Temporal and spatial variation in the growth of branching corals

Anderson, Kristen Deanna (2016) Temporal and spatial variation in the growth of branching corals. PhD thesis, James Cook University.

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Life-history traits (e.g., growth rates) of reef-building corals are fundamental in structuring populations and communities. Importantly, growth rates of corals have been shown to vary with changes in environmental gradients, such as sea surface temperature (SST), light, and aragonite saturation. Accordingly, ongoing changes in environmental conditions due to climate change (e.g., ocean warming and acidification) may be causing long-term changes in coral growth. Corals are particularly sensitive to changing temperature regimes, such that sustained increases in ocean temperatures are generally expected to have negative consequences on coral growth and survivorship. At high-latitude reefs however, projected increases in ocean temperature may actually lead to increases in annual growth rates, by relaxing constraints imposed by cool winter temperatures. This will however, depend upon on the rate and extent of declines in aragonite saturation, which is already much lower at high latitudes. Generally, it is expected that increasing temperature stress will be compounded by ocean acidification, leading to ongoing declines in abundance and survivorship of corals. However, the relative contribution of ocean warming versus ocean acidification on coral growth is poorly understood. The overarching objective of this thesis was to quantify growth rates of branching corals at multiple spatial and temporal scales on Australia's Great Barrier Reef (GBR), to understand how changing environmental conditions may impact on growth of common branching corals.

The first step towards improved understanding of the likely effects of climate change on coral growth was the compilation of extensive data on the growth of corals, testing for spatial, temporal, and taxonomic differences in linear extension versus calcification. Results from the extensive meta-analysis reveal that rates of linear extension vary greatly among coral species and morphologies, but were highest among arborescent Acropora species. Despite large variations in growth rates, they are known to vary spatially and temporally largely in response to environmental gradients, such as light, temperature, water quality, and aragonite saturation. There is also already evidence that the effects of climate change on corals are generally negative, regardless of geographical setting or habitat. However, existing data on growth rates of scleractinain corals is overwhelmingly biased towards massive coral species (e.g., Porites spp.), because it is possible to get a complete record of a colonies growth history from a single sample.

To specifically test whether growth rates of branching corals have changed in response to changing environmental conditions such as sea surface temperature and carbonate chemistry, contemporary growth rates were quantified at two locations where coral growth rates had been estimated in the 1980s or 1990s. First, linear extension rates of six scleractinian corals, Acropora yongei, Isopora cuneata, Pocillopora damicornis, Porites heronensis, Seriatopora hystrix, and Stylophora pistillata, were evaluated at a subtropical reef, Lord Howe Island, in 2010/11 and compared to equivalent data collected in 1994/95 by Harriott (1999). At Lord Howe Island there was marked interspecific variation in growth, with A. yongei growing almost twice the rate of all other species. Notably, growth rates of A. yongei and P. damicornis were 30 % less than previously recorded in 1994/95 at Lord Howe Island. However, growth rates of Porites heronensis remained unchanged.

At Davies Reef, in the central GBR, contemporary growth rates (linear extension, density, and calcification) for the staghorn coral, Acropora muricata was assessed at three different depths (5, 10, and 15 m depths) and over two years (2012- 2014). Contemporary growth rates were directly compared to equivalent measurements made in 1980-82 by Oliver (1987) at the exact same location, assessing how growth rates of A. muricata may have changed over three decades. To assist in understanding inter-annual variability in coral growth during this period (1979-2012), annual growth bands were examined for cores taken from massive Porites at Davies Reef, in 2012. Similarly, at Davies Reef, linear extension of A. muricata in 2012-14 was 5 - 58 % less compared to 1980-82. In contrast, calcification rates of massive Porites were highly variable among years and there was no consistent long-term change.

Growth parameters (linear extension, density, and calcification) of Acropora muricata, Isopora palifera, and Pocillopora damicornis were investigated along a 2,345 km latitudinal gradient from Lizard Island (14.67°S) to Davies/Trunk Reef (18.85°S), and Heron Island (23.35°S). The role of environmental factors, such as temperature, aragonite saturation, and light in controlling growth rates of corals at these locations was explored. Growth rates of A. muricata, P. damicornis and I. palifera were similar among the low latitude locations of Lizard Island and Davies/Trunk Reef. But annual linear extension rates of A. muricata, P. damicornis and I. palifera at Heron Island in the southern Great Barrier Reef were 34 %, 33 %, and 20 % less, respectively, when compared to Lizard Island. Spatial variation in growth closely corresponded with differences in the local temperature regime, while differences in carbonate chemistry and light intensity explained little of the apparent differences in growth among these locations.

The relative contribution of temperature versus carbonate saturation state in affecting coral growth cannot easily be partitioned from in situ measurements. Therefore, to determine the independent and combined effects of ocean warming and ocean acidification, two common and fast growing Acropora corals (Acropora muricata and Acropora hyacinthus) were used in a fully factorial experimental study using three temperature treatments (long-term average=26 °C, long-term summer maximum=28.5 °C and a future +2.5 °C temperature stress=31 °C) crossed with three levels of pCO₂ (current=410 μatm, mid century=652 μatm, and end of century=934 μatm). Experimental results exposed temperature-enhanced calcification for A. muricata and A. hyacinthus but by the end of the experiment, future temperature stress of +2.5 °C led to 10-50 % reduction in calcification across both coral species. End of century pCO₂ reduced survivorship and over long-term led to a 50 % reduction in calcification. Importantly, the few individual that survived the high temperature/ high pCO₂ treatment could be more tolerant genotypes suggesting the potential for some individuals to cope with future climate change scenarios.

In combination, the results from this thesis suggest that climate change is already impacting on growth of branching corals, both at tropical and subtropical locations, which is largely attributable to declines in the growth rates with increasing temperature. It is possible that changes in seawater chemistry are compounding the effects of increasing temperature, and will become increasingly important with time, but either way it appears that climate change is having generally negative effects on the growth, and therefore resilience of branching corals, across a wide range of geographical settings and habitats. We conclude that ongoing changes in environmental conditions, and particularly further increases in temperature (e.g., due to climate change) will lead to general declines in growth rates of corals, which may be further exacerbated by shifts in assemblage structure towards relatively slow-growing, thermo-tolerant species. Importantly, branching corals species are among the most sensitive corals to temperature stress and ocean acidification, but are also amongst the most important corals in contributing to habitat structure and complexity. Ongoing research and monitoring is essential to understand the cumulative effects of increasing temperature, ocean acidification and other climate-related changes in environmental conditions on key demographic processes of different corals, to develop management strategies to mitigate the effects of climate change, and to facilitate acclimation and adaptation to changing environmental conditions.

Item ID: 47479
Item Type: Thesis (PhD)
Keywords: Acropora, Acroporidae, branching corals, climate change, colony size, coral reefs, global warming, Great Barrier Reef, growth, latitudinal coral growth, life-history traits, mortality, ocean acidification, ocean warming, population, temperature
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Appendix B (publications) is not available through this repository.

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Pratchett, Morgan S., Anderson, Kristen D., Hoogenboom, Mia O., Windman, Elizabeth, Baird, Andrew H., Pandolfi, John M., Edmunds, Peter J., and Lough, Janice M. (2015) Spatial, temporal and taxonomic variation in coral growth: implications for the structure and function of coral reef ecosystems. Oceanography and Marine Biology, 53. pp. 215-295.

Chapter 3: Anderson, Kristen D., Heron, Scott F., and Pratchett, Morgan S. (2015) Species-specific declines in the linear extension of branching corals at a subtropical reef, Lord Howe Island. Coral Reefs, 34 (2). pp. 479-490.

Date Deposited: 02 Mar 2017 01:07
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 96 ENVIRONMENT > 9603 Climate and Climate Change > 960307 Effects of Climate Change and Variability on Australia (excl. Social Impacts) @ 50%
96 ENVIRONMENT > 9603 Climate and Climate Change > 960305 Ecosystem Adaptation to Climate Change @ 50%
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