ALBERT

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: This study relates the performance of an optimized one-dimensional ecosystem model to observations at three sites in the North Atlantic Ocean: the Bermuda Atlantic Time Series Study (BATS, 31N 64W), the location of the North Atlantic Bloom Experiment (NABE, 47N 20W), and Ocean Weather Ship INDIA (OWS-INDIA, 59N 19W). The ecosystem model is based on nitrogen and resolves dissolved inorganic nitrogen (N), phytoplankton (P), zooplankton (Z) and detritus (D), therefore called the NPZD-model. Physical forcing, such as temperature and eddy diffusivities are taken from an eddy-permitting general circulation model of the North Atlantic Ocean, covering a period from 1989 through 1993. When an optimized parameter set is applied, the recycling of organic nitrogen becomes significantly enhanced, compared to previously published results of the NPZD model. The optimized model yields improved estimates of the annual ratio of regenerated to total primary production (f-ratio). The annual f-ratios are 0.09, 0.31, and 0.42 for the locations of BATS, NABE, and OWS-INDIA, respectively. Nevertheless, three major model deficiencies are identified. Most conspicuous are systematic discrepancies between measured 14C-fixation rates and modeled primary production under nutrient depleted conditions. This error is primarily attributed to the assumption of a constant carbon-to-nitrogen ratio for nutrient acquisition. Secondly, the initial period of the modeled phytoplankton blooms is hardly tracked by the model. That particular model deficiency becomes most apparent at the OWS-INDIA site. The interplay between algal growth and short-term alterations in stratification and mixing is believed to be insufficiently resolved by the physical model. Eventually, the model's representation of the vertical nitrogen export appears to be too simple in order to match, at the same time, remineralization within the upper 300 meters and the biomass export to greater depths.

Description: The export of organic carbon to the deep ocean is mediated by sinking of large particles, such as marine snow, the formation of which is enhanced in the presence of transparent exopolymer particles (TEP) . TEP form from dissolved and colloidal polysaccharides by aggregation processes. Especially when running into nutrient limitation phytoplankton organisms are a source of TEP in pelagic ecosystems as the cells release a significant amount of the assimilated carbon in the form of polysaccharides. Because CO_2 concentration influences carbon assimilation rates, we hypothesized that polysaccharide exudation and aggregation into TEP is related to CO_2 concentration under nutrient limiting conditions. We tested this hypothesis in several lab and outdoor experiments with natural populations and cultures of phytoplankton exposed to various levels of CO_2 concentrations. Our results indicate that TEP production increases with CO_2 concentration and provides an enhanced sink for carbon during phytoplankton blooms.

Description: One particular task of marine ecosystem models is to simulate the biogenic transformation of dissolved inorganic carbon (DIC) into organic matter and hence to quantify the export of particulate organic carbon (POC) to deep oceanic layers. To date, environmental changes, such as increasing carbon dioxide concentrations (pCO_2) and temperature, are perceived to have an impact on the formation of organic carbon. However, well established nitrogen or phosphorus based ecosystem models are insensitive to variations in the carbonate system. In order to investigate biological responses to pCO_2 variations, ecosystem models need to distinguish between carbon, nitrogen, and/or phosphorus cycles. We present a simple biological model which decouples carbon from nitrogen fluxes such that carbon found in transparent exopolymer particles (TEP) is additionally accounted for. The model regards phytoplankton acclimation to varying environmental conditions, having included parameterizations for phytoplankton growth as proposed by Geider et al.~(1998, L&O). By means of data assimilation, an optimal parameter set is determined, which brings model results into agreement with experimental data. From the optimised model results it is infered that about 50% of dissolved organic carbon (DOC) exuded by phytoplankton is subsequently transformed into TEP, eventually influencing the amount of POC available for the export flux. Model sensitivity studies are performed at local sites and along a latitudinal transect (30^oN-60^oN at 19^oW) in the North Atlantic. As soon as CO_2 limitation for phytoplankton growth is explicitely considered in the model, the formation of POC shows great sensitivity to pCO_2 variations. Temperature variations alter remineralisation rates and growth efficiencies. With the current model version dependencies between biomass accumulation, the date of nutrient depletion to occur, and the exudation of organic compounds are acquired.

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: The overall goal of this work is to investigate the performance of ecosystem models and to relate their results to existing observations in the North Atlantic. Different data assimilation methods are applied. A variational adjoint technique and a micro-generic algorithm (mGA) are utilized to estimate model parameters, such that the misfit between model results and observations is minimised. Experiments are performed with nitrogen based ecosystem models, comprising three and four state variables (NPZ- and NPZD models): dissolved inorganic nitrogen (N), phytoplankton (P), herbivorous zooplankton (Z) and detritus (D). First, data assimilation experiments are conducted with observations from the Bermuda Atlantic Time-series Study (BATS) in order to optimise the NPZ-model. While applying the adjoint method different optimal parameter sets are obtained when starting from different initial parameter sets. It is shown that for parameter optimisation of an ecosystem model, the application of the mGA is superior to the performance of the adjoint method. Second, simultaneous assimilation experiments are performed with the NPZD-model using observational data from three locations in the North Atlantic. The parameter set retrieved from the simultaneous optimisations produces substantial differences in the biogeochemical fluxes when compared with model results using previously published parameters. The optimisation yields a best parameter set, which can be utilized for basin wide simulations in coupled physical-biological models of the North Atlantic.

Description: The overall goal of this work is to investigate the performance of ecosystemmodels and to relate their results to existing observations in the NorthAtlantic. Therefore different data assimilation methods are applied. Avariational adjoint technique and a micro-generic algorithm ($\mu$GA) areutilized to estimate model parameters, such that the misfit between modelresults and observations is minimised. Data assimilation experiments areperformed with nitrogen based ecosystem models, comprising three and fourstate variables (NPZ- and NPZD models): dissolved inorganic nitrogen (N),phytoplankton (P), herbivorous zooplankton (Z) and detritus (D). TheNPZ-model simulates mean concentrations of the different variables withinthe upper mixed layer, while the NPZD-model has a vertically resolved grid.Physical boundary conditions are obtained from three-dimensional simulationsof the ocean's circulation in the North Atlantic, with daily mean atmosphericforcing from ECMWF-reanalysis data.First, data assimilation experiments are conducted with observations fromthe Bermuda Atlantic Time-series Study (BATS) in order to optimise theNPZ-model. While applying the adjoint method different optimal parametersets are obtained when starting from different initial parameter sets. It isshown that for parameter optimisation of an ecosystem model, theapplication of the $\mu$GA is superior to the performance of the adjointmethod.Second, simultaneous assimilation experiment are performed with theNPZD-model using observational data from three locations in the NorthAtlantic: BATS, the site of the North Atlantic Bloom Experiment (NABE) andthe Ocean Weather Ship-India (OWS-INDIA). The simultaneous optimisationyields a best parameter set, which can be utilized for basin wide simulationsin coupled physical-biological (general circulation) models of the NorthAtlantic.The parameter set retrieved from the simultaneous optimisations producessubstantial differences in the biogeochemical fluxes when compared withmodel results using previously published parameters. In contrast to earliermodels the rapid cycling of organic matter for sustaining primary productionis emphasized. Furthermore, systematic discrepancies between$^{14}$C-fixation rates and modelled primary production are identified.It is suggested that carbon based primary productivity may not beadequately represented by ecosystem models when a constant nitrogento carbon conversion factor is assumed.The chosen physical boundary conditions are adequate to simulate thebiogeochemical fluxes at the BATS and NABE sites. At high latitudes(OWS-INDIA), however, the physical-biological interactions in themodel cannot represent the observed chlorophyll distribution in spring.It is suggested that during this period short-termed alterations ofstratification, rapid biological response and deep mixing of phytoplanktonare necessary in order to reproduce chlorophyllconcentrations at depths of 150-200m.