Light absorption by phytoplankton, non-algal particles and dissolved organic matter at the Patagonia shelf-break in spring and summer

Abstract

Satellite image studies and recent in situ sampling have identified conspicuous phytoplankton blooms during spring and summer along the Patagonia shelf-break front. The magnitudes and spectral characteristics of light absorption by total particulate matter (phytoplankton and detritus) and colored dissolved organic matter (CDOM) have been determined by spectrophotometry in that region for spring 2006 and late summer 2007 seasons. In spring, phytoplankton absorption was the dominant optical component of light absorption (60–85%), and CDOM showed variable and important contributions in summer (10–90%). However, there was a lack of correlation between phytoplankton biomass (chlorophyll- a concentration or [chl a]) and the non-algal compartment in both periods. A statistically significant difference was found between the two periods with respect to the CDOM spectral shape parameter ( S cdom), with means of 0.015 (spring) and 0.012 nm −1 (summer). Nonetheless, the mean S cdm values, which describe the slope of detritus plus CDOM spectra, did not differ between the periods (average of 0.013 nm −1). Phytoplankton absorption values in this work showed deviations from mean parameterizations in previous studies, with respect to [chl a], as well as between the two study periods. In spring, despite the microplankton dominance, high specific absorption values and large dispersion were found ( a* ph(440)=0.04±0.03 m 2 mg [chl a] −1), which could be attributed to an important influence of photo-protector accessory pigments. In summer, deviations from general trends, with values of a* ph(440) even higher (0.09±0.02 m 2 mg [chl a] −1), were due to the dominance of small cell sizes and also to accessory pigments. These results highlight the difficulty in deriving robust relationships between chlorophyll concentration and phytoplankton absorption coefficients regardless of the season period. The validity of a size parameter ( S f) derived from the absorption spectra has been demonstrated and was shown to describe the size structure of phytoplankton populations, independently of pigment concentration, with mean values of 0.41 in spring and 0.72 in summer. Our results emphasize the need for specific parameterization for the study region and seasonal sampling approach in order to model the inherent optical properties from water reflectance signatures.