Phytoplankton absorption in coastal waters of the Patagonian continental shelf: in situ characterization and satellite-derived variability

Abstract

The coastal zone of the northern Patagonian Continental Shelf is a biologically productive and optically complex region. This study evaluated the performance of the Quasi-Analytical Algorithm (QAA) in assessing the temporal variability and trends of the phytoplankton absorption coefficient at 443 nm (aph(443)) using an extensive in situ dataset (39°S–62.5°W; 41.5°S–60°W) and 18 years (2003–2021) of MODIS-Aqua imagery. Seventy-five samples were collected from January to March 2020 aboard artisanal fishing boats. Measurements included turbidity, suspended particulate matter (SPM), chlorophyll-a (Chl-a), phytoplankton absorption (aph(λ)), and species composition. Results showed that higher Chl-a concentrations were associated with greater turbidity and lower specific absorption (aph*(443)). Variations in aph*(443) were strongly linked to phytoplankton community structure and packaging effects, with large diatoms (e.g. Coscinodiscus, Pseudosolenia) reducing aph*(443) and causing slight overestimations in satellite-derived aph(443). Despite these biases, QAA remained robust, showing good correlation with in situ data and low sensitivity to turbidity changes. Time series decomposition using Census X-13 revealed a substantial irregular component, contributing 28–50% of the total variance, possibly due to sub-mesoscale processes and storm events. A significant monotonic increase in aph(443) was detected across the study area, aligning with previous trends observed on the Patagonian Shelf.