Project 2.5i.2

Implications of mixotrophy for coral health

Project Leader: Prof Ove Hoegh-Guldberg, UQ

Although photosynthetic rates and bleaching status are typically used as indicators to describe the “health” of corals, corals can also obtain organic carbon heterotrophically, through capture of plankton, ingestion of suspended particulate matter and uptake of dissolved organic matter. This MTSRF-funded study investigated the relationship between rates of photosynthesis, heterotrophy and growth in Acropora millepora colonies on reefs at Heron (southern Great Barrier Reef) and Lizard (northern Great Barrier Reef) islands. Coral colonies were monitored for two years, during which period maximum mean monthly maximum sea surface temperatures were breached during both summers (but by less than the NOAA Bleach Watch trigger of 1°C). Lizard Island colonies proved to be highly susceptible to whole-colony mortality, with a loss of 2 of 5 colonies monitored, while Heron Island colonies proved to be more robust, with no whole-colony mortality observed in monitored colonies. Greater rates of linear extension (growth) were observed in winter in the Heron Island colonies, than in summer in Lizard Island colonies. Interestingly, this occurred despite the fact that Heron Island colonies had relatively high levels of heterotrophy over winter. In summary, during the two years of this study, Heron Island colonies appeared to maintain their weight (as measured by areal lipid concentration) and growth (positive linear extension) despite a potential reliance on heterotrophy, while Lizard Island colonies appeared to struggle despite high rates of photosynthesis. Greater appreciation and understanding of the mixotrophic lifestyle of corals is needed to generate a more complete picture of coral health.


Heritability and genetics of thermal tolerance in corals

Project Leader: Dr Madeleine van Oppen, AIMS

The adaptive potential of corals is complicated by the complex evolutionary dynamics of two or more interacting genomes - that of the coral host and that of its symbiont(s). This MTSRF-funded project used the proportion of the variance in thermal tolerance that has a genetic basis (ie heritability) as an empirical proxy for adaptive potential in the coral Acropora millepora. High heritabilities were found for functional key traits of algal symbionts, which along with their short clonal generation time and high population sizes, could allow for rapid thermal adaptation. However, the low overall heritability of the four coral host traits investigated, along with corals’ relatively long generation time (≥4 yrs for fast growing, early maturing species such as acroporids or pocilloporids, ~20 yrs for the majority of corals), raise concern about the capacity for timely adaptation of the coral-algal symbiosis in the face of continued rapid climate warming (although further investigation of other important host traits may reveal significant heritability adaptive potential). However, there are at least two other theoretical and so far untested ways in which the coral-algal symbiosis could adapt more quickly to increasing temperatures. As most corals also reproduce asexually through fragmentation or other means, there is the potential for asexual propagation of somatic mutations within a sexual generation; in other words, it may be possible for adaptation to higher temperatures to occur via somatic mutation within the time period of a coral sexual generation. Additionally, new somatic mutations arising within symbiotic algal cells could be selected upon and, if beneficial, the new mutant could theoretically displace most of the original genotype(s) within the coral colony.


Diversity of naturally-occurring coral symbionts on the Great Barrier Reef

Project Leader: Prof Ove Hoegh-Guldberg, UQ

Relatively little is known about the characteristics of coral symbionts (Symbiodinium spp.), despite their important role in the complex responses displayed by corals to environmental disturbances, such as high sea temperatures. This MTSRF-funded project has compiled one of the most extensive Symbiodinium genetic datasets in existence worldwide. Five different Symbiodinium clades (A, B, C, D and G) were identified in 3082 samples from 65 locations on the Great Barrier Reef. The majority of samples (97%) contained clade C Symbiodinium, and ~90% of the sampled hosts contained a single detectable symbiont type rather than a mixture. Some patterns in distribution of different clades and host taxa are detectable – for example, a higher association of host species with clades C1 and D in the warmer Northern sector of the Great Barrier Reef - but the picture remains relatively incomplete at present, with sampling skewed towards particular locations and hosts. While these data are a good start, our current knowledge of Symbiodinium distribution patterns across the Great Barrier Reef, in combination with the uncertainties involved in host-symbiont flexibility in terms of acquiring more temperature-tolerant types, remains inadequate to postulate meaningful predictions concerning the health and diversity of coral reefs in the future.


Early warning of bleaching events: upwellings and intrusions

Project Leader: Dr Scarla Weeks, UQ

This MTSRF-funded investigation of the oceanography of the Great Barrier Reef has linked the intensity and persistence of upwelling with anomalously warm summers and coral bleaching events. The usefulness of a number of metrics - such as ‘upwelling days’ (the number of days of upwelling, inferred from erratic temperature variability) and diurnal variation in subsurface temperature (maxmin, 20 m depth) - for describing the duration and intensity of shelf-break upwelling events in the central Great Barrier Reef were assessed. The results show, somewhat paradoxically, that even though upwelling is associated with cold water being brought near the surface, it is linked to positive thermal anomalies on the Great Barrier Reef, both locally and regionally. Summers (December to February) with strongest upwelling occurred during the Reef-wide bleaching events of 1997-1998 and 2001-2002. Upwelling in the Great Barrier Reef is enhanced during doldrum conditions, which were a feature of these summers. During these conditions the poleward-flowing East Australian Current fl ows faster, lifting the thermocline closer to the surface and thus spilling more sub-thermocline waters onto the shelf. Doldrum conditions also result in intense local heating, stratification of the water column and, when severe, coral bleaching. Upwelling intrusions are spatially restricted (central Great Barrier Reef), generally remain subsurface, and are often intermittent, allowing Reef-wide bleaching to occur despite conditions resulting in enhanced upwelling. Intense upwelling events precede anomalous seasonal maxima by up to 2 months and bleaching by 1-3 weeks, leading to the prospect of using upwelling activity as a seasonal forecasting index of unusually warm summers and widespread bleaching risk.


Impacts of climatic variability on seabird foraging patterns and nesting success

Project Leader: Dr Brad Congdon, JCU

The foraging ecology of seabirds such as wedge-tailed shearwaters (Puffinus pacificus) is highly dependent on oceanographic dynamics. This project has sought to increase our understanding of how these dynamics will change over the coming decade and century, and to provide insight into how seabird populations will fare as the climate changes. Preliminary analyses combining satellite foraging position data, bathymetry data and meso-scale oceanographic information for the East Australian coast and Coral Sea region highlighted potential links between shearwater foraging activity and specific ocean frontal systems, currents, bathymetry and upwelling phenomena. They also identified potential overlap between shearwater foraging areas and blue-water billfish and tuna fishing activities. Further analyses combining temperature/depth foraging data from wedge-tailed shearwaters with satellite and hydrodynamic information at local inter-reef scales in the Capricorn-Bunker Group suggested that shearwaters feeding chicks foraged primarily in surface waters at temperatures below mean background levels available. This implies that shearwater foraging habitat has a restricted spatial and temporal distribution, and that sea surface temperature satellite imagery can be used to identify and map changes in the distribution of preferred foraging habitat relative to changes in oceanography. In addition, analysis of subsurface temperature changes during a thermal stress event demonstrated that decreases in the availability of both preferred foraging habitat and food availability can be linked to cold-water oceanic intrusions onto the Great Barrier Reef platform at depths >35m. These incursions appear to suppress mixing of cooler waters into near-surface layers, which traps warm water at the surface and presumably affects the distribution of shearwater prey. The delicate balancing act that is apparent within populations of nesting seabirds within the Capricorn Bunker Group suggests a high degree of vulnerability to the impacts of climate change.