This communication describes a novel respirometer for measuring the metabolism in situ of encrusting coralline algae and it provides an example of the data which can be gathered. The respirometer includes a pair of UV-transparent electrode chambers: one accommodates the specimen, the other contains a light sensor. Oxygen, pH and temperature probes monitor changes in the seawater surrounding the specimen while light is recorded concurrently in the second chamber mounted adjacent. A centrifugal pump flushes the sample chamber at regular intervals during deployment. Analog signals from the probes are transmitted via underwater cables to a data-logging device contained within a submersible pressure-resistant housing. The signals are digitized, stored in RAM and later off-loaded to a shore-based microcomputer. Oxygen and pH data are converted into rates of primary production and calcification using equations associated with the alkalinity anomaly technique and experimentally determined values for the photosynthetic and respiratory quotients. On a clear day in May 1986, on the windward reef crest at Lizard Island, Great Barrier Reef, Australia, rates of primary production and calcification were determined for a specimen of Porolithon onkodes (Heydrich) Foslie. The maximum rate of net photosynthesis was 18.9 mmol O2 · m-2 crust surface · h-1. The average rate of nocturnal respiration was 4.4 mmol O2· m-2· h-1. The ratio of maximum gross photosynthesis to respiration was 5.25. Using a photosynthetic quotient of 1.21, the data convert to a production rate of 1.5 g C · m-2 · day-1. The maximum rate of calcification during the day was 8.2 mmol CaCO3· m-2· h-1. At night the specimen dissolved at an average rate of 0.5 mmol CaCO3· m-2· h-1. Net calcification was estimated to be 8.5 g CaCO3· m-2· day-1. © 1990.
