Ecological classifications vs. taxonomic approach in the evaluation of flood-induced phytoplankton changes in a river-floodplain ecosystem

Floodplains are extremely dynamic environments in which the flood pulse is the most important environmental parameter, causing rapid change in phytoplankton composition. The aim of study was to investigate the effectiveness of different ecological classifications versus taxonomic approach in description of phytoplankton changes in a river-floodplain system. The study was conducted at Kopački Rit Nature Park (Croatia), one of the largest conserved floodplain areas of the Danube River. During eight years of investigation (2003 to 2010), extreme hydrological events significantly influenced the environmental variables of the floodplain as well as phytoplankton abundance and successional pattern. A high number of phytoplankton species (271 taxa) was found and assigned to 23 functional, 26 morpho-functional and 7 morphology-based functional groups. Redundancy analysis revealed that using ecological classifications yields a higher percentage of variance explained (axes 1 and 2 from 78.5% to 91.3%) than that by the taxonomic approach (69.4% of the variance). Furthermore, the redundancy analysis showed a clear distinction between river and lake samples. The intensity and duration of flooding was the primary cause for the separation of samples during the years. Results indicate that ecological classifications constitute a comprehensive approach to assess the flood-induced changes of phytoplankton assemblages, although the lack of sensitivity for different functional properties of dominant small centric diatoms in the river phytoplankton seems to present certain disadvantages. Altogether, it seems that there is need for certain minor integrations and arrangements of species in current ecological classifications. Flexibility in species grouping according to the specific characteristics of each individual freshwater system, along with the simultaneous application of a taxonomic approach, would aid the development of morpho-functional schemes.