Validation of MODIS-Aqua bio-optical algorithms for phytoplankton absorption coefficient measurement in optically complex waters of El Rincón (Argentina)

Abstract

El Rincón is a highly productive area in the inner-shelf of the Argentine Sea, southern Buenos Aires Province (39°S–62.5°W; 41.5°S–60°W), regarded as the habitat for commercially valuable fish species and important for local artisanal fisheries. The aims of this work were to characterize for the first time the in situ absorption coefficient of phytoplankton (aph(λ)) in relation to phytoplankton composition and chlorophyll-a (Chl-a) concentration, and to evaluate the satellite retrieval of aph(443) using two atmospheric correction algorithms and three bio-optical semi-analytical algorithms in order to define which is more suitable for the study area. A total of 28 oceanographic cruises were performed between November 2013 and March 2017 and the sampling included physical variables (temperature, salinity, suspended particulate matter, SPM), Chl-a, phytoplankton absorption and composition. The phytoplankton community was mainly dominated by chain-forming diatoms from the microplankton size class (40–80%), while a single coccolithophore, Gephyrocapsa oceanica (cell diameter <10 µm) was a key component (up to 40%) of the nanoplankon. In situ Chl-a (mean 1.8 mg m−3) and aph (443) (mean 0.052 m−1) generally covariated with no clear seasonality. Their correlation was rather low (R2 = 0.52) and with high scattering, in agreement with the high interannual variability of phytoplankton composition. Further, the relative low mean specific absorption coefficient of phytoplankton (aph*(443), 0.032 m2 (mg Chl-a)−1) and (aph*(676), 0.014 m2 (mg Chl-a)−1) denoted the presence of big cells and pigment packaging effect. Aside from the optical complexity of these waters, reasonable results were found using the Quasi Analytical Algorithm in retrieving aph coefficient (R2 = 0.55, Bias = 19%) and switching the atmospheric correction algorithm depending on the distance to the coast, the depth and turbidity. The results found in this paper present for the first time the phytoplankton composition and absorption of algal material characteristics and the derived implication on the performance of bio-optical algorithms in the optically complex waters of El Rincon.