Sensitivity of a pelagic ecosystem model to variations of process parameters within a realistic range

A one-dimensional physical-biological coupled model is constructed with ref erence to an oceanic area with a seasonal thermocline and to a pelagic trop hic network dominated by a herbivorous pathway. The sensitivity of the mode l is then tested for the effect of parameter variations within the range ac tually encountered in the literature. In the reference simulation, the phys ical component of the model reproduces reasonably well the annual cycle of the hydrographic structure. The biological component simulates a cycle wher e the spring bloom is dependent on the critical depth criterion and yields, for the state variables and the fluxes, values which are consistent with a vailable data. A comparison of the sensitivity tests within a realistic ran ge (absolute sensitivity) is effected with the more commonly performed test s over a fixed proportion of variation in parameter values (relative sensit ivity). The latter appear to essentially reflect the internal behaviour of the model and overestimate the sensitivity of the system to a number of par ameters, especially those involved in complex relationships. The absolute s ensitivity tests identify a restricted set of parameters with a major influ ence. Phytoplankton biomass and production, as well as the nitrate stock, a re highly dependent on a small number of parameters, among which the most i mportant are the coefficients of the photosynthetic curve and the sinking v elocity of one class of particles. Zooplankton biomass and production, as w ell as the ammonium stock, show a more complex sensitivity to a larger numb er of parameters, among which the same as above, plus grazing parameters an d excretion and remineralization rates. Overall, the fluxes within and from the system appear mostly dependent on the first step in the food web, and the latter, in turn, is mainly dependent on the physical forcing. (C) 1999 Elsevier Science B.V. All rights reserved.