Joint use of satellite and in-situ data for coastal monitoring

Abstract

Sea surface Temperature, Chlorophyll and turbidity are three variables of the coastal environment commonly measured by monitoring networks. The observation networks are often based on coastal stations which do not provide a sufficient coverage to val-idate the model outputs or to be used in assimilation over the continental shelf. Conversely, the products derived from satellite reflectance show generally a decreasing quality shoreward and an accurate assessment of these data is required. In this text, we show that the satellite-derived chlorophyll products, obtained through a dedicated coastal algorithm, fulfil the first requirement of a monitoring system: the ability to represent correctly the mean annual cycle. The annual cycle, mean and percentile 90 of the chlorophyll concentration, derived from MERIS/ESA and MODIS/NASA, have been compared to in-situ observations at twenty six selected stations from the Mediterranean Sea to the North-Sea. Keeping in mind the validation, the forcing or the assimilation in hydrological, sediment-transport or ecological models, the non-algal Suspended Particulate Matter (SPM) is also a parameter which is expected from the satellite imagery. However, the monitoring networks measure essentially the turbidity and a consistency between chlorophyll, representative of the phytoplankton biomass, non-algal SPM, and turbidity is required. In this study, we derive the satellite turbidity from chlorophyll and non-algal SPM with a common formula applied to in-situ or satellite observations. The distribution of the satellite-derived turbidity shows the same main statistical characteristics that measured in-situ; which satisfies our first condition to monitor the long-term changes or the large-scale spatial variation over the continental shelf and along the shore. For the first time, maps of turbidity, so useful for the surveillance of the benthic habitats, are proposed operationally from space on areas as different as the Southern North-Sea or the Western Mediterranean Sea, with validation at coastal stations.