[ARCHIVE] Baltic Sea Chlorophyll-a time series and trend from Observations Reprocessing

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'''DEFINITION'''

The time series are derived from the regional chlorophyll reprocessed (REP) product as distributed by CMEMS which, in turn, result from the application of the regional chlorophyll algorithm over remote sensing reflectances (Rrs) provided by the Plymouth Marine Laboratory using an ad-hoc configuration for CMEMS, at high resolution, of the ESA-CCI processor (ESA OC-CCI, Sathyendranath et al., 2018). The chlorophyll product is derived from a Multi Layer Perceptron neural-net (MLP) developed on field measurements collected within the BiOMaP program of JRC/EC (Zibordi et al., 2011). The algorithm is an ensemble of different MLPs that use Rrs at different wavelengths as input. The processing chain and the techniques used to develop the algorithm are detailed in Brando et al. (2021a; 2021b). Monthly regional mean values are calculated by performing the average of 2D monthly mean (weighted by pixel area) over the region of interest. The deseasonalized time series is obtained by applying the X-11 seasonal adjustment methodology on the original time series as described in Colella et al. (2016), and then the Mann-Kendall test (Mann, 1945; Kendall, 1975) and Sens’s method (Sen, 1968) are subsequently applied to obtain the magnitude of trend.

'''CONTEXT'''

Phytoplankton and chlorophyll concentration as a proxy for phytoplankton respond rapidly to changes in environmental conditions, such as light, temperature, nutrients and mixing (Gregg and Rousseaux, 2014). The character of the response in the Baltic Sea depends on the nature of the change drivers, and ranges from seasonal cycles to decadal oscillations (Kahru and Elmgren 2014). Therefore, it is of critical importance to monitor chlorophyll concentration at multiple temporal and spatial scales, in order to be able to separate potential long-term climate signals from natural variability in the short term. In particular, in the Baltic Sea phytoplankton is known to respond to the variations of SST in the basin associated with climate variability (Kabel et al. 2012).

'''CMEMS KEY FINDINGS '''

Baltic Sea shows a positive trend in the time interval 1997-2020 with a slope of 0.510.83% per year. Due to the change in chlorophyll algorithm, this trend estimate cannot be compared directly to those previously reported. Maxima values are quite variable until 2009-2010, then maxima show similar values until 2015. From 2015 onwards, maxima increase year-by-year (except for 2018). On the contrary, minima show similar values from 1997 to 2015, while from 2016 until 2020 they increase. These increments in maxima and minima values of the last five years are probably responsible of the overall positive trend of the basin.

Note: The key findings will be updated annually in November, in line with OMI evolutions.

'''DOI (product):''' https://doi.org/10.48670/moi-00197