely measured over the world oceans with two main kinds of algorithms: (i) those using the blue-to-green reflectance ratio (e.g. empirical) and (ii) the semi-analytical, e.g. those using the inherent optical properties to infer the chlorophyll concentration. Chl a is routinely measured over the world oceans with two main kinds of algorithms: (i) those using the blue-to-green reflectance ratio (e.g. empirical) and (ii) the semi-analytical, e.g. those using the inherent optical properties to infer the chlorophyll concentration The former build on the common experience that water colour spans from blue to green as Chl a increases, in open ocean (Case I waters). The latter are mathematically more complex and based on a larger number of assumptions; they are believed to be more suited for optically complex waters (known as Case II waters) where the colour of the ocean is determined by several non-covarying constituents, such as Chl a, coloured dis-
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