Water and organic carbon fluxes from an irregularly flooded brackish marsh on the upper Texas Coast, U.S.A.

Water flows, concentrations of total(TOC), dissolved(DOC), and particulate(POC) organic carbon and seston were monitored for 52 diel periods in the single creek draining a 270 ha Spartina patens-Distichlis spicata marsh on the upper Texas coast. Rainfall, creek water flows, and water levels in the creek and on the marsh were measured by recording instruments. Rainfall accounted for most marsh flooding, and water outflow was significantly correlated with both rainfall and marsh water level. Creek flows were predominantly outward because microtopographic features and dense vegetation restricted overmarsh water flows and thereby reduced tidal flooding while extending the time of precipitation runoff. Concentrations of organic carbon in water leaving the marsh were highest in spring and summer and averaged 25.62, 21.41, and 3.35 mg 1^-1 of TOC, DOC, and POC, respectively. These were 9.34, 9.93, and 0.04 mg 1^-1, respectively, higher than bay water. Most POC was 0.3-28 micrometers in diameter. Seston >28 microns leaving the marsh was 95% amorphous material; the rest was plankton, grass particles, and fecal pellets. Loss of organic carbon was directly correlated with net water flux, and thus rainfall accounted for most carbon loss. Net carbon loss averaged 196kg TOC, 150 kg DOC, and 32 kg of POC per day. Net annual loss was 2.4-5.5% of net aerial primary productivity (NAPP), or 21.55-30.09 g TOC m^-2 year^-1. Export from this marsh falls within the range found for other marshes and the data collectively indicate that coastal marshes are not losing as much organic carbon as has been suggested by indirect measurements. The discrepancy between potential and realized export is explained by the fact that export is not a simple removal of excess detritus by tidal action but is a more complicated process mediated by the interaction of additional factors such as rainfall, vegetation structure, microtopographic varition and decomposition, which can serve to reduce the amount and quality of NAPP exported.