Twenty years of satellite and in situ observations of surface chlorophyll-a from the northern Bay of Biscay to the eastern English Channel. Is the water quality improving?
|Author(s)||Gohin Francis1, Van Der Zande Dimitry2, Tilstone Gavin3, Eleveld Marieke A.4, Lefebvre Alain5, Andrieux-Loyer Francoise1, Blauw Anouk N.4, Bryère Philippe5, Devreker David6, Garnesson Philippe7, Hernandez Farinas Tania8, Lamaury Yoann9, Lampert Luis1, Lavigne Héloïse2, Menet-Nedelec Florence8, Pardo Silvia3, Saulquin Bertrand7|
|Affiliation(s)||1 : IFREMER, Laboratoire d'écologie pélagique, DYNECO PELAGOS, CS 10070-29280 Plouzané, France
2 : RBINS, 29 rue Vautier, B-1200, Brussels, Belgium
3 : PML, Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK
4 : Deltares, Marine and Coastal Information Science, P.O. Box 177, 2600, MH, Delft, the Netherlands
5 : ARGANS, Bâtiment Le Grand Large, Quai de la douane – 2ème éperon, 29200 Brest, France
6 : IFREMER, Laboratoire Environnement et Ressources de Boulogne sur Mer, 150, Quai Gambetta, 62200 Boulogne-sur-Mer, France
7 : ACRI-ST, 260 Route du Pin Montard, BP 234, 06904 Sophia-Antipolis, France
8 : IFREMER, Laboratoire Environnement et Ressources de Normandie, Av. du Général de Gaulle, BP, 32-14520 Port en Bessin, France
9 : Sorbonne Université, Campus Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France
|Source||Remote Sensing Of Environment (0034-4257) (Elsevier BV), 2019-11 , Vol. 233 , P. 111343 (17p.)|
|Keyword(s)||Satellite remote-sensing, Ocean colour, Phytoplankton, Chlorophyll-a, English Channel, Bay of Biscay, Eutrophication assessment, European marine strategy framework directive|
The variability of the phytoplankton biomass derived from daily chlorophyll-a (Chl-a) satellite images was investigated over the period 1998–2017 in the surface waters of the English Channel and the northern Bay of Biscay. Merged satellite (SeaWiFS-MODIS/Aqua-MERIS-VIIRS) Chl-a was calculated using the OC5 Ifremer algorithm which is optimized for moderately-turbid waters. The seasonal cycle in satellite-derived Chl-a was compared with in situ measurements made at seven coastal stations located in the southern side of the English Channel and in the northern Bay of Biscay. The results firstly showed that the satellite Chl-a product, derived from a suite of space-borne marine reflectance data, is in agreement with the coastal observations. For compliance with the directives of the European Union on water quality, time-series of 6-year moving average of Chl-a were assessed over the region. A clear decline was observed in the mean and 90th percentile of Chl-a at stations located in the mixed waters of the English Channel. The time-series at the stations located in the Bay of Biscay showed yearly fluctuations which correlated well with river discharge, but no overall Chl-a trend was observed. In the English Channel, the shape of the seasonal cycle in Chl-a changed over time. Narrower peaks were observed in spring at the end of the studied period, indicating an earlier limitation by nutrients. Monthly averages of satellite Chl-a, over the periods 1998–2003 and 2012–2017, exhibited spatial and temporal patterns in the evolution of the phytoplankton biomass similar to these observed at the seven coastal stations. Both the in situ and satellite Chl-a times-series showed a decrease in Chl-a in the English Channel in May, June and July. This trend in phytoplankton biomass is correlated with lower river discharges at the end of the period and a constant reduction in the riverine input of phosphorus through improvements in the water quality of the surrounding river catchments.