ltm200903-04

Dates: 23rdMarch - 9thApril 2009

Vessel: MV Elizabeth E II

Survey leaders:

Alistair Cheal

Pompey sector reefs

Whitsunday sector reefs

Image 1 and 2:These two plates show roughly the same view of the beginning of site 1 on Rebe Reef (marked by a star picket lying at an angle in the foreground) in 2007 (left plate) and after Cyclone Hamish in 2009 (right plate). This outer shelf reef was exposed to the full force of Cyclone Hamish and it is clear that even relatively large massive corals (directly in front of the star picket in 2007) were torn from the reef by wave action along with other hard and soft corals. However, some corals had retained there grip and remained in place.

Photo:AIMSLTMP

Summary

As part of ongoing surveys by the AIMS long term monitoring team, four reefs in the Pompey sector and five reefs in the Whitsunday sector of the Great Barrier Reef were surveyed using manta tows. The same five Whitsunday reefs along with four others were also surveyed using SCUBA to provide more detailed information on benthic organisms, reef fishes and agents of coral mortality. Preliminary results of the manta tow surveys and SCUBA searches for coral mortality are presented in this report.

No outbreaks of crown-of-thorns starfish (COTS) were noted during manta tow surveys and low numbers of COTS were recorded at one reef in the Whitsunday sector. Median reef-wide live hard coral cover (henceforth coral cover) in the Pompey sector was very low (0-5%) on two reefs, low (5-10%) on one reef and moderate (10-30%) on another. On all four reefs coral cover had decreased markedly since last surveyed (two to five years ago). The decreases from very high coral cover (>50%) to very low coral cover (>10%) at Creal Reef was extreme. All coral cover decreases were at least in part caused by severe Tropical Cyclone Hamish (category 4) which passed across the region 7-8 March 2009. The eye of Cyclone Hamish passed very close to all four reefs (< 50 kms) and there was clear evidence that heavy wave action had recently damaged corals, other benthic organisms and the reef framework. As is common with even the most severe tropical storms, damage was highly variable around each reef and ranged from catastrophic to barely detectable, depending on the orientation of the habitat to prevailing swells. Manta tows of reefs in the Whitsunday sector revealed that coral cover at two outer shelf reefs had also decreased markedly since last surveyed while three mid-shelf reefs had retained moderate coral cover. The two outer shelf reefs had suffered damage from Cyclone Hamish similar to that recorded in the Pompey sector, while the mid shelf reefs had suffered negligible damage.

SCUBA searches on reefs in the Whitsunday sector found no COTS but relatively high occurrence of "white syndrome" (a disease-like necrosis that particularly affects tabulate Acropora spp.) and "brown band" coral disease on a number of inner and mid shelf reefs. The significance of these increases is uncertain but may reflect a particularly stressful wet season for corals with high sea surface temperatures, major flood events and intense storm activity. Numbers of Drupella spp. (coral feeding snails) and levels of coral bleaching were within the range of previous estimates at each reef.

A summary of the results is presented in Tables 1, 2 and 3. A full list of survey reefs appears in the Long-term Monitoring Status Report Number 8 . Details of the monitoring program design, the sampling methods and a full explanation of the COTS outbreak terminology used in this report can be found on the AIMS reef monitoring website .

Pompey Sector reefs

Four reefs were surveyed using manta tows (Table 1). No COTS were recorded on any of the survey reefs. Coral cover was very low (0-5%) on two reefs, low (5-10%) on one reef and moderate (10-30%) on another and had decreased markedly on all four reefs since their last survey (two to five years ago). Decreases in coral cover from very high (>50%) to low (5-10%) at Creal Reef and from moderate (10-30%) to very low (0-5%) at Briggs Reef were extreme. All coral cover decreases were at least in part caused by severe Tropical Cyclone Hamish (category 4) which affected the region 7-8 March 2009 and reportedly produced wind speeds of 100 knots at the Creal Reef weather station before the anemometer broke. Cyclone Hamish entered the reef complex at the northern end of this sector and the eye of the storm passed very close to all four reefs (< 50 kms). Unsurprisingly there was clear evidence of recent damage to corals and other benthic organisms from heavy wave action. The clearest evidence included fragmented and rolled corals, exfoliation of the reef matrix, redistribution of old and new coral rubble, fresh green growth of turf algae over large areas and the presence of new underwater ramparts of coral rubble. Above water, the power of the storm caused new rubble cays (several metres deep) to be deposited on reef flats at Creal Reef and nearby McIntyre Reef. As with even the most severe tropical storms, damage was highly variable around each reef and ranged from catastrophic to barely detectable, depending on the orientation of the habitat to prevailing swells. Reef areas that were protected from wave action often appeared undisturbed and in several cases still retained extensive beds of fragile branching Acropora spp. hard corals.

Image 3 and 4: As is common with even the most severe tropical storms, damage was highly variable around each reef. It could range from catastrophic to undetectable, depending on the orientation of the habitat to prevailing swells. This was true at Reef 21137 in the Pompey sector: the first plate shows total destruction of fragile branching corals on a section of the reef that was exposed to cyclonic waves. The second plate, taken just around the corner from the first photograph, shows almost 100% cover of live fragile corals. Photo: AIMSLTMP
Image 5: Exposed reef areas on Briggs Reef in the Pompey sector were severely damaged by Cyclone Hamish. Here most benthic organisms were physically removed by wave action, leaving bare substratum that was rapidly colonized by green turfing algae. Photo: AIMSLTMP	Image 6: Many areas of Creal Reef in the Pompey sector previously had very high coral cover and so were hit particularly hard by Cyclone Hamish. In many areas coral colonies were reduced to rubble or were swept away leaving bare calcium carbonate. These areas were rapidly colonised by green filamentous algae that formed large clumps. As an indication of the cyclone's ferocity, wind speeds of 100 knots were reported by the Creal Reef weather station before the anemometer broke. Photo: AIMSLTMP
	Image 7: Cyclone Hamish had caused major damage on the exposed reef front and southern flank of Hyde Reef on the outer shelf of the Whitsunday sector. Here, large massive corals had been overturned and rolled across the reef. This Platygyra sp. was still alive but any patches on the colony surface that were damaged (and polyps killed) during the storm had been colonized by green algae. The fate of this coral is uncertain but dislodged massive corals have been known to survive in their new location. Photo: AIMS LTMP

Table 1. Summary of manta tow survey results for four reefs in the Pompey sector.

Five reefs were surveyed using manta tows (Table 2) and nine core reefs (including the five that were manta towed) were surveyed in more detail by SCUBA (Table 3). Manta tows surveys were prevented due to dangerous sea conditions and poor underwater visibility (<6m) at the other four core reefs targeted for survey. During manta tow surveys COTS were only recorded at one reef (Hyde Reef) in very low numbers (n = 3). This represents the first sighting of COTS during manta tows on this reef since surveys began in 1986. Coral cover at the other outer shelf reefs had decreased markedly since they were last surveyed two years ago. Slate Reef previously had high coral cover (30-50%) but this has dropped to moderate levels while coral cover at Hyde Reef has dropped from moderate to low (Table 2). These two reefs were exposed to the brunt of ocean swells produced by Cyclone Hamish that was a category 5 storm at the time and have suffered damage similar to that recorded on reefs in the Pompey sector (see previous section). One difference on these outer shelf reefs was the greater prevalence of a slimy filamentous green alga (to be identified) that formed dense mats on the substratum in many places. This alga appears to quickly colonise any recently exposed calcium carbonate surfaces. Fragments of this alga commonly became dislodged and floated at all levels in the water column. Large reef fragments had also been tossed onto the reef flat adjacent to the exposed front of Rebe Reef. In contrast, the three mid shelf reefs had suffered negligible damage from Cyclone Hamish and had retained moderate coral cover since last surveyed two years ago. The disparity in damage due to Hamish between outer and mid shelf reefs in this sector is presumably because of the path of the cyclone that tracked offshore in the Coral Sea outside the GBR. This meant that major wave forces were dissipated on outer shelf reefs while the mid shelf survey reefs were effectively shielded.

Table 2. Summary of manta tow survey results for five reefs in the Whitsunday sector.

No COTS were recorded during SCUBA searches on the intensive survey sites (Table 3), however the incidence of two coral diseases, "white syndrome" (WS, a disease-like necrosis found particularly on tabulate Acropora spp.) and "brown band" (BrB), were both clearly elevated on a number of reefs that spanned inner and mid shelf locations. The incidence of WS and BrB was the highest yet recorded on five and six reefs respectively. The significance of these increases is uncertain but may reflect a particularly stressful wet season for corals with high sea surface temperatures, major flood events and intense storm activity. Numbers of Drupella spp. (coral feeding snails) and levels of coral bleaching were within the range of previous estimates at each reef.

Table 3. Summary of results of SCUBA searches at nine reefs in the Whitsunday sector.

WS = White Syndrome, BBD = Black Band Disease, BrB = Brown Band Disease, SEB = Skeletal Eroding Band Disease. Figures are the number of starfish (COTS), number of scars (WS, BBD, BrB, SEB) or snails ( Drupella ) recorded at each reef.

References

English, S., Wilkinson, C. and Baker, V. (1997) Survey Manual for Tropical Marine Resources (2ndEdition). Australian Institute of Marine Science. Townsville.

 

This project is partially supported by the Marine and Tropical Sciences Research Facility

Dr. Hugh Sweatman , AIMS

Telephone: +61 7 4753 4470

Fax: +61 7 4753 4288

Email:h.sweatman@aims.gov.au