Measurement of oceanic chlorophyll by LIDAR, MODIS, fluorometry and above-water radiometry

Abstract

Comparisons between in situ measurements of surface chlorophyll concentration (CHL) and ocean color remote sensing estimates were conducted during an oceanographic cruise in the Brazilian Southeastern continental shelf and slope in November 2004. In situ estimates were based on fluorometry, above-water radiometry and lidar fluorosensor. Three empirical algorithms were used to estimate chlorophyll a concentration from radiometric measurements: Ocean Chlorophyll 3 bands (OC3M), Ocean Chlorophyll 4 bands (OC4v4), and Ocean Chlorophyll 2 bands (OC2v4). The satellite estimates of chlorophyll a were derived from data collected by the Moderate-resolution Imaging Spectroradiometer (MODIS) with a nominal 1.1 km resolution at nadir. Three algorithms were used to estimate chlorophyll concentrations from MODIS data: one empirical - OC3M, and two semi-analytical - Garver, Siegel, Maritorena version 01 (GSM01), and Carder. In this paper, LIDAR, MODIS and in situ above-water radiometry and fluorometry are briefly described and the estimated values of chlorophyll retrieved by these techniques are compared. Chlorophyll concentrations were fairly well estimated by all the methods. In general, the empirical algorithms applied to the satellite and in situ radiometric data showed a tendency for overestimating CHL. The semi-analytical GSM01 algorithm applied to MODIS data performed better than the Carder and the empirical OC3M algorithms.