Diel pattern with abrupt crepuscular changes of zooplankton over a coral reef

Abstract

We studied zooplankton abundance and emergence patterns over coral reefs in the Gulf of Aqaba using highresolution acoustics (multibeam sonar, FTV, 1.6 MHz, and WH600 ADCP, 614 KHz), emergence traps, and an underwater pump, supplemented with field observations on feeding behavior of zooplanktivorous fish. The zooplanktonic community over coral reefs is a complex and highly dynamic mixture of pelagic plankton advected into the reef, larvae and eggs released by benthic animals, and demersal plankton. At sunset (0 Ϯ4 min, mean Ϯ95% confidence interval) acoustic back-scattering intensity and zooplankton biomass started to increase rapidly. In contrast with this ascent, the predawn decline began much earlier during the dark (82 Ϯ5 min before sunrise) and terminated before sunrise. This light-dependent diel behavior was highly consistent throughout the year, regardless of seasonal and environmental changes. Smaller zooplankters (500-700 m) ascended first and demersal zooplankton accounted for most of the increase in zooplankton soon after sunset. Surprisingly, the emergence of zooplankton occurred while large schools of diurnal zooplanktivorous fish were still foraging in the water column. However, at that time, prey-capture efficiency by the fish is greatly reduced and corals had not yet expanded their tentacles, producing an optimal window of opportunity of low risk for emergence.Diel vertical migration (DVM) is one of the most conspicuous features of zooplankton behavior, occurring in di-

Diel variation in abundance, biomass and size composition of zooplankton community over a coral-reef in Redang Island, Malaysia

“…The mid fraction (200-335 mm) increased just after sunset, while the large fraction (Ͼ335 mm) increased most substantially 1.5-h after sunset. In a similar observation, abrupt increase at sunset was attributed to an increase in smaller zooplankton, whereas larger zooplankton emerged one hour after sunset in the Gulf of Aqaba, Red Sea (Yahel et al 2005b). High zooplankton abundance in the early hours of the night has been reported from various coral-reefs: e.g.…”

“…Some zooplankton individuals which stay in, on or near corals may emerge soon after sunset before the corals fully expand their tentacles. This active bottom avoidance may partially contribute to the intense nocturnal emergence of coral-reef zooplankton early in the night (Yahel et al 2005b). Interestingly, over a sandy bottom with only a few small patches of coral, a peak in zooplankton biomass occurred later at night, rather than at dusk at Tioman Island, Malaysia (Nakajima et al in press).…”

“…A question arises as to why zooplankton show an abrupt nocturnal increase early in the night. Yahel et al (2005b) showed using acoustic back-scattering that the abrupt emergence of reef zooplankton occurred soon after sunset when many diurnal planktivorous fish are still foraging but their prey-capture efficiency is greatly decreased and corals had not yet expanded their tentacles. Some zooplankton individuals which stay in, on or near corals may emerge soon after sunset before the corals fully expand their tentacles.…”

Neustonic copepods (Labidocera spp.) discovered living residentially in coral reefs

“…emerged at dusk and often underwent a second emergence in the middle of the night, but this migration pattern was not obviously related to the lunar cycle. From other studies in coral reefs, residential zooplankton emergence patterns appear to depend in part on circadian rhythms and the lunar cycle (Alldredge and King 1980;Ohlhorst 1982;Madhupratap et al 1991;Yahel et al 2005).…”

Abundance and distribution of nocturnal fishes over a coral reef during the night

“…Furthermore, standard methodological approaches, such as the use of flashlights and line transects, limit the relevance of the findings to specific habitats (e.g. lagoons [Marnane & Bellwood 2002] and the benthic boundary layer [Annese & Kingsford 2005]), while data on the temporal patterns of nocturnal fishes during the night are largely restricted to dusk and dawn (Fishelson et al 1971, Hobson 1972, McFarland et al 1999, Yahel et al 2005a. Therefore, a quantitative study of the spatial and temporal distributions of nocturnal fish is necessary in order to understand their role in the ecology of coral reefs.…”