Massive Porites corals as indicators of environmental changes

True, James (2004) Massive Porites corals as indicators of environmental changes. PhD thesis, James Cook University.

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Abstract

Monitoring the responses of corals to environmental change and linking responses to past environmental changes are difficult and rely on correlative proxy techniques. To date, there has been no means to directly measure biologically meaningful effects of environmental changes on corals that could both explain the consequences of sublethal stress and enhance the interpretation of proxy environmental records. Massive Porites are extremely widespread in the Indo-Pacific. While not exceptionally fecund, fast-growing or competitively dominant, they appear to possess a peculiar robustness, often becoming the dominant corals on reefs that have been heavily impacted. The longevity of Porites colonies and the existence of annual density bands in their skeletons, makes them organism of choice for Indo-Pacific researchers interested in the construction of proxy climate records. This project aimed to address the lack of knowledge about the biological responses of Porites to various types of stress and to relate these responses to proxy environmental records derived from the corals' skeletons. In addition, the project sought to develop a measure of sublethal stress which could be adapted into a general reactive monitoring protocol. The thickness of the tissue layer (TTL) of a colony of Porites is a product of linear skeletal extension at its upper surface and periodic (up to monthly) uplift of its lower margin. A survey of 20 reefs on the GBR, as well as several reefs in PNG and the Gulf of Thailand indicated that tissue thickness of massive Porites conils varies in space and time. Spatial variation in TTL is mostly a manifestation of local (habitat-level) environmental conditions rather than an indication of broad geographic trends in conditions. Seasonal trends in TTL variation of Porites on the central GBR manifest as summer maxima and winter minima, as reported for calcification and linear extension. SST and insolation are the factors most likely to be responsible for seasonal cycles in TTL. TTL variation is a sensitive indicator of incipient or sublethal stress in massive Porites. Lack of stress-specific responses precludes distinguishing between chronic and acute stress solely from the thickness of the tissue layer.

Continual mobilization of the lower portion of the tissue layer provides a unique mechanism by which massive Porites can control its response to stressful conditions. Moreover, tissue thickness and lipid levels appear to be interchangeable indications of the energy level of the polyp under stress, although extreme responses such as bleaching are more closely linked to tissue thickness than lipid levels. The response of corals to environmental impacts is dictated largely by their available energy reserve. Diminishment of a coral's ability to acquire energy leads to a reduction in linear extension as the coral adapts. Reduction of the coral's somatic reserve past a critical point leads to a bleaching response concomitant with shutdown of other functions. Mass spectroscopic analysis of skeletal cores taken from colonies used in the long term stress experiment revealed that proxy environmental reconstruction techniques based on isotope ratios within the coral skeleton are able to capture stress events in the life of a coral identified by TTL changes. Changes to the amplitude and range of /)180 and /)13C cycles during experimental shading suggest that chronic stress changes the way that corals record environmental variation. The sensitivity of isotopic techniques is diminished, however, by previous stress events. Likewise, interpretation of the nature of the stress event from the proxy record is problematic: proximal environmental variation is not recorded, and nonrelated stress events can leave identical signatures in the isotope record. Skeletal proxy techniques will therefore underestimate the frequency of environmental impacts. These findings shed light on why it has so far been difficult to reconcile proxy records from different studies. The use of TTL variation as a simple reactive monitoring technique is not readily achievable. Between-colony, habitat and seasonal variation, as well as the need to contextualize measurements to previous history, all place severe constraints on its utility as a simple technique. However, localized spatial gradients in TTL can identify environmental gradients and map their severity. As such, changes in TTL can be used as a broad-scale tool to identify habitats with sub-optimal or impacted environments.

Item ID: 7890
Item Type: Thesis (PhD)
Keywords: massive porites, corals, Great Barrier Reef, climate change monitoring, Indian Ocean, tissue thicknesses, environmental stresses, coral lipid levels, coral bleaching
Date Deposited: 22 Dec 2009 06:08
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050101 Ecological Impacts of Climate Change @ 33%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 33%
06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060303 Biological Adaptation @ 34%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 34%
97 EXPANDING KNOWLEDGE > 970105 Expanding Knowledge in the Environmental Sciences @ 33%
96 ENVIRONMENT > 9603 Climate and Climate Change > 960399 Climate and Climate Change not elsewhere classified @ 33%
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