Neubauer, S.C., W.D. Miller and I.C. Anderson. 2000. Carbon cycling in a tidal freshwater marsh ecosystem: a carbon gas flux study. Marine Ecology Progress Series 199:13-30. Keywords=Peltandra virginica, macrophyte and microalgal productivity, carbon dioxide, methane, belowground respiration, translocation End_keywords Abstract=A process-based carbon gas flux model was developed to calculate total macrophyte and microalgal production, and community and belowground respiration for a Peltandra virginica dominated tidal freshwater marsh in Virginia. The model was based on measured field fluxes of CO2 and CH4, scaled to monthly and annual rates using empirically derived photosynthesis vs. irradiance and respiration vs. 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 536-715 g C m-2 yr-1, with an additional 59 g C m-2 yr-1 fixed by sediment microalgae. Belowground respiration calculated from nitrogen mineralization was estimated to range from 516-723 g C m-2 yr-1 versus 75 g C m-2 yr-1 measured directly with sediment chambers. Methane flux (72 g C m-2 yr-1) accounted for 11-13% 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 270-477 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. End_abstract Notes= submitted by iris@vims.edu, Fri Apr 13 15:53:52 EDT 2001 End_notes