Faunal relationships among the near-reef zooplankton at three locations on heron reef,great barrier reef,and seasonal changes in this fauna

Samples of zooplankton were collected using a light trap at 5 sites in 3 locations on Heron Reef: (a) near the surface of open water 300 m south of the reef crest; (b) near the surface and at the substratum on the upper reef slope; (c) near the surface and at the substratum on a patch reef in the Heron lagoon. The collections made were analysed with respect to: (a) distribution and abundance of the taxa present; (b) faunistic relationships among samples from the 5 sites; (c) seasonal changes in both of these factors. A total of 181 taxa were recognised, many of which are identified to species, and many of which are demersal or epi-benthic in habits. At all sites, the abundance of animals increases from May to November, and faunal similarity between sites also changes. In May, reef collections are generally similar to one another and, with the exception of the slope surface collection, distinct from the open water collection. In September this pattern is enhanced, but in November slope collections more closely resemble the open water collection, while the lagoon collections are quite distinct from slope and open water collections. Lagoon surface and substratum collections also differ considerably from each other at this time. A MULTCLAS cluster analysis of the samples confirms the pattern of change in faunal relationships seen from examination of the collections. Dark-trap samples were used to assess the bias introduced by using a light to attract the animals, as well as to estimate the density of the fauna sampled. Lighttrap samples over-represent calanoid and harpacticoid copepods and gammarid amphipods, but the bias is minor and does not prevent use of a light trap as an efficient sampling tool for near-reef plankton. The density of the fauna is approximately 700 animals m^-3 at all sites. This may be a lower density than in more tropical regions. Pronounced seasonal changes occur in faunal composition of collections from open water and from surface sites. The substratum collections show more constant faunas throughout the year. Major changes are primarily in the proportions of copepods and cumaceans present. Changes in amphipod numbers are also important at lagoon sites.