DIEL VERTICAL MIGRATION - MODELING LIGHT-MEDIATED MECHANISMS

Light is generally regarded as the most likely cue used by zooplankton to regulate their vertical movements through the water column. Howeve r, the way in which light is used by zooplankton as a cue is not well understood. In this paper we present a mathematical model of diel vert ical migration which produces vertical distributions of zooplankton th at vary in space and time. The model is used to predict the patterns o f vertical distribution which result when animals are assumed to adopt one of three commonly proposed mechanisms for vertical swimming. Firs t, we assume zooplankton tend to swim towards a preferred intensity of light. We then assume zooplankton swim in response to either the rate of change in light intensity or the relative rate of change in light intensity. The model predicts that for all three mechanisms movement i s fastest at sunset and sunrise and populations are primarily influenc ed by eddy diffusion at night in the absence of a light stimulus. Dayt ime patterns of vertical distribution differ between the three mechani sms and the reasons for the predicted differences are discussed. Swimm ing responses to properties of the light held are shown to be adequate for describing diel vertical migration where animals congregate in ne ar surface waters during the evening and reside at deeper depths durin g the day. However, the model is unable to explain how some population s halt their ascent before reaching surface waters or how populations re-congregate in surface waters a few hours before sunrise, a phenomen on which is sometimes observed in the held. The model results indicate that other exogenous or endogenous factors besides light may play imp ortant roles in regulating vertical movement.