Halimeda biomass, growth rates and sediment generation on reefs in the central great barrier reef province

Abstract

The average biomass ofHalimeda per m² of solid substratum increased progressively on a series of reefs situated at increasing distances from the shore in the central Great Barrier Reef. There was none on a reef close inshore, increasing to nearly 500 g m⁻² total biomass (≃90% calcium carbonate) on an oceanic atoll system in the Coral Sea. The biomass measured contained 13 species ofHalimeda but was dominated by only two species,H. copiosa andH. opuntia, except on the atoll whereH. minima was dominant. Three sand-dwelling species were also present but did not occur anywhere in substantial quantities. Growth rates of the dominant species were measured bv tagging individual branch tips. A mean value of 0.16 segments d⁻¹ was recorded but 41% of the branch tips did not grow any new segments whilst only 1% grew more than one per day. The number of branch tips per unit biomass was very constant and has been used in conjunction with growth rates and biomass to calculate productivity rates, and thence sedimentation, in the lagoon of one of the reefs. Biomass doubling time of 15 d and production of 6.9 g dry wt m⁻² d⁻¹ are considerably higher than previously reported values forHalimeda vegetation and there was little seasonal change detected over a whole year. Those values indicate annual accretion of 184.9 g m⁻² year⁻¹ ofHalimeda segment debris over the entire lagoon floor (5.9 km²) of Davies Reef, equivalent to 0.13 mm year⁻¹ due toHalimeda alone, or 1 m every 1,892 years when other contributions to that sediment are taken into account.