Limitations to coral recovery along an environmental stress gradient.

Positive feedbacks driving habitat-forming species recovery and population growth are often lost as ecosystems degrade. For such systems, identifying mechanisms that limit the re-establishment of critical positive feedbacks is key to facilitating recovery. Theory predicts the primary drivers limiting system recovery shift from biological to physical as abiotic stress increases, but recent work has demonstrated that this seldom happens. We combined field and laboratory experiments to identify variation in limitations to coral recovery along an environmental stress gradient at Ningaloo Reef and Exmouth Gulf in northwest Australia. Many reefs in the region are coral depauperate due to recent cyclones and thermal stress. In general, recovery trajectories are prolonged due to limited coral recruitment. Consistent with theory, clearer water reefs under low thermal stress appear limited by biological interactions: competition with turf algae caused high mortality of newly settled corals and upright macroalgal stands drove mortality in transplanted juvenile corals. Laboratory experiments showed a positive relationship between crustose coralline algae cover and coral settlement, but only in the absence of sedimentation. Contrary to expectation, coral recovery does not appear limited by the survival or growth of recruits on turbid reefs under higher thermal stress, but to exceptionally low larval supply. Laboratory experiments showed that larval survival and settlement are unaffected by seawater quality across the study region. Rather, connectivity models predicted that many of the more turbid reefs in the Gulf are predominantly self seeded, receiving limited supply under degraded reef states. Overall, we find that the influence of oceanography can overwhelm the influences of physical and biological interactions on recovery potential at locations where environmental stressors are high, whereas populations in relatively benign physical conditions are predominantly structured by local ecological drivers. Such context-dependent information can help guide expectations and assist managers in optimizing strategies for spatial conservation planning for system recovery.

Gamma-irradiation of common biological samples for stable carbon and nitrogen isotope and elemental analyses.

RATIONALE: Around the world biosecurity measures are being strengthened to prevent the spread of pests and diseases across national and international borders. Quarantine protocols that involve sample sterilisation have potential effects on sample integrity. The consequences of sterilisation methods such as gamma (γ)-irradiation on the elemental and chemical properties of biological samples have not been widely examined. METHODS: We tested the effect of γ-irradiation (50 kGy) on the stable carbon and nitrogen isotope compositions (δ13 C and δ15 N values) and elemental concentrations (C % and N %) of common biological samples (fish, plants and bulk soils). The analysis used a continuous flow system consisting of a Delta V Plus isotope ratio mass spectrometer connected with a Thermo Flash 1112 elemental analyser via a ConFlo IV interface. Results were compared using two one-sided tests (TOST) to test for statistical similarity between paired samples. RESULTS: There was no change in the δ15 N values or N % of γ-irradiated samples, and only small changes to the δ13 C values of consumers (range: 0.01‰ to 0.04‰), producers (-0.02‰ to 0.04‰) and sediments (0‰ to 0.07‰). The magnitude of change in δ13 C values was greatest at low carbon concentrations but appeared negligible when measured against replicated sample analysis and the combined analytical uncertainty (i.e., 0.10‰). The C % values of irradiated samples were higher for consumers (0.23%) and lower for producers and sediments (0.04% and 0.05%, respectively) which may have implications for certain types of biological material. CONCLUSIONS: Routine γ-irradiation has little effect on the stable carbon and nitrogen isotope compositions of common biological samples and marginal effects on carbon elemental concentrations. This is unlikely to warrant concerns since the observed difference is typically of a magnitude lower than other sources of potential uncertainty.