CRC REEF RESEARCH CENTRE TECHNICAL REPORT No. 28

Variability In Growth Characteristics Of Massive Porites On The Great Barrier Reef.

J.M. Lough, Australian Institute of Marine Science
D.J. Barnes, Australian Institute of Marine Science
M.J. Devereux,Australian Institute of Marine Science
B.J. Tobin, Australian Institute of Marine Science
S. Tobin, Australian Institute of Marine Science

FOREWORD

Corals belonging to the genus Porites exhibit one of the most simple growth forms within a class of organisms which is second only to sponges in its primitive simplicity of body structure. One might think that this, together with the facts that growth is one of the most fundamental of biological processes, and corals are among the best studied of all reef animals, would mean that growth in Porites was well understood and that it could all be summarised in just a few pages. Janice Lough and Dave Barnes have made careers out of demonstrating the fallacy of this assumption. In this CRC report Lough and Barnes gather together the basic growth data for over 350 coral colonies scattered between PNG and the southern GBR. This is an extraordinary data set and represents a resource to other coral growth researchers which is often not available in such rich detail due to the constraints imposed by publishers of international science journals.

By virtue of the great age which Porites can attain, and their tendency to lay down alternate bands of high and low density skeleton, these corals have the potential to reveal an incredible amount of information about past environmental conditions. Unlocking the information contained in skeletal records requires a good understanding of how growth characteristics can change between colonies at scales from an individual reef to half a continent. The extensive data set analysed in this report indicates that growth characteristics can change significantly from north to south along the GBR and across the continental shelf. They also demonstrate the importance of temperature in determining extension and calcification rates. This knowledge is essential when extracting long-term trend data on coral health and climatic variables from small coral samples at different locations - a situation many other researchers find themselves in.

The larger research effort by Barnes & Lough, of which this report is just a small part, is making a significant contribution to our understanding of how coral are affected by long-term climatic trends, and geographic location. This information is vital to managers who need to predict how corals will react to and recover from perturbations at smaller scales caused by human activity. In essence their data provide the kind of baseline needed to put modern activities and impacts into context.

Jamie Oliver
Director, Information Support Group
Great Barrier Reef Marine Park Authority

EXECUTIVE SUMMARY

Annual density banding patterns in massive coral skeletons provide a means to "retrospectively monitor" growth characteristics of massive corals on the Great Barrier Reef (GBR). We have analysed annual growth characteristics for colonies of the major reef-building genus, Porites, collected from shallow water ( 5 m) over 14º of latitude (1500 km), and from the coast to 200 km offshore. Growth characteristics have been extracted for 357 colonies and 35 cores. This provides information about geographic variation in the performance of this major reef-building coral and the natural range in Porites performance over space and time. This provides baseline data against which the significance of observed or suspected natural and unnatural changes in reef environments can be assessed.

Major findings of this study of colonies of Porites from shallow water are:

• Growth characteristics of Porites colonies from the GBR show a high degree of variability in space and time. Despite this variability there is evidence of broad scale geographic patterns.

• Some growth characteristics varied significantly with latitude: extension and calcification were lowest and effective tissue layer thickness was highest in the southern GBR

• Linear extension and calcification were significantly and directly correlated with latitudinal variations in average sea surface temperature.

• Linear extension and calcification were not linked with latitudinal variations in solar radiation, rainfall, river flow or tropical cyclone activity. Lack of linkage with solar radiation arose because these shallow water Porites would have been light saturated through most of the day.

• Some growth characteristics varied significantly from inshore to offshore sites: effective tissue layer thickness and average skeletal density were highest at offshore sites and lowest at inshore sites whilst extension was highest at inshore and lowest at offshore sites.

• There were significant differences in some growth characteristics with colony size and age: tissue layer thickness, effective tissue layer thickness and annual extension were significantly higher and average skeletal density was significantly lower in the larger and older coral cores compared with the smaller and younger coral colonies.

• Some growth characteristics varied significantly from the top to the sides of colonies: tissue layer thickness, high-low density, extension and calcification were significantly higher when measured along the most vertical colony growth axis compared with the near horizontal track (at the side) of the colony.

• Massive Porites colonies should be identified to species level before they are used in recovery of environmental information. Porites solida is significantly denser than the 5 other species of massive Porites from the GBR. Porites mayeri tends to be the least dense species.

• Data reported here fit well with the Townsville model of Porites growth in which annual density variations are due to thickening of skeletal elements below the outside surface of colonies.

THIS PUBLICATION IS CITED AS:
Lough, J.M., Barnes, D.J., Devereux, M.J., Tobin, B.J., & Tobin, S. (1999)
Variability in growth characteristics of massive Porites on the Great Barrier Reef.
CRC Reef Research Centre Ltd
Technical Report No. 28
Townsville; CRC Reef Research Centre Ltd, 95 pp.

ISBN 1 876054 28 X

A full copy of this report may be obtained from the author(s), and through the following libraries: