Disturbance and Recovery Cycles
Long-term Monitoring on ‘Unlucky’ Inshore Fringing Reefs in the Cairns Section of the GBRMP

ince 1985, the Great Barrier Reef Marine Park Authority has contracted Sea Research to conduct regular benthic surveys on the inshore fringing reefs of the Cairns Section of the Great Barrier Reef Marine Park. Initially these surveys were confined to three locations along Cape Tribulation to monitor the effects of sediment run-off from a newly constructed dirt road running through steep rainforested hills between Cape Tribulation and the Bloomfield River. There was concern that heavy sediment run-off from the unsealed road would have adverse effects on the fringing reefs (Bonham 1985). These surveys continued until 1989.

In 1994, the Authority commissioned a broad scale survey of the fringing reefs in the Cairns Section to document fringing reef communities in the region for comparison with the Cape Tribulation study. Following on from this study, long-term monitoring of additional potential human impacts began on two island fringing reefs, Snapper Island and the Frankland Islands, and monitoring re-commenced at the Cape Tribulation sites.

In the course of these surveys the reefs have been affected by two separate bleaching events, numerous cyclones, major floods and, most recently, outbreaking populations of crown-of-thorns starfish (COTS). Thus what initially began as a program to monitor the effects of sediment run-off and anthropogenic pressures has also generated very useful data on disturbance-recovery cycles for inshore fringing reefs.

Survey Techniques and Design

The three regions were surveyed during the summer months using fixed transects. Three locations were surveyed along Cape Tribulation. Sediment run-off from old and new road construction sites and from undisturbed locations was monitored. Monitoring of the Snapper and Frankland Islands was carried out at two locations on opposite sides of each island, as each side supported slightly different coral communities. The survey history is summarised in table 1. Surveys were conducted as per Ayling & Ayling 1999.

Table 1. History of the fringing reef monitoring programs in the Cairns Section of the Marine Park

To document the effect of the 1998 bleaching event on these reefs additional surveys were carried out in March–April 1998. All Frankland Island sites and selected sites of Snapper Island and Cape Tribulation were surveyed. During these surveys the condition of all coral colonies measured along each transect was recorded so the percentage of bleached corals could be calculated.

Where COTS were encountered during the 1999 surveys, densities were estimated by counting numbers of the starfish in 20 x 2 m strips along each permanent coral transect.

Disturbance and Recovery

Collectively the three survey regions have been subjected to seven major disturbances (table 2) during the period of the surveys.

Table 2. Disturbance events which coincided with surveys of Cape Tribulation, Snapper Island and Frankland Islands fringing reefs.

	Cape Tribulation	Snapper Island	Frankland Islands
Survey Events	1985-1989 1993-2000	1994-1995 1997-2000	1994-1995 1998-2000
Cyclone Manu - 1986 Justin - 1997 Rona - 1999
Bleaching 1987 event (minor) 1998 event
Flood Daintree River, 1996
COTS Outbreak 1999-2000

Figures 1, 2 and 3 show the mean total coral cover for the three study regions during the survey period. The disturbance and recovery of these reefs in terms of overall coral cover are distinct.

Figure 1. Total hard coral cover changes on Cape Tribulation reefs. Graphs show means from five transects at four sites for each location. Approximate times of major disturbances are indicated. Error bars are standard errors. No surveys were carried out between 1989 and 1993.
Figure 2. Total hard coral cover changes on Snapper Island reefs. Graphs show means from five transects at three sites for each location. Approximate times of major disturbances are indicated. Error bars are standard errors. nr = coral cover not recorded in south habitat at this time.	Figure 3. Total hard coral cover changes on Frankland Island reefs. Graphs show means from five transects at three sites for each location. Approximate times of major disturbances are indicated. Error bars are standard errors. nr = coral cover not recorded in east habitat at this time.

Disturbance-Recovery Case Studies

Case #1
Period: 1986–1987
Disturbance: Cyclone and minor bleaching
Location: Cape Tribulation

In April 1986, cyclone Manu crossed the coast near Cooktown generating winds between 40 and 50 knots that caused extensive coral breakage to the Cape Tribulation reefs. Coral cover at the survey locations was reduced by 25%. The affected taxa consisted mainly of the dominant Acropora/Montipora species. Recovery was complicated by the advent of a minor bleaching episode the following year (1987). This event bleached 33% of the remaining corals and resulted in some coral death, particularly among the pocilloporids and Montipora spp. Coral cover was decreased by about 4% between 1986 and 1987. However, recovery of these reefs was rapid. In the absence of further disturbances, the 1988 survey showed that overall coral cover had increased by a mean of 33% from 1987, and was similar to that of the 1985 surveys. This increase was mainly driven by rapid growth of common acroporid species.

Although the cover of Acropora/Montipora may be greatly reduced during a cyclone, there were many live fragments remaining in even the worst affected areas. Growth of these fragments could have led to the rapid recovery of these communities and as such, recovery to pre-disturbance levels of coral cover may not depend entirely on new recruitment.

Case #2
Period: 1996
Disturbance: Major flood event
Location: Cape Tribulation and Snapper Island

In March 1996, the Daintree region received heavy rainfall with five-day totals of around 1500 mm. This resulted in major freshwater run-off from numerous moderate-sized creeks while the Daintree River rose up to eight meters above high spring tide level. The flood plume from the Daintree River flowed north driven by the prevailing south-east winds (Ayling & Ayling 1998a). The resulting freshwater run-off inundated Snapper Island—which is situated near the mouth of the Daintree River—while Cape Tribulation sites would probably have been inundated from run-off from various coastal creeks.

Cape Tribulation: Surveys conducted in December 1996—nine months after the flood—showed that there were many dead coral colonies that evidently had been dead for quite some time. This death probably resulted directly from the flood event. Coral cover was reduced from approximately 55% (in 1995) to 47% in the southerly site (Ayling & Ayling 1998b). These sites were situated close to the mouths of coastal creeks. There were no marked differences in mortality rates between the different coral taxa (Ayling & Ayling 1998b). Recovery was rapid with many sites recovering to pre-disturbance levels within 12–24 months.

Snapper Island: In contrast to the Cape Tribulation reefs, the January 1997 survey revealed that the shallow reefs on the southern side of Snapper Island were decimated to a depth of three metres below low tide level. Coral cover had decreased from 90% (recorded in the 1994–1995 survey) to 10% and the reef displayed the characteristic signs of flood water inundation and mortality as described by van Woesik et al. (1996). The acroporids suffered almost complete mortality and most of the surviving corals were massive poritids and favids. In contrast the northern reefs only suffered a 20% reduction in coral cover. It appears that the north side of Snapper Island escaped the flood plume and it seems likely that an upwelling of seawater on the lee of the island protected the reefs from freshwater inundation (Ayling & Ayling 1998a).

Recovery on the north face of the island has occurred with a 16% increase in coral cover between the flood and January 1997, however there has been no recovery measurable by coral cover at the southern reef sites. This may be attributed to the fact that the majority of the surviving corals are slow growing poritids, and recovery of the previously dominant acroporids to pre-flood levels will rely on new recruitment and subsequent growth. The November 1999 survey identified numerous new acroporid and pocilloporid colonies suggesting that recruitment has indeed occurred and that recovery is underway.

Case #3
Period: 1998
Disturbance: Bleaching
Location: Cape Tribulation, Snapper Island and the Frankland Islands

The coral bleaching event of February and March 1998 affected large areas of the Great Barrier Reef and all of the survey regions were affected by this event. All three regions suffered bleaching of 55–60% of the coral cover and it was evident that certain coral groups appeared to be much more vulnerable to bleaching stress than others. For example, the pocilloporids were heavily bleached and subsequently suffered almost 100% mortality from this event at all sites.

Cape Tribulation: Montiporid corals at this site were significantly affected with 74% bleached. The favids fared better with 50% bleaching while only approximately 30% of the poritids and acroporids were bleached. However, eight months after the event most of the bleached Acropora spp. and deep water corals* appeared to have recovered with only a 10% reduction in these groups (Ayling & Ayling 1999). There were no significant changes in the percentage cover of the poritids.

Snapper Island: Pocilloporids, favids and deep water corals* showed significant levels of bleaching, and 50–65% of the acroporids were bleached. Mortality was almost complete among the pocilloporids while about half of the favids and deep water corals and < 20% of the acroporids died (Ayling & Ayling 1999). Recovery was rapid and within 24 months total coral cover was close to 1997 levels. The southern Snapper Island reef, dominated by bleaching resistant poritids, was largely unaffected.

* For this study, deep water corals are those corals found below the four-metre low tide level. These corals were typically more massive species including the genera: Podabacia, Goniopora, Alveopora, Platygyra, Hydophora, Galaxea, Merulina, Lobophyllia, Symphyllia, Echinopora, Echinophylia, Oxypora, Mycedium and Pectinia.

Frankland Islands: All coral groups with the exception of the poritids were extensively bleached at these sites. Pocilloporids were the worse affected (97% bleached). Mortality among this group, as in other locations, was almost 100%. The acroporids suffered 60% mortality and the community structure has changed from being acroporid dominated to poritid dominated. The poritids were only slightly affected with < 10% bleaching. Recovery of these reefs in terms of coral cover has been slow, probably due to the (present) dominance of slow growing species.

Collectively, mortality from this bleaching event resulted in mean reductions in coral cover of 15% for Cape Tribulation, 19% for Snapper Island and 44% for the Frankland Islands. There were some differences in the mortality and recovery of corals of the same genus between different sites (e.g. Acropora between Snapper and Frankland Islands) but the reason for this is as yet, undetermined. However, numerous plausible explanations can be identified such as species specific responses to bleaching.

Case #4
Period: 1997, 1999
Disturbance: Cyclone Justin (1997), Cyclone Rona (1999)
Location: Snapper Island

Cyclone Justin crossed the coast north of Cairns in March 1997 and gave rise to 40 knot northerly winds in the vicinity of Snapper Island. This cyclone fragmented corals on the north face of Snapper Island (A.M. Ayling pers. obs.) but by the time of the next survey in December 1997 coral cover had increased by 16% over that recorded on surveys conducted in January 1997, three months before the cyclone.

Tropical cyclone Rona passed very close to Snapper Island in February 1999 generating winds of 50–70 knots. This event caused severe damage on the northern Snapper Island sites where fragile acroporid corals dominated, turning the fields of Acropora in the two eastern-most sites to rubble banks with around 90% mortality of living corals. The low level of live coral probably means that recovery of these sites may take at least several years. On the southern side of the island, dead standing corals (killed during the 1996 flood event) were broken into rubble and many massive Porites colonies knocked over. However because the corals which survived the 1996 flood were mostly these massive colonies, cyclone Rona did not reduce the overall coral cover.

Case #5
Period: 1999 – ?
Disturbance: Crown-of-thorns starfish
Location: Cape Tribulation, Frankland Islands

During the summer of the 1999–2000 survey event, COTS were recorded for the first time on Cape Tribulation and Frankland Island reefs. The density of starfish on reefs south of Cape Tribulation and the eastern Frankland Islands ranged from 50–2000 individuals per hectare and the coral communities have suffered reductions of 40–66% live coral cover.

Cape Tribulation: By the time of the February 2000 survey, very high densities of COTS in the 15–25 cm size class were found at three sites south of Cape Tribulation. There were also many feeding scars and dead standing corals evident. Coral cover at these sites had been reduced by between 40–50% from levels recorded in the 1998–1999 surveys.

Frankland Islands: Outbreaks were found on two eastern sites during the November 1999 survey. These sites supported densities of 500–2000 individuals per hectare, with diameters ranging between 10 and 20 cm. Coral cover at the south and central eastern sites was reduced by 59% and 66% respectively. The acroporids appeared to be the main target of feeding and were significantly reduced, but all other coral groups were nominally reduced.

The impact of the COTS outbreak is likely to be as severe as that of cyclone Rona or the 1996 flood event. Coral mortality is expected to be close to 100%. Subsequent recovery may take many years.

The Ups and Downs of Corals Reefs

These case studies offer a brief look into the disturbance and recovery cycles occurring on these inshore fringing reefs. Generally, they show that the effect of any particular disturbance event will depend on the nature of the disturbance and factors such as reef location and community structure. Continued monitoring of these reefs, particularly in the light of the current COTS outbreak, will make important contributions to our understanding of how reefs are affected by impacts and how they recover. At this stage, several obvious and generalised relationships are evident.

1. The effect of disturbances on coral reefs is a function of the community composition and the resilience of the corals in that community.

2. The effects of disturbances are a function the physical, hydrological and oceanographic factors occurring at that time.

3. The community composition of inshore fringing reefs at any one point in time may well depend on the recent ecological history. As coral cover may depend on community composition, the use of coral cover as an indicator of reef ‘health’ should take into account the ecological history and species flux of the reef in question.

4. Coral community composition can fluctuate dramatically from year to year. Analysis of concurrent disturbance-recovery cycles may constitute more accurate indicators of ‘reef health’ than absolute values of coral cover.

5. Recovery can be quite rapid depending on the surviving species. The presence of surviving coral fragments may greatly accelerate the recovery period. However, the respective contributions towards recovery from fragment re-growth and larval recruitment on these reefs is unknown.

6. These reefs may be able to support high levels of coral cover and diversity in spite of relatively frequent, large-scale disturbance events.

References

Ayling A.M. & Ayling A.L. 1998a, The Effect of the Daintree River Flood Plume on Snapper Island Coral Reefs, Research Publication No. 53, Great Barrier Reef Marine Park Authority, Townsville.

Ayling A.M. & Ayling A.L. 1998b, Medium-term changes in coral populations of fringing reefs at Cape Tribulation, Unpublished report to the Great Barrier Reef Marine Park Authority, Townsville.

Ayling A.M. & Ayling A.L. 1999, The dynamics of Cairns Section fringing reefs, Unpublished report to the Great Barrier Reef Marine Park Authority, Townsville.

Bonham A.J. 1985, Report on works to reduce sediment movement from the new road to the fringing reef north of Cape Tribulation, Unpublished report to the Great Barrier Reef Marine Park Authority, Townsville.

van Woesik, R., DeVantier L.M. & Glazebrook, J.S. 1996, Effects of cyclone ‘Joy’ on nearshore coral communities of the Great Barrier Reef, Marine Ecology Progress Series, 128: 261–270.