11 – Terrestrial Runoff to the Great Barrier Reef and the Implications for Its Long Term Ecological Status
111
the crown-of-thorns starfish (Acanthaster planci), a major
coral predator. Acanthaster planci outbreaks have been a
principal cause of coral mortality on the GBR (and
throughout the Indo-Pacific coral province) over the last
40 years. Its planktonic larvae feed on large phytoplankton, and experiments suggest that the successful development of these planktonic larvae is food limited: their
survivorship increases steeply with increasing availability of suitable food. Observations suggest that the location and timing of high nutrient concentrations in the
many Indo-Pacific locations correlate well with the location and times of primary outbreaks of A. planci. In the
GBR, mean summer chlorophyll concentrations in the
central and southern GBR lagoon are about twice as high
as in the far northern part of the GBR, and can be elevated
during large flood events. If experimental findings and
observations also apply to the GBR, it is likely that increased survivorship of A. planci larvae and subsequent
adult population outbreaks may be best explained by increased larval survival facilitated by the terrestrial runoff
of nutrients and sediments.
Sedimentation represents a severe disturbance for
coral reefs. Sediments can smother corals and other
reef organisms. Sedimentation reduces growth and
survival in a wide range of coral species, although responses differ substantially between species and also
between different sediment types. Coral skeletal records
can incorporate trace elements that serve as proxy for
sedimentation rates. These records suggest that the
amount of soil washed off the land has increased about
5-fold since European settlement, and that inshore corals are exposed to (and incorporate) some of this runoff
associated material. Furthermore, sediments that are
enriched with nutrients are more effective than clean
inorganic carbonate sediment in smothering corals, either through the formation of muddy marine snow or
anoxia. Sedimentation also greatly limits recruitment
and the survival of early life stages in corals: settlement
rates are near-zero on sediment-covered surfaces, and
sedimentation tolerance in coral recruits is at least one
order of magnitude lower than for adult corals. Some
of the bioeroding and space-competing groups of organisms are also sensitive to sedimentation by fine silt,
and so are crustose coralline algae, with negative consequences for coral recruitment.
Herbicide residues (especially diuron, atrazine, simazine, ametryn, hexazinone and tebuthiuron) are
commonly being detected in river water in high flow
conditions and in river flood plumes. More sensitive
monitoring tools are now deployed in the GBR Lagoon,
where residues of some pesticides are also being detected, presently in quite low concentrations, during
non-flood (dry season) periods. Herbicide residues
(e.g. diuron) can disrupt photosynthetic activity in marine plants including mangroves, seagrasses, coral
zooxanthellae and crustose coralline algae. In addition,
pesticide residues of various commonly used and detected types (herbicides, insecticides, fungicides) can
directly affect the physiology of coral, especially at the
reproductive, larval and juvenile stages.
The type and severity of response to terrestrial runoff at any particular location depends on whether
changes occurred predominantly in sedimentation,
turbidity, POM, dissolved inorganic nutrients or pesticides. The severity of response also depends on the
physical, hydrodynamic, spatial and biological properties of a location. Reefs that are surrounded by a shallow sea floor, reefs in poorly flushed bays or lagoons,
deeper reef slopes, and frequently disturbed reefs are
likely to experience changes even at low levels of pollution, in particular when populations of herbivores are
low. In contrast, well-flushed shallow reef crests surrounded by deep sea floors or in areas of moderate
tides are likely to have the highest level of resistance
and resilience, especially when inhabited by healthy
populations of herbivores that protect against overgrowth by sediment-trapping macroalgae. In most
places, reduced recruitment success in corals, together
with the promotion of macroalgae and A. planci, arguably represent the most significant direct effect of terrestrial runoff on coral reefs. In severe conditions, the
overall outcome is reduced reef calcification, shallower
photosynthetic compensation points (see Chapter 7),
changed coral community structure, and greatly reduced species richness. Hence, reef ecosystems increasingly simplify with increasing exposure to terrestrial
runoff, compromising their ability to maintain essential
ecosystem functions and to recover from disturbances,
including the presently increasing level of humaninduced disturbances.
111
the crown-of-thorns starfish (Acanthaster planci), a major
coral predator. Acanthaster planci outbreaks have been a
principal cause of coral mortality on the GBR (and
throughout the Indo-Pacific coral province) over the last
40 years. Its planktonic larvae feed on large phytoplankton, and experiments suggest that the successful development of these planktonic larvae is food limited: their
survivorship increases steeply with increasing availability of suitable food. Observations suggest that the location and timing of high nutrient concentrations in the
many Indo-Pacific locations correlate well with the location and times of primary outbreaks of A. planci. In the
GBR, mean summer chlorophyll concentrations in the
central and southern GBR lagoon are about twice as high
as in the far northern part of the GBR, and can be elevated
during large flood events. If experimental findings and
observations also apply to the GBR, it is likely that increased survivorship of A. planci larvae and subsequent
adult population outbreaks may be best explained by increased larval survival facilitated by the terrestrial runoff
of nutrients and sediments.
Sedimentation represents a severe disturbance for
coral reefs. Sediments can smother corals and other
reef organisms. Sedimentation reduces growth and
survival in a wide range of coral species, although responses differ substantially between species and also
between different sediment types. Coral skeletal records
can incorporate trace elements that serve as proxy for
sedimentation rates. These records suggest that the
amount of soil washed off the land has increased about
5-fold since European settlement, and that inshore corals are exposed to (and incorporate) some of this runoff
associated material. Furthermore, sediments that are
enriched with nutrients are more effective than clean
inorganic carbonate sediment in smothering corals, either through the formation of muddy marine snow or
anoxia. Sedimentation also greatly limits recruitment
and the survival of early life stages in corals: settlement
rates are near-zero on sediment-covered surfaces, and
sedimentation tolerance in coral recruits is at least one
order of magnitude lower than for adult corals. Some
of the bioeroding and space-competing groups of organisms are also sensitive to sedimentation by fine silt,
and so are crustose coralline algae, with negative consequences for coral recruitment.
Herbicide residues (especially diuron, atrazine, simazine, ametryn, hexazinone and tebuthiuron) are
commonly being detected in river water in high flow
conditions and in river flood plumes. More sensitive
monitoring tools are now deployed in the GBR Lagoon,
where residues of some pesticides are also being detected, presently in quite low concentrations, during
non-flood (dry season) periods. Herbicide residues
(e.g. diuron) can disrupt photosynthetic activity in marine plants including mangroves, seagrasses, coral
zooxanthellae and crustose coralline algae. In addition,
pesticide residues of various commonly used and detected types (herbicides, insecticides, fungicides) can
directly affect the physiology of coral, especially at the
reproductive, larval and juvenile stages.
The type and severity of response to terrestrial runoff at any particular location depends on whether
changes occurred predominantly in sedimentation,
turbidity, POM, dissolved inorganic nutrients or pesticides. The severity of response also depends on the
physical, hydrodynamic, spatial and biological properties of a location. Reefs that are surrounded by a shallow sea floor, reefs in poorly flushed bays or lagoons,
deeper reef slopes, and frequently disturbed reefs are
likely to experience changes even at low levels of pollution, in particular when populations of herbivores are
low. In contrast, well-flushed shallow reef crests surrounded by deep sea floors or in areas of moderate
tides are likely to have the highest level of resistance
and resilience, especially when inhabited by healthy
populations of herbivores that protect against overgrowth by sediment-trapping macroalgae. In most
places, reduced recruitment success in corals, together
with the promotion of macroalgae and A. planci, arguably represent the most significant direct effect of terrestrial runoff on coral reefs. In severe conditions, the
overall outcome is reduced reef calcification, shallower
photosynthetic compensation points (see Chapter 7),
changed coral community structure, and greatly reduced species richness. Hence, reef ecosystems increasingly simplify with increasing exposure to terrestrial
runoff, compromising their ability to maintain essential
ecosystem functions and to recover from disturbances,
including the presently increasing level of humaninduced disturbances.