Regulation of zooplankton vertical migrations by light, food, and fish

The regulation of zooplankton vertical migration has been attributed to the physical environment, and, more recently, to biological factors. This study evaluated the influence of light, food and fish on vertical migration. Laboratory experiments quantified changes in vertical migrations of Daphnia pulex in tall columns in response to short term changes in food concentration and the presence of a fish, under a wide range of natural light conditions. A stimulus-threshold model predicted the time of the start of migration from a threshold rate of relative light intensity change. Hypotheses that migrations follow isopleths of light intensity, and that velocity of migration is directly correlated to relative rate of light intensity change, were not supported. Concentrations of Chlorella sp. had little influence on the timing, amplitude, or velocity of migration. There was evidence of a sensory response of Daphnia pulex to the short term presence of a fish in the columns. With a fish present, pre-migration population depths were deeper. This response ceased within 24 hours after removing the fish. Juvenile Daphnia pulex started their migrations from shallower depths in the columns, and otherwise migrated similarly to adults. Field studies of zooplankton vertical migration were conducted on four New Hampshire lakes. The lakes varied in food concentrations, and all contained planktivorous fish. The field results were consistent with an avoidance of ultra-violet radiation by zooplankton. Pre-migration depths were deeper in waters more transparent to visible light. Food stratification was correlated with amplitude and direction of vertical migration, and the direction of migration was apparently influenced by the phytoplankton composition. Although planktivorous fish were present in the lakes, the zooplankton did not take advantage of possible light refugia. Fish density did not significantly affect vertical migrations. Invertebrate predation was not sufficient to explain reverse migrations