Interactions among chronic and acute impacts on coral recruits: the importance of size-escape thresholds

Newly settled recruits typically suffer high mortality from disturbances, but rapid growth reduces their mortality once size-escape thresholds are attained. Ocean acidification (OA) reduces the growth of recruiting benthic invertebrates, yet no direct effects on survivorship have been demonstrated. We tested whether the reduced growth of coral recruits caused by OA would increase their mortality by prolonging their vulnerability to an acute disturbance: fish herbivory on surrounding algal turf. After two months' growth in ambient or elevated CO levels, the linear extension and calcification of coral (Acropora millepora) recruits decreased as CO partial pressure (pCO) increased. When recruits were subjected to incidental fish grazing, their mortality was inversely size dependent. However, we also found an additive effect of pCO such that recruit mortality was higher under elevated pCO irrespective of size. Compared to ambient conditions, coral recruits needed to double their size at the highest pCOto escape incidental grazing mortality. This general trend was observed with three groups of predators (blenny, surgeonfish, and parrotfish), although the magnitude of the fish treatment varied among species. Our study demonstrates the importance of size-escape thresholds in early recruit survival and how OA can shift these thresholds, potentially intensifying population bottlenecks in benthic invertebrate recruitment

The use of detached kelp (Ecklonia radiata) by seagrass-associated mesograzers in temperate South-Western Australia

The movement of nutrients and organisms between habitats provide important spatial subsidies on local and regional scales, resulting in increased primary and secondary production, especially where inputs supplement habitats of relatively low levels of comparable resources. In coastal south-western Australia, the brown kelp, Ecklonia radiata, is produced in large quantities on offshore reefs from where it detaches and passes through neighbouring habitats. This allochthonous resource is present in large quantities in seagrass meadows and thereby potentially influences the trophic dynamics of this habitat, providing an additional food source for grazers to those produced in situ. This study investigated the effects of the large detached kelp on mesograzer trophic dynamics in Posidonia and Amphibolis seagrass meadows. Laboratory choice and no-choice feeding experiments tested whether preferential consumption of the kelp occurred in comparison to autochthonous resources by two locally abundant gastropods, Pyrene bidentata and Cimtharidus lepidus. Results from the feeding experiments demonstrated that both species of gastropod did not preferentially consume fresh or aged kelp, but the rates of consumption were generally similar to locally abundant periphyton and red algae that are epiphytes on seagrass leaves. In comparison, the gastropods consistently avoided consumption of seagrass leaves. Field experiments were conducted at four sites during winter to measure the effects of mesograzer consumption of kelp in different dominant seagrass habitats and at different proximities to reef to encompass a range of landscape-scale effects on seagrass meadows. There were no significant effects of any main factor in the field experiment, due to an apparent lack of grazing of the large detached kelp by the mesograzers in the seagrass meadows. High variability influenced the consistency of the results, which may have been a result of cage artefacts, bacterial decomposition, or physical abrasion. Overall, the results suggest that, although seagrass-inhabiting mesograzers are capable of consuming detached E. radiata, consumption was either absent or not detected, possibly due to the high availability of autochthonous resources in seagrass meadows. Thus, it is possible that this allochthonous food source provides a negligible spatial subsidy to mesograzers in a habitat where comparable food resources are relatively unlimited, matching empirical thought. However, additional studies during different seasons and at different locations are necessary to further investigate these conclusions, to assess if allochthonous resources influence seagrass meadow trophic dynamics when in situ food limitation occurs

Assessing the Image: Creative Practice in an Online Learning Environment

Academics who work in the field of online art education are frequently asked the question, ‘How do you assess art online?’ The view that art cannot be assessed through a digital interface stems in part from a Kantian perception of art as a product that is aesthetically evaluated via the senses in a physical space. In this presentation, we will establish that assessing art in an online learning environment requires an understanding of the multiple roles the digital image assumes. This will be illustrated through a consideration of the digital image as a repository of data that must visually promote doing and thinking. In the context of art online, doing and thinking ‘are acts (and forms) of critique’ by which the assessor can evaluate competence of skill and judgment. As evidence of experimentation, reflection and interpretation of creative practice, the image acts as proof of the existence of the work and the authenticity of the author. Under the colloquium’s theme of ‘Images in the Service of Learning’, the digital image will be positioned as a vital contemporary tool which has the power to shift and enhance how we see, experience and contextualize art in an eLearning environment

Tania Doropoulos (Art Forum)

Tania discusses the way she entered curatorial practice and focuses on two key projects she has curated this year. She is the Curatorial Coordinator at Sherman Galleries, N.S.W

Hidden giants: The story of Bolbometopon muricatum at ningaloo reef

Bolbometopon muricatum (bumphead parrotfish, Valenciennes, 1839) is a conspicuous, iconic and ecologically important coral reef fish species. B. muricatum plays an important role in the bioerosion of the reef framework and as a result has been described as both an ecosystem engineer and keystone species. Despite the complete absence of B. muricatum from 32 years of scientific surveys across the Ningaloo Reef World Heritage Area, we recorded a total of 155 individuals of B. muricatum across 63.2 ha of reef crest surveys, equating to mean density of 2.38 ind/ha. Our observations represent the first record of this iconic species in scientific surveys at Ningaloo and in combination with qualitative observations of B. muricatum by expert witnesses, indicate B. muricatum is likely to have been present in ecologically relevant densities since 2006. The densities of B. muricatum observed at northern Ningaloo in 2021 suggest this species is removing an estimated 13.42 tonnes/ha or 1.34 kg/m2 of calcium carbonate per year, which is broadly comparable with estimates of total parrotfish bioerosion across many reefs in the central Indian and Pacific Oceans. Although not currently afforded elevated conservation status within management plans, B. muricatum possess many life-history characteristics that make them vulnerable to overfishing and may justify consideration for increased protection within the world heritage listed Ningaloo Reef Marine Park

High rates of erosion on a wave-exposed fringing coral reef

Erosion is a key process in shaping the physical structure of coral reefs, yet due to erosion being semi-cryptic and difficult to quantify, information remains limited. Here, we investigate erosional processes along Ningaloo Reef, an extensive fringing coral reef in Western Australia. We employed both direct and indirect methods to measure erosion in wave-exposed reef slopes and protected lagoonal habitats. Direct measurements of erosion on coral blocks were among the highest found globally, with total erosion of 3.07 kg m−2 yr−1 (4% from micro, 0.6% from macro, and 94% from external), whilst indirect rates were estimated at 2.4 ± 0.20 kg m−2 yr−1 (78% from parrotfish, 22% from urchins). Indirect erosion rates were influenced by the species and size of parrotfish, with Chlorurus microrhinos removing 0.44 ± 0.19 kg m−2 yr−1 (22% of parrotfish erosion). Scanning electron microscopy and computed tomography show that micro and macroborer erosion contributions to direct erosion were low, most likely due to heavy grazing by parrotfish and the short deployment period of experimental substrates. A substantial portion of external erosion on blocks (0.53 ± 0.23 kg m−2 yr−1) could not be attributed to bioeroders and was poorly correlated with wave exposure, suggesting processes not quantified contribute to this unaccounted aspect of erosion. Our results confirm that bioerosion by parrotfish is especially significant at Ningaloo Reef, and large-bodied individuals of C. microrhinos are key in conserving this key ecological process