Water quality dynamics of floodplain lakes in relation to river flooding and cattle grazing

Abstract

Floodplain water quality is naturally regulated by river floods, but is increasingly affected by anthropogenic activities. Accordingly, we evaluated the combined influence of river flooding (pre- and post-flood periods) and rotational grazing (cattle presence or absence) on water quality of floodplain lakes. We hypothesised that river floods decrease the concentrations of most water constituents and dilute grazing effects on water quality, and that differences in cattle pressure (i.e. grazing duration) promote differences in water quality among lakes. Nutrient concentration decreased after the flood through washing-out and dilution, but other water constituents showed different responses. Contrary to expectations, water-quality changes in relation to cattle presence or absence occurred only after the flood, with higher turbidity and phosphorus during periods with than without cattle. Finally, water-quality variability among lakes was attributed to their different sizes (and therefore dilution capacities), but not to differences in cattle pressure. Our results suggest that river–floodplain connectivity and grazing location in areas with large lakes are important for preserving water quality. Pre-flood changes in water quality in relation to cattle presence or absence could have been obscured by wind-driven sediment resuspension, which has important effects during shallow-water stages, and by high phosphorus concentration, which can prevent removal during periods without cattle.