Control mechanisms of diel vertical migration: Theoretical assumptions

We explore control mechanisms underlying the vertical migration of zooplank ton in the water column under the predator-avoidance hypothesis. Two groups of assumptions in which the organisms are assumed to migrate vertically in order to minimize realized or effective predation pressure (type-I) and to minimize changes in realized or effective predation pressure (type-II), re spectively, are investigated. Realized predation pressure is defined as the product of light intensity and relative predation abundance and the part o f realized predation pressure that really affects organisms is termed as ef fective predation pressure. Although both types of assumptions can lead to the migration of zooplankton to avoid the mortality from predators, only th e mechanisms based on type-II assumptions permit zooplankton to undergo a n ormal diel vertical migration (morning descent and evening ascent). The ass umption of minimizing changes in realized predation pressure is based on co nsideration of DVM induction only by light intensity and predators. The ass umption of minimizing changes in effective predation pressure takes into ac count, apart from light and predators also the effects of food and temperat ure. The latter assumption results in the same expression of migration velo city as the former one when both food and temperature are constant over wat er depth. A significant characteristic of the two type-II assumptions is th at the relative change in light intensity plays a primary role in determini ng the migration velocity. The photoresponse is modified by other environme ntal variables: predation pressure, food and temperature. Both light and pr edation pressure are necessary for organisms to undertake DVM. We analyse t he effect of each single variable. The modification of the phototaxis of mi gratory organisms depends on the vertical distribution of these variables. (C) 2001 Academic Press.