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THE GREAT BARRIER REEF MARINE PARK AUTHORITY
© Sea Research ABN 51 594 242 720
A.M. and A.L. AylingPO Box 810
This publication may be referred to as:
LIST OF TABLES
LIST OF FIGURES
Depth Stratification of Fringing Reefs
Cairns Region Fringing Reefs
Appendix 1. Summary of Benthic Organism Abundance on Cairns Section Fringing Reefs
List of Tables
List of Figures
THE DYNAMICS OF CAIRNS SECTION FRINGING REEFS: PART II.
Draft Report to the Great Barrier Reef Marine Park Authority.
From Sea Research: A.M. and A.L. Ayling.
Monitoring of fringing reefs in the Cairns Section began in 1985 in the Cape Tribulation area with a program to look at the effects of sediment run-off from a newly constructed dirt road. This program, based on annual surveys in three locations, with four sites of five fixed transects in each location, ran from 1985-88 and again from 1994-99. A more general survey of 17 Cairns fringing reefs was carried out in 1994-95, and long-term fixed transect monitoring programs were established on two of these reefs, Snapper Island and the Frankland Islands, in 1997. These two programs were based on annual surveys of three sites of five transects on each face of these islands.
A number of natural impacts affected these reefs during these programs. In 1986 a small cyclone damaged the Cape Tribulation reefs, reducing overall coral cover by 25%. This was followed by a moderate bleaching event in 1987 that gave rise to a further 4% reduction in coral cover. With no disturbance in 1988, overall coral cover increased by 33% back to 1985 levels. There was a further impact on some of these reefs in 1996 following a flood event resulting from five day rain falls of around 1,500 mm. Coral cover in location 1 was reduced by 16% following this episode. Coral cover on these reefs was higher in 1997 than at any time during the 15 years of this program, with mean cover of almost 64%. The Great Barrier Reef (GBR) wide bleaching event of early 1998 also affected these reefs with 55% of coral cover bleached. This resulted in an overall reduction in coral cover of 15%, with the highest mortality being in pocilloporid and Montipora corals. Cyclone Rona in February 1999 did not affect these reefs significantly but Acanthaster planci outbreaks on the three sites south of Cape Tribulation reduced coral cover by 40-50% at these sites between 1998 and 1999.
The monitoring program on Snapper Island began with the general Cairns Section survey in 1994, and at this time coral cover on both faces of the island was almost 90%. The reefs on the south face were badly damaged by the 1996 flood event described above, with an almost 90% reduction in hard coral cover from the 1994 level. Massive poritids dominated the remaining hard corals. There was a more moderate 20% reduction in coral cover on the northern reefs. Snapper Island reefs were also affected by the 1998 bleaching event with 60% of corals on the northern face being bleached. As a result there was a reduction in coral cover on these northern reefs of 20%, although there was no change on the poritid dominated southern reefs. Cyclone Rona severely affected the northern Snapper Island reefs with a mean reduction in coral cover of 75%. Massive corals on the southern Snapper Island reefs were not affected by this cyclonic episode, although the reef structure was impacted.
Fringing reefs on both faces of the Frankland Islands were also 60% bleached by the 1998 event. Coral mortality from this episode was much higher on the Frankland Islands than in the other two programs, with almost 45% coral cover reduction on both faces of this island group. As acroporids and pocilloporids suffered much higher levels of mortality than poritids and faviids, the coral community structure of the western reefs on these islands shifted from acroporid dominance to poritid dominance as a result of this bleaching event. The eastern Frankland Island reefs were also impacted by Acanthaster in 1999 with an overall reduction in coral cover of 54%.
Recovery of these fringing reefs from these impacts was very rapid in most instances. Reductions in coral cover of 5-25% were made up or surpassed within 12 months in the absence of any further disturbance. It is suggested that this is a result of the dominance of the shallow sections of these reefs by fast growing acroporid corals. However, recovery from the severe impact caused by the flood event on southern Snapper Island reefs will probably take at least a decade. Acroporid corals were almost extinct in the affected depth strata and recovery will await the recruitment and growth of new colonies. Recovery from the severe impact of Acanthaster on the southern Cape Tribulation and eastern Frankland Island reefs is also anticipated to take many years. However, recovery from the severe cyclone damage on the northern Snapper Island reefs may be much more rapid due to the remaining presence of large numbers of live coral fragments.
Coral communities on the study reefs supported similar levels of coral cover to that recorded on other fringing reefs between Cape Flattery and the Keppel Islands, but were much higher than on offshore GBR reefs. No changes attributable to human induced impacts were recorded during this study.
There are numerous fringing reefs in the Cairns Section of the Great Barrier Reef (GBR) Marine Park, both on the mainland coast and around near-shore islands. Most of these reefs share a variety of physical characteristics. Fringing reefs in this region are narrow and shallow (A.M. Ayling personal observations). Reef width, between low tide level and the edge of the soft sediment ranges from a few metres to a maximum of around 50 m. Maximum water depth where the reef slope meets the sand ranges from 2 to 6 m below low tide spring level for coastal reefs, and 4-10 m for island reefs. Many of these fringing reefs have formed on the outer edge of sediment banks around creek mouths and much of the reef structure is unconsolidated (Partain and Hopley 1989). As a result all but the larger and more massive reef corals are easily moved and even moderate strong wind episodes can cause considerable damage (Ayling and Ayling 1991).
Run-off from terrestrial catchments has introduced a thick wedge of terrigenous sediment into the near shore environment (Hopley et al. 1990), and onshore wave action generated by the predominant SE trade winds resuspends this sediment. As a result the inshore water mass around these fringing reefs is usually very turbid. Throughout much of the year underwater visibility is less than a metre and sedimentation rates are very high. Hopley et al (1990) recorded mean sedimentation rates of 110 mg sediment per sq cm per day on the Cape Tribulation fringing reefs. Only during the summer months, when the SE trade winds are less consistent and long calm periods can occur, does turbidity reduce and the water visibility improve. During calm periods at this time of the year underwater visibility of 5-10 m may be recorded in shallow water, although it is usually only 2-4 m below about 4 m depth.
Fringing reefs can be divided into two broad categories based on the extent of development of algal forests on the reef. The majority of both coastal and island fringing reefs are dominated by a forest of Sargassum spp. and other macroalgae in the upper 2-4 m. There may be some hard corals underneath the algal canopy, or patches of coral dominated substratum within the forest, but rich coral communities are generally restricted to the reef area below this algal fringe (Ayling and Ayling 1991). On some fringing reefs this algal fringe is either poorly developed or absent, and hard coral dominated communities cover the entire reef slope.
If terrestrial human activities are degrading reefs in the GBR region then this effect is most likely to be felt on these fringing reefs. Impacts of most concern are nutrient enrichment from fertiliser and sewage run-off, as well as increased siltation and increased turbidity from disturbed river catchments, but anchor damage is increasingly seen as a threat to fringing reefs in heavily used areas. Some fringing reef areas are close to large centres of population and are likely to be subjected to general industrial pollution in addition to these more widespread impacts. There is increasing concern that fringing reefs are being degraded by human impacts. Hopley et al. (1990) suggested that the Cape Tribulation reefs were suffering siltation stress and would be unable to recover from any future disturbances. In a related study Partain and Hopley (1989) reported that these reefs ceased prograding about 5,000 years ago and claimed that this was because sedimentation levels had passed beyond the threshold that allowed for active reef growth.
With support from the Great Barrier Reef Marine Park Authority (GBRMPA) we have attempted to document changes on a number of fringing reefs in the Cairns Section where there was concern that human impact may have resulted in reef degradation. In late 1984 a coastal dirt road was constructed through steep rainforested hills between Cape Tribulation and the Bloomfield River 20 km to the north (Craik and Dutton 1987) (figure 1). There was concern that heavy sediment run-off from this unconsolidated road was damaging the many fringing reefs in this area (Bonham 1985) and we were asked to instigate a monitoring program to document changes to the fringing reef communities. This program was compromised by the lack of any pre-road construction data, but the use of suitable controls and a long time scale for the study minimised the problems associated with this serious shortcoming (Ayling and Ayling 1991).
A broad-scale survey of Cairns Section fringing reefs was carried out in 1994-95 to document fringing reef communities from a wide range of areas for comparison with the more detailed Cape Tribulation study (Ayling and Ayling 1995a). Following on from this study, long term monitoring of potential impacts was initiated on two island fringing reefs in this area: Snapper Island (16°18’S) and the Frankland Islands (17°14’S) (figure 1). On Snapper Island the program was designed to look at recovery rates after large areas of the fringing reef were badly damaged by freshwater flood inundation in March 1996 (Ayling and Ayling 1997). It was suggested that if the fringing reefs of this area were under stress from human impacts then recovery of the badly damaged sections of the reef would be affected. The Snapper and Frankland Island programs were also both intended to document the effects of the heavy recreational use of these areas by small boats.
These studies have been documented in a number of reports to the GBRMPA (Ayling and Ayling 1991, 1995a, 1997, 1998a, 1998b, 1999). Although no human induced impacts have been noted to date, a number of natural disturbances have had a profound effect on these reefs during the course of these studies. These included a moderate cyclonic episode in 1986, a moderate coral bleaching event in 1987, flood rains in 1996, a severe bleaching event in 1998, cyclone Rona in February 1999 and an Acanthaster outbreak at some sites during 1999. The aim of this document is to summarise the effects of these events on the fringing reefs of this area and to present the results of the 1999 impacts in detail.
All the fringing reef studies reported here have been based on groups of five 20 m line intersect transects run haphazardly within an area of approximately 60 x 10 m. Two to four such sites have been surveyed within each location of interest. Surveys to document coral communities or to establish broad patterns usually used temporary random transects, but those used for monitoring changes over the medium to long-term used fixed transects that were repeatedly re-surveyed. A fibreglass surveyors tape was used to delineate the transects. Transects were usually run along a depth contour and were straight unless the reef topography decreed otherwise. The transects at each site usually covered a depth range of between 2-5 m below low tide spring level, and were positioned below the algal dominated fringe where that occurred.
Depth stratification was established using groups of five random transects in four depth strata at one Cape Tribulation site.
Permanent transects were marked with five 100 cm long 15 mm reinforcing rod stakes driven deep into the reef structure at 5 m intervals. Each site was mapped and the position of transect stakes relative to reef topography and distinctive coral colonies noted. This enabled lost stakes to be replaced with very little error. Stakes were replaced approximately every five years to ensure transects could be reliably relocated. During each survey a fibreglass tape was stretched tightly between the 5 marker stakes and the corals beneath measured.
For both temporary and fixed transects the estimated length in cm of all hard corals and other encrusting organisms directly beneath the tape was measured. The following organisms, or groups of organisms, were measured: the brown macroalgae Sargassum spp.; all turfing algae grouped; all sponges grouped; Millepora spp.; Heliopora spp.; all hard corals to genus level, or to structural growth form grouping if more appropriate eg Acropora staghorn, Acropora tabulate etc.; all soft corals to genus level. Intersects were totalled for each group and converted to percentage covers.
Coral abundance is documented in this report as percentage cover of the substratum. Coral community composition of the different coral groups is reported as a relative percentage of the total hard coral cover. Any change in coral cover is also reported as a relative percentage change of the original coral cover.
The broad survey of Cairns region fringing reefs used two sites on the outermost part of each coastal reef and five random 20 m line transects were surveyed at each site. The Cape Tribulation fringing reef study used a design of three locations, each comprising about 8 km of coastline and a number of separate fringing reefs (figure 2). Location 1 was adjacent to a long established coastal road; location 2 was adjacent to a newly constructed section of coastal road and location 3 was adjacent to undisturbed catchments. Four sites of 5 fixed 20 m transects were set up in each location (table 1). Studies on the fringing reefs around Snapper Island and the Frankland Islands were subdivided by habitat as the two sides of the islands supported slightly different coral communities. Three sites of five fixed 20 m transects were established in each habitat on these islands (table 1). Surveys were usually carried out during the summer period October-March when winds were light and underwater visibility adequate for locating the transect markers reliably. For most sites surveys were repeated annually.
To document the effect of the 1998 bleaching event on these reefs additional surveys were carried out in March/April 1998 on all Frankland Island sites, the north Snapper sites and on 8 of the 12 Cape Tribulation sites. During these surveys the condition of all coral colonies measured along the transects was recorded so the percentage of bleached corals could be calculated.
Where Acanthaster planci were encountered during the 1999 surveys densities were estimated by counting numbers in 20 x 2 m strips along each permanent coral transect.
Data from these surveys were analysed using anova techniques. For the fixed transects a repeated measures analysis design was used (table 2).
3. i Depth Stratification on Fringing Reefs.
The majority of fringing reefs in the GBR region are algal dominated on the outer reef flat and the upper few metres of the reef slope (figure 3). Extensive stands of Sargassum spp., with a dense understory of smaller algal species, cover over 50% of the substratum in this algal band (Ayling and Ayling 1991). Hard corals generally cover only around 5% of the substratum on the reef flat. Hard coral cover increases rapidly down the reef slope, and at a depth of 5 m below low spring tide level the cover of living corals is usually between 40 and 80% (figure 3).
The composition of the hard coral community also changed with depth. The few corals beneath the Sargassum forest on the reef edge were mainly faviids in the genus Goniastrea. On the upper reef slope encrusting Montipora species made up 80% of coral cover; this was the main coral group found beneath the algal canopy. Explanate and whorl-forming Montipora corals along with corymbose plate and staghorn Acropora species dominated the third strata, 2-4 m below low tide level, in which the study sites were located. Where the deeper reef slope strata were present, the coral communities were equally rich but they were dominated by a suite of more massive corals including the following species and genera: Pachyseris speciosa, Podabacia crustacea, Goniopora, Alveopora, Platygyra, Hydnophora excesa, Galaxea, Merulina ampliata, Lobophyllia, Symphyllia, Echinopora, Echinophyllia, Oxypora, Mycedium elephantotus, Pectinia lactuca. These deep water species (referred to as deep water corals in this report) accounted for about 80% of corals in this deeper strata. On reefs that lacked the algal fringe the composition of the coral community showed similar depth stratification to that described above (A.M. Ayling personal observations).
3. ii Cairns Region Fringing Reefs.
Mean hard coral cover from the 17 Cairns Section locations recorded during this study was 81%, with a range from 65 to 93% (appendix 1). Acroporids made up about 85% of this coral cover, about equally split between Acropora spp. and Montipora spp. Although poritids were common at a few locations the overall mean accounted for only about 5% of coral cover, and other coral groups were even less important. Soft corals were not common with a mean cover of only 3% (range 0-9).
3. iii Cape Tribulation.
There have been a number of impacts on Cape Tribulation coral communities over the 14 years of this study (figure 4). A small cyclone that crossed the coast near Cooktown in April 1986 gave rise to winds between 40-50 knots in this region and caused extensive coral breakage on the study reefs. This resulted in a 25% overall reduction in coral cover, mainly of the dominant Acropora/Montipora species, between 1985 and 1986. In early 1987 a coral bleaching episode bleached a mean of around 33% of the corals on these fringing reefs and resulted in some coral death, especially of pocilloporids and Montipora spp. There was an average 4% decrease in overall coral cover between 1986 and 1987, probably as a result of this bleaching event. Both these disturbances affected all locations equally and coral cover remained similar at all locations. In the absence of disturbance coral cover on the Cape Tribulation reefs increased by a mean of 33% in 1988, back to 1985 levels. This increase was driven by rapid growth of the common acroporid species.
In March 1996 this region received flood rains, with 24 hr falls of between 500-1,000 mm and five day totals of around 1,500 mm. This gave rise to huge fresh water run-off from local catchments. There are a number of moderate sized creeks that may impinge on the Cape Tribulation study area, including Myall Creek, Emmagen Creek, Tachalbadga Creek, Donovan Creek and Melissa Creek. The Daintree River flood plume flowed north from the mouth, and we may assume northward movement of the much smaller plumes from the Cape Tribulation catchments given the SE winds that prevailed at the time. In that case the sites at most risk would be sites 1-3, site 5 and site 6. At the time of the 1996 survey, 9 months after this event, we noted many dead coral colonies at sites 1-3 and site 5. These corals had been dead for some time and may have resulted from freshwater inundation during the March flood event. Coral cover decreased by between 5-32% at these four sites between 1995 and 1996 (mean 16%) and this was responsible for the reduced coral cover recorded in location 1 (sites 1-4) in 1996 (figure 4).
The 1998 bleaching event resulted in the bleaching of 55% of the coral cover on the Cape Tribulation reefs, almost double the effect of the 1987 event. As has been documented previously (Ayling and Ayling 1991, Fisk and Done 1985) not all coral groups were equally affected by bleaching. Almost all pocilloporids were bleached, along with 74% of Montipora spp., but only half of faviids and deep-water corals were bleached and only 30% of poritids and Acropora spp. The different hard coral groups also showed differing reactions to bleaching. Almost all the bleached pocilloporids appeared to have died, but only about a third to a half of poritids and Montipora spp. Most of the bleached Acropora spp., faviids and deep-water corals on these reefs appear to have recovered from the bleaching, with only about 10% reduction in the cover of these groups being recorded 8 months after the bleaching event (Ayling and Ayling 1999). As a result the 1998 bleaching episode gave rise to an overall 15% reduction in hard coral cover on the Cape Tribulation reefs, with similar levels of mortality in all locations (figure 4).
Cyclone Rona crossed the coast near the Daintree River mouth in February 1999. Very strong S to E winds were experienced between Snapper Island and Port Douglas prior to the eye crossing the coast. As the eye passed inland strong W to N to NE winds devastated forests in the Cow Bay to Cape Tribulation area. It was expected that these winds would have caused some damage to the Cape Tribulation fringing reefs but in fact damage was minimal with some minor structural damage to some sites and either very little or no coral cover reduction (figure 4).
However, some sites had been impacted at the time of the February 2000 survey by outbreaks of Acanthaster planci. Very high densities of small Acanthaster 15-25 cm in diameter were found in the three sites south of Cape Tribulation itself, along with many feeding scars and a number of dead standing coral colonies. Coral cover had been reduced by between 40-50% at these three sites between the 1998 and 1999 surveys (figure 10A) and damage was on-going. As a result coral cover had been significantly reduced in Location 1 but had increased in the other two locations giving a significant time x location interaction (table 3). No Acanthaster were observed in the fourth Location 1 site immediately north of Cape Tribulation and coral cover at that site was not affected (figure 10A).
3. iv Snapper Island.
We surveyed a preliminary set of five transects on the northern Snapper Island reefs in February 1988 as part of another study. Mean hard coral cover at this time was 56% (sd = 8). By the time of the 1994 random transect survey on Snapper Island coral cover was almost 90% on both the south and north facing habitats (figure 5). Reefs on the north habitat were dominated by bottlebrush and staghorn Acropora spp (84% of hard corals) and Montipora spp. (12% of hard corals), while those on the south were dominated by staghorn and corymbose plate Acropora (59% of hard corals) and poritids (22% of hard corals). Faviid corals were not recorded on the northern reefs but accounted for almost 8% of hard corals on the southern reefs.
The shallow reefs on the south side of Snapper Island were almost totally destroyed to a depth of about 3 m below low tide level by the March 1996 Daintree River flood episode that resulted from a fall of about 1,350 mm of rain in five days. The flood plume was pushed north of the river by the SE winds prevailing at the time but although it surrounded Snapper Island the northern reefs were largely protected by an up-welling of seawater in the lee of the island (Ayling and Ayling 1997). Coral cover on the southern reefs decreased from almost 90% to around 10% (figure 6). Almost all the surviving corals were massive poritids along with a few faviids, while acroporids showed almost complete mortality. Some corals were damaged on the northern reefs but the reduction in total coral cover was only 20%, from 88% down to 70.5%. Coral cover on the north face increased by 16% over the next 11 months to December 1997 (figure 6), but remained static on the southern reefs where slow growing poritids dominated the flood damaged coral community.
At the time of the December 1997 survey no bleaching was evident on these reefs but by the time of a survey in March 1998 a mean of almost 60% of coral cover was bleached. As on the Cape Tribulation reefs bleaching did not affect all coral groups to the same extent. Pocilloporids were badly affected, as were poritids, faviids and deep water corals, although the latter three groups were rare on the northern Snapper Island reefs (<1% cover). Half to two thirds of acroporids were bleached. Most bleached pocilloporids died, but only about half of the deep water corals and faviids, and less than 20% of acroporids (Ayling and Ayling 1999). Mean hard coral cover reduced by about 20% on the northern reefs of Snapper Island due to this bleaching event (figure 6). Turf algae cover increased, probably due to the reduction in coral cover. The southern reefs, dominated by bleach-resistant poritids, were not affected by this event (figure 6).
As mentioned above cyclone Rona impacted this area in February 1999. Wind records for Low Isles, about six miles SE of Snapper Island, were obtained from the Bureau of Meteorology for the passage of Rona. These indicated that the eye passed just north of Low Isles. Winds for the two hours prior to the eye passing ranged from 35-60 knots with gusts to 75 knots while swinging from the south to the east. After the eye had passed winds increased to over 70 knots with 85 knot gusts before decreasing to below 30 knots three hours later while swinging from east to north. Given this regime it is clear that strong winds affected reefs on both sides of Snapper Island which is aligned roughly SE-NW.
Reef communities at all six sites were impacted by these strong winds. On the south face the dead standing acroporid colonies remaining from the 1996 flood episode were broken up into rubble and some of the remaining massive poritid and faviid colonies were slightly damaged. Many of the massive poritid colonies were knocked over and some turned upside down. Because the community was dominated by these massive colonies the coral cover was not reduced at these three sites (figure 6, 7). On the north face where the coral communities were dominated by acroporids the impact was severe with a strong gradient in damage severity from the most impacted eastern site to the least affected western site (figure 10B). The eastern and central northern sites were reduced to rubble banks with a few remaining live corals and live broken fragments, with 94% and 88% live coral reduction respectively between 1998 and 1999. The west site, which is open to the NW and north rather than to the NE was more protected and only showed a 40% coral cover reduction (figure 10B). Nominal reductions occurred in all coral groups and soft corals (figure 7), although these were only significant for the dominant Acropora and Montipora spp. (table 4).
Over the past six years Snapper Island fringing reefs have been affected by floods, bleaching and a cyclone with a resulting change in overall coral cover from a mean of 88.2% to a mean of 14.5%.
3. v Frankland Islands.
At the time of the initial random transect survey in early 1995 hard coral cover on the west face of the Frankland Islands was similar to that recorded on the southern Snapper Island reefs, with a grand mean of almost 80% (figure 8). Coral communities on these western reefs were dominated by bottlebrush Acropora spp. (60% of hard corals) and poritids (21% of hard corals). Soft corals, primarily, Clavularia and Sinularia, covered 7.3% of the substratum. Benthic organisms were not recorded on the eastern reefs at this time. Both faces of these Islands were surveyed using fixed transects at the height of the 1998 bleaching event in April 1998. At this time coral cover on the western reefs was similar to that recorded in 1995 (figure 8), although community composition was slightly different. In early 1998 these western reefs were dominated by both poritid corals (Porites rus and Porites finger primarily) accounting for 46% of hard corals, and Acropora species (primarily A. longicyathus and A. formosa) accounting for 41% of hard corals. Deep water species (Ayling and Ayling 1991) such as Pachyseris, Hydnophora and Echinopora, were also common (11% of hard corals). Soft corals, mainly Sinularia species, covered 8% of the substratum. Pocilloporids, Montipora species and faviids each covered about 1% in this community.
Overall hard coral cover was lower on the eastern reefs that were exposed to the predominate SE winds (53% compared to 80%). Community composition was also different on these eastern reefs, being dominated by corymbose plate and staghorn Acropora spp. (33% cover), and Montipora spp. (11% cover) with much lower cover of poritids and deep water corals (figure 9). Tropical Cyclone Joy passed by this area in December 1990 and caused extensive coral damage on these eastern reefs and this was probably responsible for the overall coral cover differences between eastern and western reefs in 1998 (A.M. Ayling personal observations in early 1991).
Over 60% of the hard corals on these reefs were bleached during the 1998 event. All coral groups except poritids were extensively bleached, with pocilloporids being most affected (over 97% bleached). Poritids were only slightly affected, with less than 10% bleaching. Coral death resulting from this bleaching event caused significant reductions in the cover of all coral groups (Ayling and Ayling 1999). Pocilloporids were most severely affected with almost 100% mortality, but even poritids showed a significant reduction in cover of over 15%. Overall hard coral cover was reduced by almost 45% on the Frankland Island fringing reefs as a result of the 1998 bleaching event, with similar reductions on both the east and west reefs (figure 8). Over 70% of soft corals (primarily Sinularia spp.) were bleached on these reefs but these apparently recovered without mortality and soft coral cover increased slightly between the 1998 and 1999 surveys (Ayling and Ayling 1999). Due to differential mortality of the various coral groups, poritids are now the dominant coral group on the western Frankland Island reefs, accounting for almost 70% of hard coral cover (figure 9).
The strong winds from cyclone Rona did not affect the Frankland Islands but an outbreak of small Acanthaster planci similar to those found on some of the Cape Tribulation reefs was affecting two of the eastern sites at the time of the November 1999 survey. No evidence of Acanthaster presence on these reefs was seen in Jan 1999 at the time of the previous survey. The southern and central eastern sites supported densities of over 2000 and over 500 Acanthaster per ha respectively, with diameters ranging from 10 to just over 20 cm. All were actively feeding on coral colonies and cover at these two sites had been reduced by 66% and 59% respectively (figure 10C). The preferred acroporids were significantly reduced (table 5, figure 9) but all coral groups were nominally reduced. Overall coral cover on the eastern reefs reduced to around 14%, down from 53% two years ago. On the non-impacted western reefs coral cover increased nominally to over 46% (figure 8).
A major feature of many fringing reefs is the presence of an algal forest in the first few metres below low tide level. This is usually a dense stand of Sargassum spp. brown algae, but may contain a number of other species such as Cystoseira trinodis and Hormophysa triquetra, as well as an understory of a variety of smaller red and brown algae. All the coastal reefs visited as part of this study supported such an algal fringe. Most of the reefs around the inshore islands, with the exception of Double Island, lacked an algal forest, although they usually had a scattering of the large brown algae Turbinaria ornata on the reef flat. The reason for this distinction is not clear. Many of the coastal reefs are on the outer edge of huge sediment wedges and lie over 2 km from the shore itself, whereas some of the inshore islands (eg Snapper Is., Black Rocks) are only a kilometre or less from the shore. All the study reefs with the possible exception of the Frankland Islands are in the turbid coastal water mass; it does not appear to be differences in water quality that determine the presence or absence of the algal fringe. It is possible that the maximum depth the reef reaches has some bearing on this question, as the inshore island reef slopes generally extend deeper before reaching the sand (also with the exception of Double Island).
Of the reefs monitored through time as part of this study only the Cape Tribulation fringing reefs supported an algal fringe. Most of the survey sites were in the Acropora/Montipora dominated depth strata immediately beneath this algal fringe and a mean of 1.3% of the transects at these sites were covered by Sargassum spp. If the cover of Sargassum forest on these reefs was increasing then this cover may be expected to have increased over the 15 years of this study. There was no evidence of this (Ayling and Ayling 1998a), and there was a nominal decrease in Sargassum cover between 1998 and 1999 from 1.3% to 0.8% .
The fringing reefs of the Cairns Section of the GBR Marine Park were generally rich in terms of hard coral cover. During the 1994-95 survey of 17 locations from Cedar Bay to the Frankland Islands the reef slope mean hard coral cover was over 80%. Fast growing Acropora spp and Montipora spp. corals dominated these shallow water fringing reef coral communities, accounting for almost 85% of total coral cover. With few exceptions coastal reefs supported significantly higher levels of Montipora spp. corals than Acropora spp. (mean cover 51.4% compared with 16.8%), whereas on reefs around inshore islands Acropora spp. were more abundant than Montipora spp. (59.1% compared with 8.7%). As a general rule the reefs that were Montipora dominated were those that had a well developed algal fringe in the upper few metres, while Acropora dominated reefs lacked this algal fringe. Although other coral groups such as poritids and faviids were generally rare in these shallow water fringing reef communities, a few of the study reefs around inshore islands supported large areas of poritids, primarily Porites finger and Porites rus. The high level of natural disturbance in the shallow fringing reef coral communities probably accounts for the dominance of fast growing acroporid corals on these reefs.
There have been a number of severe natural impacts on these reefs over the period covered by this study. The Cape Tribulation reefs were affected by a small cyclone that crossed the coast near Cooktown in April 1986, generating 40-50 knot winds in this area. The waves resulting from this event fragmented a large number of Acropora/Montipora colonies in the study sites and caused an overall reduction in coral cover of 25%. More massive colonies such as poritids and faviids were minimally affected by this episode (Ayling and Ayling 1991). Although recovery of the coral communities after this cyclone was delayed by a bleaching event in February 1987, an increase of 33% between November 1987 and November 1988 saw hard coral cover reach pre-cyclone levels again. Although cover of Acropora/Montipora coral may be reduced by a cyclone there are still many living fragments remaining, even in badly damaged sections of the reef. Growth of these fragments can lead to rapid recovery of these communities following such an event; recovery is not dependent on settlement of new coral individuals.
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 event fragmented corals on the north face of Snapper Island (A. M. Ayling personal observations) but by the time of the next survey in December 1997 coral cover had increased by 16% over that recorded prior to the cyclone in January 1997. It is possible this level of wave-fragmentation encouraged coral growth and resulted in a greater increase in coral cover than might otherwise have been
Tropical cyclone Rona passed very close to Snapper Island in February 1999 generating sustained winds of 50-70 knots. This event caused severe damage on the northern Snapper Island sites were fragile acroporid corals dominated, turning the fields of Acropora in the two eastern most sites to rubble banks with around 90% death of living corals. Although there are many live acroporid fragments remaining in this rubble the low level of live coral probably means that recovery of these sites will take at least five years. The western site was not as severely affected and should recover rapidly.
The destructive effect of tropical cyclones on coral reef communities has been well documented (Done et al. 1986; Done et al. 1991; Woodley et al. 1981). Few follow-up studies of recovery rates have been made but Done et al. (1991) made predictions of recovery times of a decade or two for severely damaged front reef slope communities. The present study suggests that moderate cyclone damage on fringing reefs may be repaired in only a few years. It will be interesting to see how long it takes the severely affected Snapper Island northern sites to recover following the impact of cyclone Rona.
Damage to fringing reefs caused by freshwater inundation after flood rains was also documented during this study. The 1996 flood in this region was initiated by 24 hr falls of 500-1,000 mm of rain and 5 day totals of 1,300 to over 1,500 mm. Sediment laden flood plumes extended several miles from the coast and were carried north by the strong SE winds prevailing at the time (Ayling and Ayling 1997). Freshwater inundation caused almost total coral mortality over the entire south face of Snapper Island, and probably around 20% mortality on the northern reefs. It seems likely that 4 of the study sites on the Cape Tribulation reefs suffered coral mortality of 5-32% from this flood event. The sites that were only slightly damaged by this flood recovered to previous hard coral levels within 12 months, but the impact was more profound on the southern Snapper Island reefs that were devastated during this event.
Over 85% of coral cover on the southern Snapper Island reefs died as a result of this event, including almost all acroporid corals. The remaining hard coral community was almost totally dominated by slow growing poritids (72% of hard coral cover). As a result coral cover was almost unchanged at the time of the November 1999 survey, 3 years and 8 months after the flood. Recovery of the once-dominant acroporids will rely on settlement of new coral colonies and may take ten years or so. By November 1999 there were numerous new acroporid and pocilloporid recruits on these reefs, with the largest colonies 15-25 cm in diameter and it appears that recovery is well underway in spite of some scouring of these colonies by cyclone Rona (A.M. Ayling personal observations).
Coral bleaching due to sustained high water temperatures can also have an impact on coral reefs (Harriott 1985; Fisk and Done 1985). In February 1987 high water temperatures bleached about 33% of corals in the Cape Tribulation study sites. This caused some mortality, especially of pocilloporid and acroporid corals (Ayling and Ayling 1991), and gave rise to an overall reduction in coral cover of 4%. This effect was short lived, with a coral cover increase of 33% recorded during the subsequent 12 months.
In 1998 many reefs on the GBR experienced a severe bleaching event during February and March (Berkelmans and Oliver 1999). The aerial survey carried out by Berkelmans and Oliver (1999) showed that the most intense and widespread bleaching occurred on fringing reefs, with most of the GBR inshore region being affected. Results from our study areas supported this contention. All three were similarly affected, with 55-60% of coral cover being bleached. Resultant coral mortality was much higher than during the 1987 event, and differed in the different locations, even though levels of bleaching were similar. Overall reductions in hard coral cover were 15% for Cape Tribulation, 19% for northern Snapper Island reefs, and 44% for the Frankland Islands. Pocilloporids suffered almost 100% mortality at all locations, and Montipora spp. corals were also significantly reduced at all locations. Poritids and faviids were least affected, with no significant changes in cover on Snapper and Cape Tribulation reefs, and relatively low reductions on the Frankland reefs. On Cape Tribulation reefs Acropora spp. were unaffected by this bleaching event, on Snapper Island the reduction was about 20%, but on the Frankland Island reefs mortality of this group was over 60%. Differential mortality of the major coral groups changed the community structure at the Frankland Islands from acroporid dominated to poritid dominated. In addition, pocilloporids reduced from 3.1% to 0.1% cover on these reefs. Because poritids were not common on Cape Tribulation or Snapper Island reefs, community structure was similar before and after the bleaching event. Pocilloporids accounted for less than 2% cover at these sites and the near extinction of this group did not have a major effect on community structure. Recovery from this bleaching event was rapid on the Cape Tribulation Location 1 and 2 reefs with cover back close to 1997 levels less than 24 months after the event. However recovery could take considerably longer on the Frankland Islands, especially on the western reefs where acroporid cover reduced from 33% to less than 6%.
During the 1999 surveys we encountered Acanthaster planci outbreaks on fringing reefs for the first time. At the present time outbreaks of sub-adult Acanthaster 10-25 cm in diameter are affecting many mid-shelf reefs in the Cairns area but they have not previously been reported from inshore reefs. The densities measured on the reefs south of Cape Tribulation and on the eastern Frankland Island reefs ranged from 50 to over 2000 per ha and the coral communities have already suffered reductions of 40-66% in live coral cover. The impact from these outbreaks is clearly going to be as severe as that of cyclone Rona and the Snapper Island 1996 flood event. It is anticipated that coral mortality on affected reefs will be close to 100% and that recovery will take many years.
Natural impacts of this type apparently have a disproportionate effect on acroporids and pocilloporids compared with other hard coral groups. It was suggested earlier that regular disturbance may encourage dominance of fast growing coral groups and it seems likely that repeated small to moderate impacts may indeed lead to acroporid dominance. However, repeated severe impacts, such as the flood effect on southern Snapper Island reefs and the bleaching event on the western Frankland Island reefs, may change community composition markedly by removing much or all of the acroporids and pocilloporids. This may eventually lead to the type of community present on the south face of Snapper Island and the west face of the Frankland Islands where poritids are present at much higher levels of cover than on most other fringing reefs.
These studies have been confined to the shallow depth strata usually dominated by acroporid corals, because deeper strata were not present at all sites. Our observations during this study suggest that the deeper coral strata, dominated by more massive coral species, were not as severely affected by the above impacts we have documented in the Acropora/Montipora depth strata. Deep-water corals within the Cape Tribulation study sites were not reduced by the moderate cyclonic episode in 1986 (Ayling and Ayling 1991). The deeper strata were also beneath the 3-4 m below low tide spring level that was the limit of the flood impact on Snapper Island. In addition, these species were not as severely affected by the bleaching events. It is suggested that in future some monitoring sites be set up in these deeper coral strata to document changes within this apparently more stable community.
Pre-bleaching coral cover on the study reefs is comparable with that recorded from studies on other fringing reefs in the GBR region (table 6). Comparisons were made with the pre-bleaching surveys from this study because it was assumed that the coral cover from the comparative locations would have been reduced by an unknown amount following the bleaching event. Coral cover is usually high on the reef slope of fringing reefs, with the exception of reefs within the area of big tides between Mackay and Port Clinton, where maximum tide range is more than 5 m. Grand mean coral cover ranged from 46% to over 66% between Cape Flattery and Keppel Islands but averaged only 25% within the big tide area, although individual sites from this area also had over 60% cover (Ayling et al. 1998). The reasons for the lower coral cover in the big tide area are unclear, but probably relate to turbidity and siltation caused by strong tidal currents, rather than to lower temperatures (Ayling et al. 1998; Van Woesik 1992)
A comparison of fringing reef coral cover measurements with those recorded on offshore reefs is interesting (table 7). Grand mean coral cover from 330 sites recorded throughout the GBR region over the past 5 years is only around 30%, half of that from our fringing reef study sites, and only marginally higher than that from fringing reefs in the strong tide area. It seems likely that it is the input of terrigenous organic matter that makes fringing reefs so rich in comparison with offshore reefs, providing extra food for the corals and encouraging rapid growth.
The Cape Tribulation fringing reef monitoring study was originally established to look for potential impacts resulting from sediment run-off from a newly constructed dirt road. Although a number of natural impacts have affected the coral communities over the 14 years of this study, there was no evidence that the road-derived sediment input had any impact on the Cape Tribulation fringing reefs (Ayling and Ayling 1991, 1998a). There is considerable concern that nutrient enrichment of the coastal water mass caused by run-off from agricultural catchments is causing degradation of fringing reef communities. The most likely effect of nutrient enrichment would be an increase in algal cover on the fringing reefs. On the Cape Tribulation reefs there were significant fluctuations in Sargassum and algal turf cover but no evidence to date of any overall increase in these groups (Ayling and Ayling 1998a).
During the summer months turfing algae cover most available free space in fringing reef communities and turf cover usually increases as more space becomes available following coral mortality. When coral cover on the southern Snapper Island reefs was almost 90% in 1994, turfing algal cover was only 2%. Following the 1996 flood event coral cover was reduced by almost 90%, and as a result turf algal cover increased to almost 40%. Similarly algal turf cover on the northern Snapper Island reefs increased by almost 80% following the 19% reduction in hard coral cover that resulted from the 1998 bleaching event. On the western Frankland Island reefs this bleaching event caused a reduction in hard coral cover from 80% down to 45% and as a result turf algal cover increased from 4% to over 30%. Similar increases in algal turf were seen following the cyclone Rona damage on Snapper Island and the Acanthaster impacts on Cape Tribulation and Frankland Island reefs. It has been suggested that dense algal turf inhibits the settlement of corals (Fisk and Harriott 1989). A longer-term monitoring program will be necessary before the effects of the increases in turfing algae on these fringing reefs can be determined.
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Ray Berkelmans, Ken Anthony and Dave Wachenfeld of the GBRMPA provided much assistance with the planning and paper work for these surveys and their help is appreciated. Warren and Melba Nott provided valuable assistance in the field for some of these surveys.
Summary of Benthic Organism Abundance: Cairns Section Fringing Reefs.
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