Bathymetric and seasonal changes in photosynthesis and respiration of the phototrophic sponge <i>Phyllospongia lamellosa</i> in comparison with respiration by the heterotrophic sponge <i>Ianthella basta</i> on Davies Reef, Great Barrier Reef

Abstract

The phototrophic sponge Phyllospongia lamellosa is found to depths of 30 m on Davies Reef. Studies of the photophysiology show that this corresponds to the depth at which the sponge–symbiont system can meet 80% of its daily respiratory carbon needs photosynthetically. Net 24-h production was constant to a depth of 20 m (20 µmol O2 g-1 fresh weight day -1 ) and then decreased to compensatory levels at 25 m. The maintenance of net 24-h production to a depth of 20 m was characterized by reductions in the sub-saturating light intensity ( I k ), indicating increased efficiency of light usage at depth. At depths greater than 20 m the changes in I k could not compensate for the reduced light intensity. The respiration rate of Phyllospongia (3–5 µmol O2 g-1 fresh weight h-1 ) was significantly greater than that of the heterotrophic sponge Ianthella (2.0–3.6 µmol O2 g-1 fresh weight h-1 ) to an extent that depended on season and location. Respiration rates for both species changed similarly between seasons, being higher in summer. There was no evidence for increased respiration rates in shallower water (&amp;lt;10 m), suggesting that this is not a cause for the reduced occurrence of Phyllospongia in shallow waters.