The Roles of Predator Avoidance and Light in the Diel Vertical Migration of Zooplankton

Almost all zooplankton species undergo diel vertical migration (DVM) at some time in their life cycle. The most commonly observed type of DVM is a nocturnal pattern which consists of a single daily ascent with minimum depth reached between sunset and sunrise and maximum depth attained during the day (Forward and Hettler 1992). Migrating animals spend the day in deep waters and stay near the surface at night. Zooplankton may either migrate up and down together in a narrow band or occur stratified in deep waters during the day. The animals then may be spread throughout the entire water column in darkness at night (Lampert 1989).

The adaptive value of diel vertical migration has remained an important and consistent question, but prevalent and ongoing research on the phenomenon has produced conceivable explanations. The idea of migrating from warm, food-rich surface waters to stay in colder water with much lower food abundancy suggests that such a cost should indicate some adaptive value for this behavior. Zooplankton feed on phytoplankton, which must remain closer to the surface in the photic zone of the ocean to receive the necessary amount and intensity of light to undergo photosynthesis. Leaving this food-rich zone and remaining in cold water is considered a cost for zooplankton, not only because of the reduced food availability, but due to the fact that this also results in slower growth and lower fecundity of the organisms (Lampert 1989). Migrating up and down through the depths of the water column also requires a large amount of energy. Ongoing research on this unique behavior has revealed that light and food are thought to be key environmental factors influencing diel vertical migration in zooplankton and evidence for predation avoidance being the ultimate factor in DVM is increasing (Jensen et al. 1999).

One important concept that explains diel vertical migration in zooplankton is the dangerous direct effects of light on the animals. Ultraviolet radiation is a hazardous factor for zooplankton in the surface layers of marine and freshwater environments, and has been suspected as a cause of daytime downward migration (Rhode et al. 2001). Recent experiments have shown that ambient levels of solar ultraviolet radiation are potentially lethal to zooplankton and negative phototactic behavior, or the movement away from light, has been demonstrated in the laboratory (Leech and Williamson 2001). Research by Rhode, Pawlowski and Tollrian (2001) suggest that all Daphnia species escaped into significantly deeper water layers under ultraviolet radiation, and the extent of this response was linked to their pigmentation. One of the most common type of zooplankton, daphnids are tiny crustaceans that feed on microscopic photosynthetic algae. These animals occur in unpigmented and pigmented or melanized forms. Both types of daphnids exhibited a significant downward migration into colder water when exposed to light with a natural ultraviolet radiation. However, melanin pigmentation increased the ultraviolet tolerance of this species because those individuals displayed a less pronounced downward migration. Pigmented Daphnia stayed closer to the surface than unpigmented species because they are more able to tolerate the deleterious and harmful effects of ultraviolet light. The downward migration of both forms of Daphnia during the day indicates that ultraviolet radiation is a factor in depth selection and diel vertical migration of zooplankton (Rhode et al. 2001).

Despite possessing the dangerous effects of ultraviolet exposure, light is still important in the aggregation behavior of zooplankton, more specifically, Daphnia. An experiment by Jensen, Kleiven and Jakobsen (1999) demonstrated that daphnids aggregated irrespective of food level because such behavior reduces vulnerability to predation. When exposed to a normal day and night pattern of light, Daphnia not only aggregated, but also exhibited diel vertical migration even at a high food concentration. This behavior indicates that light is an important factor in the formation of aggregates (Jensen et al. 1999).

The concept of vertical migration as a predator evasion is the most straightforward deduction. Animals avoid surface waters during the day because it is dangerous to stay in the light and be exposed to visual predation. In these surface waters, zooplankton are visible to planktivorous fish during they day, so they descend and escape into darker and deeper waters. These animals then migrate upwards into this food-rich layer of