An experimental analysis of factors controlling the standing crop of the epilithic algal community on a coral reef

Abstract

The hypotheses that grazing losses and/or ambient inorganic nitrogen concentrations control the standing crop of the epilithic algal community were tested in two habitats at One Tree Reef (Great Barrier Reef, Australia). Short (12–15 days) and long (167–306 days) multifactorial experiments using grazer exclusion and nitrogen fertilization treatments were used to partition variance in algal community biomass on portable segments of natural reef substratum during 1980. On outer reef slopes, inorganic nitrogen limited algal community growth, but the standing crop was determined by grazing losses. In the subtidal lagoon inorganic nitrogen and grazing alternated seasonally in controlling standing crop. The recolonization of cleared natural substratum was followed at two additional sites. The algal standing crop in subtidal habitats reached control levels within 4 months, while that in an intertidal reef habitat took up to 14 months. The standing crop of benthic algae on natural reef substrata was monitored in all habitats over 2 yr. In shallow and intertidal habitats, the standing crop was three to five times higher than in deeper areas, and showed a spatial and seasonal variation apparently controlled by factors other than grazing intensity, despite high levels of yield to grazers. Seasonal variation was much less in subtidal habitats. It is concluded that only within limited temporal and spatial scales is grazing intensity alone an adequate predictor of benthic algal standing crop.