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

Abstract

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 herbivory obtained on a cruise in the NE Atlantic during 20 days of the spring bloom. Rates of water-column-integrated primary production and heterotrophic bacterial production were used to force the model, and the model parameters were determined by optimizing the model to the remaining observations. A unique model solution was not possible in the absence of dissolved organic carbon (DOC) measurements. By setting a weak constraint on the DOC concentrations (that they should remain broadly constant), a solution was obtained that predicted a small DOC increase during the bloom period; this increase was similar in magnitude to observations in other areas and produced a good fit with the other data. Major conclusions: (1) gross primary production (GPP) was over twice the measured 14 C production estimates; (2) phytoplankton exudation was a major source of DOC, and 35% of the GPP flowed through the DOC and was utilized by the bacteria, with a low gross growth efficiency (17%); (3) 62% of the community respiration came from organisms Ͻ5 m in size; (4) the major source of food for microzooplankton was phytoplankton 1-5 m in size, but the mesozooplankton ingested more microzooplankton than phytoplankton; and (5) only 8% of the total net primary production was grazed by mesozooplankton. These results provide quantitative support to the emerging consensus on the relative roles of large and small organisms in the euphotic zone and demonstrate the necessity of measuring DOC to constrain the carbon budget within the euphotic zone.A prominent objective of biological oceanography is to determine and understand the processes controlling the fluxes of carbon in the ocean over time. The most important of these is the fate of CO 2 taken up by phytoplankton in the euphotic zone. We wish to quantify how this carbon passes through the food web to its eventual fate as carbon respired within the euphotic zone or carbon exported to the deeper ocean. An attempt to budget carbon was one of the aims of the Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment (NABE) carried out in 1989 at a number of stations along the 20ЊW meridian. The most extensive coverage was at 47ЊN, where Bender et al. (1992) calculated a carbon budget for the upper water column (0-50 m) over a 13-d period during the spring bloom. Of the total gross primary production of 1.83 mmol C m Ϫ2 , they estimated that 63% was respired, 16% was stored as particulate organic carbon (POC) within the upper 50 m, and 18% was sedimented out to depths Ͼ50 m. Bender et al. (1992) did not attempt to unravel the details 1 Corresponding author: mjf@soc.soton.ac.uk. AcknowledgmentsWe thank those colleagues in the UK BOFS Program who provided data for this study and also the staff of the British Oceanographic Data Centre for assistance with accessi...

The magnitude of spring bacterial production in the North Atlantic Ocean

“…Reliable DOC data do not exist for JGOFS-NABE, preventing a comparison of variations in the dissolved semilabile pool, which accumulates and degrades on seasonal timescales (e.g., Anderson and Williams 1998), with BP (Fasham et al 1999). We compared mean euphotic zone BP with mean POC concentrations (POC stock divided by the EZ depth) to remove the variation in stock contributed by changing EZ depth (Fig.…”

“…We then examine possible additional sources of organic matter to support BP, such as degrading stocks of previously accumulated POC or DOC, as a means of explaining the BP : PP estimates higher than levels that can be supported by shortterm production of fresh DOC by foodweb processes. The approach is complementary to the top-down strategy used by Fasham et al (1999), who forced a linear flow analysis model of the NABE observations with measured BP and community respiration estimates and determined what levels of gross primary production (GPP) and phytoplankton exudation (PER) were required to support the observations. Our bottom-up method starts by generating the DOM supply from foodweb processes and asking what BP : PP it would support, given various levels of BGE and PP.…”

“…Our bottom-up method starts by generating the DOM supply from foodweb processes and asking what BP : PP it would support, given various levels of BGE and PP. Fasham et al (1999) concluded that either a large amount of phytoplankton exudation (drawn from increased GPP, a fresh DOC scenario) or use of a large preexisting DOC pool (older DOC alternative) was required to simulate the observed respiration levels. The lack of reliable DOC measurements precluded evaluation of these two alternative scenarios.…”

“…Application of models incorporating ecosystem data from each specific study can be used to constrain the possible BP rates for a given set of conditions. Fasham et al (1999) addressed the same region in the period just before that considered here, forcing a linear flow, difference equation model with observed primary and bacterial production. One implication of their model was that observed bacterial production and community respiration required large inputs of DOC, either from an existing pool or from (unmeasured) phytoplankton exudation.…”

“…Their model indicates that if BGE is as low as 0.1-0.2 then, at least at steady state, fluxes of DOC arising directly or indirectly from PP would be expected to generate BP : PP in the range 5-20% depending on recycling parameters and the magnitude of phytoplankton exudation. Fasham et al (1999) took the BP estimates as given and did not consider uncertainty in them. In this paper we reconsider the original NABE BP data and the derivations of the NABE BP estimates.…”