Modelling ecosystem dynamics in the turbulent surface layers of the ocean

Abstract

A size-dependent ecological model based on the original developed by Fasham et al. (1990) was used to investigate the ecosystem dynamics in the North Atlantic at 47^oN 20^oW, as this location has been the site of intensive oceanographic and biological studies so an important background of data was available. The model has eight different compartments: diatom and non-diatom phytoplankton, micro and mesozooplankton, nitrate, ammonium, silicate and detritus. It was calibrated using data provided by the North Atlantic Bloom Experiment for 1989 (NABE89). An optimisation technique based on Powell's method (Press et al, 1992) was applied to estimate unknown parameters by fitting the model output to NABE observations. The uncertainty and correlation in the optimal model parameters were estimated by analysing the cost function. Finally, tests on the choice of the "initial guess" for parameters are well as on the time-stepping technique used were carried out.

A series of twin experiments using 'synthetic' data of the same type and frequency (weekly data throughout the year and daily data during the spring bloom only) were performed to investigate the role of sparse observations on parameter recovery and on reproducing the North Atlantic annual cycle. The same experiments were also used to estimate model parameters when either noise-free or noisy data were assimilated as model observations. The sensitivity to the model structure was also tested.

The ecosystem model was embedded into a 1-D physical model, the Miami Isopycnic Co-ordinate Ocean Model (MICOM) so as to study the effect of realistic physical forcing on the development of the spring bloom and the seasonal plankton cycle. The coupled model was forced with physical fields provided by the NCEP group from 1988 to 1996, which permitted the study of the intra and interannual variability of the ecosystem. The inclusion of an extra compartment (detrital biogenic silica) and the parameterisation of nitrification processes were needed in order to accurately reproduce the vertical gradient of nutrients in the ocean. The coupled model was tuned to the NABE area (using the NABE89 data set) and validated with data from the German JGFOS phase for 47^oN and 20^oN during 1996.

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