Very High Productivity of the C₄ Aquatic Grass Echinochloa polystachya in the Amazon Floodplain Confirmed by Net Ecosystem CO₂ Flux Measurements

Fluxes of CO₂ and H₂O vapour from dense stands of the C4 emergent macrophyte grass Echinochloa polystachya were measured by eddy covariance in both the low water (LW) and high water (HW, flooded) phases of the annual Amazon river cycle at Manaus, Brazil. Typical clear-sky midday CO₂ uptake rates by the vegetation stand (including detritus, sediment or water surface) were 30 and 35 μmol CO₂ (ground) m-2s-1in the LW and HW periods, respectively. A rectangular hyperbola model fitted the responses of "instantaneous" (20- or 30-min average) net CO₂ exchange rates to incident photosynthetic photon flux densities (PFD) well. Stand evaporation rates were linearly related to PFD. The major difference in CO₂ uptake rates between the two periods was the larger respiration flux during LW due to the CO₂ efflux from sediment, roots and litter. Integrated 20- or 30-min fluxes were used to derive relationships between daily CO₂ and H₂O vapour fluxes and incident radiation. The daily CO₂ fluxes were almost linearly related to incident radiation, but there was evidence of saturation at the highest daily radiation totals. Annual productivity estimated from the daily model in 1996-1997 agreed closely with that previously estimated for 1985-1986 from a leaf-scale photosynthetic model, but were some 15% less than those derived at that time from biomass harvests. Both CO₂ uptake and water use efficiency were comparable with those found in fertilised maize fields in warm temperate conditions.