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

Abstract

A coral reefs carbonate budget strongly influences reef structural complexity and net reef growth potential, and thus is increasingly recognized as a key "health" metric. Despite this, understanding of habitat specific budget states, how these scale across reef platforms, and our ability to quantify both framework and sediment production values remains limited. Here, we use in-situ census data from an atoll rim reef platform in the central Maldives to quantify rates of both reef framework and sediment production and loss within different platform habitats, and then combine these data with high-resolution habitat maps to quantify contributions to platform wide carbonate budgets. The net reef framework budget for the entire platform is extremely low (0.12 G, where G = Kg CaCO 3 m −2 year −1 ), with a very high proportion (143,745 kg or 65.1%) of total framework production generated within the platform margin reef zones, despite these comprising only ∼8% of platform area. Net platform-scale sediment budgets are higher (1.04 G), but most is produced in the reef and platform margin hardground habitats, of which ∼80% derives from parrotfish bioerosion. Significant quantities of new sediment (up to ∼1 G derived from the calcareous green algae Halimeda) are produced only in one habitat. All lagoonal habitats have negative or neutral net carbonate budgets. These data demonstrate the marked inter-habitat differences in reef carbonate budgets that occur across reef platforms, and the major dampening effect on overall platform scale budgets when rates are factored for habitat type and size. Furthermore, the data highlights the disproportionately important role that relatively small areas of reef habitat can have on the maintenance of net positive platform scale budgets. Because of the intrinsic link between carbonate production rates and reef-associated landform development and maintenance, these findings also have implications for understanding reef-associated landform stability. In this context the reef island at this site has been highly mobile over the last ∼40 years, and we hypothesize that such instability may be being exacerbated by the measured low overall rates of framework and sediment generation.

Coral reef carbonate budgets and ecological drivers in the central Red Sea – a naturally high temperature and high total alkalinity environment

“…The central Red Sea G budget data presented here are within the range of contemporary reef carbonate budgets from the Atlantic (2.55±3.83 kg CaCO 3 m −2 yr −1 ) and Indian (1.41± 3.02 kg CaCO 3 m −2 yr −1 ) oceans (Perry et al, 2018). Notably, these data are below the suggested "optimal reef budget" of 5-10 kg CaCO 3 m −2 yr −1 observed in "healthy", high coral cover fore reefs (see data in Perry et al, 2018, and comparisons therein;Vecsei, 2001Vecsei, , 2004.…”

“…Furthermore, the use of carbonate budgets provided valuable insight into the reef growth trajectories in the Seychelles, where surveys conducted since the 1990s provide important ecological baseline data that were employed in reef growth calculations (Januchowski-Hartley et al, 2017). Most recently, carbonate budget data were used to explore the relation of vertical reef growth potential and trends in sea level rise, suggesting that reef submergence poses a threat as long as climate-driven and human-made perturbations persist (Perry et al, 2018). Other studies highlight the susceptibility of marginal coral reefs to ocean warming and acidification (Couce et al, 2012).…”

Habitat‐specific biogenic production and erosion influences net framework and sediment coral reef carbonate budgets

“…The organisms and processes that influence production and erosion vary spatially across reef habitats and systems. Across entire reef systems, reef slope framework carbonate budgets are generally greater than those of lagoonal environments (e.g., reef flat) (Eakin 1996; Hart and Kench 2007; Browne et al 2013; Perry et al 2017; Ryan et al 2019), particularly when dominated by fast‐growing acroporid corals (Hart and Kench 2007; Browne et al 2013; Perry et al 2015; Van Woesik and Cacciapaglia 2018). Likewise, gross bioerosion is also greatest in reef slope environments, with lower rates of bioerosion by parrotfishes and other macrobioeroders in lagoonal habitats (Hutchings et al 1992; Hoey and Bellwood 2008).…”

Massive corals maintain a positive carbonate budget of a Maldivian upper reef platform despite major bleaching event

“…Despite their recognized importance, carbonate budgets have been estimated for few reefs globally, most of which are located in the Caribbean 17–19,22,31,32 . However, recent work in the past couple of decades has expanded the growing knowledge of reef carbonate budgets to the central Indian Ocean 13,16,33–35 and elsewhere in the Indo-Pacific 22,29,36,37 . A recent global analysis of carbonate budget data 12 indicates that net carbonate budgets of reefs in the Indo-Pacific and tropical western Atlantic are currently low and are sub-optimal for reef accretion potential.…”