High rates of net primary production and turnover of floating grasses on the Amazon floodplain: implications for aquatic respiration and regional CO₂ flux

Abstract

We investigated whether rates of net primary production (NPP) and biomass turnover of floating grasses in a central Amazon floodplain lake (Lake Calado) are consistent with published evidence that CO 2 emissions from Amazon rivers and floodplains are largely supplied by carbon from C4 plants. Ground-based measurements of species composition, plant growth rates, plant densities, and areal biomass were combined with low altitude videography to estimate community NPP and compare expected versus observed biomass at monthly intervals during the aquatic growth phase (January-August). Principal species at the site were Oryza perennis (a C3 grass), Echinochloa polystachya, and Paspalum repens (both C4 grasses). Monthly mean daily NPP of the mixed species community varied from 50 to 96 g dry mass m À2 day À1 , with a seasonal average (AE1SD) of 64 AE 12 g dry mass m À2 day À1 . Mean daily NPP (AE1SE) for P. repens and E. polystachya was 77 AE 3 and 34 AE 2 g dry mass m À2 day À1 , respectively. Monthly loss rates of combined above-and below-water biomass ranged from 31% to 75%, and averaged 49%. Organic carbon losses from aquatic grasses ranged from 30 to 34 g C m À2 day À1 from February to August. A regional extrapolation indicated that respiration of this carbon potentially accounts for about half (46%) of annual CO 2 emissions from surface waters in the central Amazon, or about 44% of gaseous carbon emissions, if methane flux is included.

Globally significant greenhouse-gas emissions from African inland waters

“…Similarly, a relationship between CH 4 , p CO 2 and %O 2 with wetland fraction was also found across river systems (Fig. 4), consistent with findings in the Amazon basin, where CO 2 emissions from wetland lakes and river channels have been attributed to organic C from wetlands [11][12][13] that also sustain intense CH 4 evasion 17 .…”

“…Alternatively, if most of the CO 2 emissions from SSA river channels are derived from wetland C, the net balance would be nearly neutral as it is balanced by the atmospheric CO 2 fixation by the emerged vegetation, as shown in the Amazon lowland areas [11][12][13] . Untangling the relative contributions of wetland and upland C in sustaining CO 2 and CH 4 emissions from inland waters is essential to better understand the role of tropical inland waters in the global C cycle and related potential feedbacks on a warming climate.…”

“…The CO 2 emissions from inland waters have been traditionally interpreted as fuelled by organic C from upland terrestrial biomass 1,14 . In the Amazon basin, CO 2 emissions from floodplain lakes 11 and from river channels 12,13 have been attributed to organic and inorganic C from wetlands (flooded forest and macrophytes). Finally, recently recognized biases in computed CO 2 data traditionally used in inland water studies 15,16 highlight the requirement for careful data-quality checking and future emphasis on high-quality direct CO 2 measurements.…”

“…In particular, there is a need to further understand the link between inland water GHG fluxes and catchment characteristics, in particular regarding their connectivity with upland terrestrial 9,10 and wetland [11][12][13] C production and stocks. The CO 2 emissions from inland waters have been traditionally interpreted as fuelled by organic C from upland terrestrial biomass 1,14 .…”

Factors controlling water‐column respiration in rivers of the central and southwestern Amazon Basin

“…Isotopic work suggests C 4 vegetation (mainly grasses within river-corridors and floodplains) may be an important substrate for respiration within the central Amazon Basin (Quay et al 1992;Engle et al 2008). For example, the d 13 C of respiration-derived CO 2 (CO 2 resp ) indicates that C 4 grasses can support up to 40% of water-column respiration on the Solimõ es River during the early rising-water stage (Quay et al 1992) within the central Amazon Basin.…”