Dynamics of marginal coral reef ecosystems: Historical responses to climatic and anthropogenic change

Lybolt, Matthew (2012). Dynamics of marginal coral reef ecosystems: Historical responses to climatic and anthropogenic change PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Lybolt, Matthew
Thesis Title Dynamics of marginal coral reef ecosystems: Historical responses to climatic and anthropogenic change
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2012-07
Thesis type PhD Thesis
Supervisor John Pandolfi
David Neil
Jian-xin Zhao
Total pages 198
Total colour pages 48
Total black and white pages 150
Language eng
Subjects 060205 Marine and Estuarine Ecology (incl. Marine Ichthyology)
060206 Palaeoecology
050101 Ecological Impacts of Climate Change
Formatted abstract
The nature and consequences of declines in coral habitats worldwide are well documented, and in rare cases the causes of these declines are understood on timescales longer than a few decades. This historical range of variation provides valuable information allowing us to better predict ecosystem responses to future environmental conditions. This is particularly important for sub-tropical marginal reefs because many consequences of global climate change will promote sub-tropical marginal reefs at the expense of ‘classical’ tropical reefs. Without historical information natural resource managers might respond to the wrong driver, for example enacting climate change abatement to resolve a problem caused by habitat destruction. Sub-tropical marginal reefs extend along Australia’s coastline well south of the GBR. Prominent among these are the sub-tropical coral communities of Moreton Bay in southeast Queensland, and the central aim of this thesis is to understand the long-term dynamics of this marginal coral reef ecosystem. Pursuant to this; I aim to investigate reef accretion and coral assemblage composition; establish the chronology of changes in the system; and use this new historical range of variation to suggest certain historically relevant goals and criteria to natural resource managers.

The chronological foundation of this study is based on 122 radiometric U-series dates of corals (n=65 from cores and 57 from the superficial death assemblage). I took cores through Moreton Bay reefs and explored temporal patterns of reef initiation, termination, and vertical accretion rates; and examined what elements of reef composition drove patterns in vertical accretion rate. I examined how accretion rates changed with time, sea level change, natural climate change, and differences in the primary inputs to coral reef accretion; namely coral abundance, coral taxonomic composition, and terrestrial sediment inputs. The average vertical reef accretion rate in Moreton Bay was 4.0 m ka-1 (n = 13 cores) ± 1.1 standard error. The minimum overall accretion rate was 0.6 m ka-1 (1.0 m total vertical accretion) and the maximum was 14.9 m ka-1 (2.0 m total vertical accretion). I found that reef accretion rates in Moreton Bay are de-coupled from Holocene sea level and climate changes, and are de-coupled from the primary factors associated with reef composition. Surprisingly, factors influencing the input of sediments to the reef do not predict reef accretion rates (e.g., proportion of corals relative other reef sediments, the characteristics of the sedimentary setting, and the abundance of Acropora (the most prolific and fast-growing reef-building coral genus).

Moreton Bay coral assemblages were strikingly stable and persistent over space and time, despite episodic growth during periods of sea level and natural climate change. Results of this coring study were confirmed by a parallel sampling of the superficial coral death assemblage. The timing of reef growth episodes correlates with regional and global drivers, including sea-level change and the onset of the modern ENSO regime. There were no significant changes in coral assemblage composition over 7 000 years; save two exceptions. One exception is a change in depth zonation patterns over time, and one is an unprecedented recent change in coral assemblage composition from Acropora to massive corals. Occurring between ~200 and ~50 years ago this significant change followed anthropogenic alterations to the bay and its catchments resulting in degraded water quality and increased sedimentation. Natural historical instability of reefs suggests that Moreton Bay could function as refuge habitat for GBR reef species for decades to centuries, but not longer. This trajectory of degradation, however is not monotonic. There are more living coral taxa in the Bay today than at any time in the past 7 000 years. While this increase in Moreton Bay coral diversity is not statistically significant, the reversal of a trajectory of anthropogenic degradation is very encouraging.

The key benefit of applied palaeoecology is improved natural resource management planning and setting goals in a historically relevant context. Reefs in Moreton Bay grew episodically over 7 000 years with no significant change in community composition or accretion rate. However in the past 200 years reefs changed significantly, and for the first time in 7 000 years reefs of Moreton Bay persist in a degraded state caused by increased sediment and nutrient runoff from anthropogenic land-use changes. This means that natural resource managers targeting reversal of this degraded state should target any prescription which reduces sediment and nutrient loads onto the reefs. The historically relevant indicator of success, on the short-term, is any increase in the abundance of Acropora (i.e., increases in the abundance of Acropora indicate the success of natural resource management actions to improve water quality in the catchment). Even a marginal reef habitat such as Moreton Bay has a history of recovery and rapid reef growth, and rapid recovery may yet be possible if the causes of anthropogenic degradation are reversed.
Keyword Marginal reef
high-latitude reef
Moreton Bay (Qld)
Radiometric Dating
u-series dates
Refugial habitats
reef accretion
Additional Notes 7, 22, 33, 34, 49-54, 61-63, 91-97, 109-119, 126-128, 149, 152, 155, 157, 159, 161, 163, 165, 167, 169, 171, 173, 176

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Created: Mon, 02 Jul 2012, 22:39:01 EST by Mr Matthew Lybolt on behalf of Scholarly Communication and Digitisation Service