Michael J. R. Fasham

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

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.

Modelling the Marine Biota

Marine biota play an important role in the natural carbon cycle of the ocean. Biogenic particles are exported from the euphotic zone to the deep ocean and this ‘biological’ pump, in conjunction with the ‘solubility’ pump, maintains the vertical gradient of dissolved inorganic carbon (DIC). Model studies (Bacastow & Maier-Raimer, 1990; Shaffer, this volume) and analysis of GEOSECS data (Volk & Hoffert, 1985) suggest that the biological pump contributes between 60%-83% of the total DIC pump on a world-wide basis. If the marine biota were removed, the atmospheric pCO2 would increase from its present values of 335 ppmv to 460 ppmv (Shaffer, this volume). However, when modelling the oceanic uptake of anthropogenic CO2 it is generally assumed that the marine biota play no role in this process (Sarmiento et al., 1992). The reason for this assumption is that the high concentration of bicarbonate ions in seawater implies that marine plants should not be limited by CO2 (Fogg, 1975) and that, therefore, the anthropogenic increase in surface water DIC will not produce any increase in primary production. This situation is in contrast to the terrestrial biosphere where there is evidence for a CO2 fertilisation effect (Gifford, this volume).

Towards a model of ocean biogeochemical processes

Themes in modelling ocean biogeochemical processes.- Global extrapolation.- Fluctuations: a task package for the physicists.- Trophic resolution.- Modelling growth and light absorption in the marine diatom Skeletonema costatum.- Carbon: a phycocentric view.- Towards a general description of phytoplankton growth for biogeochemical models.- Modelling zooplankton.- Microbial processes and the biological carbon pump.- Dissolved organic matter in biogeochemical models of the ocean.- Modelling particle fluxes.- The significance of interannual variability.- Some parametric and structural simulations with a three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone.- Data assimilation for biogeochemical models.- An annotated bibliography of marine biological models.- List of participants.

Mesoscale and upper ocean variabilities during the 1989 JGOFS bloom study

Abstract Altimetric data from Geosat and some critical hydrographic measurements were used to estimate in real time the mesoscale physical oceanographic environment surrounding the Joint Global Ocean Flux Study (JGOFS) 1989 North Atlantic Bloom Experiment. Three cyclonic eddies, including an exceptionally large one, evolved and interacted over the 10 weeks of observations. Subsequent analysis of all available hydrographic data confirmed the real time estimates and provided further quantitative information concerning the mesoscale and submesoscale structure of the upper ocean. Remotely sensed indicators of near-surface chlorophyll content reveal significant biological variability on these wavelengths. The altimetric and hydrographic data have been assimilated into a dynamical model to produce optimal estimates of physical fields of interest as they evolve in time for use in physical and biological process studies.

Eddies and Biological Processes

Surprisingly, there has been little biological sampling done at space or time scales which will detect changes associated with mesoscale features in the open ocean (by mesoscale we mean scales of tens to hundreds of km and 1 to 3 years). Repeated sampling has either been in a localised area (e.g., Angel 1969, Angel et al. 1982), or in labelled parcels of water (e.g., Climax experiment, see McGowan and Walker 1980, for full references), or in highly advective regions (e.g., CALCOFI investigations), or the data are not in a form amenable to such analysis (e.g., Continuous Plankton Recorder Programme, Colebrook, pers. comm.), or have too gross a scale to detect the features (e.g., many zoogeographical surveys).

An Examination of the “Continental shelf pump” in an open ocean general circulation model

In a recent study of the shelf region of the East China Sea, Tsunogai et al. [1999] estimated that a combination of air-sea exchange and biological and physical transport processes could transfer carbon from the shelf region into the open ocean at a rate of 35 g Cm-2 yr-1. Contrasting with the solubility and biological pumps of the open ocean, they described this collective activity as the ‘‘continental shelf pump’’ and suggested that if this pump operated throughout the world’s shelf regions, it could be responsible for ocean uptake of ~1 Gt C yr-1 (~50% current ocean uptake of anthropogenic CO2). In this work a general circulation model (GCM) is used to explore the potential strength of this pump across the world’s shelves. Since the GCM does not represent the continental shelf regions explicitly, a parameterization of the pump has been used. Results of simulations find modeled pump activity very variable between shelf regions, with the East China Sea shelf behaving very similarly to the global average. Storage of pump carbon is particularly high in the Atlantic Ocean and other regions where deep water is formed. A considerable reservoir of pump carbon becomes trapped under the Arctic ice sheet. Simple extrapolations from the results suggest that should shelf regions absorb CO2 at the rate of the East China Sea, the pump would account for a net oceanic uptake of 0.6 Gt C yr-1.

Global fields of sea surface dimethylsulfide predicted from chlorophyll, nutrients and light

Abstract The major difficulty in estimating global sea–air fluxes of dimethylsulphide (DMS) is in interpolating measured seawater DMS concentrations to create seasonally resolved gridded composites. Attempts to correlate DMS with variables that can be mapped globally, e.g. chlorophyll, have not yielded reliable relationships. A comprehensive database of DMS measurements has recently been assembled by Kettle et al. [Global Biogeochem. Cycles 13 (1999) 399]. This database, which contains chlorophyll as a recorded variable, was extended by merging nutrients and light from globally gridded fields. A new equation was developed whereby DMS is predicted from the product of chlorophyll ( C , mg m −3 ), light ( J , mean daily shortwave, W m −2 ) and a nutrient term ( Q , dimensionless) using a “broken-stick” regression: DMS =a, log 10 (CJQ)≤s DMS =b[ log 10 (CJQ)−s]+a, log 10 (CJQ)>s where Q =N/( K N +N), N is nitrate (mmol m −3 ) and K N is the half saturation constant for nitrate uptake by phytoplankton (0.5 mmol m −3 ). Fitted parameter values are: a =2.29, b =8.24, s =1.72. Monthly maps of global DMS were generated by combining these equations with ocean color data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The resulting high DMS concentrations in high latitude, upwelling and shelf areas are consistent with observed patterns. Predicted global seasonally averaged mean DMS is 2.66 nM. The further application of gas transfer equations to these fields leads to estimates of globally integrated DMS fluxes from ocean to atmosphere of 0.86 and 1.01 Tmol S year −1 for two formulations of piston velocity. The simplicity of the new relationship makes it suitable for implementation in global ocean general circulation models. The relationship does not however resolve DMS variability in low-DMS areas, which constitute large tracts of the open ocean, and should therefore be used with caution in localized studies.

Dissolved Organic Matter in Biogeochemical Models of the Ocean

Recent developments in high temperature catalytic oxidation techniques (Sugimura and Suzuki, 1988) have raised a number of questions about dissolved organic carbon (DOC). Several key aspects of the DOC pool are still unknown, including its size (Martin & Fitzwater, 1992; Ogawa & Ogura, 1992) and turnover rates (Kirchman et al., 1991; Kepkay & Wells, 1992; Bauer et al., 1992). Even the low estimates of DOC concentrations, however, imply that DOC is large enough (700 × 109 tons C) to rival the terrestrial biomass and atmospheric CO2. Thus the dissolved carbon pool needs to be included in biogeochemical models. Yet uncertainty about concentrations and many key aspects of DOC production and consumption does limit our ability to include DOC in these models. Furthermore, the lack of data allows a model to evoke DOC with few restrictions in order to explain otherwise unresolved issues. Because of these uncertainties, the DOC working group spent more time discussing what is known about DOC than about modelling it. What emerged from that discussion are some recommendations for future experimental research in order to provide data that would aid in modeling more accurately the role of DOC in biogeochemical processes.

The impact of mesoscale eddies on plankton dynamics in the upper, ocean

Abstract A coupled QuasiGeostrophic mixed-layer ECOsystem model (QGECO) is used to investigate the impact of the underlying mesoscale eddy field on the spatial and temporal scales of biological production and on overall rates of primary productivity. The model exhibits temporal trends in the biological and physical fields similar to those observed in the North Atlantic; i.e. the mixed layer shallows in spring causing a rapid increase in phytoplankton concentrations and a corresponding decline in nutrient levels. Heterogeneity is produced in the mixed layer through Ekman pumping velocities resulting from the interaction of windstress and surface currents. This variability impacts on biological production in two ways. Firstly, spatial variations in the depth of the mixed layer affect the photosynthetically active radiation (PAR) availability and hence production rates, and secondly, eddy enhanced exchange between the surface water and those at depth bring additional nutrients into the euphotic zone. These processes result in significant spatial and temporal heterogeneity in the ecosystem distributions. Investigation of the spatial heterogeneity of the biological system finds variability to be significantly greater than that of the mixed layer. The relationship between the eddy field and the ecosystem is investigated. The structure and correlation of the biogeochernical fields change with time. The biological fields are found to have a shorter horizontal scale, but whiter spectrum than the underlying eddy field. Overwinter conditions are found to have a profound effect on the variability, size and timing of the following spring bloom event. Variations in the nitrate levels are primarily responsible for the variability in the biological system in the first year. In subsequent years the variation in the overwintering population is found to be dominant.

Phytoplankton production and community structure in an unstable frontal region

Abstract Results are presented for a two-phytoplankton model of an unstable frontal ecosystem. Vertical transport results in both an increased flux of nitrate to surface waters and to a transport of phytoplankton communities at a rate too rapid to track equilibrium. The former leads to an increase in primary production within the region by ∼10%. The latter is responsible for transient heterogeneity in distributions, especially of phytoplankton ratios. Although the inclusion of two size classes of phytoplankton does not appear to change significantly the total primary production compared to a single phytoplankton model, it does allow for a dynamic partitioning of phytoplankton biomass and production between the classes. This partitioning is controlled by biological responses to transport, dictated here by inter-class differences in nutrient limitation and mortality. Biological responses to upwelling are also shown to be dependent on the background nitrate profile.