Diel Movements of Resident and Transient Zooplankters Above Lagoon Reefs at Enewetak Atoll, Marshall Islands.

Abstract

Of those zooplankters above reefs on the lagoon shelf at Enewetak Atoll at some time during the diel cycle, the vast majority of those larger than about 1.5 mm were there only at night. Many of these larger forms were local residents that by day sheltered in or near shelf substrata, or in swarms close to these substrata, and at night made purposeful forays above the shelf. Many others, however, were transients from the deeper regions of the lagoon, or from the open sea outside the atoll, and these were above the shelf at night by chance. The residents included various polychaetes, cypridinacean ostracods, copepods, mysids, tan aids, isopods, amphipods, and carideans. The transients were mostly holoplankters that included halocyprid ostracods, calanoid copepods, euphausids, and chaetognaths. Both residents and transients were above the shelf at night as a result of diel vertical migrations. The residents were adapted to stay within reach of their diurnal habitats while in the nocturnal water column, often by avoiding currents, and so were readily able to return to those habitats at dawn. The open-water transients, however, lacked such adaptations, and, as a result, probably many were stranded in the shallows above the shelf at dawn, unable to return to their daytime depths and probably vulnerable to planktivorous fishes.