Environmental Engineering Reference
In-Depth Information
Rates of biotic accretion & erosion
Accretion (+)
Macroboring (-)
Grazing (-)
1kg -1 m -2
in 4 a
Composition of macroborers
100%
Sponge
50%
Worm
Bivalve
0%
Inshore
Mid-shelf
Outer barrier
Oceanic
Reef location
Snap.
Low
Liz.
Rib.
Har.
Osp.
Turbidity
High
Low
FIGURE 3 Biotic accretion and degradation along an offshore transect of the Great Barrier Reef.
Experiments using dead coral substrate exposed over a 4-year period demonstrate the rates of car-
bonate accretion by algae, and carbonate erosion by grazing and macroboring fauna. Offshore sedi-
ment gradient and increasing water clarity extend from the coast at Snapper Island (Snap.) to Osprey
Reef (Osp.), Coral Sea. See Osorno et al. (2005) for full results and map of reef localities . Abbre-
viations: Low, Low Isle; Liz., Lizard Island; Rib., Ribbon Reef No. 3; Har., Harrier Reef.
which illustrate that these rates vary considerably, and while rates increase on
anthropogenically impacted sites, these are not uniform.
An example of reef recovery from grazing, for example, Crown of Thorns
plague, can be seen in areas of the Great Barrier Reef. After the plague has
moved on, new coral recruits coming from adjacent reefs can settle on the newly
created space and—provided that the water quality is good with low sediment
loads and healthy fish populations present—within 5-10 years these reefs can
make a full recovery ( Emslie et al., 2008 ). However, if other factors are working
against this, such as poor water quality and high sediment loads, recovery will
be slow and/or incomplete.
5. CONCLUSIONS
The process of bioerosion is inextricably linked with Phanerozoic reef ecosys-
tems. The intensity and diversity of bioeroding agents has generally increased
over time. Dominance among macroboring assemblages has shifted from worm
borings (Cambrian-Triassic) to bivalve borings (Jurassic-Eocene) to sponge
borings (Oligocene-Recent). Boring has evolved several times within each
of the major groups of borers and may have produced similar-looking trace
fossils.
The lack of substrate specificity among most bioeroders, and their eurytopic
synecology, may be significant contributors to their relative prosperity and
behavioral conservatism through times of biotic crisis, at the same time as their
reef coralline substrates undergo significant taxonomic turnover. By contrast,
Search WWH ::




Custom Search