The ecological role of sediments on coral reefs

Goatley, Christopher Harry Robert (2013) The ecological role of sediments on coral reefs. PhD thesis, James Cook University.

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Sediments are deleterious to coral reefs, yet our understanding of how they actually damage reefs is somewhat limited. While effects on individual organisms, particularly those on the benthos, are well known (e.g. smothering, shading and abrasion) the role of sediment in mediating ecological processes, on which coral reefs depend, is less well understood. This thesis assesses the role of benthic sediments in mediating herbivory and detritivory by coral reef fishes, two vital processes on modern coral reefs.

Algal turfs can stabilise benthic sediments on coral reefs, yet sediment in these algal turfs can affect their palatability to fishes. In the first data chapter (Chapter 2), it seemed appropriate to assess the abundance of algal turfs, which together with detritus, infauna and sediment comprise the epilithic algal matrix (EAM). The simple question of how much EAM is present on a coral reef was rapidly confounded by the presence of 3-dimensional structure in the form of canopies. Canopies are common among autotrophs, increasing their access to light and thereby increasing competitive abilities. If viewed from above, however, canopies may conceal objects beneath them, creating a 'canopy effect'. Due to complexities in collecting 3-dimensional data, most ecosystem monitoring programmes reduce dimensionality when sampling, resorting to planar views. The resultant 'canopy effects' may bias data interpretation, particularly following disturbances. Canopy effects are especially relevant on coral reefs where coral cover is often used to evaluate and communicate ecosystem health. Canopies were found to hide benthic components including massive corals and, perhaps more importantly, algal turfs. These turfs stabilise benthic sediments on coral reefs and are often ignored by standard benthic sampling methods. As planar views are almost ubiquitously used to monitor disturbances, the loss of vulnerable canopy-forming corals may bias findings by presenting pre-existing benthic components as an altered system. Our reliance on planar views in monitoring ecosystems, especially coral cover on reefs, needs to be reassessed if we are to better understand the ecological consequences of ever more frequent disturbances.

In the second data chapter (Chapter 3), benthic sediment reductions were used to assess the role of sediment in suppressing reef fish herbivory across a depth gradient (reef base, crest and flat). Sediment suppressed herbivory across all reef zones. Even slight reductions on the reef crest, which had 35 times less sediment than the reef flat, resulted in over 1800 more herbivore bites (h⁻¹ m⁻²). The Acanthuridae (surgeonfishes) were responsible for over 80 per cent of all bites observed, and on the reef crest and flat took over 1500 more bites (h⁻¹ m⁻²) when sediment load was reduced. These findings highlight the role of natural sediment loads in shaping coral reef herbivory and suggest that changes in benthic sediment loads could directly impair reef resilience.

While conducting these experimental habitat manipulations to assess the roles of herbivorous reef fishes, green turtles (Chelonia mydas) showed responses remarkably similar to those of herbivorous fishes. Furthermore, while deploying macroalgal bioassays hawksbill turtles (Eretmochelys imbricata) were observed feeding. Reducing the sediment load of the epilithic algal matrix on a coral reef resulted in a forty-fold increase in grazing by green turtles. Hawksbill turtles were also observed to browse transplanted thalli of the macroalga Sargassum swartzii in a coral reef environment. These responses not only show strong parallels to herbivorous reef fishes, but also highlight that marine turtles actively, and intentionally, remove algae from coral reefs. When considering the size and potential historical abundance of marine turtles it appears that these potentially valuable herbivores may have been lost from many coral reefs before their true importance was understood.

In the third data chapter (Chapter 4), an experimental combination of caging and sediment addition treatments were used to investigate the effects of sediment pulses on herbivory and EAMs, and to determine whether sediment addition could trigger a positive-feedback loop, leading to deep, sediment-rich turfs. A 1-week pulsed sediment addition resulted in rapid increases in algal turf length with effects comparable to those seen in herbivore exclusion cages. Contrary to the hypothesised positive-feedback mechanism, benthic sediment loads returned to natural levels within 3 weeks, however, the EAM turfs remained almost 60% longer for at least 3 months. While reduced herbivore density is widely understood to be a major threat to reefs, acute disturbances to reef sediments may elicit similar ecological responses in the EAM. With reefs increasingly threatened by both reductions in herbivore biomass and altered sediment fluxes, the development of longer turfs may become more common on coral reefs.

In the fourth data chapter (Chapter 5), off-reef sediment transport by the surgeonfish Ctenochaetus striatus (Acanthuridae) was quantified on the reef crest at Lizard Island, Great Barrier Reef. Three independent methods were implemented to estimate sediment ingestion rates. These considered (1) the bite rate and bite volume, (2) the defecation rate and faecal pellet size, and (3) the average gut contents and throughput rate. The 3 methods provided a broad range of estimates of sediment ingestion from 8.8 ± 2.4, to 66.1 ± 14.4 g fish⁻¹ d⁻¹ (mean ± SE). Nevertheless, these estimates were comparable to rates of sediment ingestion by parrotfishes (Labridae), the other major sediment-moving group on reefs. Overall, 36.5% of all sediment ingested was transported from the upper reef crest into deeper water, equating to a removal rate of 28.6 ± 6.2 kg 100 m⁻² yr⁻¹ at the study site. By brushing the reef, C. striatus reduces the sediment loading in the epilithic algal matrix (EAM) while causing little damage to the algal turf. Reducing sediments in EAMs provides favourable settlement surfaces for benthic organisms and increases the palatability of the EAM to herbivorous reef fishes, thus supporting reef resilience. The ecological importance of C. striatus, which is abundant on reefs throughout the Indo-Pacific, appears to have been underestimated, particularly when considering reef sediment dynamics.

This thesis highlights the importance of benthic sediments in mediating grazing by both herbivorous fishes and macroherbivores. By making EAMs less palatable to herbivores, sediments reduce top down pressure controlling growth of algal turfs in the EAM and allows for the development of longer, apparently stable EAMs. By feeding on the reef crest and defecating off reef, some herbivorous and detritivorous reef fishes may reduce the levels of benthic sediment in reef crest EAMs, however, these fishes are also affected by the presence of sediments, and as yet the thresholds at which this process will be affected are unknown. The roles of benthic sediments in mediating ecological processes on coral reefs appear far subtler yet more damaging than previously thought. While inundation by large quantities of sediment is undoubtedly deleterious to reefs, this thesis suggests that far smaller increases can have considerable impacts. By affecting ecological processes on which coral reefs rely (e.g. grazing), slight increases in benthic sediments erode the resilience of coral reefs. With sediment as such an important factor in maintaining ecological processes on coral reefs, the impacts of sediment can no longer be ignored when managing for coral reef resilience.

Item ID: 39228
Item Type: Thesis (PhD)
Keywords: coral reef; ecological; ecology; Lizard Island; marine; North Queensland; Queensland; reefs; sediments
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Appendix 2.1: Goatley, Christopher H.R., and Bellwood, David R. (2011) The roles of dimensionality, canopies and complexity in ecosystem monitoring. PLoS ONE, 6 (11). pp. 1-8.

Appendix 3.1.1: Goatley, Christopher H.R., and Bellwood, David R. (2012) Sediment suppresses herbivory across a coral reef depth gradient. Biology Letters, 8 (6). pp. 1016-1018.

Appendix 3.2.1: Goatley, Christopher H.R., Hoey, Andrew S., and Bellwood, David R. (2012) The role of turtles as coral reef macroherbivores. PLoS One, 7 (6). pp. 1-7.

Appendix 3.2.2. Green turtle (Chelonia mydas) feeding on algal turfs. The darker area to the left of the frame is cleared of sediment. See video at:

Appendix 3.2.3: Hawksbill turtle (Eretmochelys imbricata), feeding on Sargassum swartzii assay at the back reef (see Fig. 3.2.1). See video at:

Appendix 3.2.4: Hawksbill turtle (Eretmochelys imbricata), feeding on Sargassum swartzii assay at the sheltered reef base (see Fig. 3.2.1). See video at:

Appendix 4.1: Goatley, Christopher H.R., and Bellwood, David R. (2013) Ecological consequences of sediment on high-energy coral reefs. PLoS ONE, 8 (10). pp. 1-7.

Appendix 5.1: Goatley, Christopher H.R., and Bellwood, David R. (2010) Biologically mediated sediment fluxes on coral reefs: sediment removal and off-reef transportation by the surgeonfish Ctenochaetus striatus. Marine Ecology Progress Series, 415. pp. 237-245.

Date Deposited: 24 Jun 2015 00:00
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060202 Community Ecology (excl Invasive Species Ecology) @ 33%
06 BIOLOGICAL SCIENCES > 0699 Other Biological Sciences > 069902 Global Change Biology @ 34%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 50%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
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