Diel vertical migration: modelling light-mediated mechanisms

Light is generally regarded as the most likely cue used by zooplanktonto regulate their vertical movements through the water column.However, the way in which light is used by zooplankton as acue is not well understood. In this paper we present a mathematicalmodel of diel vertical migration which produces vertical distributionsof zooplankton that vary in space and time. The model is usedto predict the patterns of vertical distribution which resultwhen animals are assumed to adopt one of three commonly proposedmechanisms for vertical swimming. First, we assume zooplanktontend to swim towards a preferred intensity of light. We thenassume zooplankton swim in response to either the rate of changein light intensity or the relative rate of change in light intensity.The model predicts that for all three mechanisms movement isfastest at sunset and sunrise and populations are primarilyinfluenced by eddy diffusion at night in the absence of a lightstimulus. Daytime patterns of vertical distribution differ betweenthe three mechanisms and the reasons for the predicted differencesare discussed. Swimming responses to properties of the lightfield are shown to be adequate for describing did vertical migrationwhere animals congregate in near surface waters during the eveningand reside at deeper depths during the day. However, the modelis unable to explain how some populations halt their ascentbefore reaching surface waters or how populations re-congregatein surface waters a few hours before sunrise, a phenomenon whichis sometimes observed in the field. The model results indicatethat other exogenous or endogenous factors besides light mayplay important roles in regulating vertical movement.