MODELING THE PHOTOSYNTHETIC PRODUCTION BY SPONGES ON DAVIES-REEF, GREAT-BARRIER-REEF

Sponge populations on Australia's Great Barrier Reef (GBR) may contain a mix of both phototrophic and heterotrophic species. The distribution of many of these sponges on reefs is assumed to be determined by light. A model was developed to investigate how the distribution of phototrophic sponges over depth is restricted by the availability of photosynthetically active radiation. Estimates of the balance between photosynthetic production and the total respiratory demand of entire sponge communities on Davies Reef (a middle-shelf reef of the Great Barrier Reef) are provided. These estimates are based upon published data for community composition and biomass, whilst photokinetic parameters have been determined for a variety of sponge species from oxygen-exchange measurements. Phototrophic sponges on the fore-reef slope are predicted to exist at or above a state of net 24 h compensation (i.e., photosynthetic oxygen production by sponges balances or exceeds respiration over a 24 h period) to a depth of 30 m. It is proposed that phototrophic sponges are obligate phototrophs because the availability of light for photosynthesis corresponds with the lower depth limit of their distribution. Sponge communities (including both phototrophs and heterotrophs) from the fore-reef and lagoon exist close to a state of net 24 h compensation to a depth of 10 to 15 m. This balance shows diurnal variations, associated with the activity of phototrophs, such that instantaneous compensation of the community may occur to depths of 20 to 25 m when light is maximal.