Very high productivity of the C-4 aquatic grass Echinochloa polystachya inthe Amazon floodplain confirmed by net ecosystem CO2 flux measurements

Fluxes of CO2 and H2O vapour from dense stands of the C4 emergent macrophyt e grass Echinochloa polystachya were measured by eddy covariance in both th e low water (LW) and high water (HW, flooded) phases of the annual Amazon r iver cycle at Manaus, Brazil. Typical clear-sky midday CO2 uptake rates by the vegetation stand (including detritus, sediment or water surface) were 3 0 and 35 mu mol CO2 (ground) m(-2) s(-1) in the LW and HW periods, respecti vely. 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 rel ated to PFD. The major difference in CO2 uptake rates between the two perio ds was the larger respiration flux during LW due to the CO2 efflux from sed iment, roots and litter. Integrated 20- or 30-min fluxes were used to deriv e relationships between daily CO2 and H2O vapour fluxes and incident radiat ion. The daily CO2 fluxes were almost linearly related to incident radiatio n, but there was evidence of saturation at the highest daily radiation tota ls. Annual productivity estimated from the daily model in 1996-1997 agreed closely with that previously estimated for 1985-1986 from a leaf-scale phot osynthetic model, but were some 15% less than those derived at that time fr om biomass harvests. Both CO2 uptake and water use efficiency were comparab le with those found in fertilised maize fields in warm temperate conditions .