Abstract:
Carbon fluxes in tidal marshes vary spatially and temporally because of vegetation cover, subsurface biogeochemical processes, and environmental forcing. The objective of this study was to examine how ecosystem carbon exchange changes along an estuarine salinity gradient. I measured greenhouse gas fluxes, methane (CH_4) and carbon dioxide (CO_2), from three marshes along a salinity gradient (0-32 ppt) in the Mobile Bay estuary, Alabama, USA. CH_4 flux ranged from 1.2 to 2.4 mmol CH_4 m^-2 d^-1 with no significant differences across sites. Soil temperature, dissolved inorganic nitrate and nitrite, and ecosystem respiration of CO_2, not salinity, were correlated to CH_4 emissions. Midday net ecosystem exchange was greatest at the most fresh site (-4.8 ± 0.3 μmol CO_2 m^-2 s^-1), followed by the saline (-2.8 ± 1.0 μmol CO_2 m^-2 s^-1) and brackish (-1.4 ± 0.6 μmol CO_2 m^-2 s^-1) sites. However, net ecosystem exchange integrated over a diurnal time period using a shade cloth technique revealed each marsh to be a net CO_2 source to the atmosphere as a result of high ecosystem respiration with no difference across the fresh (105.5 ± 28.9 mmol CO_2 m^-2 d^-1), brackish (100.1 ± 36.5 mmol CO_2 m^-2 d^-1), and salt marsh (78.3 ± 28.6 mmol CO_2 m^-2 d^-1) sites. These findings lead to the conclusion that either the marshes are losing carbon or that they receive a subsidy of respirable carbon, possibly via tidal deposition. The extent to which sedimentation from tidal deposition contributes carbon to these ecosystems, however, remains unknown. Without such a subsidy, marshes in the study area will not be able to keep up with sea level rise.
