SPATIAL AND TEMPORAL VARIABILITY IN CARBON DIOXIDE FLUXES AT THREE COASTAL MARSHES ALONG A SALINITY GRADIENT IN THE NORTHERN GULF OF MEXICO: HOW SUSCEPTIBLE ARE COASTAL MARSHES IN THE REGION TO FUTURE WARIMING?

Abstract:

Carbon gas fluxes in tidal marshes vary spatially and temporally because of vegetation cover, subsurface biogeochemical processes, and environmental forcing and predicting the impact of climate change on greenhouse gas fluxes from wetlands remains challenging. We examined how ecosystem carbon gas exchange varies along a salinity gradient (0-32 ppt) in three marshes along an estuary in the northern Gulf of Mexico, USA. Midday net ecosystem exchange (where a negative rate indicates net carbon assimilated through photosynthesis) was greatest at the most freshwater site (4.8 ± 0.3 µmol CO₂ m^-2 s^-1), followed by the saline (2.8 ± 1.0 µmol CO₂ m^-2 s^-1) and brackish (1.4 ± 0.6 µmol CO₂ m^-2 s^-1) sites. However, net ecosystem exchange integrated diurnally revealed each marsh to be a net CO₂ source to the atmosphere as a result of high ecosystem respiration with no significant difference across the fresh (105.5 ± 28.9 mmol CO₂ m^-2 d^-1), brackish (100.1 ± 36.5 mmol CO₂ m^-2 d^-1), and salt marsh (78.3 ± 28.6 mmol CO₂ m^-2 d^-1) sites. The large loss of carbon from these ecosystems is suggested to be a contributing factor to the disappearances of marshes in the region. Fifty percent of coastal Alabama wetlands, for examples, have disappeared from 1780 to 1980, and between 1955 and 1979 the percent loss (29%) in the region has exceeded the national average by a factor of three. While future warming is not expected to impact carbon assimilation significantly, our warming simulations suggest that carbon loss in these ecosystems can be enhanced by 12 to 26%, potentially exasperating the loss of marshes in the region.