Modeling the relative contributions of autotrophs and heterotrophs to carbon flow at a Lagrangian JGOFS station in the Northeast Atlantic: The importance of DOC

A size-structured ecosystem model was developed and fitted to an extensive set of observations of size-fractionated biomass, primary and heterotrophic bacterial production, respiration, particle flux, and zooplankton herbivor y obtained on a cruise in the NE Atlantic during 20 days of the spring bloo m, Rates of water-column-integrated primary production and heterotrophic ba cterial production were used to force the model, and the model parameters w ere determined by optimizing the model to the remaining observations. A uni que model solution was not possible in the absence of dissolved organic car bon (DOC) measurements. By setting a weak constraint on the: DOC concentrat ions (that they should remain broadly constant), a solution was obtained th at predicted a small DOC increase during the bloom period, this increase wa s similar in magnitude to observations in other areas and produced a good f it with the other data. Major conclusions: (1) gross primary production (GP P) was over twice the measured C-14 production estimates; (2) phytoplankton exudation was a major source of DOC, and 35% of the GPP Rowed through the DOC and was utilized by the bacteria, with a low gross growth efficiency (1 7%); (3) 62% of the community respiration came from organisms <5 mu m in si ze; (3) the major source of food for microzooplankton was phytoplankton 1-5 mu m in size, but the mesozooplankton ingested more microzooplankton than phytoplankton; and (5) only 8% of the total net primary production was graz ed by mesozooplankton. These results provide quantitative support to the em erging consensus on the relative roles of large and small organisms in the euphotic zone and demonstrate the necessity of measuring DOC to constrain t he carbon budget within the euphotic zone.