Methodological recommendations for assessing scleractinian and octocoral recruitment to settlement tiles

Quantifying recruitment of corals is important for evaluating their capacity to recover after disturbances through natural processes, yet measuring recruitment rates in situ is challenging due to the minute size of the study organism and the complexity of benthic communities. Settlement tiles are widely used in studies of coral recruitment because they can be viewed under a microscope to enhance accuracy, but methodological choices such as the rugosity of tiles used and when and how to scan tiles for recruits post-collection may cause inconsistencies in measured recruitment rates. We deployed 2,880 tiles with matching rugosity on top and bottom surfaces to 30 sites along the Florida Reef Tract for year-long saturations during a three year study. We scanned the top and bottom surfaces of the same tiles for scleractinian recruits before (live scans) and after treating tiles with sodium hypochlorite (corallite scans). Recruit counts were higher in corallite than live scans, indicating that scleractinian recruitment rates should not be directly compared between studies using live scans and those scanning tiles which have been processed to remove fouling material. Recruit counts also were higher on tile tops in general, but the proportion of settlement to the top and bottom surfaces varied significantly by scleractinian family. Thus, biases may be introduced in recruitment datasets by differences in tile rugosity or by only scanning a subset of tile surfaces. Finally, we quantified octocoral recruitment during live scans and found they preferentially settled to tile tops. We recommend that recruitment tile studies include corallite scans for scleractinian skeletons, deploy tiles with matching rugosity on top and bottom surfaces, and scan all tile surfaces.

First insights into coral recruit and juvenile abundances at remote Aldabra Atoll, Seychelles

Coral recruitment and successive growth are essential for post-disturbance reef recovery. As coral recruit and juvenile abundances vary across locations and under different environmental regimes, their assessment at remote, undisturbed reefs improves our understanding of early life stage dynamics of corals. Here, we first explored changes in coral juvenile abundance across three locations (lagoon, seaward west and east) at remote Aldabra Atoll (Seychelles) between 2015 and 2019, which spanned the 2015/16 global coral bleaching event. Secondly, we measured variation in coral recruit abundance on settlement tiles from two sites (lagoon, seaward reef) during August 2018–August 2019. Juvenile abundance decreased from 14.1 ± 1.2 to 7.4 ± 0.5 colonies m-2 (mean ± SE) during 2015–2016 and increased to 22.4 ± 1.2 colonies m-2 during 2016–2019. Whilst juvenile abundance increased two- to three-fold at the lagoonal and seaward western sites during 2016–2018 (from 7.7–8.3 to 17.3–24.7 colonies m-2), increases at the seaward eastern sites occurred later (2018–2019; from 5.8–6.9 to 16.6–24.1 colonies m-2). The composition of coral recruits on settlement tiles was dominated by Pocilloporidae (64–92% of all recruits), and recruit abundance was 7- to 47-fold higher inside than outside the lagoon. Recruit abundance was highest in October–December 2018 (2164 ± 453 recruits m-2) and lowest in June–August 2019 (240 ± 98 recruits m-2). As Acroporid recruit abundance corresponded to this trend, the results suggest that broadcast spawning occurred during October–December, when water temperature increased from 26 to 29°C. This study provides the first published record on coral recruit abundance in the Seychelles Outer Islands, indicates a rapid (2–3 years) increase of juvenile corals following a bleaching event, and provides crucial baseline data for future research on reef resilience and connectivity within the region.

Increased Coral Larval Supply Enhances Recruitment for Coral and Fish Habitat Restoration

Loss of foundation reef-corals is eroding the viability of reef communities and ecosystem function in many regions globally. Coral populations are naturally resilient but when breeding corals decline, larval supply becomes limiting and natural recruitment is insufficient for maintaining or restoring depleted populations. Passive management approaches are important but in some regions they are proving inadequate for protecting reefs, therefore active additional intervention and effective coral restoration techniques are needed. Coral spawning events produce trillions of embryos that can be used for mass larval rearing and settlement on degraded but recoverable reef areas. We supplied 4.6 million Acropora tenuis larvae contained in fine mesh enclosures in situ on three degraded reef plots in the northwestern Philippines during a five day settlement period to initiate restoration. Initial mean larval settlement was very high (210.2 ± 86.4 spat per tile) on natural coral skeleton settlement tiles in the larval-enhanced plots, whereas no larvae settled on tiles in control plots. High mortality occurred during early post-settlement life stages as expected, however, juvenile coral survivorship stabilised once colonies had grown into visible-sized recruits on the reef by 10 months. Most recruits survived and grew rapidly, resulting in significantly increased rates of coral recruitment and density in larval-enhanced plots. After two years growth, mean colony size reached 11.1 ± 0.61 cm mean diameter, and colonies larger than 13 cm mean diameter were gravid and spawned, the fastest growth to reproductive size recorded for broadcast spawning corals. After three years, mean colony size reached 17 ± 1.7 cm mean diameter, with a mean density of 5.7 ± 1.25 colonies per m–2, and most colonies were sexually reproductive. Coral cover increased significantly in larval plots compared with control plots, primarily from A. tenuis recruitment and growth. Total production cost for each of the 220 colonies within the restored breeding population after three years was United States $17.80 per colony. A small but significant increase in fish abundance occurred in larval plots in 2018, with higher abundance of pomacentrids and corallivore chaetodontids coinciding with growth of A. tenuis colonies. In addition, innovative techniques for capturing coral spawn slicks and larval culture in pools in situ were successfully developed that can be scaled-up for mass production of larvae on reefs in future. These results confirm that enhancing larval supply significantly increases settlement and coral recruitment on reefs, enabling rapid re-establishment of breeding coral populations and enhancing fish abundance, even on degraded reef areas.

Factors Affecting Coral Recruitment and Calcium Carbonate Accretion Rates on a Central Pacific Coral Reef

<p>Coral recruitment and calcium carbonate (CaCO₃) accretion are fundamental processes that help maintain coral reefs. Many reefs worldwide have experienced degradation, including a decrease in coral cover and biodiversity. Successful coral recruitment helps degraded reefs to recover, while CaCO₃ accretion by early successional benthic organisms maintains the topographic complexity of a coral reef system. It is therefore important to understand the processes that affect coral recruitment and CaCO₃ accretion rates in order to understand how coral reefs recover from disturbances. The aim of this thesis was to determine how biophysical forcing factors affect coral recruitment, calcification and bioerosion on a pristine coral reef. I used artificial settlement tiles to measure coral recruitment and CaCO₃ accretion at ten sites (four on the fore reef, four on the Western Reef Terrace and two at the Entrance Channel) at Palmyra Atoll. Fungia skeletons and pieces of dead coral rock were used to measure bioerosion rates, which were combined with the CaCO₃ accretion rates to obtain a net CaCO₃ budget of the reef substratum. Interactions between coral recruits and other benthic organisms on the settlement tiles were recorded to determine the settlement preferences and competitive strength of coral recruits. The settlement preference of Pocillopora damicornis for divots shaped like steephead and bumphead parrotfish bites marks was determined by adding P. damicornis larvae to a container with a settlement tile with the aforementioned divots. I found that coral recruitment and CaCO₃ accretion are influenced by biophysical forcing factors. Most pocilloporids likely recruit close to their parents while the origin of poritid larvae is much more distant. Pocilloporid recruitment rates were also significantly correlated with the successional stage of the benthic community on the settlement tiles, especially the cover of biofilm and bryozoa. Biofilm and crustose coralline algae (CCA) were preferred as settlement substrata by coral larvae, however both pocilloporids and poritids settled on a large number of different benthic substrata. P. damicornis larvae showed a significant settlement preference for divots shaped like parrotfish bite marks over a flat settlement surface. Coral recruits were good competitors against encrusting algae but were often outcompeted by filamentous and upright algae. Settlement tiles were almost entirely colonised by benthic organisms within three to twelve months of deployment. The mass of CaCO₃ deposited onto the settlement tiles negatively correlated with herbivore grazing pressure on the benthic community. Bioerosion rates within pieces of coral (internal bioerosion) increased over time but overall bioerosion rates (internal and external) rarely exceeded CaCO₃ deposition by CCA. My results show how variability in biophysical forcing factors leads to natural variation in coral recruitment and CaCO₃ accretion. This thesis highlights the importance of measuring herbivore grazing, CCA and turf algae cover to gain a better understanding of reef resilience. I conclude that models constructed for Caribbean reefs may not be suited to predict resilience in Pacific reefs and that within the Pacific, two different kinds of resilience models need to be constructed, one for human-inhabited coral reefs and one for uninhabited coral reefs.</p>

Factors Affecting Coral Recruitment and Calcium Carbonate Accretion Rates on a Central Pacific Coral Reef

<p>Coral recruitment and calcium carbonate (CaCO₃) accretion are fundamental processes that help maintain coral reefs. Many reefs worldwide have experienced degradation, including a decrease in coral cover and biodiversity. Successful coral recruitment helps degraded reefs to recover, while CaCO₃ accretion by early successional benthic organisms maintains the topographic complexity of a coral reef system. It is therefore important to understand the processes that affect coral recruitment and CaCO₃ accretion rates in order to understand how coral reefs recover from disturbances. The aim of this thesis was to determine how biophysical forcing factors affect coral recruitment, calcification and bioerosion on a pristine coral reef. I used artificial settlement tiles to measure coral recruitment and CaCO₃ accretion at ten sites (four on the fore reef, four on the Western Reef Terrace and two at the Entrance Channel) at Palmyra Atoll. Fungia skeletons and pieces of dead coral rock were used to measure bioerosion rates, which were combined with the CaCO₃ accretion rates to obtain a net CaCO₃ budget of the reef substratum. Interactions between coral recruits and other benthic organisms on the settlement tiles were recorded to determine the settlement preferences and competitive strength of coral recruits. The settlement preference of Pocillopora damicornis for divots shaped like steephead and bumphead parrotfish bites marks was determined by adding P. damicornis larvae to a container with a settlement tile with the aforementioned divots. I found that coral recruitment and CaCO₃ accretion are influenced by biophysical forcing factors. Most pocilloporids likely recruit close to their parents while the origin of poritid larvae is much more distant. Pocilloporid recruitment rates were also significantly correlated with the successional stage of the benthic community on the settlement tiles, especially the cover of biofilm and bryozoa. Biofilm and crustose coralline algae (CCA) were preferred as settlement substrata by coral larvae, however both pocilloporids and poritids settled on a large number of different benthic substrata. P. damicornis larvae showed a significant settlement preference for divots shaped like parrotfish bite marks over a flat settlement surface. Coral recruits were good competitors against encrusting algae but were often outcompeted by filamentous and upright algae. Settlement tiles were almost entirely colonised by benthic organisms within three to twelve months of deployment. The mass of CaCO₃ deposited onto the settlement tiles negatively correlated with herbivore grazing pressure on the benthic community. Bioerosion rates within pieces of coral (internal bioerosion) increased over time but overall bioerosion rates (internal and external) rarely exceeded CaCO₃ deposition by CCA. My results show how variability in biophysical forcing factors leads to natural variation in coral recruitment and CaCO₃ accretion. This thesis highlights the importance of measuring herbivore grazing, CCA and turf algae cover to gain a better understanding of reef resilience. I conclude that models constructed for Caribbean reefs may not be suited to predict resilience in Pacific reefs and that within the Pacific, two different kinds of resilience models need to be constructed, one for human-inhabited coral reefs and one for uninhabited coral reefs.</p>

Impact of the local environmental variability on the patterns of coral recruitment on Indo-Pacific reefs

<p>Coral reefs are threatened by a range of human activities at both local and global scales. The result of these impacts has resulted in a worldwide decline in the coral reef ecosystems. Corals are the principle reef builders and the maintenance of their populations is fundamental for healthy reef ecosystems. Local environmental factors are critically important in shaping coral populations, particularly at the post-settlement phase, when young coral colonies are most vulnerable to disturbances. In this context, understanding the environmental factors that drive coral recruitment and affect coral survivorship in the early life history stages is vital to effectively manage coral reefs. In this thesis I began by investigating the effect of abiotic and biological factors on coral recruitment and juvenile coral life history stages using settlement panels deployed in the Wakatobi Marine National Park (SE Sulawesi, Indonesia). My objectives were to assess the spatio-temporal variability in coral recruitment rates and juvenile abundance. I used a modelling approach to identify the environmental factors that affected the distribution and abundance patterns of corals. Then, I focused on the main environmental factors, identified from previously published research, affecting coral recruitment. I conducted a caging experiment to assess the impact of fish predation on coral juveniles. Finally, I analysed the development of the benthic community and the interactions between corals and benthic organisms in the first two years of colonisation of artificial bare surfaces. I found high spatial and temporal variability in recruitment rates over seven years of data, values were lower than on other Indo-Pacific reefs and ranged from 9.6 (±8.21 SE) to 317.19 (±12.76 SE) rec. m⁻²; while juvenile abundance ranged from 4.2 (±1.49 SE) to 33 (±6.36 SE) juv. m⁻². The local characteristics of the sites, such as coral cover, influenced the distribution of coral colonies in early life history stages; furthermore differences in coral density between the two life history stages (juvenile and recruits) were consistent over time. However, no single or combination of factors adequately explained abundance patterns for either recruits or juveniles. Fish predation did not appear to be the main cause of coral post-settlement mortality in the Wakatobi and it affected only 10.8% of the coral juveniles in the experiment. In contrast, 58.51% of the coral juveniles were found to be overgrown by algae and other invertebrates, however only turf and green encrusting algae affected coral survivorship. Coral colony abundance and the number of interactions with other benthic organisms, especially crustose coralline algae (CCA) and sponges, increased over time on panels and they were different between the front and back side of the panels, which was attributed to differences in light and predation regimes. Coral recruitment was higher on older benthic communities, although none of the known coral recruitment promoters, such as CCA, or competitors, such as turf algae, were correlated with coral abundance. My results show that changes in coral populations between the recruit and juvenile stages are likely driven by small-scale processes. The site characteristics determine the final patterns, which vary over time following temporal fluctuations in environmental factors. The effect of the interactions between algae and sponges with coral recruits and their influence on juvenile survivorship suggests these organisms having a role in coral recruitment success and highlight their importance as a focus for reef management. Furthermore, the use of long term studies allowed a better understanding of the high variability present in coral recruitment and the trends of the recruitment process, which are useful information for conservative purposes.</p>

Impact of the local environmental variability on the patterns of coral recruitment on Indo-Pacific reefs

<p>Coral reefs are threatened by a range of human activities at both local and global scales. The result of these impacts has resulted in a worldwide decline in the coral reef ecosystems. Corals are the principle reef builders and the maintenance of their populations is fundamental for healthy reef ecosystems. Local environmental factors are critically important in shaping coral populations, particularly at the post-settlement phase, when young coral colonies are most vulnerable to disturbances. In this context, understanding the environmental factors that drive coral recruitment and affect coral survivorship in the early life history stages is vital to effectively manage coral reefs. In this thesis I began by investigating the effect of abiotic and biological factors on coral recruitment and juvenile coral life history stages using settlement panels deployed in the Wakatobi Marine National Park (SE Sulawesi, Indonesia). My objectives were to assess the spatio-temporal variability in coral recruitment rates and juvenile abundance. I used a modelling approach to identify the environmental factors that affected the distribution and abundance patterns of corals. Then, I focused on the main environmental factors, identified from previously published research, affecting coral recruitment. I conducted a caging experiment to assess the impact of fish predation on coral juveniles. Finally, I analysed the development of the benthic community and the interactions between corals and benthic organisms in the first two years of colonisation of artificial bare surfaces. I found high spatial and temporal variability in recruitment rates over seven years of data, values were lower than on other Indo-Pacific reefs and ranged from 9.6 (±8.21 SE) to 317.19 (±12.76 SE) rec. m⁻²; while juvenile abundance ranged from 4.2 (±1.49 SE) to 33 (±6.36 SE) juv. m⁻². The local characteristics of the sites, such as coral cover, influenced the distribution of coral colonies in early life history stages; furthermore differences in coral density between the two life history stages (juvenile and recruits) were consistent over time. However, no single or combination of factors adequately explained abundance patterns for either recruits or juveniles. Fish predation did not appear to be the main cause of coral post-settlement mortality in the Wakatobi and it affected only 10.8% of the coral juveniles in the experiment. In contrast, 58.51% of the coral juveniles were found to be overgrown by algae and other invertebrates, however only turf and green encrusting algae affected coral survivorship. Coral colony abundance and the number of interactions with other benthic organisms, especially crustose coralline algae (CCA) and sponges, increased over time on panels and they were different between the front and back side of the panels, which was attributed to differences in light and predation regimes. Coral recruitment was higher on older benthic communities, although none of the known coral recruitment promoters, such as CCA, or competitors, such as turf algae, were correlated with coral abundance. My results show that changes in coral populations between the recruit and juvenile stages are likely driven by small-scale processes. The site characteristics determine the final patterns, which vary over time following temporal fluctuations in environmental factors. The effect of the interactions between algae and sponges with coral recruits and their influence on juvenile survivorship suggests these organisms having a role in coral recruitment success and highlight their importance as a focus for reef management. Furthermore, the use of long term studies allowed a better understanding of the high variability present in coral recruitment and the trends of the recruitment process, which are useful information for conservative purposes.</p>

Recruitment of hard coral after coral bleaching in Krueng Raya Water, Aceh Besar

Krueng Raya waters is an area that has a distribution of coral reef ecosystems. This study aims to determine the recruitment of hard corals after coral bleaching Krueng Raya waters, Aceh Besar District. This research was conducted in April 2017 in Krueng Raya, Aceh Besar. Data were collected at three observation stations using a systematic sampling method using a 1x1 m quadratic transect. Based on the study results, coral recruitment was obtained from eight families and 13 genera (Acropora, Cetenactis, Diploastrea, Favia, Favites, Galaxea, Goniastrea, Helliopora, Leptastrea, Montastrea, Pavona, Pocillopora, and Porites). Where the most abundant coral genus found was from the genus Pocilopora and Porites (16 coloni/10m2). while the station with the highest colony abundance was at station 3 with 28 colonies/10m2.