Accreting coral reefs in a highly urbanized environment

Abstract

Globally, many coral reefs have fallen into negative carbonate budget states, where biological erosion exceeds carbonate production. The compounding effects of urbanization and climate change have caused reductions in coral cover and shifts in community composition that may limit the ability of reefs to maintain rates of vertical accretion in line with rising sea levels. Here we report on coral reef carbonate budget surveys across seven coral reefs in Singapore, which persist under chronic turbidity and in highly disturbed environmental conditions, with less than 20% light penetration to 2 m depth. Results show that mean net carbonate budgets across Singapore’s reefs were relatively low, at 0.63 ± 0.27 kg CaCO3 m−2 yr−1 (mean ± 1 SE) with a range from − 1.56 to 1.97, compared with the mean carbonate budgets across the Indo-Pacific of 1.4 ± 0.15 kg CaCO3 m−2 yr−1, and isolated Indian Ocean reefs pre-2016 bleaching (~ 3.7 kg CaCO3 m−2 yr−1). Of the seven reefs surveyed, only one reef had a net negative, or erosional budget, due to near total loss of coral cover (< 5% remaining coral). Mean gross carbonate production on Singapore’s reefs was dominated by stress-tolerant and generalist species, with low-profile morphologies, and was ~ 3 kg m−2 yr−1 lower than on reefs with equivalent coral cover elsewhere in the Indo-Pacific. While overall these reefs are maintaining and adding carbonate structure, their mean vertical accretion potential is below both current rates of sea level rise (1993–2010), and future predictions under RCP 4.5 and RCP 8.5 scenarios. This is likely to result in an increase of 0.2–0.6 m of water above Singapore’s reefs in the next 80 yr, further narrowing the depth range over which these reefs can persist.

Light Limitation and Depth-Variable Sedimentation Drives Vertical Reef Compression on Turbid Coral Reefs

“…Further declines in water quality can dramatically compress the zone of active reef growth (Figure 8C), as suspended particles adsorb light and greater sediment accumulation on reef slopes convert more benthic surfaces to soft-sediment. Under higher future sea levels associated with global climate change (Figure 8D), estimated deficits between vertical reef accretion by living coral communities (1.29 ± 0.20 mm year −1 ) and projected sea level rise (3.0 ± 1.3 mm year −1 ) in Singapore suggest water depths could increase by 20-60 cm in the next 80 years (Tkalich et al, 2013;Januchowski-Hartley et al, 2020). Raising of the euphotic depth by this amount will compress available reef habitat by 8-12% assuming no additional declines in water quality.…”

“…(1) reduced coral growth and calcification; (2) smothering of benthic communities and recruitment substrates; and (3) excessive particle loading on coral surfaces (Fabricius, 2005;Erftemeijer et al, 2012;Risk, 2014;Jones et al, 2016;Bainbridge et al, 2018). As a result, coral community compositions on human-impacted reefs have shifted to favor slow-growing and stress-tolerant taxa (Cleary et al, 2016), influencing rates of reef calcification and bioconstruction (Perry and Alvarez-Filip, 2018;Januchowski-Hartley et al, 2020). Localized environmental change is therefore a major present and future issue for SE Asian coral reefs, because the region supports high marine biodiversity (Bellwood and Hughes, 2001) and a very populous coastal zone (Hinrichsen, 2016).…”

Extensive remineralization of peatland-derived dissolved organic carbon and acidification in the Sunda Shelf Sea, Southeast Asia

“…We collected biogeochemical timeseries data at two sites in the Singapore Strait, Kusu Island (1.226°N, 103.860°E) and Hantu Island (1.227°N, 103.747°E). Both sites have coral reefs with >70 species of hard coral and coral cover of around 40%-50% (Bauman et al, 2017;Huang et al, 2009;Januchowski-Hartley et al, 2020). The Singapore Strait is located in the center of the Sunda Shelf Sea, and is close to the coastal peatlands on Sumatra (Figure 1), with water depth mostly less than 40 m. The annual average circulation runs from the South China Sea into the Indian Ocean through the Java Sea and the Malacca Strait (Gordon et al, 2012).…”

Insights from extreme coral reefs in a changing world

“…Importantly, impacts from turbidity are often compounded by additional anthropogenic pressure from eutrophication, pollution, fishing pressure, and related stressors (Heery et al 2018;Burt and Bartholomew 2019;Todd et al 2019;Figueroa-Pico et al 2020). While these extreme environmental conditions come at a cost (e.g., low carbonate accretion and coral growth rates, Browne et al 2015;Januchowski-Hartley et al 2020), urban reefs are typically heavily dominated by robust, stress-tolerant corals that are relatively resistant to bleaching and/or are able to recover rapidly from stressors (Guest et al 2016b;Brown et al 2020).…”

“…Beyond thermally extreme and turbid reefs, coral communities associated with ojos and volcanic CO 2 vents have drawn attention due to potential acclimation of resident corals to ocean acidification. While volcanic reefs have long been studied for successional processes and disturbance/recovery dynamics (Grigg and Maragos 1974;Tomascik et al 1996;Starger et al 2010;Vroom and Zgliczynski 2011;Smallhorn-West et al 2019), it is only recently that the focus has shifted toward using acidified waters near volcanic CO 2 vents as natural laboratories to understand how tropical reef organisms may respond to future ocean acidification (Hall-Spencer et al 2008;Fabricius et al 2011;Inoue et al 2013;Enochs et al 2015;Januar et al 2017). These studies have shown various ecological consequences of exposure to acidified waters near these vents, including shifts from hard corals to soft coral or macro-algal dominance (Inoue et al 2013;Enochs et al 2015), reduced coral diversity (Fabricius et al 2011;Enochs et al 2015), and enhanced colonization by bioeroders (Enochs et al 2016a, b), providing insights into possible future changes to tropical reefs under ocean acidification.…”