BATHYMETRIC AND SEASONAL-CHANGES IN PHOTOSYNTHESIS AND RESPIRATION OFTHE PHOTOTROPHIC SPONGE PHYLLOSPONGIA-LAMELLOSA IN COMPARISON WITH RESPIRATION BY THE HETEROTROPHIC SPONGE IANTHELLA-BASTA ON DAVIES REEF, GREAT-BARRIER-REEF

The phototrophic sponge Phyllospongia lamellosa is found to depths of 30 m on Davies Reef. Studies of the photophysiology show that this cor responds to the depth at which the sponge-symbiont system can meet 80% of its daily respiratory carbon needs photosynthetically. Net 24-h pr oduction was constant to a depth of 20 m (20 mu mol O-2 g(-1) fresh we ight day(-1)) and then decreased to compensatory levels at 25 m. The m aintenance 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 int ensity. The respiration rate of Phyllospongia (3-5 mu mol O-2 g(-1) fr esh weight h(-1)) was significantly greater than that of the heterotro phic sponge lanthella (2.0-3.6 mu mol O-2 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 summe r. There was no evidence for increased respiration rates in shallower water (<10 m), suggesting that this is not a cause for the reduced occ urrence of Phyllospongia in shallow waters.