Linking reef ecology to island building: Parrotfish identified as major producers of island-building sediment in the Maldives

Abstract

Reef islands are unique landforms composed entirely of sediment produced on the surrounding coral reefs. Despite the fundamental importance of these ecological-sedimentary links for island development and future maintenance, reef island sediment production regimes remain poorly quantified. Using census and sedimentary data from Vakkaru island (Maldives), a sand-dominated atoll interior island, we quantify the major sediment-generating habitats, the abundance of sediment producers in these habitats, and the rates and size fractions of sediment generated by different taxa. The estimated annual sediment production is 685,000 kg (or 370 m 3), ~75% of which is produced on the narrow outer reef flat, despite composing only 21% of the total platform area. Approximately 65% of the platform acts solely as a sediment sink. Census data identify parrotfish as the major sediment producers, generating >85% of the 5.7 kg m-2 of new sand-grade sediment produced on the outer reef flat each year. Halimeda (macroalgae) produce a further 10%, most as gravelgrade material. Comparisons between production estimates and sedimentary data indicate that reef ecology and island sedimentology are tightly linked; reef flat and lagoon sediments are dominated by coral and Halimeda, although fine-to medium-grained coral sand is the dominant (~59%) island constituent. The generation of sediment suitable for maintaining this reef island is thus critically dependent on a narrow zone of high-productivity reef, but most especially on the maintenance of healthy parrotfish populations that can convert reef framework to sand-grade sediment.

Atoll-scale comparisons of the sedimentary structure of coral reef rim islands, Huvadhu Atoll, Maldives

“…The prevalence of sand-and rubble-sized coral within the rim islands is therefore likely attributable to parrotfish grazing, 0.7 ± 0.1 1.4 ± 0.1 29.4 ± 6.8 68.9 ± 6.6 1.7 ± 0.2 1.4 ± 0.1 < 1 cm 3 2A 12 1.2 ± 0.2 1.6 ± 0.1 15.3 ± 6.8 82.6 ± 6.7 2.1 ± 0.7 1.3 ± 0.1 < 1 cm -2B 14 0.8 ± 0.2 1.4 ± 0.1 21.4 ± 4.6 76.2 ± 4.4 2.4 ± 0.3 1.6 ± 0.1 < 1 cm -3A 18 0.6 ± 0.2 1.9 ± 0.1 40.6 ± 4.9 56.1 ± 4.8 3.3 ± 0.6 1.9 ± 0.1 < 4 cm 4 3B 14 0.4 ± 0.1 1.8 ± 0.1 47.7 ± 3.6 49.6 ± 3.4 2.7 ± 0.4 1.9 ± 0.1 < 12 cm 3 Leeward 1 10 1.2 ± 0.1 1.2 ± 0.1 1.8 ± 0.6 96.6 ± 0.6 1.7 ± 0.2 1.1 ± 0.0 < 1 cm -2A 10 1.1 ± 0.1 1.2 ± 0.1 1.8 ± 0.4 97.4 ± 0.4 0.8 ± 0.3 0.9 ± 0.1 < 1 cm -2B 10 0.7 ± 0.1 1.0 ± 0.1 9.4 ± 1.9 89.2 ± 1.9 1.4 ± 0.3 1.2 ± 0.1 < 1 cm -3A 9 0.2 ± 0.1 1.4 ± 0.0 45.6 ± 3.1 53.1 ± 3.1 1.3 ± 0.2 1.7 ± 0.0 < 4 cm 3 3B 4 -0.1 ± 0.2 1.7 ± 0.1 60.2 ± 6.8 38.2 ± 6.8 1.6 ± 0.4 1.7 ± 0.0 < 12 cm 4 and physical erosion of the reef framework by low-frequency high-magnitude events, respectively. A similar provenance for the coral-dominated sands has recently been suggested for Maldivian atoll interior islands (Perry et al, 2015). Likewise, the high proportions of coral sand and rubble are consistent with Woodroffe's (1992) descriptions of Feydhoo island, Addu Atoll.…”

“…Of equal importance is the process by which coral is converted from reef framework into material for reef island building. Four processes may cause this conversion, which may be inferred from sediment texture: (1) physical erosion of the reef framework produces sand-sized sediments via abrasion (though this is unlikely to be a dominant process given the high durability of coral - Ford and Kench, 2012;Perry et al, 2015) or rubble grade clasts; (2) endolithic sponge bioerosion produces silt-sized (<63 m) material, though is evidently of minimal significance for reef island building as silt-sized material accounted for only 2.11 ± 0.57% of each sample; (3) urchin grazing produces predominantly silt-sized material and is thus, likewise, unlikely to represent a dominant process; (4) sand-sized sediments are produced as a by-product of parrotfish grazing (Hoey and Bellwood, 2008;Perry et al, 2015). The prevalence of sand-and rubble-sized coral within the rim islands is therefore likely attributable to parrotfish grazing, 0.7 ± 0.1 1.4 ± 0.1 29.4 ± 6.8 68.9 ± 6.6 1.7 ± 0.2 1.4 ± 0.1 < 1 cm 3 2A 12 1.2 ± 0.2 1.6 ± 0.1 15.3 ± 6.8 82.6 ± 6.7 2.1 ± 0.7 1.3 ± 0.1 < 1 cm -2B 14 0.8 ± 0.2 1.4 ± 0.1 21.4 ± 4.6 76.2 ± 4.4 2.4 ± 0.3 1.6 ± 0.1 < 1 cm -3A 18 0.6 ± 0.2 1.9 ± 0.1 40.6 ± 4.9 56.1 ± 4.8 3.3 ± 0.6 1.9 ± 0.1 < 4 cm 4 3B 14 0.4 ± 0.1 1.8 ± 0.1 47.7 ± 3.6 49.6 ± 3.4 2.7 ± 0.4 1.9 ± 0.1 < 12 cm 3 Leeward 1 10 1.2 ± 0.1 1.2 ± 0.1 1.8 ± 0.6 96.6 ± 0.6 1.7 ± 0.2 1.1 ± 0.0 < 1 cm -2A 10 1.1 ± 0.1 1.2 ± 0.1 1.8 ± 0.4 97.4 ± 0.4 0.8 ± 0.3 0.9 ± 0.1 < 1 cm -2B 10 0.7 ± 0.1 1.0 ± 0.1 9.4 ± 1.9 89.2 ± 1.9 1.4 ± 0.3 1.2 ± 0.1 < 1 cm -3A 9 0.2 ± 0.1 1.4 ± 0.0 45.6 ± 3.1 53.1 ± 3.1 1.3 ± 0.2 1.7 ± 0.0 < 4 cm 3 3B 4 -0.1 ± 0.2 1.7 ± 0.1 60.2 ± 6.8 38.2 ± 6.8 1.6 ± 0.4 1.7 ± 0.0 < 12 cm 4 and physical erosion of the reef framework by low-frequency high-magnitude events, respectively.…”

Reef Habitat Type and Spatial Extent as Interacting Controls on Platform-Scale Carbonate Budgets

“…In a few cases this budget work has also been extended to considerations of future reef growth potential as a function of contemporary budget states (Perry et al, 2015b;Perry and Morgan, 2017), and to the modeling of carbonate budgets under future change scenarios (Alvarez-Filip et al, 2013;Kennedy et al, 2013). Finally, there has been some recent work aimed at using census-based approaches to quantify rates of reef-scale carbonate sediment generation, using habitat specific census-data to explore the linkages between reefs and adjacent reef islands (Perry et al, 2015a;Morgan and Kench, 2016a), and remotely sensed imagery to up-scale production rates for specific sediment producing taxa (e.g., Doo et al, 2017).…”

Investigating the evolution and formation of coastlines and the response to sea-level rise

“…For example in the Maldives, 75% of the estimated annual sand-sized sediment budget on the reef flat was produced on the reef-flat rim (ocean-side) (Perry et al, 2015). The rate of motu formation on the atolls varies greatly from decadal to millennial timescales (Kench et al, 2014a;Woodroffe and Morrison, 2001;Woodroffe et al, 2007;Ford and Kench, 2014).…”