Water Hyacinth’s Effect on Greenhouse Gas Fluxes: A Field Study in a Wide Variety of Tropical Water Bodies

Water hyacinth is able to sequester large amounts of carbon dioxide (CO 2 ) in wetlands. At the same time, the high production of organic matter combined with the plant’s capacity to limit the diffusion of oxygen from the atmosphere into the water creates favorable conditions for the production of methane (CH 4 ). The combination of these mechanisms challenges the prediction of water hyacinth’s net effects on greenhouse gas (GHG) emissions. To unravel the impact of water hyacinth on GHG fluxes, we performed an extensive fieldwork study encompassing 22 sites dominated by water hyacinth in the Pantanal and Amazon during two different seasons. The highest CH 4 emissions from water hyacinth beds occurred in shallow systems where sediment rooting enabled plant-mediated CH 4 transport (307 ± 407 mg CH 4 m −2 day −1 in waters shallower than 1 m, as opposed to 6.1 ± 10.6 mg CH 4 m −2 day −1 in deeper waters). When CO 2 uptake rates are added to the GHG budget (in terms of global warming potential), the water bodies were usually a GHG sink (− 5.2 ± 10 gCO 2 eq m −2 day −1 ). The strength of the sink is highest in deeper systems where even a low water hyacinth coverage may already offset open water emissions. This dual effect of strong CO 2 uptake—and at least temporal carbon storage in biomass—in combination with a high CO 2 –to-biomass-to-CH 4 (and possibly back to CO 2 ) conversion highlights the necessity to include vegetation characteristics in relation to depth when estimating GHG fluxes for tropical wetlands.