The standing stock and CaCO3 contribution of Halimeda macroloba in the tropical seagrass-dominated ecosystem in Dongsha Island, the main island of Dongsha Atoll, South China Sea.

Calcareous green alga in the genus Halimeda are important contributors to the marine carbonate budget. Dongsha Island is located in the northernmost South China Sea and is a seagrass-dominated ecosystem with intermixed Halimeda macroloba patches, making it an excellent system to better examine the extent of carbonate contribution by H. macroloba in such an ecosystem. To this end, we examined the standing stock and actual CaCO₃ contribution of H. macroloba in the seagrass-dominated ecosystem (herein Dongsha Island) and compared them with those in Halimeda-dominated ecosystems. The density, growth rate, calcification rate and CaCO₃ content of H. macroloba at four life stages were investigated. The mean density of H. macroloba was around 8.82 ± 1.57 thalli m⁻² and the estimated standing stock was 61,740 to 72,730 thalli. Thalli produced 1 to 2 new segments day⁻¹, giving a growth rate of 0.003 ± 0.001 g dry weight thallus⁻¹ day⁻¹. Calculated algal biomass and annual areal production were 0.03 g m⁻² and 9.66 g m⁻² year⁻¹. In each square metre of this area, H. macroloba produced 8.82 to 17.64 new segments day⁻¹, accumulating 0.002 ± 0.001 g CaCO₃ thallus⁻¹ day⁻¹ or around 6.44 g CaCO₃ m⁻² year⁻¹. Mean CaCO₃ content was 0.32 ± 0.05 g thallus⁻¹. As expected, the growth rate and CaCO₃ production of H. macroloba in Dongsha Island were lower than in other studies from Halimeda tropical ecosystems. Overall, this work provides the baseline of carbonate production of H. macroloba in Dongsha Island and relevant systems where the ecosystem is dominated by seagrasses. [ABSTRACT FROM AUTHOR]