Coral Reefs (1988) 6:195-205
Studies on Halimeda
I. The distribution and species composition of Halimeda meadows throughout the Great Barrier Reef Province*
Edward A Drew and Kay M Abel
Australian Institute of Marine Science, PMB No 3, MSO, Townsville,
Queensland 4810, Australia
*Contribution No 367 from the Australian Institute of Marine Science
Abstract
Large areas of the inter-reefal seabed in the Great Barrier Reef are carpeted with vegetation composed almost entirely of the green calcareous alga Halimeda. These meadows occur principally in the northern sections between 11^30' and 15^35'S at depths of 20 to 40 m, but there are also some in the central and southern sections, where they have been found at depths down to 96 m. The vegetation is dominated by the same sprawling Halimeda species that are common on coral reefs in this region. However, on reefs these species grow on solid substrata, not soft sediments like the Halimeda-rich gravels that underlie the meadows. A total of 12 Halimeda species, together with two Udotea and one Penicillus species, are characteristic components of the shallow meadows. Below 50 m depth, species composition is restricted to only two major components. One, H. copiosa, is also important shallower, but the other is an unusually large and heavily calcified form of H. fragilis, a species that is normally a minor, fragile component of the shallower meadows. The maximum biomass found in these meadows was 4637 g/m2 of calcareous algae, although the mean for vegetated areas was 525 g/m2. These meadows are confined to the nutrient-depleted waters of the outer continental shelf just inside the outer barrier reefs, and are usually associated with distinct shoaling of the seabed caused by accumulation of thick deposits of calcareous Halimeda segments. The meadows are probably supported by very localized upwelling of nutrients from the adjacent Coral Sea onto the shelf, where they enrich the otherwise nutrient-depleted waters.