Accretion versus bioerosion on the Maputaland reefs in South Africa - The major processes.
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Date
2011
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Abstract
The development of coral reefs is largely restricted to areas within the tropics where favourable
conditions for both coral and reef growth prevail. There is, however, a continuum from these typical,
accretive reefs in the tropics to marginal, non-accretive, coral-dominated reef communities which
occur at higher latitudes. High-latitude reefs function similarly in many regards to their tropical
counterparts and are regulated by similar processes to a varying degree. In this study, the major
biological and physico-chemical processes were assessed which directly or indirectly prevent the
continued persistence of reefal frameworks and thus hinder reef accretion on high-latitude reefs in
the iSimangaliso Wetland Park. These reefs have a high diversity of hard and soft corals with
significant reef coverage, yet little evidence of any biogenic accretion has been observed. The
scleractinian coral, Acropora austera, is one of the few corals which may be responsible for reef
framework production. It exhibits a gregarious growth pattern, forming large, monospecific stands
with an interlocking framework characteristic of the early stages of reef accretion. The framebuilding
potential of A. austera and the continued persistence of such frameworks were thus determined by
in situ monitoring of coral growth, mortality, bioerosion and several physico-chemical parameters.
Growth rate and mortality of A. austera branches were measured at three sites of differing stand
size and apparent age. This was achieved by repeated image analysis and by staining branches with
the vital stain, Alizarin Red S. Both measures of growth yielded a similar linear extension rate of 24.5
mm/yr (n = 467), comparable to related species at similar latitudes. Mean branch mortality was as
high as 50%, with clear differences manifested between each A. austera stand. Branch extension
rates and branch mortality were inversely related between sites. Small, young stands exhibited
significantly faster coral growth rates, lower mortality and a net increase in overall branch length
over the study period, whilst the opposite was true of larger, more developed stands. In addition,
bioerosion was determined at each site to assess its potential for carbonate removal and its
destabilizing effect on reef frameworks. Bioerosion intensity was recorded as “percentage area
damage” within cross-sections and “frequency of occurrence” of bioeroding organisms in coral
rubble fragments (n = 120). The level of bioerosion was found to be substantial (up to 11.5% loss in
weight of coral fragments over the 12-month study period) and was found to decrease significantly
with a reduction in size of each A. austera stand.
Aragonite saturation state is considered a major factor that limits the geographical range of coral
reefs globally. Although previously thought to be limiting in Maputaland, mean ΩArag values of
4.40±0.29 were measured on the reefs in summer and 4.33±0.21 in winter and thus would not have
limited reef development. Past studies have noted the turbulence on South African east coast reefs
and its adverse effect on reef development. This was corroborated in this study with the
measurement of considerable sediment re-suspension (0.17 g cm⁻² day⁻¹) and regular damage to both
living coral and the reef framework caused by large swells.
These results lead to the theory that Acropora austera stands senesce with increasing size and age.
Although large coral frameworks are found on the Maputaland reefs, they do not persist in the long
term. High rates of sediment re-suspension prevent infilling of the interstitial spaces and eventual
cementation, while high levels of bioerosion lead to framework instability over time. Rough seas
further hamper accretion by physical removal of both living coral and the coral-derived framework,
thus removing recent growth. This process is suspected to cause an imbalance in the carbonate
budget of these marginal reefs, ultimately favoring carbonate removal over carbonate deposition.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2011.
Keywords
Corals--KwaZulu-Natal--Maputaland., Theses--Marine biology., Coral reefs and islands--KwaZulu-Natal--Maputaland.