IMPACT ASSESSMENT OF HERON ISLAND
BUND WALL ON CORAL COMMUNITIES

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Ray Berkelmans¹, Jamie Oliver¹, Grahame Byron² and John Olds^2
1 Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville Qld 4810
² Queensland Department of Environment, PO Box 3130, Rockhampton Shopping Fair, Rockhampton Qld 4701

When the reef rim of Heron Island's reef was breached in 1945 to allow boat access to the resort, a substantial change occurred to the hydrodynamics of the reef-flat. Water drained from the reef-flat during low tide; tidal currents formed through the hole in the reef rim carrying sediment from the reef-flat and cay; the sand spit on the north-west side of the cay moved to the south-west side and was almost completely eroded and the overall minimum water on the reef-flat appeared to drop significantly. A study examining the changes to the island's reef since 1989 has illustrated positive impacts on the reef's biota resulting from the re-design and re-building of the harbour's bund wall in 1993.

Photo 1. Heron Island harbour channel in 1979, prior to the construction of bund walls. Large erosion scars are evident as a result of water draining through the channel at low tide.

In 1966 a one-metre high rubble wall was built along either side of the harbour channel in an attempt to reduce erosion. The rubble wall is believed to have caused an increase in the low tide level on the reef-flat. The walls were breached by cyclone Emily in April 1972. In the following 20 years continued attempts to remedy the effects of the breached rim failed. A series of cyclones exacerbated the problems and caused extensive sand loss from the island. Channel dredging and bund wall reconstruction were repeatedly undertaken with limited success.

In 1993-94, however, the bund wall was engineered properly for the first time and built as an interlocking series of concrete blocks, set on a concrete base to a height of 0.95 m above MLWS (mean low water springs). As a result of the construction of this new bund wall the minimum water level on the reef-flat was raised between 8 cm and 14 cm at sites close to the channel (Gourlay and Jell 1996) but no effect was detected more than 400 m from the harbour (Hacker and Gourlay, in press).

to view this map larger, open the image directly Figure 1. Heron Island and reef showing coral monitoring sites

Figure 2. Total hard coral cover at the outer reef-flat sites near the reef crest. Error bars indicate +/- SE

On the inner reef-flat, adjacent to the cay, positive impacts of the bund wall are also evident, however these appear more variable between sites. A large increase at site A1 was detected (from 1.3 ± 0.7% in 1993 to 8.0 ± 1.4% in 1995), however, contrary to expectations, no significant change in percent coral cover occurred at site N1 north of the harbour suggesting that other processes limiting coral growth may be acting in this area. On the lower reef-slope a substantial increase in coral cover was detected across all sites and averaged 7.4% between 1993 and 1994 and 8.3% between 1994 and 1995. However, the similar pattern of increase across all sites from site H4 north of the harbour to H3, more than 1 km south of the harbour suggests that this growth cannot be attributed to the harbour bund wall development, but rather is likely the result of the reef recovering from the degradation caused by cyclone Fran in March 1992.

Photo 2. Large Montipora sp. colony on the north side of the harbour. The outer margin of the colony has reached the new growth limit while the inner portion of the colony is still undergoing active vertical growth.

The positive effect of the bund wall is also evident from the growth morphology of large Montipora and Acropora colonies, especially north of the harbour. In many colonies, the margin around the colonies are 10-15 cm higher than the centre of the colonies (photo 2). These observations are consistent with a rise in the minimum low tide level of 14 cm north of the harbour which has been observed by Gourlay and Jell (1996) following the harbour wall development in 1994. The area north of the harbour is characterised by extensive stands of staghorn Acropora colonies and until 1994, these were interspersed with large open sandy areas. The open sandy areas are now closing as the margins of the staghorn colonies undergo rapid horizontal growth. This growth may be accelerated by the increase in the vertical growth limit of these corals.

Photo 3. Corals on the reef-flat south of the harbour are mainly composed of slower growing massive species such as Porites. Many colonies are showing signs of increased vertical growth, evident by a thick lip of new growth around the perimeter of the colonies. Photo taken at site A1 in November 1995. Note that this is probably the second vertical growth release, with the white space in the middle of the microatoll being the original vertical growth limit of the old colony.

South of the harbour, coral communities are dominated by slower growing Porites microatolls. These microatolls also show signs of vertical growth, which is evident by a thick lip of new growth around the perimeter of the colonies adjacent to their characteristically flat surface of dead skeleton. These flat surfaces most likely represent the vertical growth limit of the colonies prior to the 1994 bund wall reconstruction (photo 3). The height of this new growth lip of around 20 mm (as at November 1996) is consistent with growth rates reported in the literature for this species of around 9 mm per year (Veron 1993). A further interesting feature of the Porites microatolls immediately south of the harbour is that they also show a second platform in the centre of the colonies which is around 100 mm lower than the outer platform (photo 3).

Photo 4. There are no raised margins on this Porites microatoll in the vicinity of the present harbour back in 1950 (photo courtesy of Isobel Bennett). See text for details.

It is possible that these lower platforms in the middle of the microatolls represent the growth limit prior to 1987, when the height of the minimum low tide level inside the lagoon was raised (M. Prekker, pers. comm.). Fisk (1991) also attributes an increase in coral cover he observed between 1989 and 1990 to a raising of the minimum low tide levels. Adding further credence to this interpretation is a photo by Isobel Bennett, taken in 1950 in the vicinity of the current harbour, which shows no raised growth margins around a large Porites microatoll (photo 4). The lack of these raised growth margins on Porites microatolls away from the harbour is consistent with the results of tide monitoring since September 1993 which indicate that the effects of the increase in minimum low tide levels is reduced to almost zero approximately 400 m away from the harbour (Hacker and Gourlay, in press).

Figure 3. Mean number of corals in a 50 cm belt transect at each site on the inner reef-flat between 1989 and 1996

A positive effect of the 1993 bund wall development may also be evident in the number of coral colonies on the inner reef-flat south of the harbour. There was a steady decline in the number of colonies between 1989 and 1994 across all sites (figure 3). The trend levelled out in 1995 and 1996 at sites B1, C1 and D1, but was reversed at site A1 in these last two years. This is indicative of a possible positive effect of the harbour bund wall on the number of coral colonies, mainly through coral recruitment and growth of fragments of branching corals in the 0-25 cm² size class (figure 4).

Figure 4. Size frequency distribution of corals in a 50 cm belt transect at site A1

Overall, coral communities on the Heron Island reef-flat appear to have responded positively to increased minimum tide levels brought about by the re-construction of bund walls along the harbour channel. Photographs of the sites show increased vertical growth, and both coral cover and numbers of colonies have increased close to the harbour due to the raised lagoonal tidal level. Such results have provided valuable information on effects of bund wall construction near degraded sites and an insight into the ability and processes of reefs to recover from human impacts.

References Christie, C.A., Bass, D.K., Neale, S.J., Osborne, K. and Oxley, W.G. 1996, Surveys of sessile benthic communities using the video transect technique, Standard Operational Procedure No. 2, Australian Institute of Marine Science, Townsville. Fisk, D.A. 1991, Final report on the resurvey of Heron Island reef flat monitoring stations - April 1990, Report to the Great Barrier Reef Marine Park Authority, 27 pp. Flood, P.G. 1984, Changes in the shoreline position of six coral cays, Capricornia Section, Great Barrier Reef Marine Park: A record to 1984, University of New England, Department of Geology and Geophysics, 67 pp. Gourlay, M.R. and Jell, J.S. 1996, Impact of reconstruction of Heron Island boat harbour bund walls upon reef-top hydraulic and sediment transport processes, Draft Report to the Great Barrier Reef Marine Park Authority, 39 pp. Hacker, J.L.F. and Gourlay, M.R. in press, Harbour bund wall construction at Heron Island: coral response, in The Great Barrier Reef: Science, Use and Management, Townsville November 1996, Proceedings Volume 2. Veron, J.E.N. 1993, Corals of Australia and the Indo-Pacific, University of Hawaii Press, Honolulu, 644 pp.

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