Abstract
Sediment loads have long been known to be deleterious to corals, but the effects of turbidity and settling particles have not previously been partitioned. This study provides a novel approach using inert silicon carbide powder to partition and quantify the mechanical effects of sediment settling versus reduced light under a chronically high sedimentary regime on two turbid water corals commonly found in Singapore (Galaxea fascicularis and Goniopora somaliensis). Coral fragments were evenly distributed among three treatments: an open control (30% ambient PAR), a shaded control (15% ambient PAR) and sediment treatment (15% ambient PAR; 26.4 mg cm−2 day−1). The rate of photosynthesis and respiration, and the dark-adapted quantum yield were measured once a week for four weeks. By week four, the photosynthesis to respiration ratio (P/R ratio) and the photosynthetic yield (Fv/Fm) had fallen by 14% and 3–17% respectively in the shaded control, contrasting with corals exposed to sediments whose P/R ratio and yield had declined by 21% and 18–34% respectively. The differences in rates between the shaded control and the sediment treatment were attributed to the mechanical effects of sediment deposition. The physiological response to sediment stress differed between species with G. fascicularis experiencing a greater decline in the net photosynthetic yield (13%) than G. somaliensis (9.5%), but a smaller increase in the respiration rates (G. fascicularis = 9.9%, G. somaliensis = 14.2%). These different physiological responses were attributed, in part, to coral morphology and highlighted key physiological processes that drive species distribution along high to low turbidity and depositional gradients.
Highlights
Singapore’s diverse coral reef system hosts 255 hard coral species [1,2] but pressure from coastal reclamation and dredging operations have resulted in the loss of at least 60% of its original coral reef area [3]
The decline in photosynthesis was significant for both coral species (p, 0.022; Table 2), and post hoc analysis revealed that the most significant decline occurred between weeks 3 and 4 (p,0.001; Table 3)
In the shaded control and sediment treatment, the respiration rates increased from,0.9 in week 1 to 0.9–1.00 mmol cm22 hr21 measured in week 4 for G. fascicularis, and from,0.96 to, 1.10 mmol cm22 hr21 for G. somaliensis (Fig. 3; Table 1)
Summary
Singapore’s diverse coral reef system hosts 255 hard coral species [1,2] but pressure from coastal reclamation and dredging operations have resulted in the loss of at least 60% of its original coral reef area [3]. Sedimentation rates and turbidity decline with increasing distance from Singapore’s mainland, resulting in spatial variations in coral species composition and reef health [10]. High sediment loads usually result in greater turbidity, limiting light availability and reducing photosynthetic yield by symbiotic zooxanthellae. This results in a decrease in net productivity and lower carbon gain. Anthony and Hoegh-Guldberg [25] showed that it is possible for zooxanthellae to photo-acclimatize by increasing the number and size of chloroplasts This response varies among species with some corals being better able to photoacclimate, and capable of growing at deeper sites or on turbid reefs typically found close to shore
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