A NUMERICAL STUDY OF PRIMARY PRODUCTION RELATED TO VERTICAL TURBULENT-DIFFUSION WITH SPECIAL REFERENCE TO VERTICAL MOTIONS OF THE PHYTOPLANKTON CELLS IN NUTRIENT AND LIGHT FIELDS

Assuming stationary physical processes, in particular the light field and turbulent activity [K(z)], we described steady-state and convergen t solutions obtained from a simple time-dependent vertical model of ph ytoplankton dynamics. Simulations included vertical turbulent motions experienced by the cells in the light and nutrient fields. Parallel si mulations made with a classical formulation of phytoplankton growth, i .e., neglecting vertical turbulent motions, are discussed. From two ty pical situations of stratification in the Western Mediterranean, we id entified two distinct systems of new production, as the consequence of Low (LTR) and High Turbulent Regime (HTR) in the photic zone respecti vely. Data from the Prolig-II (1985) and Almofront-I (1991) cruises su pported the LTR system of new production. The results of the second pa rt of the Mediprod-I (1969) cruise show several patterns that specific ally appeared in the HTR simulation. Regenerated production was not in fluenced by the turbulent activity situation. In natural conditions, r egenerated production depends on the specific phytoplankton-grazers sy stem that develops according to the level of new production; such ecol ogical dynamics were not considered in our model. Differences with the reference model changed the relationships between the vertical distri butions of biomass and new production. Particularly, the HTR simulatio n led to distinct vertical distribution of biomass and new production. Such a pattern did not occur with the reference model. Although the v ertical turbulent motions affected both the level and vertical distrib ution of new production, a significant effect on the depth-integrated production finally depends on how the phytoplankton biomass interacts with its environmental conditions. It is shown that the minimum of K(z ) in the euphotic zone determined the system of new production, wherea s its-values below the euphotic zone scaled the production and biomass levels. The two distinct systems of new production, LTR and HTR, may be diagnosed from simple cast measurement by examining the relationshi ps between the distributions of parameters implicated in new productio n (biomass maximum, nitracline and oxycline) and the density profile.