A physical-biological coupled model for algal dynamics in lakes

A coupled model is presented for simulating physical and biological dynamic s in fresh water lakes. The physical model rests upon the assumption that t he turbulent kinetic energy in a water column of the lake is fully containe d in a mixed layer of variable depth. Below this layer the mechanical energ y content is assumed to vanish. Additionally, the horizontal currents are i gnored. This one-dimensional two-layered model describes the internal conve rsion of the mechanical and thermal energy input from the atmosphere into a n evolution of the mixed layer depth by entrainment and detrainment mechani sms. It is supposed to form the physical domain in which the simulation of the biological processes takes place. The biological model describes mathematically the typical properties of phy to- and zooplankton, their interactions and their response to the physical environment. This description then allows the study of the behaviour of Lag rangian clusters of virtual plankton that are subjected to such environment s. The essence of the model is the dynamical simulation of an arbitrary num ber of nutrient limited phytoplankton species and one species of zooplankto n. The members of the food web above and below affect the model only static ally. The model is able to reproduce the typical progression of a predator-prey i nteraction between phyto- and zooplankton as well as the exploitative compe tition for nutrients between two phytoplankton species under grazing pressu re of Daphnia. It suggests that the influence of the biological system on t he physical system results in a weak increase of the surface temperature fo r coupled simulations, but a considerably higher seasonal thermocline in sp ring and a lower one in autumn. (C) 1999 Society for Mathematical Biology.