Dataset: Effects of fish grazing and damselfish territoriality on algal community structure at Britomart Reef, Great Barrier Reef


In November 1979, settlement plates made from the coral Pachyseris speciosa, a suitable substratum for algal settlement, were deployed under a series of experimental conditions on Britomart Reef. Plates were placed within a depression directly behind the reef crest, where the territorial damselfish, Hemiglyphidodon plagiometopon, was known to be abundant.

Racks of twelve settlement plates were subjected to each of the following four experimental conditions.

  1. fully exposed to grazing by fish
  2. within the territories of Hemiglyphidodon plagiometopon
  3. within cages
  4. beneath a shading top covered with mesh similar to that of the cages

Exposed and shaded racks, were placed in areas of low habitat preference for damselfish. Three complete sets of replicate treatments were deployed at depths between 9 and 11m.

Two plates per rack were removed in February and October 1980 and visually assessed for percent cover of algae and community structure, using a non-destructive sampling technique. A 9.2 x 18.5cm grid, divided into 98 equally sized squares was placed over each plate and the percentage cover of flora or fauna scored for each square. Subsamples of major algal species and their microassociates were frozen for later identification.

Four plates per rack were removed in March and December 1980 and frozen for later estimation of algal biomass. Thawed plates were examined with a dissecting microscope and all epibenthic algae (excluding crustose coralline algae) were removed with forceps. The algae was decalcified with 5% HCl, dried at 55°C for approximately 24 hours and weighed.

Underwater irradiance within treatments was measured with a diver operated PAR radiometer, which measured light energy in the 400 to 700nm wavelength range. Measurements were made in mid-April 1980 between 1130 and 1230 hrs.

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