The population structure and trophic composition of free-living nematodes from carbonate sands within different reef zones (reef crest, reef flat and lagoon) at Davies Reef in the Great Barrier Reef were examined.
Seven benthic stations (C-I) were sampled across the southeastern side of the reef in the following reef zones: within an open basin at the fore-reef crest (C); in bare sandy hollows on the reef flat (D, E, F); in an unprotected shallow lagoon area (G); in a protected shallow lagoon area (H) and in a deeper lagoon area among sediment mounds constructed by ghost shrimps (Callianassa spp.) (I). Sediments at all stations were carbonate sands consisting of coral, mollusc and crustacean shell, coralline algae and Halimeda debris as well as foraminiferan and radiolarian tests.
At each station, 6 cores (2.5 cm diameter and 5 cm length) were collected by hand by SCUBA divers. At (I), 6 cores each were taken from the top, side and the area between the Callianassa mounds. Each core was placed into a plastic jar while under water and later preserved in a 1:500 mixture of Rose Bengal-buffered seawater:5% formalin. The depth at which cores were collected was also recorded.
Sediments were passed through 500 µm and 45 µm sieves. Nematodes retained in the 45 µm sieve were identified to species level when possible and counted. Each species was also classified according to a feeding-type scheme: selective deposit feeder (1A); non-selective deposit feeder (1B); epistrate feeder (2A); predator/omnivore (2B).
Separate sediment cores were taken at each station for grain size analysis. Median grain size, sorting coefficient and percentage sand, silt and clay (by weight) were determined. Total organic matter (% by weight) was measured.
