Description: The Joint Global Ocean Flux Study (JGOFS) has completed a decade of intensive process and time-series studies on the regional and temporal dynamics of biogeochemical processes in five diverse ocean basins. Its field program also included a global survey of dissolved inorganic carbon (DIC) in the ocean, including estimates of the exchange of carbon dioxide (CO2) between the ocean and the atmosphere, in cooperation with the World Ocean Circulation Experiment (WOCE). This report describes the principal achievements of JGOFS in ocean observations, technology development and modelling. The study has produced a comprehensive and high-quality database of measurements of ocean biogeochemical properties. Data on temporal and spatial changes in primary production and CO2 exchange, the dynamics of of marine food webs, and the availability of micronutrients have yielded new insights into what governs ocean productivity, carbon cycling and export into the deep ocean, the set of processes collectively known as the "biological pump." With large-scale, high-quality data sets for the partial pressure of CO2 in surface waters as well for other DIC parameters in the ocean and trace gases in the atmosphere, reliable estimates, maps and simulations of air-sea gas flux, anthropogenic carbon and inorganic carbon export are now available. JGOFS scientists have also obtained new insights into the export flux of particulate and dissolved organic carbon (POC and DOG), the variations that occur in the ratio of elements in organic matter, and the utilization and remineralization of organic matter as it falls through the ocean interior to the sediments. JGOFS scientists have amassed long-term data on temporal variability in the exchange of CO2 between the ocean and atmosphere, ecosystem dynamics, and carbon export in the oligotrophic subtropical gyres. They have documented strong links between these variables and large-scale climate patterns such as the El Nino-Southern Oscillation (ENSO) or the North Atlantic Oscillation (NAO). An increase in the abundance of organisms that fix free nitrogen (N-2) and a shift in nutrient limitation from nitrogen to phosphorus in the subtropical North Pacific provide evidence of the effects of a decade of strong El Ninos on ecosystem structure and nutrient dynamics. High-quality data sets, including ocean-color observations from satellites, have helped modellers make great strides in their ability to simulate the biogeochemical and physical constraints on the ocean carbon cycle and to extend their results from the local to the regional and global scales. Ocean carbon-cycle models, when coupled to atmospheric and terrestrial models, will make it possible in the future to predict ways in which land and ocean ecosystems might respond to changes in climate.

Description: An eddy‐permitting coupled ecosystem‐circulation model with accurate descriptions of advection and turbulent mixing is used to estimate the nitrate supply to the euphotic zone in the North and equatorial Atlantic Ocean. The simulated annual mean input of nitrate into the euphotic zone is separated into different supply routes, namely, turbulent vertical mixing, vertical advection, and horizontal transport. Vertical mixing is found to be the dominant supply mechanism in the subpolar North Atlantic, while horizontal advection provides most of the simulated nitrate input into the subtropical gyre. Contributions by vertical advection are largest in the equatorial upwelling region and in the subpolar gyre. A comparison with observational estimates of nitrate flux into euphotic zone of the subtropical gyre reveals that the model can simultaneously fit the estimates by Lewis et al. [1986] and Jenkins [1988] that were previously thought to be contradictory. The simulated nitrate supply is, on the other hand, not consistent with estimates of export production based on oxygen consumption [Jenkins, 1982]. The model results are used to investigate to what extent the advective input of organic matter could possibly explain a local imbalance between new and export production. It turns out that the simulated advective input of organic matter alone, which in the model provides nitrogen at a rate similar to that arising from nitrate supply, is not sufficient to explain observed oxygen consumption rates.

Description: Light abundance is a major prerequisite for primary production in pelagic ecosystems, influencing the evolution of the marine environment. Realistic simulations of planktonic ecosystems therefore require an appropriate representation of the underwater light field. Taking a look at the different biogeochemical models discussed in literature, one finds a variety of descriptions for the distribution of light, or more specific, photosynthetically available radiation (PAR) in the water column. This paper compares the effect of different parameterizations of PAR on the primary production and phytoplankton evolution at the European Station for Time-Series in the Ocean Canary Islands (ESTOC) station (29°10′N, 15°30′W) north of Gran Canaria, Canary Islands. Observations from two cruises in 1997 are used to illustrate the winter and spring situation at the time-series site. Four alternative PAR descriptions are used in a one-dimensional coupled biological–physical model of the upper ocean driven by daily forcing fields over a 5-year period. The biological model is a simple nitrate-phytoplankton–zooplankton-detritus model. Although the different descriptions are found to have only a small effect (±3%) on the annual primary production, we observe significant changes in the vertical distribution of simulated phytoplankton. The large variation (±32%) in the near-surface chlorophyll contents will be of particularly crucial importance when using satellite ocean-color sensors for model validation and parameter estimation. For future three-dimensional biogeochemical models, a computationally efficient and accurate parameterization of the light field will be particularly relevant.

Description: An optimization experiment is performed with a vertically resolved, nitrogen-based ecosystem model, composed of four state variables (NPZD-model): dissolved inorganic nitrogen (N), phytoplankton (P), herbivorous zooplankton (Z) and detritus (D). Parameter values of the NPZD-model are optimized while assimilating observations at three locations in the North Atlantic simultaneously, namely at the sites of the Bermuda Atlantic Time-Series Study (BATS; 31N 64W), of the North Atlantic Bloom Experiment (NABE; 47N 20W), and of Ocean Weather Ship-India (OWS-INDIA; 59N 19W). A method is described for a simultaneous optimization which effectively merges different types of observational data at distinct sites in the ocean. A micro-genetic algorithm is applied for the minimization of a weighted least square misfit function. The optimal parameter estimates are shown to represent a compromise among local parameter estimates that would be obtained from single-site optimizations at the individual locations. The optimization yields a high estimate of the initial slope parameter of photosynthesis (alpha), which is shown to be necessary to match the initial phases of phytoplankton growth. The estimate of alpha is well constrained by chlorophyll observations at the BATS and OWS-INDIA sites and likely compensates for a deficiency in the parameterization of light-limited growth. The optimization also points toward an enhanced recycling of organic nitrogen which is perceived from a high estimate for the phytoplankton mortality/excretion rate.

Description: In the literature, an inconsistency exists between estimates of biotically-effected carbon export inferred from large-scale geochemical studies (Jenkins 1982; 47 gC m−2 a−1) and local measurements of turbulent nutrient supply (Lewis et al. 1986; 4 gC m−2 a−1) in the eastern subtropical North Atlantic. Nutrient supply to the upper ocean by turbulent mixing is reexamined using local standard oceanographic measurements and high-resolution vertical profiles of nutrients averaged over a large region directly comparable to that investigated by Jenkins (1982).

Description: Assimilation experiments with data from the Bermuda Atlantic Time-series Study (BATS, 1989¯1993) were performed with a simple mixed-layer ecosystem model of dissolvedinorganic nitrogen (N), phytoplankton (P) and herbivorous zooplankton (H). Our aim is to optimize the biological model parameters, such that the misfits between model results andobservations are minimized. The utilized assimilation method is the variational adjoint technique, starting from a wide range of first-parameter guesses. A twin experiment displayedtwo kinds of solutions, when Gaussian noise was added to the model-generated data. The expected solution refers to the global minimum of the misfit model-data function, whereasthe other solution is biologically implausible and is associated with a local minimum. Experiments with real data showed either bottom-up or top-down controlled ecosystemdynamics, depending on the deep nutrient availability. To confine the solutions, an additional constraint on zooplankton biomass was added to the optimization procedure. Thisinclusion did not produce optimal model results that were consistent with observations. The modelled zooplankton biomass still exceeded the observations. From the model-datadiscrepancies systematic model errors could be determined, in particular when the chlorophyll concentration started to decline before primary production reached its maximum. Adirect comparision of measured 14C-production data with modelled phytoplankton production rates is inadequate at BATS, at least when a constant carbon to nitrogen C : N ratio isassumed for data assimilation.

Description: A coupled ecosystem-circulation model of the North Atlantic Ocean is used to investigate the impact of radiative heating by biotically induced absorption of solar radiation on the ocean's heat budget, on water column stability and circulation, and on biological production itself. For fixed atmospheric conditions, the local sensitivity of the nonsolar heat flux to changes in sea surface temperature leads to a net cooling of the ocean by the biota at a rate of about 1 W m−2. As a result, simulated winter mixed-layer depths are deeper by more than 100 m in parts of the subpolar gyre, whereas upper-ocean stratification is enhanced in the tropics and subtropics, and coastal upwelling and associated nutrient supply are reduced by about 10% compared to a model run with optical properties of clear seawater. Simulated chlorophyll concentrations increase, indicating a positive feedback, only in subpolar regions that exhibit a pronounced phytoplankton spring bloom. Here biotically induced trapping of heat closer to the sea surface leads to a faster shoaling of the mixed layer and a more intense spring bloom in the model. On the basin average, simulated surface chlorophyll concentrations, however, decrease by 3%, constituting a weak negative feedback of 0.03 W m−2, when heating by biotic absorption of solar radiation is accounted for. These findings are based on the approximation of the atmosphere as a passive heat buffer and will have to be tested against results from fully coupled atmosphere-ocean models with interactive marine biology.