On the sensitivity of ecosystem box model simulations on mixed-layer depth estimates

Abstract

A certain class of pelagic ecosystem box models uses the mixed layer depth as the main physical forcing. This paper investigates the influence of the definition and derivation of the mixed layer depth on the simulation results. For this investigation the model of Fasham et al. (1990) ( Journal of Marine Research, 48, 591–639) is used to simulate the phytoplankton bloom during the Fladenground Experiment in spring 1976 (FLEX '76). The time of establishment of the thermocline, the mean depth and the temporal variability of the mixed-layer depth differ substantially between the estimates under consideration. Exactly those features influence the model results fundamentally. All processes and state variables of the ecosystem model examined for a series of representations of mixed layer development during FLEX '76 differ widely from each other and from observations. Temporal variability of the mixed-layer depth causes a variability of the onset of the plankton bloom of about two weeks. Consequently, this class of ecological models cannot reproduce the dynamics of the spring bloom ecosystem appropriately; for this purpose vertical mixing processes need to be represented explicitly.