A process-based carbon gas flux model was developed to calcuIate total macrophyte and microalgal production, and community and belowground respiration, for a Peltandra virginica dominated tidaI freshwater marsh in Virginia. The model was based on measured field fluxes of CO2 and CH,, scaled to monthly and annual rates using empirically derived photosynthesis versus irradiance, and respiration versus temperature relationships. Because the gas exchange technique measures whole system gas fluxes and therefore includes turnover and seasonal translocation, estimates of total macrophyte production will be more accurate than those calculated from biomass harvests. One limitation of the gas flux method is that gaseous carbon fluxes out of the sediment may underestimate true belowground respiration if sediment-produced gases are transported through plant tissues to the atmosphere. Therefore we measured gross nitrogen mineralization (converted to carbon units using sediment C/N ratios and estimates of bacterial growth efficiency) as a proxy for belowground carbon respiration. We estimated a total net macrophyte production of 536 to 715 g C m-' yr-', with an additional 59 g C m-' yr-' fixed by sediment microalgae. Belowground respiration calculated from nitrogen mineralization was estimated to range from 516 to 723 g C m-2 yr-' versus 75 g C m-' yr-l measured directly with sediment chambers. Methane flux (72 g C m-' yr-') accounted for 11 to 13 % of total belowground respiration. Gas flux results were combined with biomass harvest and literature data to create a conceptual mass balance model of macrophyte-influenced carbon cycling. Spring and autumn translocation and re-translocation are critical in controlling observed seasonal patterns of above and belowground biomass accumulation. Annually, a total of 270 to 477 g C m-2 of macrophyte tissue is available for deposition on the marsh surface as detritus or export from the marsh as particulate or dissolved carbon.

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