Plankton community composition, production, and respiration in relation todissolved inorganic carbon on the West Florida Shelf, April 1996

In April 1996 the Florida Shelf Lagrangian Experiment examined dissolved in organic carbon (DIC) dynamics on the West Florida Shelf. DIC concentrations increased over 2 weeks at an average rate of 1 mu mol kg(-1) d(-1) in a pa tch of the intentionally released tracers 3 (He-3). Approximately 20% of th e increase was due to sulfur hexafluoride (SF6) and helium air-sea exchange with the remaining 80% attributed to plankton respiration [Wanninkhof et a l., 1997]. Here we present particulate matter concentrations, phytoplankton production, and community respiration rates from the tracer patch that sug gest that heterotrophs dominated the community after the termination of a s pring bloom. During the experiment, chlorophyll a and phaeopigment concentr ations declined from > 1.5 to < 0.5 mu g L-1, with 75-85% of total chloroph yll a in the < 5 mu m size fraction. Particulate matter composition, with m ean ratios of particulate organic carbon:chlorophyll a > 200 and particulat e organic nitrogen:chlorophyll a > 100, suggests that phytoplankton were a minor component of the plankton biomass. Rates of daily gross primary produ ction estimated by the (H2O)-O-18 method averaged 69 +/- 5 mmol C m(-2) d(- 1) (n = 3) while dark respiration rates, estimated from dark bottle incubat ions, were approximately, proximately - 40 +/- 3 mmol C m(-2) d(-1). Net co mmunity production rates (6 +/- 6 mmol C m(-2) d(-1)) were much lower than respiration rates. Thus respiration rates nearly balanced phytoplankton pro duction. 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 autotrophi c carbon fixation, were more than sufficient to account for the observed in crease of DIC within the tracer patch.