Settlement patterns of the coral Acropora millepora on sediment-laden surfaces

Abstract

Successful recruitment in corals is important for the sustenance of coral reefs, and is considered a demographic bottleneck in the recovery of reef populations following disturbance events. Yet several factors influence larval settlement behaviour, and here we quantified thresholds associated with light attenuation and accumulated sediments on settlement substrates. Sediments deposited on calcareous red algae (CRA) directly and indirectly impacted coral settlement patterns. Although not avoiding direct contact, Acropora millepora larvae were very reluctant to settle on surfaces layered with sediments, progressively shifting their settlement preference from upward to downward facing (sediment-free) surfaces under increasing levels of deposited sediment. When only upward-facing surfaces were presented, 10% of settlement was inhibited at thresholds from 0.9 to 16mgcm (EC), regardless of sediment type (carbonate and siliciclastic) or particle size (fine and coarse silt). These levels equate to a very thin (<150μm) veneer of sediment that occurs within background levels on reefs. Grooves within settlement surfaces slightly improved options for settlement on sediment-coated surfaces (EC: 29mgcm), but were quickly infilled at higher deposited sediment levels. CRA that was temporarily smothered by sediment for 6d became bleached (53% surface area), and inhibited settlement at ~7mgcm (EC). A minor decrease in settlement was observed at high and very low light intensities when using suboptimal concentrations of a settlement inducer (CRA extract); however, no inhibition was observed when natural CRA surfaces along with more realistic diel-light patterns were applied. The low deposited sediment thresholds indicate that even a thin veneer of sediment can have consequences for larval settlement due to a reduction of optimal substrate. And while grooves and overhangs provide more settlement options in high deposition areas, recruits settling at these locations may be subject to ongoing stress from shading, competition, and sediment infilling.

Density of coral larvae can influence settlement, post-settlement colony abundance and coral cover in larval restoration

“…Of the 61 colonies persisting on our tiles until 8 months, over 80% began as spat that initially settled on a vertical edge or underhang tile surface. Settlement in these semi-sheltered locations would have provided good access to light for photosynthesis 34 and only limited sedimentation 52 , with underhang locations potentially having some protection from predation by corallivorous fish 53 . In contrast, corals that settled on upward facing surfaces would have had abundant light but also higher sedimentation and possibly increased risk of predation, while corals settled on inner bottom surfaces may not have thrived due to insufficient light.…”

Sexual production of corals for reef restoration in the Anthropocene

“…A range of toxicants, including metals, pesticides, petroleum products, and other industrial products, have been shown to impact larval settlement (Negri & Heyward 2000, Reichelt-Brushett & Harrison 2000, Negri et al 2005, Lam et al 2015, Hudspith et al 2017, and the physical blocking of cues can impede settlement as well. A very thin layer of sediment, for example, can prevent settlement on substrates with strong cues, and when removed, a legacy impact on the inductive capacity of the substrate may remain (Ricardo et al 2017). Elevated temperature and pCO 2 exposure can also directly reduce settlement rates and increase post-settlement mortality (Randall & Szmant 2009a,b, Albright et al 2010, Heyward & Negri 2010, Albright & Langdon 2011, Doropoulos et al 2012a, but see Putnam et al 2008), but can also negatively impact the biota that induce coral settlement, causing the broad-scale impairment of natural recruitment (Kuffner et al 2008, Doropoulos et al 2012a, Webster et al 2013, Fabricius et al 2015.…”

“…The shape and topography of artificial substrates can be designed and manufactured to produce an optimal settlement habitat. Larvae are drawn to small crevices and areas free of sediments and grazers (our Section 5.1; Babcock & Davies 1991, Doropoulos et al 2016, Ricardo et al 2017. Several organisations are developing 'seeding units' designed to maximise recruitment success by incorporating natural microrefugia (e.g.…”

A 3D perspective on sediment accumulation in algal turfs: Implications of coral reef flattening

“…The state of algal turfs is, in‐turn, intimately linked to the sediments they contain (Clausing et al, ; Goatley et al, ; Tebbett et al, ). By mediating key processes such as herbivory (Goatley et al, ; Tebbett, Goatley, & Bellwood, ), coral settlement (Birrell, McCook, & Willis, ; Ricardo, Jones, Nordborg, & Negri, ; Speare, Duran, Miller, & Burkepile, ) and benthic productivity (Tebbett et al, ) sediments can mediate many critical processes of reef systems. This recent recognition of the ecological importance of turf‐bound sediments appears timely, given that more than 50% of reefs are forecast to be at risk of increased terrestrial sediment inputs (Burke, Reytar, Spalding, & Perry, ).…”