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

Brief description

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 fish2. within the territories of Hemiglyphidodon plagiometopon3. within cages4. beneath a shading top covered with mesh similar to that of the cagesExposed 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. This study was initiated to investigate the effects of fish grazing and territoriality of the damselfish, Hemiglyphidodon plagiometopon, on the structure of the benthic algal community. Dominant algal species identified during the experiment:Polysiphonia subtilissimaCentroceros clavulatumGracilariopsis rhodotrichaLyngbya aestuariiLyngbya convervoidesSymploca hydnoidesPseudendoclonium sp.Oscillatoria aff. amphibiaOscillatoria aff. nigroviridis

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Statement: Preparation of settlement plates:Colonies of the scleractinian coral, Pachyseris speciosa, were collected, deep frozen for approximately 12 hours and then exposed to full sunlight for several hours. Plates measuring approximately 10x20cm were cut from the coral, using a diamond bladed rock saw cooled with seawater. Dead tissue and epibiota were left attached to the plates. Plates were then attached to galvanised steel racks with nylon cable ties and plastic tips were placed over plate supports, so that plates were not in direct contact with the steel.Cage description:Cages were 100x130x25 cm in size and made of 5x5 cm galvanized steel mesh. The cage was then covered with 2.54 cm² plastic mesh attached by cable ties and the structure further supported by 4 steel reinforcement bars. Cages were cleared of algae by brushing at monthly intervals.Non-destructive sampling techniques are described in: Sammarco PW (1980) Effects of grazing by Diadema antillarum Philippi (Echinodermata: Echinoidea) on algal diversity and community structure. J exp mar Biol Ecol 65:83-105.The PAR radiometer is described in:Drew EA (1983) Light. In Earll R, Edwin D (eds). The sublittoral environment of the British Isles - in perspective. Oxford University Press, London.

Notes

Credit
Sammarco, Paul W, Dr (Principal Investigator)