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

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 <0.5 μg L−1, with 75–85% of total chlorophyll a in the < 5μm size fraction. Particulate matter composition, with mean ratios of particulate organic carbon:chlorophyll a >200 and particulate organic nitrogerr.chlorophyll a >100, suggests that phytoplankton were a minor component of the plankton biomass. Rates of daily gross primary production estimated by the H2 18O method averaged 69±5 mmol C m−2 d−1 (n = 3) while dark respiration rates, estimated from dark bottle incubations, were approximately ‐ 40±3 mmol C m−2 d−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−1 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.