The natural context of changes in the Great Barrier Reef - The Great Barrier Reef: An Environmental History - page 36

juvenile stage to the mature and senile stages occurs under conditions of eustatic
and isostatic stability: periods when tectonic movements of the continental crust
are limited and sea level is relatively constant. During those periods of stability,
coral reefs grow upwards to reach sea level and subsequently develop horizontally.
During the mature phase of reef development, lagoons are formed; in the senile
stage, the growth of live coral is restricted to the edge of the reef, while sediment
infill occurs on the reef flat. Therefore, this model suggests that, in periods of
tectonic and sea level stability, the deterioration of coral reefs occurs naturally
as reefs progress through the juvenile and mature stages to reach a condition of
senility.
Hopley (1994, p319) acknowledged that the Great Barrier Reef has experienced
tectonic and eustatic stability since around 6,000 years ago, when many of its
coral reefs commenced the transition to maturity and senility. Therefore, he
suggested, many reefs of the Great Barrier Reef have declined from a juvenile
state, in which rapid vertical coral growth took place, and instead have become
characterised by sediment-covered reef flats, extensive patches of dead coral and
comparatively small margins of live coral growth. Using this model, the coral
reefs of the Great Barrier Reef can be divided into four main groups:
(a) fringing and nearshore reefs that have been severely impacted by
sedimentation, displaying high mortality and limited recovery;
(b) fringing and nearshore reefs that have been significantly impacted by
sedimentation, but that display ecological change, spatial patchiness in
mortality, and some capacity to recover;
(c) mid-shelf reefs that have experienced terrestrial impacts, resulting in
increased vulnerability to bioerosion and displaying increased rates of coral
rubble formation; and
(d) offshore reefs which may have been affected by terrestrial influences, but for
which degradation is only detectable using geochemical analysis techniques.
This framework is illustrated in Table 3.1, together with some examples of coral
reefs that display those characteristics.
The significance of this morphogenic approach lies in the possibility that
many reefs - particularly some fringing and nearshore reefs - may now be
characterised by extreme vulnerability to other impacts; they may exist naturally
close to ecological thresholds beyond which recovery from further degradation is
very difficult, if not impossible. Therefore, for geomorphological reasons, along
the Queensland coast - and particularly in the nearshore zone - the thresholds
that determine whether or not coral reefs can recover from stresses are now
likely to be relatively easy to cross. As a consequence, historical human impacts
could easily have exceeded critical ecological thresholds, especially on some
vulnerable, nearshore reefs. For those reefs, the effects of even comparatively
slight historical human activities may have caused ecological phase shifts.
Furthermore, once coral growth has been inhibited, for whatever reason, it may