TEMPERATURE EFFECTS ON STOCKS AND STABILITY OF A PHYTOPLANKTON-ZOOPLANKTON MODEL AND THE DEPENDENCE ON LIGHT AND NUTRIENTS

A model of a closed phytoplankton-zooplankton ecosystem was analyzed f or effects of temperature on stocks and stability and the dependence o f these effects on light and total nutrient concentration of the syste m. An analysis of the steady state equations showed that the effect of temperature on zooplankton and POM biomass was levelled when primary production is nutrient limited. Temperature increase had a generally n egative effect on all biomasses at high nutrient levels due to increas ed maintenance costs. Nutrient limitation of net primary production is the main factor governing the effect of stocks and flows as well as t he stability of the system. All components of the system, except for p hytoplankton biomass, are proportional to net production and thus to t he net effect of light on photosynthesis. However, temperature determi nes the slope of that relationship. The resilience of the system was m easured by calculating the eigenvalues of the steady state. Under olig otrophic conditions, the system can be stable, but an increase in temp erature can cause instability or a decrease in resilience. This conclu sion is discussed in the face of recent models that take spatial heter ogeneity into account and display far more stable behavior, in better agreement to empirical data. Using simulations, we found that the ampl itude of fluctuations of the herbivore stock increases with temperatur e while the mean biomass and minimum values decrease in comparison wit h steady state predictions. (C) 1997 Elsevier Science B.V.