Sediment generation by Halimeda on atoll interior coral reefs of the southern Maldives: A census-based approach for estimating carbonate production by calcareous green algae

• Here we test a new census-based method for quantifying Halimeda sand generation.
• Based on data from the Maldives we show Halimeda produce around 70 g CaCO3 m− 2 yr− 1.
• A high proportion (~ 75–90%) of segments rapidly degrade to mud-grade carbonate.
• Such sediment fractions are volumetrically unimportant within nearby reef islands.
• A marked discrepancy occurs between Halimeda production and island sediment supply.

Methods for quantifying rates and size fractions of carbonate sand production on reefs remain limited, despite the urgent need for such data to support assessments of reef island and tropical beach-dominated shoreline resilience. Here we present a census-based approach that supports estimates of sediment generation by the calcareous green alga Halimeda spp., which is an often conspicuous reef and lagoon substrate coloniser. Based on data from Kandahalagala (South Huvadhoo atoll, southern Maldives), we present carbonate sediment production data for the two dominant Halimeda spp. (Halimeda macrophysa and Halimeda micronesica) that occur on the reef flat and reef slope habitats. Whilst total mean production rates by Halimeda spp. are similar in both habitats (reef flat average, 67.49 g CaCO3 m− 2 yr− 1; reef slope, 70.89 g), individual species contributions differ markedly. H. micronesica dominates on the reef flat (annual mean 41.91 g CaCO3 m− 2 yr− 1, compared to 25.08 g by H. macrophysa), whilst production is dominated by H. macrophysa on the reef slope (H. macrophysa 40.49 g, H. micronesica 29.01 g CaCO3 m− 2 yr− 1,). In terms of sediment generation we show that these species also contribute very differently to the sediment reservoir. Whilst the sedimentary breakdown products from H. micronesica are somewhat bimodal (~ 17% is in the medium to very coarse sand fraction, and ~ 76% in the silt and clay fraction), almost all (> 90%) of the segments produced by H. macrophysa rapidly degrade to silt and clay sized sediment. Based on our census data this suggests that Halimeda spp. will contribute only between 7 and 9 g m− 2 yr− 1 of sand grade sediment on the reef flat and shallow slope habitats, but ~ 55–60 g m− 2 yr− 1 of mud grade sediment. Scaled to the total area of combined reef habitat around Kandahalagala (~ 130,583 m2) this equates to Halimeda spp. producing ~ 2192 kg of sand-grade sediment, but ~ 15,181 kg of mud-grade sediment per year. However, sediment compositional data suggests that Halimeda sp. are actually a very minor constituent of reef and islands sediments, especially in the fine sediment fractions. This 1) suggests that much of the predominantly finer-grained sediment generated by Halimeda has little actual relevance to the later stages of reef island development, and 2) highlights the potential for marked discrepancies in terms of sediment generating reef species abundance and resultant sediment generation rates, especially in terms of the types and sizes of sediment that are appropriate to supply adjacent beaches and islands.