http://www.marine.usf.edu/reefslab/documents/evol_ecol2007/Fabricius_terrest-runoff_2005.pdf

Effects of terrestrial runoff on the ecology of corals and coral reefs: review and synthesis

Ocean acidification represents a key threat to coral reefs by reducing the calcification rate of framework builders. In addition, acidification is likely to affect the relationship between corals and their symbiotic dinoflagellates and the productivity of this association. However, little is known about how acidification impacts on the physiology of reef builders and how acidification interacts with warming. Here, we report on an 8-week study that compared bleaching, productivity, and calcification responses of crustose coralline algae (CCA) and branching (Acropora) and massive (Porites) coral species in response to acidification and warming. Using a 30-tank experimental system, we manipulated CO2 levels to simulate doubling and three- to fourfold increases [Intergovernmental Panel on Climate Change (IPCC) projection categories IV and VI] relative to present-day levels under cool and warm scenarios. Results indicated that high CO2 is a bleaching agent for corals and CCA under high irradiance, acting synergistically with warming to lower thermal bleaching thresholds. We propose that CO2 induces bleaching via its impact on photoprotective mechanisms of the photosystems. Overall, acidification impacted more strongly on bleaching and productivity than on calcification. Interestingly, the intermediate, warm CO2 scenario led to a 30% increase in productivity in Acropora, whereas high CO2 lead to zero productivity in both corals. CCA were most sensitive to acidification, with high CO2 leading to negative productivity and high rates of net dissolution. Our findings suggest that sensitive reef-building species such as CCA may be pushed beyond their thresholds for growth and survival within the next few decades whereas corals will show delayed and mixed responses.

/pdf/ocean-acidification-causes-bleaching-and-productivity-loss-3k3w0wab5h.pdf

Ocean acidification causes bleaching and productivity loss in coral reef builders

Phase-shifts from one persistent assemblage of species to another have become increasingly commonplace on coral reefs and in many other ecosystems due to escalating human impacts. Coral reef science, monitoring and global assessments have focused mainly on producing detailed descriptions of reef decline, and continue to pay insufficient attention to the underlying processes causing degradation. A more productive way forward is to harness new theoretical insights and empirical information on why some reefs degrade and others do not. Learning how to avoid undesirable phase-shifts, and how to reverse them when they occur, requires an urgent reform of scientific approaches, policies, governance structures and coral reef management.

https://www.cell.com/trends/ecology-evolution/pdf/S0169-5347(10)00182-5.pdf

Rising to the challenge of sustaining coral reef resilience.

Corals of the world

https://nsuworks.nova.edu/cgi/viewcontent.cgi?article=1332&context=occ_facarticles

Environmental Impacts of Dredging and Other Sediment Disturbances on Corals: A Review

Effects of algal turfs and sediment on coral settlement.

The ecological resilience of coral reefs depends critically on the capacity of coral populations to re-establish in habitats dominated by macroalgae. Coral reefs globally are under rapidly increasing pressure from human activities, especially from climate change, with serious environmental, social and economic consequences. Coral mortality is usually followed by colonisation by benthic algae of various forms, so that algae dominate most degraded and disturbed reefs. The capacity of coral populations to re-establish in this algal-dominated environment will depend on direct and indirect impacts of the algae on the supply of coral larvae from remnant adults, on settlement of coral larvae and on the post-settlement survival and growth of juvenile corals. The effects of benthic algae on coral replenishment vary considerably but the thick mats or large seaweeds typical of degraded reefs have predominantly negative impacts. Some algae, mostly calcareous red algae, may enhance coral settlement on healthy reefs. Algal effects on coral replenishment include reduced fecundity and larval survival, pre-emption of space for settlement, abrasion or overgrowth of recruits, sloughing or dislodgement of recruits settled on crustose algae, and changes to habitat conditions. There is a serious lack of information about these effects, which are likely to cause bottlenecks in coral recovery and significantly reduce the resilience of coral reefs.

/pdf/effects-of-benthic-algae-on-the-replenishment-of-corals-and-2nx7om1q4y.pdf

Effects of benthic algae on the replenishment of corals and the implications for the resilience of coral reefs

Sharks are considered the apex predator of coral reefs, but the consequences of their global depletion are uncertain. Here we explore the ecological roles of sharks on coral reefs and, conversely, the importance of reefs for sharks. We find that most reef-associated shark species do not act as apex predators but instead function as mesopredators along with a diverse group of reef fish. While sharks perform important direct and indirect ecological roles, the evidence to support hypothesised shark-driven trophic cascades that benefit corals is weak and equivocal. Coral reefs provide some functional benefits to sharks, but sharks do not appear to favour healthier reef environments. Restoring populations of sharks is important and can yet deliver ecological surprise.

The Ecological Role of Sharks on Coral Reefs

Recovery of degraded reefs is dependent on the settlement of coral larvae into habitats typically dominated by benthic algae, so that benthic algae may play pivotal roles in coral settlement and reef recovery. Here we demonstrate that waterborne influences of macroalgae could affect coral settlement before larvae contact reef substrata and that such effects vary between macroalgae. We tested for waterborne effects of algae on both pre-settlement behaviour and settlement of larvae of the coral Acropora millepora onto live fragments of the crustose coralline alga Hydrolithon reinboldii. Treatments comprised seawater collected from aquaria that had previously contained 1 of 3 macroalgae common on degraded reefs. The foliose brown macroalga, Lobophora variegata, enhanced coral settlement by 40% relative to substratum control treatments. In contrast, the filamentous green macroalga Chlorodesmis fastigiata ('turtle weed'), hindered coral settlement by delaying settlement of larvae, although final settlement was similar to that in control treatments. Padina sp., a foliose brown macroalga closely related to L. variegata, reduced coral settlement by 30% compared with substratum controls. The demonstration of waterborne effects suggests that macroalgae can influence coral settlement before larvae reach reef substrata, even on a crustose coralline alga known to induce settlement, and even where the immediate settlement location is free of macroalgal cover. These results demonstrate the complexity in the mechanisms underlying the effects that over-abundant macroalgal growth may have on reef recovery. These effects have critical implications for the ecological resilience of coral reefs, especially as climate change increases the frequency and severity of disturbances to reefs.

/pdf/chemical-effects-of-macroalgae-on-larval-settlement-of-the-4lw0tkpm3j.pdf

Chemical effects of macroalgae on larval settlement of the broadcast spawning coral Acropora millepora

In our recent review of the ecological roles of sharks on coral reefs [1], we concluded that the evidence to support hypothesised shark-driven trophic cascades on coral reefs was weak and equivocal. In their response to our review, Ruppert et al. [2] assert that a major issue with our approach was that we primarily reviewed evidence from correlative observational studies to reach this conclusion. This is incorrect, because our conclusion is based upon multiple lines of evidence, including observational studies, food-web models, dietary and stable isotope analysis, trophic position, habitat use, and behavioural evidence [1].

Chico L. Birrell

Papers

Effects of algal turfs and sediment on coral settlement.

Effects of benthic algae on the replenishment of corals and the implications for the resilience of coral reefs

The Ecological Role of Sharks on Coral Reefs

Chemical effects of macroalgae on larval settlement of the broadcast spawning coral Acropora millepora

Reassessing Shark-Driven Trophic Cascades on Coral Reefs: A Reply to Ruppert et al.