Microbial respiration and chemical composition of different sediment fractions in waterbodies of the upper Paraná River floodplain, Brazil.

Four size fractions of the sediment of six environments of the upper Paraná River floodplain were analyzed for carbon, nitrogen, and phosphorus contents and microbial respiration (oxygen consumption). Particle size did not affect nitrogen and phosphorus content or microbial activity, but did affect carbon content (F = 4.274, df = 3; 20, p = 0.020). The carbon concentration of ultra-fine particles was significantly lower than that of other sizes of sediment particles. Microbial respiration values were well predicted by sediment chemical composition, as shown by multiple regression (microbial respiration = -0.39 - 0.210C + 0.108N + 0.796P; F = 7.0495, p = 0.0022). However, phosphorus was the element which best explained the microbial respiration (partial coefficient = 0.796, p = 0.0039, n = 23). Considering that i) phosphorus was the best predictor of microbial respiration; ii) phosphorus is trapped in the series of reservoirs located upstream from the section of the floodplain studied; and iii) microbial respiration is a measure of decomposition rates and nutrient cycling, we hypothesize that the long-term accumulation of litter detritus and reduction of nutrient cycling in environments of the upper Paraná River floodplain are probable impacts of this decrease in phosphorus caused by the upstream reservoirs.