Phytoplankton morphological response to the underwater light conditions in a vegetated wetland

Abstract

This study analyses the influence of the underwater light climate on the morphometric characteristics of the phytoplankton from a South American floodplain at the population and community level. The differences in the light condition across the wetland were mainly defined by the occurrence of floating macrophytes and the concentration of humic acids. A morphometric response at the community level to the underwater PAR was registered. Sites with strong light constraints were characterised by assemblages mainly composed by non-flagellated organisms or with a small proportion of unicellular flagellates. Short organisms (<10µm) with a unit volume of less than 1000 µm3 and a high surface:volume ratio (SA/V>2) correspond to the morphotypes better adapted to poorly illuminated environments. Moreover, organisms showed more attenuated forms under these limiting conditions. This pattern was different to that registered in well-illuminated sites where longer and larger organisms with a smaller SA/V and frequently flagellated, coexisted with the previously mentioned organisms. The autotrophic picoplancton, the smallest phytoplankton fraction, revealed lower abundances in the sites with higher humic substances. Short term changes in the dominant species were studied by means of mesocosm experiments simulating different light climates. Intraspecific morphological plasticity was observed in relation to the filament length and the vacuolization of the cells. The improvement of light conditions produced a decrease in the presence of vacuoles and a lengthening of the filaments.