5. Chapter 5 –Discussion
5.5. Conclusion
The results of this study suggest that there are differing stages of development in each monospecific Acropora austera stand. This would be comparable with unpublished results of Montoya-Maya (pers. comm.) who is showing self-seeding in these coral stands. Of the three sites sampled, there were clear differences in coral growth rates, levels of mortality, framework depth (here assumed to be analogous to complexity) and net change in branch lengths over the study period. However, further research over a longer timeframe would elucidate such trends and provide further insight into the dynamics of these monospecific coral stands and their potential for reef accretion.
I hypothesize that these monospecific stands of Acropora austera are regulated by high summer mortalities and, in particular, mass mortality events arising from wave damage and facilitated by high levels of bioerosion. The results showed high growth rates and low mortality in young stands with little to no underlying framework and substantially higher mortality at sites that seemed to be older, with a deep, highly open framework. Mortality in the latter instance seemed to result predominantly from the collapse or removal of underlying framework structure. In addition, (presumably) younger A. austera stands (Sites 2 and 3) manifested a positive overall change in branch extension over time, as opposed to an older stand (Site 1) which yielded an effectively zero net percentage change at the end of the study period. This leads to the assumption that there is a maximum size for such monospecific coral stands where net growth is outweighed by loss through mortality due to collapse of the underlying framework structure before sufficient reef consolidation can occur. Under environmental circumstances in the Caribbean similar to those in South Africa, coral fragments broken by disturbance events quickly become incorporated into the reef structure by the binding effect of coralline algae and contribute significantly to reef accretion (Blanchon et al.
1997; Perry 1999). No such evidence for the permanent reincorporation of broken coral fragments into the reef structure was observed at Sodwana Bay. Collapsed areas in the coral stands either tended to erode further and expand, or were completely washed away during large storms. A similar relationship between disturbance by wave stress and reef development was seen in both Hawaii (Grigg 1998) and Lord Howe Island where evidence of reef accretion was observed only on the protected, leeward side of the island (Harriott 1999).
Although future predictions of elevated SSTs may provide a competitive advantage for a subset of corals due to increased growth rates and the opportunity expand their range, under temperature extremes (both above and below normal) corals may expel their symbiotic zooxanthellae and succumb to thermal-bleaching (Muller-Parker & D’Elia 1997; Lough 2000). Such a response by corals is often aggravated by high irradiance (Gleason &Wellington, 1993). Future predictions in SST from general circulation models (GCM) coupled with thermal limits for corals derived from various sources (summary in Crabbe 2008) indicate the frequency of bleaching events will rise rapidly. Such events are predicted to become annual in all oceans by 2040, and sooner in areas such as the Caribbean and Southeast Asia (Crabbe 2008). Increasing SSTs have been documented at Sodwana Bay over the past decade but they do not reach hazardous levels for any protracted period of time (Schleyer & Celliers 2003). Few instances of coral bleaching have been observed on these reefs, and their severity has been far less than that in the tropics and unlikely to pose a real threat at this latitude (Schleyer & Celliers 2003).
Future predictions on the reduction of aragonite saturation state in ocean waters indicate a gradual loss of carbonate accretion through reduced cementation and reef structure stabilization, and possibly slower coral growth rates (Kleypas et al. 2001; Guinotte et al. 2003; Schleyer & Celliers 2003). It has been suggested that this will result in fewer net accretionary reefs and community shifts towards more non-frame-building corals (Kleypas et al. 2001; Guinotte et al. 2003), leading them to resemble high-latitude, marginal reef systems such as those found off the Maputaland coastline of South Africa. However, aragonite saturation states were found to be non-limiting at this locality and yet these reefs fail to accrete.
This failure to accrete seems to result from a combination of high summer mortality and low winter growth rates which, when combined, limit framework development. Framework persistence is further limited by the export of CaCO3 material through bioerosion, breakage and sediment resuspension. The results presented here suggest that seasonality is the primary cause that limits reef growth, and framework destructive processes are the primary factor that limit framework
persistence. These processes regulate reef development, rather than any single environmental variable placing an absolute limit on accretion on the Maputaland reefs of South Africa.
The growth pattern of Acropora austera, one of the important reef-building corals on the Sodwana reefs, thus generates an essential framework necessary for reef growth. Yet, without sediment infilling and subsequent cementation by crustose coralline algae, these coral-derived frameworks do not remain stable and their large scale removal ensues. These findings suggest avenues for future research such as growth rates and the cementation potential of crustose coralline algae, further investigations on rates of bioerosion, and more refined measurement of sedimentation and sediment infilling.