A simple model has the potential to resolve the long-running debate amongst oceanographers over whether nitrogen or phosphorus exerts overall control on oceanic primary production. A representation of the competition between nitrogen-fixing and other phytoplankton is inserted into a two-box global model of the oceanic nitrogen and phosphorus cycles. Homeostatic regulation of both nitrate and phosphate concentrations results, with surface waters more deficient in nitrate than phosphate in the steady state, but with external phosphate inputs controlling longer-term primary production in the global ocean.

The relative influences of nitrogen and phosphorus on oceanic primary production

Despite the impressive advances that have been made in assessing the diversity of marine microorganisms, the mechanisms that underlie the participation of microorganisms in marine food webs and biogeochemical cycles are poorly understood. Here, we stress the need to examine the biochemical interactions of microorganisms with ocean systems at the nanometre to millimetre scale--a scale that is relevant to microbial activities. The local impact of microorganisms on biogeochemical cycles must then be scaled up to make useful predictions of how marine ecosystems in the whole ocean might respond to global change. This approach to microbial oceanography is not only helpful, but is in fact indispensable.

Microbial structuring of marine ecosystems

The biological pump is a process whereby CO(2) in the upper ocean is fixed by primary producers and transported to the deep ocean as sinking biogenic particles or as dissolved organic matter. The fate of most of this exported material is remineralization to CO(2), which accumulates in deep waters until it is eventually ventilated again at the sea surface. However, a proportion of the fixed carbon is not mineralized but is instead stored for millennia as recalcitrant dissolved organic matter. The processes and mechanisms involved in the generation of this large carbon reservoir are poorly understood. Here, we propose the microbial carbon pump as a conceptual framework to address this important, multifaceted biogeochemical problem.

Microbial production of recalcitrant dissolved organic matter: long-term carbon storage in the global ocean.

The ecology of Cytophaga–Flavobacteria in aquatic environments

Microscopic estimation of bacterial biomass requires determination of both biovolume and biovolume-to-biomass conversion. Both steps have uncertainty when applied to the very small bacteria typically found in natural seawater. In the present study, natural bacterioplankton assemblages were freshly collected, passed through 0.6-μm-pore-size Nuclepore filters to remove larger particulate materials, and diluted for growth in 0.22-μm-pore-size Millipore filter-sterilized unenriched seawater. This provided cells comparable in size and morphology to those in natural seawater, but the cultures were free of the interfering particulate detritus naturally present. Cells were collected on glass-fiber GF/F filters, and biovolumes were corrected for cells passing these filters; C and N were measured with a CHN analyzer. Our criteria for size measurement by epifluorescence photomicrography were confirmed with fluorescent microspheres of known diameters. Surprisingly, in six cultures with average per-cell biovolumes ranging from 0.036 to 0.073 μm3, the average per-cell carbon biomass was relatively constant at 20 ± 0.08 fg of C (mean ± standard error of the mean). The biovolume-to-biomass conversion factor averaged 0.38 ± 0.05 g of C cm−3, which is about three times higher than the value previously estimated from Escherichia coli, and decreased with increasing cell volume. The C:N ratio was 3.7 ± 0.2. We conclude that natural marine bacterial biomass and production may be higher than was previously thought and that variations in bacterial size may not reflect variations in biomass per cell. Images

Relationships between Biovolume and Biomass of Naturally Derived Marine Bacterioplankton.

A nitrogen-based model of plankton dynamics in the oceanic mixed layer

A seven-component upper ocean ecosystem model of nitrogen cycling calibrated with observations at Bermuda Station “S” has been coupled to a three-dimensional seasonal general circulation model (GCM) of the North Atlantic ocean. The aim of this project is to improve our understanding of the role of upper ocean biological processes in controlling surface chemical distributions, and to develop approaches for assimilating large data sets relevant to this problem. A comparison of model predicted chlorophyll with satellite coastal zone color scanner observations shows that the ecosystem model is capable of responding realistically to a variety of physical forcing environments. Most of the discrepancies identified are due to problems with the GCM model. The new production predicted by the model is equivalent to 2 to 2.8 mol m−2 yr−1 of carbon uptake, or 8 to 12 GtC/yr on a global scale. The southern half of the subtropical gyre is the only major region of the model with almost complete surface nitrate removal (nitrate<0.1 mmol m−3). Despite this, almost the entire model is nitrate limited in the sense that any addition of nitrate supply would go predominantly into photosynthesis. The only exceptions are some coastal upwelling regions and the high latitudes during winter, where nitrate goes as high as ∼10 mmol m−3.

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A seasonal three‐dimensional ecosystem model of nitrogen cycling in the North Atlantic Euphotic Zone

Flows of Energy and Materials in Marine Ecosystems

DIRECT calculation of the air-sea flux of CO2 requires detailed knowledge of the partial pressure of carbon dioxide (
 ) and gas-transfer velocities at the surface of the global ocean1, with the available observations of surface suggesting that it varies in a smooth manner with season and position over the major ocean regions2-5. In spring 1989 we mapped surface , total inorganic carbon (TIC), chlorophyll, temperature and salinity at several locations between 47° N and 60° N in the northeast Atlantic near 20° W, observing large variations in on spatial scales of ≪100 km, Correlated with plankton chlorophyll, surface temperature and TIC. The variation of with latitude was in the opposite sense to that previously reported for this region3-5. Thus, in this ocean area and season at least, the air-sea flux is strongly modulated by biological activity and variable on short spatial scales. The inhomogeneity observed suggests that estimates of the oceanic sink for fossil fuel inferred from existing data (relatively sparse even in the North Atlantic) will be subject to significant error.

Spatial variability in the sink for atmospheric carbon dioxide in the North Atlantic

One important theme of modern biological oceanography has been the attempt to develop models of how the marine ecosystem responds to variations in the physical forcing functions such as solar radiation and the wind field. The authors have addressed the problem by embedding simple ecosystem models into a seasonally forced three-dimensional general circulation model of the North Atlantic ocean. In this paper first, some of the underlying biological assumptions of the ecosystem model are presented, followed by an analysis of how well the model predicts the seasonal cycle of the biological variables at Bermuda Station s' and Ocean Weather Station India. The model gives a good overall fit to the observations but does not faithfully model the whole seasonal ecosystem model. 57 refs., 25 figs., 5 tabs.

M. J. R. Fasham

Papers

A nitrogen-based model of plankton dynamics in the oceanic mixed layer

A seasonal three‐dimensional ecosystem model of nitrogen cycling in the North Atlantic Euphotic Zone

Flows of Energy and Materials in Marine Ecosystems

Spatial variability in the sink for atmospheric carbon dioxide in the North Atlantic

Ecosystem behavior at Bermuda Station “S” and ocean weather station “India”: A general circulation model and observational analysis