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An anoxic event and other biogeochemical effects of the Pantanal wetland on the Paraguay River

The Paraguay River was sampled throughout the annual cycle at two sites located downriver from most of the Pantanal, and major tributaries were occasionally sampled close to their entry into the Pantanal. The floodplains strongly modulate the discharge regime and substantially reduce runoff by enhancing evapotranspirative losses. Contact of the river water with the floodplain results in depletion of dissolved O2, oversaturation of CO2 and CH4 loss of suspended sediments, and reduced export of N and P. Oxygen depletion and associated chemical changes are most marked when river water first contacts the floodplain; in 1995, the water remained anoxic throughout the river channel for 6 weeks, causing massive fish mortality. Bottle incubations showed that the rate of O2 consumption by the river water during the anoxic event was high at first (7.2 µM h−1) but declined exponentially over several hours and was not stimulated by organic carbon and nutrient additions. The O2 demand may be due largely to bacterial CH4 oxidation; the concentration of CH4 in the river was particularly high (∼31 µM) at the time of the incubations. The O2 depletion corresponded with higher concentrations of most major ions, Si, and dissolved organic carbon but was not accompanied by elevated concentrations of inorganic N and P, H2S, or most of the 46 dissolved trace elements that were measured. In contrast to O2 depletion and associated chemical changes, sediment retention by the floodplains was greatest at maximum river stages. Most chemical weathering of minerals seems to take place in the upland drainage basins rather than on the floodplains, and most major solutes display conservative mixing in the river‐floodplain system.