Effects of fish grazing and damselfish territoriality on coral reef algae. II. Nitrogen fixation

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Abstract
N fixation was determined for plates of dead coral substratum placed at 10 m depth behind the reef crest on Britomart Reef, Great Barrier Reef [Australia]. These plates were maintained for 12 mo. under 3 different experimental conditions: exposed to natural levels of fish grazing; within the territories of the damselfish Hemiglyphidodon plagiometopon which aggressively excludes most other grazing fish; and within fish exclusion cages. N fixation rates (determined by acetylene reduction technique) were highest on plates exposed to fish grazing which had the lowest algal biomass and were lowest on caged plates with the highest algal biomass. Plates within damselfish territories were intermediate for both parameters. Grazing fish reduced algal biomass and shifted the algal community structure from dominance by red algae to dominance by rapidly colonizing and growing blue-green algae. N fixation was directly proportional to the extent of fish grazing and inversely proportional to total algal biomass.