Carbon Dioxide Assimilation, Photosynthetic Efficiency, and Respiration of a Chesapeake Bay Salt Marsh

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Abstract
Net CO2 exchange and photosynthetically active radiation (PAR; 400-700 nm) were measured periodically between Aug. 1974 and Sept. 1979 in 2 salt marsh communities on Chesapeake Bay, USA. One community had the 2 C4 grasses, Spartina patens and Distichlis spicata, and the other included these grasses and the C3 sedge, Scirpus olneyi. The mean value (.permill.) of photosynthetic efficiency from June-Aug. of the grass community was significantly greater than that of the mixed community but the year-to-year differences within both communities were not statistically significant. The maximum value for daytime net CO2 assimilation per unit area of salt marsh, between sunrise and sunset, occurred in late June. During June, July and Aug., net CO2 assimilation by both communities accounted for 3.5 .+-. 0.9% of the incident PAR. Whole-season community net CO2 assimilation was estimated using a regression equation for the seasonal course of photosynthetic efficiency and a model for incident solar radiation. The seasonal (May-Oct.) daytime net assimilation of CO2 exceeded seasonal (April-Dec.) nighttime net loss of CO2. The carbon balance in the salt marsh was positive and accumulation of CO2 equivalent to a biomass of about 740 g m-2/yr was available for storage within the system or for export to the adjacent estuary. Though the photosynthetic efficiency between June-Aug. was 1.9%, the net ecosystem efficiency, allowing for CO2 losses, for utilization of PAR during May-Oct. was 0.8% equivalent to 0.4% of total solar radiation.