Dominant phytoplankton morphotypes in relation to light conditions in a South American wetland

O'FARRELL, INES; DE TEZANOS PINTO, PAULA; IZAGUIRRE, IRINA

Sapanka, Turquia

Congreso; Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP); 2005

International Association of Phytoplankton Taxonomy and Ecology (IAP)

Light influence on phytoplankton cell and individual morphological characteristics was analyzed for the assemblages from the Parana River floodplain during two different periods. From 1998 to 2000 a dense floating macrophyte cover determined a poor underwater climate in the profusely vegetated relictual oxbow lakes (ROLs). In these environments the bulk of individuals were not flagellated, mostly unicellular and filamentous in a minor proportion. On the contrary, in the lake, characterized by better light conditions, up to 30% of the phytoplankton density corresponded to flagellated forms, and even though unicellular individuals also predominated, filament forms were rare and coenobial specimens were frequent. Regarding size, more than 80% of the individuals at ROL1 were shorter than 10 µm, whereas at the lake this percentage was achieved with a GALD (Greatest Axial Lineal Dimension) of less than 30 µm. A similar pattern was registered for biovolume, as individuals of less than 1000 µm3 represented almost the entire assemblage at ROL1, whereas at the lake mean percentage dropped to less than 80%. The more relevant parameter is perhaps the surface to volume ratio that remains higher in the ROLs, particularly of ROL1, as compared to the lake. A more extensive sampling carried out during spring 2004 and summer 2005 comprised the previous sampling sites together with other environments. The floating macrophyte cover was considerably less developed than in the previous study, thus establishing minor light constraints, which were mostly occasioned by high humic acids content. The ordination produced by a principal component analysis revealed that organisms with low surface to volume ratios, large volume and greatest axial lineal dimensions were associated to the highest light intensities, whereas the opposite characteristic typified the phytoplankton of environments with a poor light quality such as the ROLs and the littoral areas of the Laguna Grande. In this occasion the occurrence of the smallest fraction (picoplankton) was also analyzed under epifluorescence microscopy.