Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 741

Studies at Lizard Island, Great Barrier Reef, showed that rates varied across
the reef, with subtidal reef slopes and lagoonal sites experiencing higher rates of
grazing than deeper sites and reef flats, and these differences were maintained in
experimental substrates exposed for 9 years ( Kiene and Hutchings, 1994a,b ).
The density of grazing by scarids in these habitats is suggested to control the
bioerosion rates. Recruitment of boring organisms was highly seasonal and
variations occurred between years ( Hutchings and Murray, 1982 ), with prevail-
ing wind patterns important in successful larval recruitment. Sediment loads in
the water column were also very important for inshore sites, especially those
near river mouths experiencing higher levels of boring than grazing compared
to sites in clear water ( Hutchings et al., 2005; Osorno et al., 2005 ; Fig. 3 ). Again,
the distribution of grazing fish was important and this was related to the amount
of endolithic algae in the surface layers of the substrate. Inshore sites had lower
levels of these algae than offshore sites due to levels of sedimentation on the
surface of the blocks, which reduced light penetration necessary for algal
growth.
Experimental sites subjected to increased nutrients without sediment load,
but with healthy grazing fish populations, also exhibited variations in the levels
of microborers ( Osorno, 2005 ; Table 1 ) and no changes in the levels of grazing.
In contrast, eutrophic sites also subjected to overfishing experienced high levels
of grazing ( Pari et al., 2002 ; Table 1 , Fig. 2H ). This was because the high
levels of nutrients in the water column encouraged dense epilithic algal popula-
tions which facilitated the development of large populations of grazing echi-
noids. Such populations could develop as severe overfishing had occurred
and therefore little or no predation of the juvenile echinoids occurred. Grazing
levels were so high that little boring occurred in the experimental substrates at
this polluted site.
Increasing incidences of bleaching, which may lead to death of coral colo-
nies, are substantially increasing the amount of dead coral substrate available
and can therefore lead to localized increases in bioerosion ( Eakin, 1992 ; see
Table 1 ). Substrates heavily bored may be more susceptible to physical removal
during storm events, the intensity of which is predicted to increase with climatic
warming ( Przeslawski et al., 2008 ). In some cases, though, sponge boring may
allow dead coral colonies to bend with the water movement with the presence of
the sponge enmeshed in the coral skeleton strengthening the colony, making
them less susceptible to being dislodged during storm events ( Chaves-Fonnegra
and Zea, 2007; L ´ pez-Victoria and Zea, 2004 ). Increased ocean acidification is
also likely to influence rates and agents of bioerosion with reduced densities of
coral substrate ( Przeslawski et al., 2008 ).
A variety of factors such as depth, levels of sedimentation, water quality,
bleaching, overfishing, and plagues of Crown of Thorns affect the rates of
bioerosion as well as the ratio of grazing to boring, and often these act syner-
gistically and are difficult to separate. In Table 1 , the rates of grazing and boring
at selected sites subjected to varying environmental conditions are shown,