ALBERT

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: 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.