Seasonal and ontogenetic variation in diel vertical migration of Chaoborus flavicans and its effect on depth‐selection behavior of other zooplankton

Abstract

We analyzed diel vertical migration (DVM) of zooplankton in June, July, and September in Lake Kärnsjön, Sweden. In this lake, the density of the invertebrate predator Chaoborus flavicans was high, and they performed normal DVM. However, the migration pattern of Chaoborus differed between its four larval instars. Larger instars were found at greater mean depths during the day than were the smaller instars. In both June and July, very few observations of the fourth instar were made at a depth of 1 m during the night. Hence, in June, when only the fourth instar was present, the upper 1‐m layer was a Chaoborus‐free refuge for other zooplankton during the night. This was not the case in July, when the smaller instars (1‐3) hunt at that depth, or in September, when instar four was present there. Crustacean zooplankton showed great flexibility in their DVM behavior. In September, both Bosmina and small Daphnia tended to perform reverse DVM (i.e., they resided closer to the surface during the day). However, no migration was observed in July. In June, most species performed normal DVM by crowding in the Chaoborus‐free stratum close to the surface during the night. Both this pattern of normal DVM of zooplankton in June and the reverse DVM in September indicate that invertebrate predation has a strong effect on the zooplankton community in this fish‐rich lake, showing that ontogenetic changes in migration pattern of an invertebrate predator may influence the seasonal variation in the depth‐selection behavior of other zooplankton.