Status of Near-Shore Reefs of the Great Barrier Reef 2004

Research Report

Hugh Sweatman, Angus Thompson, Steve Delean, Johnston Davidson and Steve Neale

Australian Institute of Marine Science

CRC Reef Research Centre Project C1.14 - Status of near-shore reefs of the Great Barrier Reef 2003-04

ISBN 9781921359019
Published June 2007

Executive Summary

Near-shore reefs of the Great Barrier Reef (GBR) cover only a small part of the World Heritage Area, but they have disproportionate significance as signals of the condition of the ecosystem. These reefs are the most accessible to coastal communities and they are the most at risk from runoff. While there have been many studies of near-shore reefs, there has not been a large-scale systematic assessment of their status. This study had dual aims: (1) to assess the current status of a large sample of near-shore reefs along the GBR coast; and (2) to assemble a list of past studies and incorporate their findings as appropriate.

In 2004, we surveyed the benthic communities at 33 reefs in six regions between Cape Tribulation (16°S) and Keppel Island (23°S). Where topography allowed, two depths were surveyed at replicate sites at each reef giving a total of 63 locations. Surveys measured benthic cover, community composition, diversity of coral species and size-structures of coral communities.

Principal findings were as follows:

• The near-shore reef communities were very variable in 2004.

• Coral cover was extremely high in some locations: shallow parts of the reef slopes on the backs of Middle Island, Halfway Island and Humpy Islan (Keppel region) had more than 80% cover of living hard coral. Nearly a quarter of the locations had more than 50% cover of hard corals. Coral cover was less than 10% at ten locations. Over all, the average cover of living hard coral was 33%. This is slightly higher than the average cover of 30% from 36 reefs in middle and outer regions of the GBR lagoon that were surveyed by the Australian Institute of Marine Science (AIMS) in 2004/2005.

• The number of species of hard corals ranged widely. There was an average of 22 spp. per location on the shallow slopes of some reefs in the Keppel region whereas there were more than 100 spp. in some locations in the Whitsunday region. The overall average number of species was 69.

• Densities of large coral colonies (definition of “large” was species-specific) varied by a factor of more than forty. Densities were lowest on the shallow slope of Wentworth Reef (Cairns region) and the deeper slope on the front of the Frankland Island (Innisfail region). The highest densities of large colonies were found on shallow slopes of reefs at Cape Tribulation (Cairns region) and Nelly Bay (Townsville region).

• Densities of small colonies (<10 cm maximum dimension, density corrected for the area of suitable substrate) ranged from a mean of less than one per square metre on the shallow reef slope on the back of Keswick Island (Mackay region) to more than forty per square metre in deeper parts of the reef slope of King Reef (Innisfail region) and at the back of Dunk Island (Innisfail region). The overall mean density was 15.6 small colonies per square metre.

• Three broad community types were recognised: Acropora dominated communities, Porites dominated communities and mixed communities. Communities dominated by Acropora were common in the Keppel region while Porites communities were most common in the Innisfail region. Variation in community structure was correlated with the grainsize of sediment at the locations (an indicator of the resuspension/ deposition regime). There was only a weak relationship with an estimate of risk of exposure to runoff. The divergent communities in the Keppel and Innisfail regions contributed substantially to both these relationships.

• After correcting for differences in structure of coral communities among regions, there were substantial differences in community composition between shallow and deeper sites on reef slopes. This difference was influenced by variation in either settlement or early survival as the differences in abundance of a number of coral genera between depths was due to variation in the numbers of small colonies.

• Past studies giving information on status of near-shore reefs are collated in an appendix. Examination of the few long-term data sets on near-shore reefs showed that coral cover changed dramatically in many of the sites. Most of the changes in communities over time were due to changes in cover of Acroporidae and, to a lesser extent, Pocilloporidae. Rates of recovery varied widely, depending on the disturbance. After a cyclone, coral cover on reefs at Cape Tribulation recovered quickly, increasing by more than five percent a year, presumably through regrowth of damaged colonies and the growth of fragments. In other instances, recovery was minimal after several years, presumably because recovery depended on recruitment of new individuals. It is clear that the coral bleaching in 1998 had widespread and severe effects on coral communities of near-shore reefs. Mortality associated with bleaching has been partially responsible for the decline in condition of some near-shore reefs in recent years. By killing corals over a wide area, bleaching is also likely to have reduced the regional supply of potential recruits on which recovery depended. In 2004 many near-shore reefs had substantial densities of small colonies that would have recruited after 1998. This suggests that general recruitment failure, which is one of the predicted results of excessive exposure to polluted runoff, was not widespread.

• Surveys in 2004 provided a baseline for assessing future changes in coral communities on near-shore reefs, but also raise many questions about factors that determine the structure of the coral communities, about their dynamics and their likelihood of persistence in the long term. Accurate measurements of biophysical variables at the survey sites, in combination with information on disturbance history, could explain some of the fine-scale variation in community structure. The few long-term data on coral communities on near-shore reefs show strong effects of several kinds of disturbance. For coral communities to persist they must recover during intervals between disturbances. Rates of recovery will vary with the kind of disturbance but there are few estimates of the rates of recovery that can be expected under different conditions. Recruitment of new colonies is essential for community resilience, so the presence of numbers of small colonies of several genera in many sites is a positive sign, but without information on survival and growth rates under truly representative conditions it is impossible to say whether even the highest densities of recruits that were recorded will be sufficient to replace adults in the long term. The Reef Water Quality Protection Plan provides an opportunity to monitor near-shore reefs over several years so as to relate community dynamics to local environmental conditions and to disturbance in order to provide answers to these questions.