Spectrometric constraint in analysis of benthic diatom biomass using monospecific cultures

Among microphytobenthic species (unicellular algae and cyanobacteria), benthic diatoms are often the most common photosynthetic organisms colonizing shallow marine environment such as an intertidal flat. As such areas, particularly mudflats of limited access for point sampling, remote sensing techniques have the potential to map diatom biomass. This study used two monospecific cultures of the benthic diatoms Navicula ramosissima and Entomoneis paludosa, isolated from mudflats in Bourgneuf Bay (France), to obtain a biomass range. Exponential and senescent stages of microalgal growth were also tested. The objective was to relate the spectral reflectance obtained by spectroradiometry to the composition and amount of principal and accessory pigments measured by high-performance liquid chromatography (HPLC). Analysis of pigments vs. chlorophyll a ratios revealed differences between these two diatom species, mainly due to fucoxanthin and diadinoxanthin. These interspecific variations among the class of Bacillariophyceae were also detected by spectroradiometry. No differences were observed between the two growth stages, and cell senescence had no effects on spectral properties. The chlorophyll pigments used to establish a relationship between diatom biomass and spectral reflectance gave the most interesting results for an absorption band at 632 nm corresponding to chlorophyll a and chlorophyll c. This band, contrary to that at 673 nm related to chlorophyll a only and traditionally used for biomass estimations in green plants, provided the largest range of variations and was not saturated at a high biomass concentration.