Seasonal evolution of particulate and dissolved absorption coefficients in a subtropical estuary

Abstract

The seasonal evolution of colored dissolved organic matter (CDOM), phytoplankton, and non-algal particulate absorption coefficients were measured for the first time along a salinity gradient within the Paranaguá Estuarine Complex (PEC), in southern Brazil. CDOM (mostly of autochthonous origin) inside the estuary exhibited a conservative behavior that dominates the absorption budget at lower wavelengths (~380 nm). On average, absorption by non-algal particles was more important than absorption by phytoplankton throughout the year and across all wavelengths, except for the maximum absorption band of chlorophyll-a (665–675 nm), where the algal component was predominant year-round. Phytoplankton optical properties were generally consistent with variability in phytoplankton cell counts, particularly during bloom periods. Simultaneous condition of high suspended matter concentrations and low turbidity conditions associated with sampling of mucilaginous materials released by centric diatoms exemplified the complex relationships between biogeochemical and optical properties in this region. The relationship between chlorophyll-a concentration and phytoplankton absorption exhibited seasonal oscillations related to the variation of algal assemblages, ultimately determining the magnitude of the packaging effect. Finally, separation of the signals of these three components in the visible domain is possible only in the near-ultraviolet range (380 nm), associated with CDOM dominance, and at 665 nm where the phytoplankton signature can be isolated.