Monitoring Geoffrey Bay reef flat corals

It is often remarked by long-term residents of Magnetic Island that the local reefs have deteriorated over the years. Concerns about sedimentation and the deterioration of Magnetic Island reef health caused by dredging in Cleveland Bay go way back and are reported by Brown (1972). Brown documents the history of dredging and sediment disposal in Cleveland Bay, and the impacts of suspended sediment from dredging on Townsville beaches, and Middle and Magnetic Island reefs over the period 1961 to 1972. Brown’s systematic and detailed scientific studies over the period led him to conclude that dredge spoil was responsible for the mass mortality of corals in Geoffrey Bay (as well as other Magnetic Island reefs), resulting in 80%-85% mortality of branching and tabulate forms, between 1967 and 1972 while dredge spoil was dumped adjacent to Magnetic Island’s eastern bays. While Brown (1972) reports that by mid 1972 there were some signs of coral regrowth, there is little doubt that the baseline for reef monitoring is likely to have already moved substantially before the first quantitive studies by Bull (1977) and Morrissey (1978, 1980) over the period 1976 to 1978 of the Geoffrey Bay reef .

When challenged by those accused of contributing to the decline, namely the Townsville Port Authority, the response is frequently scepticism and a request to see the empirical evidence. It is a reasonable request but somewhat surprising that after so many years of expressed concern and the scientific study by Brown (1972) that all relevant data have not been compiled and an appropriate, quantitive monitoring programme established  by authorities. There is, without a doubt, a long overdue need to begin more systematic monitoring of Magnetic Island coral reefs in the face of threats from climate change and the proposed expansion of the Townsville Port, particularly given the reefs’ economic importance to local tourism and World Heritage status.

The Geoffrey Bay reef is amongst Magnetic Island’s most diverse coral-algal reef flat communities (see previous blog posts about Geoffrey Bay). Typical of inshore reefs, they are dominated by corals and fleshy, filamentous and coralline algae. The reef flat varies markedly from the eastern to western ends in the width of the reef flat proper and presence of an extensive sedimentary accumulation zone towards the eastern end, lying inshore of the reef flat proper.

RF1 cropped
Looking seawards across the western end of the Geoffrey Bay reef flat. The very tail end of the sedimentary accumulation zone is evident in the foreground. The reef flat proper comprises identifiable inner, middle and outer reef flat coral assemblages.

There are difficulties in quantitatively comparing the early research by Bull and Morrissey of Geoffrey Bay corals with what is currently observed. The study of coral reefs was still in its infancy during the 1970s and efforts, although quantitative, were focused on identifying and describing patterns of zonation. The sampling sites were not accurately located (remember this was before GPS) and there was no replication of sample transects to establish natural variability. In this blog, I limit discussion to a qualitative comparison of the current reef flat ecology to research undertaken in the second half of the 1970s by Bull and Morrissey.

Both Bull and Morrissey lay line transects across the reef flat towards the western end of Geoffrey Bay where the reef flat is widest and most diverse in its coral assemblage. In this area, the reef flat is approximately 120-150m wide. Morrissey (1980), in contrast to Bull (1977), documented the algal component as well as the coral assemblage.

Inner Reef Flat

Currently, the inner reef flat (closest to the shore) at the western end of Geoffrey Bay has relatively low coral cover, few coral species but quite extensive algal and seagrass beds. Montipora ramosa is most common but colonies of Goniastrea aspera, Porites lobata and, possibly, a fourth species (Turbinaria auricularis) are also present. Bull (1977) recorded five species of coral, M. ramosa, P. australiensis (possibly P. lobata but misidentified), Platygyra sinensis, G. aspera and Turbinaria auricularis over the inner reef flat, with M. ramosa being particularly abundant within the transects with a reported coral cover of 5-6%. In contrast, Morrissey (1980) only recorded two species of coral, M. ramosa and P. lobata over the inner reef flat with a reported cover of 1-2% of the transects. This discrepancy is most likely related to the length of the transects, Bull (1977) sampled double that of Morrissey (1980), and the very heterogeneous habitat. Such high coral cover as observed by Bull (1977) is not apparent now but large rubble banks comprised of old M. ramosa colonies are evident.

Macroalgae, turf and coraline algae, and seagrass dominate community structure over the inner reef flat. Morrissey (1980) found that algae and seagrass covered 89% of the inner reef flat transects.

The inner reef flat does not appear to be qualitatively different to that observed by Bull and Morrissey, except possibly for a lower abundance of M. ramosa than noted by Bull (1977).

Middle Reef Flat

The middle reef flat has the appearance of having a more abundant and diverse coral assemblage than the inner reef flat. Bull (1977) identified M. ramosaG. aspera, P. sinensis and Porites sp. (australiensis and lobata) as particularly widespread and relatively abundant across the entire reef flat in Geoffrey Bay. This remains so. M. ramosa, G. aspera and micro-atolls of Porites lobata all become more common across the middle reef flat. Furthermore, Bull (1977) found up to 9 coral species from 7 genera within his middle reef flat transects. These included the five species found across the inner reef flat but also included Symphyllia recta. The other additional species are not specifically identified by name.

Coral cover is evidently greater in the middle reef flat compared to the inner zone, although it is highly variable. Morrissey (1980) recorded relatively high cover (~13%) on only one of 3 middle reef transects, while Bull (1977) recorded a pretty uniform 5-6% coral cover.

What is notable is the presence of isolated colonies of a digitate Acropora sp., while large banks of Acropora sp. can be seen towards the outer reef flat. Colonies of the genus Acropora were not noted by either Bull or Morrissey and have probably grown since the 1998 bleaching event.

Outer Reef Flat

The outer reef flat, much like the middle reef flat, is a very heterogeneous or patchy environment where coral abundance can vary markedly over quite short distances. Currently there are some extensive areas of relatively high coral cover, but not everywhere. Colonies of Acropora sp. and P. lobata are increasingly common, as is encrusting and foliose Montipora sp. within lagoons. G. aspera and P. sinensis were reported to be relatively more common along the outer reef flat transects by Bull (1977) and remain so. Bull (1977) found that species richness was slightly lower on the outer reef flat but that it rapidly increased over the reef crest and down the reef slope.

Interestingly, while 10 species of the genus Acropora were recorded on the reef slope, no reference is made by Bull (1977) to any colonies being present on the reef flat at all. Foliose Montipora sp. were only found on the reef slope also. Both Bull (1977) and Morrissey (1980) found coral cover across the outer reef flat transects to be between 4 and 7%. It is apparent that current coral cover greatly exceeds these percentages in some locations.

 Conclusions

It is not evident that coral diversity and cover is currently lower than that observed by Bull (1977) and Morrissey (1978, 1980) in the mid 1970s. However, given the extreme patchiness or heterogeneity over the reef flat, the paucity of transects and lack of transect replication, comparison is fraught with difficulties. It should also be remembered that channel dredging in Cleveland Bay had already been long implicated in coral reef degradation around Magnetic Island when the research was undertaken in the mid 1970s. That is, the coral reef in Geoffrey Bay was already degraded prior to the surveys by Bull and Morrissey.

Current observations and the reef monitoring by both Bull and Morrissey suggest that an extreme disturbance resulting in mass mortality (e.g. by coral bleaching as occurred in 1998) could be quite easily identified and documented on the basis of current knowledge. However, the impacts of sedimentation from continuing and/or further dredging due to Townsville Port expansion is likely to be less obvious. This would require a concerted effort and a focussed, quantitative monitoring programme that establishes natural variability within the reef flat community. Furthermore, the impacts of continued dredging may be less direct by slowing coral recovery after extreme events such as coral bleaching.

The horse may have already bolded on answering the question whether Magnetic Island reefs are currently deteriorating as a consequence of Townsville Port Authority dredging. The impacts of climate change and associated coral bleaching, though, remain the elephant in the room.

References:

Brown, T.W. (1972) Silt pollution – The destruction of Magnetic Island’s fringing coral reefs. Published by the author, Roseville, NSW.

Bull, G.D. (1977) Comparative analysis of community structure of two fringing reefs of Magnetic Island (North Queensland). Masters (coursework) thesis, James Cook University.

Morrissey, J.I. (1978) Community structure and zonation, with particular reference to macro algae and hermatypic corals, on a fringing reef flat of Magnetic Island (North Queensland). B.Sc. Thesis, James Cook University of North Queensland. 173pp.

Morrissey, J. (1980) Community structure and zonation of macro algae and hermatypic corals on a fringing reef flat of Magnetic Island (Queensland, Australia. Aquatic Botany, 8: 91-139.

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