Shallow, seaward portions of modern coral reefs produce about 4 kilograms of calcium carbonate per square meter per year, and protected areas produce about 0.8 kilogram per square meter per year. The difference is probably largely a function of water motion. The more rapid rate, equivalent to a maximum vertical accretion of 3 to 5 millimeters per year, places an upper limit on the potential of modern coral reef communities to create a significant vertical structure on a rising sea.

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Heron I., in the Australian Great Barrier Reef, is a small sand cay situated near the western end of a reef 6.5 miles long. The area investigated was south-west of the cay and approximately level, with extensive areas of living coral in the seaward half. Larger algal species were common near the cay but were not in evidence in the areas of active coral growth. Oxygen levels were determined without sampling using a membrane-enclosed polarographic probe. Results are presented as profiles across the platform and also as time sequences in the main draining channel. Some deep water results are included. Oxygen production and consumption were associated primarily with areas of rich coral growth and hence larger algae seemed unimportant in the overall oxygen exchange. Low tide oxygen levels in less than 1 ft of water ranged from 2.1 mg O2/l for a spring tide after midnight to more than 10.8 mg O2/l (the limit of the instrument) in the early afternoon. High tide levels in more than 6 ft of water ranged from 9.0 mg O2/l in the early afternoon to 6.4 mg O2/l 2hr after sunset. High tide readings were not taken late at night. There is some evidence that water already reduced or enriched in oxygen content on the previous tide was returned in significant quantities to the reef platform.

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The development and mode of action of enclosed electrode systems for the polarographic measurement of oxygen concentration or tension is reviewed and discussed. The improved electrode system has a cathode of 99.99% pure gold, an anode of 99.99% pure silver and is enclosed by a membrane of polyethylene, polypropylene or Teflon. The merits of these various membrane materials are discussed. Polypropylene is one of the best all‐purpose membranes. Other aspects of design and construction are discussed in detail. This probe is most suitable for measurement of oxygen tension, rather than oxygen concentration, in an agitated or flowing environment of liquid or gas. The performance characteristics of the probe compare very favourably with other available published data and it has all the requirements for use in industrial control loops. Only 5 min. are required to stabilize after switching on. Calibration against oxygen tension is linear. Response to change in oxygen tension may be as rapid as 5 sec. for 90% response to a step change from zero to 100% saturation. Much shorter times are required for response to the smaller changes normally encountered. The residual current of the system is normally less than 1% of the air saturation current, and can be completely nulled for use in very low oxygen tension ranges. The probe is unaffected by extremely high levels of suspended solids.

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