Satellite and In Situ Monitoring of Chl-a, Turbidity, and Total Suspended Matter in Coastal Waters: Experience of the Year 2017 along the French Coasts

Francis Gohin,Alain Lefebvre,Philippe Bryère,Pascal Conan,Thierry Cariou,Tania Hernández Fariñas,Nicolas Savoye,Sylvain Coudray,Aude Piraud,Pierre-Guy Sauriau,Patrick Raimbault,Michael Rétho,Gaelle Courtay,Maud Lemoine,Vincent Vantrepotte,Sylvaine Françoise,Anne Goffart,Yann Bozec

doi:10.3390/jmse8090665

Abstract

The consistency of satellite and in situ time series of Chlorophyll-a (Chl-a), Turbidity and Total Suspended Matters (TSM) was investigated at 17 coastal stations throughout the year 2017. These stations covered different water types, from relatively clear waters in the Mediterranean Sea to moderately turbid regions in the Bay of Biscay and the southern bight of the North-Sea. Satellite retrievals were derived from MODIS/AQUA, VIIRS/NPP and OLCI-A/Sentinel-3 spectral reflectance. In situ data were obtained from the coastal phytoplankton networks SOMLIT (CNRS), REPHY (Ifremer) and associated networks. Satellite and in situ retrievals of the year 2017 were compared to the historical seasonal cycles and percentiles, 10 and 90, observed in situ. Regarding the sampling frequency in the Mediterranean Sea, a weekly in situ sampling allowed all major peaks in Chl-a caught from space to be recorded at sea, and, conversely, all in situ peaks were observed from space in a frequently cloud-free atmosphere. In waters of the Eastern English Channel, lower levels of Chl-a were observed, both in situ and from space, compared to the historical averages. However, despite a good overall agreement for low to moderate biomass, the satellite method, based on blue and green wavelengths, tends to provide elevated and variable Chl-a in a high biomass environment. Satellite-derived TSM and Turbidity were quite consistent with in situ measurements. Moreover, satellite retrievals of the water clarity parameters often showed a lower range of variability than their in situ counterparts did, being less scattered above and under the seasonal curves of percentiles 10 and 90.

Highlights

Since the launch of SeaWiFS in August 1997, marine reflectance data at about 1 km resolution have been provided by space agencies without interruption
The aim of this work was not the calibration of the satellite algorithms, a slight modification of some parameters in the Total Suspended Matter (TSM) algorithm has been carried out. This modification has been made in the equations of the non-algal SPM algorithm, in order to obtain better fits for low TSM values, as those encountered in the Mediterranean Sea
Graphs of the time series of Chl-a, Turbidity and TSM were generated for the years 2016 to 2018 but only graphs of the year 2017 are shown for all stations (Figures 3–12)

Summary

Introduction

Since the launch of SeaWiFS in August 1997, marine reflectance data at about 1 km resolution have been provided by space agencies without interruption. Among them the Ifremer OC5 algorithm, derived from OC4, has been largely applied to SeaWiFS, MODIS, VIIRS, MERIS marine reflectance with numerous validations in coastal waters of the eastern Atlantic Ocean [2,3,4,5] and in the western Mediterranean Sea [6,7]. It is currently applied at global scale through the OC-CCI (Ocean Color-Climate Change Initiative) and GlobColour products of the CMEMS (Copernicus Marine Environment Centre) [8,9]. The effective contribution of the satellite retrievals to the knowledge of the short term (weekly) dynamics of the coastal environment has not been evaluated in the context of existing conventional in situ networks

Objectives

Methods

Results

Discussion

Conclusion