Abstract
Coral reefs experienced the third global bleaching event in 2015–2016 due to high sea-surface temperature (SST) anomalies. Declines in net carbonate production associated with coral bleaching are implicated in reef structural collapse and cascading impacts for adjacent coral reef islands. We present the first carbonate budget study of a reef platform surface (reef crest and reef flat) in the southern Maldives and the first record of upper reef flat condition in the central Indian Ocean post the 2015–2016 coral bleaching event. Scleractinian corals were the primary carbonate producers, with live coral cover averaging between 11.1 ± 6.5 and 31.2 ± 21.8% and dominated by massive corals. Gross carbonate production rates averaged 5.9 ± 2.5 G (kg CaCO3 m2 yr−1). Bioerosion was estimated at 3.4 ± 0.4 G, resulting in an average net carbonate production rate of 2.5 ± 2.4 G. Comparison of results with a study of the fore-reef slope highlights major differences in post-bleaching carbonate budget state between the fore-reef slope and the reef platform surface. The positive reef flat carbonate budget is attributed to the persistence of massive corals (Porites spp. and Heliopora spp.) through the bleaching event.
Highlights
Tropical coral reefs provide important biological functions and ecosystem goods and services, including dissipation of ocean wave energy, food and building resources, and income through tourism[1,2,3]
Dead coral colonies were often covered in filamentous turf algae and/or crustose coralline algae, and this was characteristic of all eco-geomorphic zones where dead coral was recorded
Massive corals were the dominant contributor to live coral cover, comprising 66.3% of all live coral recorded on the reef flat (Fig. 4)
Summary
Tropical coral reefs provide important biological functions and ecosystem goods and services, including dissipation of ocean wave energy, food and building resources, and income through tourism[1,2,3]. These factors include: hydrodynamics and the rate of sea-level rise; the contribution of reef flat carbonate production to overall reef accretion; sediment delivery, retention and removal[28,29] (including storm deposits); biological reef-binding processes (crustose coralline algae); microbial cementation; algal rim accretion rates; the role of carbonate production on sand flats, patch reefs and seagrass meadows; and biochemical dissolution of carbonate framework and sediment[30] Despite their recognized importance, carbonate budgets have been estimated for few reefs globally, most of which are located in the Caribbean[17,18,19,22,31,32]. The fore-reef slope environments play a key functional role in coral reef ecosystems, the potential for vertical reef accretion and the associated impact of theoretically higher water levels on reef flats under sea-level rise scenarios is largely dependent on carbonate production (coral growth and sediment production) and removal processes (including sediment losses, bioerosion and coral mortality) operating at the reef crest and upper reef flat. The reef crest and reef flat are critical components of the carbonate factory for sediment supply to reef islands[42,43], while sediment produced on the fore-reef slope is largely exported from the reef sytem[44,45]
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