Seasonal variability in phytoplankton light absorption properties: Implications for the regional parameterization of the chlorophyll a specific absorption coefficients

Data of in vivo absorption of particulate material in water column obtained from multiple research cruises performed in the Alboran Sea (Western Mediterranean Sea) were used to investigate seasonal variability of the phytoplankton light absorption properties Specific absorption coefficients of phytoplankton, a_ph^*() were calculated and used to determine the performance of a_ph^*() parameterizations based on chlorophyll a concentration for each season. a_ph^*() in the blue spectral bands for the euphotic layer decreased by 20% in spring and summer compared with autumn and winter. These changes might reflect stronger dominance of diatoms during these periods, since these cells are characterized by a high degree of pigment packaging that leads to flattening of the absorption spectrum. Surface temperature and salinity, which are proxies for modifications of the surface layer by vertical mixing, explained more than 50% of the variability in the absorption properties, suggesting that the communities also presented high photo-acclimation capacity in response to short-term hydrological variability. The biases between measured and parameterized a_ph^*() decreased when the seasonal relationships between chlorophyll a concentration and a_ph^*() were used for estimating the parameterization coefficients instead of using the whole dataset. The regional parameterization using seasonal data performed better that parameterizations using global data. These seasonal and regional parameterizations can be useful for developing more refined bio-optical models of primary production for the study area, although it has to be taken into account that a significant percentage of variability in a_ph^*() related to short-term hydrological changes remained still unexplained by the seasonal parameterization.
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