PHYTOPLANKTON WORKSHOP Phytoplankton morphological response to the underwater light conditions in a vegetated wetland Ine ´ s O’Farrell Æ Paula de Tezanos Pinto Æ Irina Izaguirre Ó Springer Science+Business Media B.V. 2007 Abstract This study analyses the influence of the underwater light climate on the morphometric characteristics of the phytoplankton at the pop- ulation and community levels. The differences in light conditions across the floodplain were mainly defined by the patchiness of floating macrophytes and humic acids concentration. A morphometric response at the community level to the underwa- ter PAR was registered. Sites with strong light constraints were characterised by non-flagellated organisms or with a small proportion of unicellu- lar flagellates. Short organisms (<10 lm) with a unit volume of less than 1,000 lm 3 and a high surface:volume ratio (S/V >2) were the morpho- types related to poorly illuminated environments. Moreover, the organisms showed forms more slender under these limiting conditions. This pattern was different to that registered in well- illuminated sites where longer and larger organ- isms, with a smaller S/V and frequently flagel- lated, coexisted with the previously mentioned organisms. The autotrophic picoplankton, the smallest phytoplankton fraction, revealed lower abundances at sites with higher humic substances. Short term morphological changes were addition- ally studied for the dominant species by means of mesocosm experiments simulating different light climates. Intraspecific morphological plasticity was observed with respect to the filament length and the vacuolization of cells. Keywords Phytoplankton Á Morphology PAR Á Macrophytes Á South American wetland Introduction Reynolds (1984) relates the success of the algae in exploiting the potential advantages offered by pelagic habitats with a certain degree of pre- adaptation to a dispersed existence. This capabil- ity is connected to the low level of structural organization and the morphological plasticity of algal cells, and is evidenced by the existence of a diversity of form, function and adaptive strate- gies. Algal growth depends upon intercepting sufficient light energy to perform the photosyn- thesis needed to sustain carbon fixation. The Guest editors: M. Albay, J. Padisa ´ k & L. Naselli-Flores Morphological plasticity of phytoplankton under different environmental constraints. Electronic supplementary Material Supplementary material is available for this article at http://dx.doi.org/ 10.1007/s10750-006-0434-3 and is available for authorized users. I. O’Farrell (&) Á P. de Tezanos Pinto Á I. Izaguirre Lab. Limnologı´a, Dpto. Ecologı´a, Gene ´tica y Evolucio ´ n, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires. CONICET, Buenos Aires, Argentina e-mail: ines@ege.fcen.uba.ar 123 Hydrobiologia (2007) 578:65–77 DOI 10.1007/s10750-006-0434-3