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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−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.

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Journal of Geophysical Research, v. 105, issue C3, p. 6579-6589

Copyright 2000 by the American Geophysical Union.