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

Abstract

Turbid coral reefs experience high suspended sediment loads and low-light conditions that vertically compress the maximum depth of reef growth. Although vertical reef compression is hypothesized to further decrease available coral habitat as environmental conditions on reefs change, its causative processes have not been fully quantified. Here, we present a high-resolution time series of environmental parameters known to influence coral depth distribution (light, turbidity, sedimentation, currents) within reef crest (2–3 m) and reef slope (7 m) habitats on two turbid reefs in Singapore. Light levels on reef crests were low [mean daily light integral (DLI): 13.9 ± 5.6 and 6.4 ± 3.0 mol photons m–2 day–1 at Kusu and Hantu, respectively], and light differences between reefs were driven by a 2-fold increase in turbidity at Hantu (typically 10–50 mg l–1), despite its similar distance offshore. Light attenuation was rapid (KdPAR: 0.49–0.57 m–1) resulting in a shallow euphotic depth of <11 m, and daily fluctuations of up to 8 m. Remote sensing indicates a regional west-to-east gradient in light availability and turbidity across southern Singapore attributed to spatial variability in suspended sediment, chlorophyll-a and colored dissolved organic matter. Net sediment accumulation rates were ∼5% of gross rates on reefs (9.8–22.9 mg cm–2 day–1) due to the resuspension of sediment by tidal currents, which contribute to the ecological stability of reef crest coral communities. Lower current velocities on the reef slope deposit ∼4 kg m2 more silt annually, and result in high soft-sediment benthic cover. Our findings confirm that vertical reef compression is driven from the bottom-up, as the photic zone contracts and fine silt accumulates at depth, reducing available habitat for coral growth. Assuming no further declines in water quality, future sea level rise could decrease the depth distribution of these turbid reefs by a further 8–12%. This highlights the vulnerability of deeper coral communities on turbid reefs to the combined effects of both local anthropogenic inputs and climate-related impacts.

Dissolved organic matter from tropical peatlands impacts shelf sea light availability on coral reefs in the Singapore Strait, Southeast Asia

“…The strong light attenuation, and resulting shallow Z10% depth that we observed in our time series is consistent with previous reports of strong vertical attenuation of photosynthetically active radiation (PAR) in the Singapore Strait (Dikou & van Woesik 2006, Chow et al 2019, Morgan et al 2020. Our data additionally show that the seasonal input of terrigenous CDOM to the Singapore Strait clearly contributes significantly to the extinction of PAR with depth, and also alters the spectral quality of the available light.…”

“…The Singapore Strait is home to >100 different scleractinian coral species, but their depth range is restricted to within the upper 10 m (Huang et al 2009). This shallow depth distribution is attributed chiefly to a combination of sediment stress and light limitation (Dikou & van Woesik 2006, Guest et al 2016, Chow et al 2019, Morgan et al 2020, and matches quite closely with the average Z10% of around 7 m that we measured. Sedimentation and light limitation together appear to have driven a process of vertical reef compression (Morgan et al 2020), with coral reef monitoring data suggesting that coral cover at depths of 6-7 m (but not at 3-4 m) has decreased since the 1980s (Guest et al 2016).…”

“…It has been suggested that coral communities in shallow low-light environments such as Singapore should be considered as mesophotic coral ecosystems, like those found below 30-40 m depth in clear, "blue-water" environments (Laverick et al 2020, Morgan et al 2020.…”

Physiological and ecological consequences of the water optical properties degradation on reef corals

“…Two main stress-related factors are: (a) interference of nutrient enrichment in the host's capacity to control populations of algal symbionts and opportunistic microorganisms (e.g., Symbiodiniaceae and bacteria) (Shantz and Burkepile 2014;Morris et al 2019) and (b) disease prevalence and energetic losses resulting from the physical disturbance of particle abrasion and deposition on the coral tissue (Junjie et al 2014;Pollock et al 2014). In areas exposed to terrestrial runoff or dredging-related activities, recent evidence suggests that the energetic imbalance produced by the reduction in light availability and symbiotic algae photosynthesis, as well as the process of particle clearance, compromises reef-building capacity, coral survivorship and coral reef structure (Junjie et al 2014;Bessell-Browne et al 2017;Fisher et al 2019;Omachi et al 2019;Morgan et al 2020). Moreover, that these responses to water turbidity alone or in combination with other type of stressors may be species-specific and mediated by the duration and intensity of the exposure.…”

Environmental impact on the mechanical properties of Porites spp. corals

“…Scleractinian corals build structures that provide complex three-dimensional habitat of significant ecological and economic value (Moberg and Folke 1999;Pratchett et al 2008;Graham and Nash 2012). Globally, corals are threatened by increased sea surface temperature (SST), changes in storm intensity, ocean acidification, excess sediment and nutrient input, and sea level rise (Hoegh-Guldberg et al 2007;Morgan et al 2020;Hughes et al 2018;Hoegh-Guldberg et al 2017;Silbiger et al 2018). Such changes in environmental conditions can affect mechanical and microstructural properties of the coral, including calcification rate, skeletal density, and porosity, which may ultimately affect coral health and survival (Fantazzini et al 2015).…”