Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 740

Fig. 2G ). During these experiments, which were conducted over several years,
microborers were also investigated as their importance was increasingly recog-
nized by how they alter substrate and facilitate recruitment of macroborers and
encourage grazing ( Chazottes et al., 2002; Tribollet et al., 2002 ).
All these experiments, by default, were also measuring rates of loss by
chemical and physical erosion. Most of the experimental blocks were laid in
relatively quiet waters to mitigate chemical and physical erosion, although
some at Lizard Island, Great Barrier Reef, were laid on the reef front subjected
to South East trade winds ( Kiene and Hutchings, 1994a,b ).
Grazing rates have been determined for echinoids in the Galapagos ( Glynn,
1988; Reaka-Kudla et al., 1996 ), in French Polynesia ( Peyrot-Clausade et al.,
2000 ) and in the Indian Ocean ( Peyrot-Clausade et al., 2000 ); chitons on the
Great Barrier Reef ( Barbosa et al., 2008 ), and scarid fish on the Great Barrier
Reef ( Bellwood and Choat, 1990 ), in the Indian Ocean ( Peyrot-Clausade et al.,
2000 ), and in the Caribbean ( Rotjan and Lewis, 2006 ). Attempting to compare
these figures is difficult as different methods have been used, but a substantial
loss of reef framework can occur especially from echinoids (see Table 1 , espe-
cially in the Galapagos and Faaa in Tahiti; Fig. 2H ).
Most of the quantitative studies discussed here are limited by their short
duration (

5 years) and use of a single type of coral, Porites , as the substrate.
Rates are known to vary according to the type and density of coral skeleton
( Hutchings, 1986 ). These studies reveal considerable variations within and
between reefs, and over time. Boring rates do not continue to increase over time
but rather slow down over time as boring communities mature. This is partic-
ularly true for sponges ( R¨ tzler, 1975; Sch¨nberg, 2008 ). The agents of boring
change over time with a distinct succession of borers, from short-lived poly-
chaete species to long-lived polychaete species, followed by sponges, sipuncu-
lans, and bivalves after a delay of 2-3 years. Therefore, caution is warranted
before extrapolating these quantitative results through time and by facies.
In spite of these limitations, all quantitative studies show that bioerosion is a
major process occurring on modern reefs and that the structure and form of reefs
is the result of interactions between reef growth and reef destruction. Just as past
climatic and evolutionary episodes have reshaped reef ecosystems over the eras,
modern climate change and anthropogenic factors are contributing to the
balance between productivity and destruction in reef carbonate budgets.

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4. ENVIRONMENTAL CONTROLS ON BIOEROSION
What it means to be a “healthy” reef is challenging to quantify and probably will
remain a subjective concept; however, the carbonate budget of a reef system can
capture the dynamics of reef expansion, stability, or contraction. Studies on
bioerosion from anthropogenically affected reefs versus more pristine reefs
allow identification of environmental factors that control

the bioerosion

process.