A regional correction model for satellite surface chlorophyll concentrations, based on measurements from sea water samples collected around Iceland

Abstract

Near-surface chlorophyll $a$ concentration is a fundamental component of marine ecological processes, and its changes reflect the phytoplankton growth (primary productivity as well as loss due to grazing and sinking) feeding into higher trophic levels. Time series of measurements from several satellite sensors since late 1997 can be used as a proxy of chlorophyll $a$ concentrations after calibrating against direct sea water measurements from oceanographic surveys. Previous studies indicate a need for a regional correction model in specific ‘case 2’ areas, where the relationship between satellite measurements and in situ measurements is different from the relationship in the general ‘case 1’ areas, due to complex environmental characteristics in different areas. Subarctic and boreal North Atlantic, including the waters around Iceland, have been considered case 2 waters, but a regional correction model has not been developed until now. We collated all relevant measurements of near-surface chlorophyll $a$ from sea water samples, available in the Marine Research Institute database, and matched by date and location with satellite chlorophyll records, i.e. the GSM CHL1 records offered by the GlobColour Project. A multiple linear regression model was fitted to the observed in situ chlorophyll measurements, based on the satellite chlorophyll values (CHL1) and physical covariates: day of the year, sun elevation, and ocean depth. The resulting parsimonious model converts the satellite measurements to estimates that are in much better agreement with in situ measurements ($R^2$ increases from 0.2 to 0.5), and is therefore proposed for calibration of regional corrections to the GlobColour Project’s GSM chlorophyll parameter, CHL1.