Phytoplankton diversity analysis in a floodplain wetland along a ten-year period: different scales and approaches

O'FARRELL I.

Meise

Simposio; Botanical diversity. Exploration, understanding and use; 2010

Royal Academy of overseas Sciences and National Botanic Garden od Belgium

Wetlands are important conservation sites; notwithstanding, the relevance of micro algae to the current debate on biodiversity and the identification of the mechanisms driving their occurrence and distribution in such systems are still not assessed. For the past decade we studied the phytoplankton in a vegetated floodplain wetland of the Lower Parana River using different approaches and temporal and spatial scales. We conducted field surveys along the different habitats of the wetland on a fortnightly, monthly and annual basis and performed experiments using in situ micro and mesocosms, In the diversity analysis of phytoplankton we considered the levels of species and higher taxonomic ranks, their functional groups and physiological and morphological traits. Substantial differences were evident along the different habitats of this wetland as characterised by the degree of dominance of the macrophyte vegetation. Floating plants dynamics was extremely important in shaping the spatial and temporal changes of the composition and structure of phytoplankton; seasonal floods homogenised such differences. Poor underwater light climate determined the dominance of shade adapted slow growing species with high surface:volume ratio, mainly Oscillatoriales. Conversely light availability favoured small fast growing unicellular green algae or centric diatoms and flagellates (cryptophytes and euglenophytes). By mimicking short-term (few days) wind driven displacements of floating plants we observed different responses regarding phytoplankton diversity. Light conditions also caused differences in the photosynthetic picoplankton structure among systems: in the dark anoxic environments anaerobic anoxygenic photosynthetic bacteria dominated over phycocyanin rich picocyanobacteria and picoeukaryotes. On the other hand, the floating macrophyte cover was strongly affected by water level fluctuations in the long-term (over the decade) and produced a gradual and slow increase in phytoplankton abundance that was accompanied by a notable change in its composition and structure. The diverse flora in the largest floodplain lake of the wetland was replaced by few big-sized species of Cyanobacteria after a prolonged low water period. A combination of factors, namely nutrient concentrations, light conditions and prevailing pH values account for the occurrence and final dominance ofN-fixing Nostocales.