Marine Biology 41, 317-333 (1977) MARINE BIOLOGY 9 by Springer-Verlag 1977 Distribution, Abundance, and Substrate Preferences of Demersal Reef Zooplankton at Lizard Island Lagoon, Great Barrier Reef A.L. AIIdredge I and J.M. King2 1 Department of Biological Sciences, University of California; Santa Barbara, California, USA and 2Marine Science Institute, University of California,Santa Barbara, California, USA Abstract Demersal zooplankton, those plankton which hide within reef sediments during the day but emerge to swim freely over the reef at night, were sampled quantitatively using emergence traps placed over the substrate at Lizard Island Lagoon, Great Barrier Reef. Densities of zooplankton emerging at night from 6 substrate types (fine, medium, and coarse sand, rubble, living coral and reef rock) and from 5 reef zones (seaward face, reef flat, lagoon, back reef, and sand flat) were deter- mined. A large population of nocturnal plankton including cumaceans, mysids, ostracods, shrimp, isopods, amphipods, crustacean larvae, polychaetes, foraminif- erans and copepods are resident members of the reef community at Lizard Island. The mean density of plankton emerging throughout the reef was 2510 • 388 (standard error) zooplankton/m2 of substrate. Biomass averaged 66.2 • 5.4 mg ash-free dry weight/m2 of substrate. Demersal zooplankton exhibited significant preferences for substrate types and reef zones. The highest mean density of zooplankton emerged from coral (11,264 • 1952 zooplankton/m 2) while the lowest emerged from reef rock (840 • 106 zooplankton/m2). The density of demersal plankton was six times greater on the face than in any other zone, averaging 7900 • 1501 zooplankton/m 2. Copepods dominated samples collected over living coral and rubble while foraminiferans, ostracods and decapod larvae were most abundant from sand. Plankton collected with nets at night correlated only qualitatively with plankton collected in emergence traps from the same location. Although abundant, demersal plankton were not numer- ous enough to meet the metabolic needs of all corals at Lizard Island Lagoon. De- mersal plankton appear especially adapted to avoid fish predation. The predator- avoidance strategies of demersal plankton and maintenance of position on the reef are discussed. Our results indicate that much of the zooplankton over coral reefs actually lives on the reef itself and that previous studies using standard net sam- pling techniques have greatly underestimated plankton abundance over coral reefs. Introdu~ion revealed a distinctive resident fauna greatly differing in species composition Zooplankton are an important source of from samples collected in open water out- food and nutrients for coral reef commu- side the lagoons. These resident zoo- nities, especially for corals (Johannes plankton include mysids, amphipods, cuma- et al., 1970; Porter, 1974) and fish ceans, polychaetes, many crustacean lar- (Davis and Birdsong, 1973; Hobson, 1974). vae and distinct species of copepods not Although it is generally accepted that most zooplankton drift across the reef from surrounding oceanic waters, Emery (1968) has suggested that many zooplank- ters may be resident members of the reef community itself. Evidence supporting a strictly resident planktonic fauna on coral reefs is now substantial. Plankton samples collected with plankton nets (Johnson, 1954; Bakus, 1964; Tranter and George, 1972) and with light traps (Sale et al., 1976) within reef lagoons have found outside the reef. Emery (1968) observed zooplankton near reefs while SCUBA diving. Many were capable of maintaining their position on the reef through active swimming and by utilizing crevices, caves and coral heads as protection from predators and currents. Emery also noted that large numbers of resident plankton appeared only at night. Many other investigators have also noted the increased abundance of zooplankton over reefs at night (Jo-