In April 1996 the Florida Shelf Lagrangian Experiment examined dissolved inorganic carbon (DIC) dynamics on the West Florida Shelf. DIC concentrations increased over 2 weeks at an average rate of 1 μmol kg-1 d-1 in a patch of the intentionally released tracers sulfur hexafluoride (SF6) and helium 3(3He). Approximately 20% of the increase was due to air-sea exchange with the remaining 80% attributed to plankton respiration <Wanninkhof et al., 1997>. Here we present particulate matter concentrations, phytoplankton production, and community respiration rates from the tracer patch that suggest that heterotrophs dominated the community after the termination of a spring bloom. During the experiment, chlorophyll a and phaeopigment concentrations declined from >1.5 to 200 and particulate organic nitrogen: chlorophyll a>100, suggests that phytoplankton were a minor component of the plankton biomass. Rates of daily gross primary production estimated by the H218O method averaged 69¿5 mmol C m-2d-1 (n=3) while dark respiration rates, estimated from dark bottle incubations, were approximately -40¿3 mmol C m-2d-1. Net community production rates (6¿6 mmol C m-2 d-1) were much lower than respiration rates. Thus respiration rates nearly balanced phytoplankton production. Light respiration rates were estimated from gross production minus net community production (-51¿8 mmol C m-2 d-1) and exceeded dark respiration. Plankton community respiration rates, corrected for autotrophic carbon fixation, were more than sufficient to account for the observed increase of DIC within the tracer patch. ¿ 2000 American Geophysical Union