Estimation of phytoplankton size structure in coastal waters using simultaneous laser diffraction and fluorescence measurements

The size structure of phytoplankton reveals relevant information on the structure and response of aquatic systems to environmental drivers. To estimate the phytoplankton size distribution, we combined simultaneous particle size measurements obtained with a laser in situ scattering and transmissometry instrument (LISST) and fluorescence profiles using canonical correlation analysis (CCA). We tested the performance of this method with a series of synthetic laboratory samples generated by mixing cultured phytoplankton cells and natural sediments with grain sizes within the size range of phytoplankton. Our results revealed that the method yields reliable estimation of the algal fraction allowing the discrimination of phytoplankton size distribution when algal cells constitute >25% of total suspended particulate matter. Uncertainties arise in very turbid waters and when samples are composed of two phytoplankton species with very different contribution to chlorophyll in terms of cell size. Application of the proposed method in a survey of 37 stations carried out in Palma Bay (Mediterranean Sea) effectively represented the patterns of four main size fractions dominating coastal phytoplankton biomass, demonstrating the utility of the method. Compared to previous published methods, the proposed method is more robust because it is not impeded by the presence of non-algal components.