The Great Barrier Reef
88
techniques, including algal transplantation (e.g. from areas of low to high herbivore density), herbivore enclosure and exclusion experiments, herbivore and algae
removals, and so-called ‘natural experiments’ such as the
die-off of the Caribbean herbivorous sea urchin, Diadema
antillarum. All of these approaches have their strengths
and weaknesses. It is clear that both top-down and bottom-up effects are important, and they are difficult to
separate since overfishing of herbivorous fishes and declining water quality often go hand in hand. No-Take Areas (NTAs), where fishing is prohibited, are an increasingly common management tool for sustaining targeted
species and maintaining their ecological roles (Box 9.2).
Runoff from land
Runoff from land causes elevated nutrient loads and increased turbidity from suspended sediments (see Chapter 11). Excessive levels of sedimentation are caused by
activities such as soil erosion from agriculture, dredging,
and drilling for oil and gas. The most widespread of
these is soil erosion, due to changes in land use practices,
into rivers that flow onto coastal reefs. Throughout the
tropics, there has been widespread deforestation and
land clearing for agriculture, aquaculture and urbanisation. Turbidity influences the physiology, growth and
survival of corals in several ways (see also Chapter 7).
First, corals are forced to expend energy cleaning themselves of sediment and repairing damaged tissues. Second, the amount of light reaching a coral colony is
reduced by increased turbidity, slowing their growth.
Third, turbid waters tend to be enriched in organic matter and nutrients, which can boost the energy intake of
some hardy corals. However, in general, too much sediment is bad for reefs. It is especially damaging to juvenile corals that are easily smothered by silt (Fig. 9.2).
Turbidity and sedimentation on the GBR, particularly
inshore, are high compared to many reefs elsewhere.
Muddy sediment from land has built up along the inner
part of the shallow continental shelf off Queensland since
sea level stabilised close to its present level about 6500
years ago (Chapter 3), and it is easily resuspended in
windy weather. Visibility (that depends on the amount of
BOX 9.2 NO-TAKE AREAS
No-Take Areas (NTAs) are an important tool for protecting fish stocks and for maintaining the critical ecological functions of fishes. No-Take Areas also provide a refuge from
harvesting and destructive fishing practices (e.g. damage from poisons, dynamite and
fishing gear), and of course they allow targeted species to grow older and larger. These
big individuals typically have disproportionately higher reproductive success, and their
offspring spill over into adjoining areas that have lower levels of protection, influencing
the seascape as a whole. Many tourist operations are located at NTAs, so that snorkelers
and divers can view larger fish.
Once new NTAs are established, the recovery of severely depleted fish stocks continues for many years. In the Philippines, a long term study revealed that the biomass of
predatory fishes increased exponentially (by 17-fold), and showed no sign of slowing
after 18 years of continuous protection. On the GBR, researchers are also working on
new NTAs (green zones) established in 2004 (see Chapter 12) where the size and number
of coral trout and other targeted species is already increasing. NTAs are seen as important tools for managing the resilience of reefs, that is, their capacity to cope with natural
disturbances and human impacts (Box 9.3). Grazing by herbivorous fish that are protected within NTAs can facilitate recruitment by corals after a disturbance, by preventing blooms of seaweed.
88
techniques, including algal transplantation (e.g. from areas of low to high herbivore density), herbivore enclosure and exclusion experiments, herbivore and algae
removals, and so-called ‘natural experiments’ such as the
die-off of the Caribbean herbivorous sea urchin, Diadema
antillarum. All of these approaches have their strengths
and weaknesses. It is clear that both top-down and bottom-up effects are important, and they are difficult to
separate since overfishing of herbivorous fishes and declining water quality often go hand in hand. No-Take Areas (NTAs), where fishing is prohibited, are an increasingly common management tool for sustaining targeted
species and maintaining their ecological roles (Box 9.2).
Runoff from land
Runoff from land causes elevated nutrient loads and increased turbidity from suspended sediments (see Chapter 11). Excessive levels of sedimentation are caused by
activities such as soil erosion from agriculture, dredging,
and drilling for oil and gas. The most widespread of
these is soil erosion, due to changes in land use practices,
into rivers that flow onto coastal reefs. Throughout the
tropics, there has been widespread deforestation and
land clearing for agriculture, aquaculture and urbanisation. Turbidity influences the physiology, growth and
survival of corals in several ways (see also Chapter 7).
First, corals are forced to expend energy cleaning themselves of sediment and repairing damaged tissues. Second, the amount of light reaching a coral colony is
reduced by increased turbidity, slowing their growth.
Third, turbid waters tend to be enriched in organic matter and nutrients, which can boost the energy intake of
some hardy corals. However, in general, too much sediment is bad for reefs. It is especially damaging to juvenile corals that are easily smothered by silt (Fig. 9.2).
Turbidity and sedimentation on the GBR, particularly
inshore, are high compared to many reefs elsewhere.
Muddy sediment from land has built up along the inner
part of the shallow continental shelf off Queensland since
sea level stabilised close to its present level about 6500
years ago (Chapter 3), and it is easily resuspended in
windy weather. Visibility (that depends on the amount of
BOX 9.2 NO-TAKE AREAS
No-Take Areas (NTAs) are an important tool for protecting fish stocks and for maintaining the critical ecological functions of fishes. No-Take Areas also provide a refuge from
harvesting and destructive fishing practices (e.g. damage from poisons, dynamite and
fishing gear), and of course they allow targeted species to grow older and larger. These
big individuals typically have disproportionately higher reproductive success, and their
offspring spill over into adjoining areas that have lower levels of protection, influencing
the seascape as a whole. Many tourist operations are located at NTAs, so that snorkelers
and divers can view larger fish.
Once new NTAs are established, the recovery of severely depleted fish stocks continues for many years. In the Philippines, a long term study revealed that the biomass of
predatory fishes increased exponentially (by 17-fold), and showed no sign of slowing
after 18 years of continuous protection. On the GBR, researchers are also working on
new NTAs (green zones) established in 2004 (see Chapter 12) where the size and number
of coral trout and other targeted species is already increasing. NTAs are seen as important tools for managing the resilience of reefs, that is, their capacity to cope with natural
disturbances and human impacts (Box 9.3). Grazing by herbivorous fish that are protected within NTAs can facilitate recruitment by corals after a disturbance, by preventing blooms of seaweed.