A LAGRANGIAN MODEL OF PHYTOPLANKTON GROWTH DYNAMICS FOR THE NORTHERN ADRIATIC SEA

This paper deals with the strong algal blooms in the Northern Adriatic Sea. The seasonal change in hydrographic conditions and the monitoring data of Regione Emilia-Romagna are discussed. A numerical model of phytoplankton growth dynamics is presented for the simulation of spring blooming and autumn eutrophication for the Northern Adriatic Sea. The model considers horizontal advection by mean field flows and local sources. The transport equations for phytoplankton biomass, herbivorous zooplankton biomass and nutrients, are calculated in a Lagrangian framework using dynamically passive but interactive tracers. The model is forced by Steele's light function and by Michaelis-Menten uptake kinetics. Comparisons of model results with conventional in situ measurements as well as CZCS-pictures show, qualitatively, good agreement. The reason for autumn eutrophication seems to be a combination of physical and biological processes: a strong salinity front in autumn prevents an exchange of water masses and keeps high nutrient concentrations close to the coastline; net primary production is larger in autumn due to higher water temperatures; the depth of optimum photosynthesis in relation to the water depth or the depth of thermocline yields to reduced growth conditions in spring and to favourable growth conditions in autumn.