Abundance, distribution and trophic potential of cryptofauna at Davies Reef, Great Barrier Reef

Full description

The major groups of motile cryptofauna inhabiting dead coral substrata across a transect at Davies Reef were quantified in terms of abundance, biomass and density.Samples of dead corals of similar size were collected by enclosing a piece of protruding dead coral substrate with a plastic bag and detaching a piece with a masonry hammer. Collections were made at 7 locations in November 1985: on the front slope at 10 m depth; on the front crest; on the main flat; where the main flat borders the lagoon at 10 m depth; on the outer back reef on the flat; and on the outer back reef at 10 m depth. Five replicate samples were collected at each of 6 sites (3 inside, 3 outside damselfish territories) where possible, at each location. Stegastes apicalis territories were present in the reef flat and reef crest locations. The front slope location contained Plectroglyphidodon lacrymatus territories. Two locations did not have damselfish territories. Sampling was repeated in August 1986 at 2 locations (reef flat and reef crest), to provide an estimate of seasonal variation in cryptofaunal abundance. Ten replicate samples were collected both within and outside fish territories at each location.Cryptofauna were extracted in the laboratory and any animals too large to be washed through a sieve of 1 cm mesh were excluded from the analysis. Individuals within each of the 5 most common taxonomic groups (amphipods, copepods, other crustaceans, polychaetes and molluscs) were counted. Separate counts of crustaceans were also made to a finer taxonomic level (gammarideans, caprellids, isopods, cumaceans, tanaids, ostracods, decapods). Biomass was measured (ash-free dry weight) for the 5 common taxonomic groups for the November 1985 samples only.Chlorophyll a was used as an index of algal biomass present on the coral rock. Following pigment extraction, the surface areas (SA) of coral blocks were estimated using a latex-coating method. Blocks were photocopied, and their projected surface area (PSA) measured by digitising those images. The ratio of PSA to SA was taken as a measure of surface complexity.The expected food consumption rate of cryptofauna for each sample was calculated using the published allometric relationships between measured ingestion rate, individual body mass and the organic fraction of the food source (epilithic algal community), for a range of small macroinvertebrates representative of the taxa collected in the study. Ingestion rates were converted to cryptofaunal group rates based on the average density of that group for all locations.
This research was undertaken to:1. determine if there were variations in density and biomass of major groups of motile cryptofauna across environmental and biotic zones on a coral reef2. determine if there were variations in cryptofauna within and outside damselfish territories3. assess the trophic importance of motile cryptofauna as grazers on coral reefs4. provide preliminary estimates on the grazing rates of reef cryptofauna
Cryptofauna refers to small motile invertebrates inhabiting the epilithic algae and crevices on the surface of the coral rock.

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Maintenance and Update Frequency: notPlanned

Statement: Statement: Photosynthetic pigment analysis:The photosynthetic pigments of coral blocks were extracted in 90% acetone (with magnesium carbonate) for 24 hours. All extractions were carried out at 0°C and in the dark. Pigment optical density was read at 665 nm in a Varian Superscan 3 spectrophotometer before and after acidification (1N HCl), to convert chlorophylls to phaeopigments. The amount of chlorophyll a present in each sample was calculated using the formula for periphyton as given in Vollenweider (1974). Serial extractions of the first 25 samples revealed an extraction efficiency of 70% in the first two 24-h extraction periods; subsequent samples were thus extracted for only 2 consecutive 24-h periods and the data corrected for extraction efficiency.Estimation of surface area of coral blocks:The surface areas (SA) of the coral blocks were estimated using a latex-coating method. First the broken surface of each block, where it had been detached from the substrate, was coated with plaster of paris, thus leaving a smooth surface to facilitate the subsequent removal of latex from this part. The blocks were then dried in an oven at 60°C for 48 hours and weighed. Then the blocks were immersed in a solution of rubber latex and re-dried. Prior to reweighing the blocks, the latex adhering to the plaster of paris surface was peeled off, leaving a coating of latex only on the originally exposed coral surface. Regression of weight increment against surface area for standards (a series of geometrically-regular coral blocks) of which the surface area had been calculated, gave a correlation coefficient value of 0.96. The blocks were then photocopied, and their projected surface area (PSA) measured by digitising the photocopied images. The ratio of PSA to SA was taken as a measure of surface complexity.Vollenweider RA (1974) Sampling techniques and methods for estimating quantity and quality of biomass. In: Vollenweider RA (ed.) A manual on methods for measuring primary production in aquatic environments. Blackwell Scientific Publications. Oxford, p. 46-50.The allometric relationships used to calculate food consumption rates are described in:Cammen LM (1980) Ingestion rate: an empirical model for aquatic deposit feeders and detritivores. Oecologia (Berl.) 44: 303-310.

Notes

Credit
Klumpp, David W, Dr (Principal Investigator)

Credit
Skuza, Michele (Custodian)