The relative importance of water quality, sediment composition and floating vegetation in explaining the macrobenthic community structure of floodplain lakes (Parana River, Argentina)

The bottom macroinvertebrates of three floodplain lakes were sampled during a period of 15 months to evaluate the relative importance of water quality and sediment-vegetation variables in explaining patterns of variation in community structure. Water quality variables were associated with hydrological and climatic cycles, while sediment-vegetation variables were related to local environmental conditions. The relative weights of both sets of variables were measured by performing several Partial Canonical Correspondence Analyses (PCCA), followed by a variance partitioning technique. In all lakes, the greatest macrobenthic biomass was observed during the low water phase. During that period, Chironomus and burrowing mayflies (Campsurus) were dominants in a lake having 60% cover of Eichhornia crassipes whereas sphaeriid clams (Pisidium and Eupera), burrowing mayflies, and gastropods (Littoridina) were more abundant in a lake without floating plants. A lake completely covered with E. crassipes had the lowest mean biomass. Biomass decreased sharply during the high water period, and community structure became similar in all lakes, with dominance of Oligochaeta and Ostracoda. The most relevant environmental variables associated with spatial and temporal variations were dissolved oxygen, calcium, clay, floating vegetation, and bottom organic matter. Environmental variables explained about 74% of total variance in community structure, with similar relative contributions of physical and chemical variables of the water (16%), and substrate-vegetation variables (14%). Both sets of variables shared 44% of the explained variance. These results suggest that in the studied lakes, the flood is the major 'batch' process affecting macrobenthos, mainly through changes in water quality. However, local environmental conditions may be as important as water quality in explaining among-lakes variations in macrobenthos at low water periods.