Effects of decreasing light availability due to
turbidity
Benthic irradiance is an important factor determining the
ability of organisms to conduct photosynthesis. At
a given water depth, it is strongly governed by sediment
resuspension, phytoplankton productivity, and other forms
of particulate and dissolved organic matter. Sediments
with large grain sizes settle near the source (e.g., in river
deltas), but the smallest sediment fractions (clay and silt
particles) remain suspended for days to weeks, during
which times they may be transported over tens to hundreds
of kilometers. Compared with larger grain sizes, these
small particles absorb more light (Moody et al., 1987)
and carry more nutrients and pesticides (Gibbs et al.,
1971). Historic data on water clarity in marine systems
are, however, sparse, and the conditions leading to longterm changes in water clarity in tropical coastal systems
are poorly understood. Exceptions are records of reduced
water clarity around a sewage outfall site in Kanehoe
Bay, Hawaii (Hunter and Evans, 1995), and around floating fish farms in the Northern Red Sea (Loya et al., 2004).
Benthic irradiance is an essential environmental factor
for corals, seagrasses, and algae. Corals can acclimatize
to changing irradiance by altering their pigment density.
As photoacclimation takes about 5–10 days, it is too slow
to compensate for energy losses at fluctuating turbidity
(Anthony and Hoegh-Guldberg, 2003). Shading from turbidity, therefore, leads to slower coral calcification and
thinner tissues. Coastal coral reefs in turbid water, therefore, tend to be restricted to the upper 4–10 m compared
to >40-m depths in clear oceanic waters (Yentsch et al.,
2002). Reef development tends to cease at around 4–8%
mean surface irradiance (Yentsch et al., 2002; Cooper
et al., 2007). Coral cover and species richness decline with
increasing depth at high turbidity, with the most
phototrophic species being the first to disappear. Importantly, macroalgae are also missing at deeper depths
despite the high availability of nutrients, as they are
quickly light limited (Hillebrand, 2005). On these dark
reef slopes, filter-feeder communities may be prolific. In
turbid but shallow water, both light and nutrient levels
are high and coral cover and coral growth rates may be
very high – but only if herbivores are effective in keeping
macroalgae in bay and if hydrodynamics prevent the accumulation of sediments. If herbivores are missing, the slowgrowing corals tend to be outcompeted by fast-growing
macroalgae.
Fishes play an essential role in the ecology of coral
reefs, with low abundances of herbivorous fish potentially
leading to a proliferation of macroalgae and declining
coral cover (Hawkins and Roberts, 2004). Importantly,
the abundance of herbivorous fish on coral reefs may
potentially decline not only from overfishing, but also
from nutrient pollution. Wolanski et al. (2004) reported
a negative relationship between water clarity and the
abundance of herbivores on the Great Barrier Reef, and
a similar result was found along a water-quality gradient
in the Caribbean (Mallela et al., 2007). On the Great Barrier Reef, herbivorous fish are not usually targeted by fishers suggesting minimal fishing pressure, yet macroalgae
cover extensive areas on many inshore reefs (De’ath and
Fabricius, 2010), suggesting that control by herbivores is
ineffective. The mechanisms for the apparent negative
relationship between abundances of herbivorous fishes
and poor water quality is still poorly understood; they
may include an avoidance of turbidity (Wolanski et al.,
2004) or a reduced food palatability, if algae trap large
amounts of sediments (Mallela et al., 2007). If such negative relationship was more widespread than presently realized, then water quality would not only directly but also
indirectly promote macroalgae, by releasing both nutrient
limitation and grazing pressure. A systematic investigation of the potential relationships between water quality
and the abundance herbivorous fishes is urgently needed.
Effects of altered sediment properties
Coastal nutrient pollution is often linked to altered sediment properties or sedimentation regimes. Coarse sediment grains usually settle within a few kilometers of the
discharge source, often contributing to forming river
deltas. However, fine particles including detritus, clay,
and silt particles may remain suspended for prolonged
periods of time. These fine particles carry a large proportion of the total river load of nutrients and pesticides,
which eventually settle onto the seafloor and onto the benthic organisms, sometimes tens to hundreds of kilometers
away from the source.
Previous studies have mostly focused on assessing the
effects of increased rates of sedimentation on coral reefs.
Sedimentation rate is a strong environmental driver for
coral reefs. It is known to reduce coral recruitment rates
and coral biodiversity, and many sensitive species are
missing or underrepresented in areas of high sedimentation (Rogers, 1990; Fabricius, 2005). Photophysiological
stress in corals increases linearly as the product of the
duration and amount of sediment exposure, i.e., two units
of sediment deposited on the coral for one time unit, exerts
a similar decline in photosynthetic yields as half the
amount deposited for twice the time (Philipp and
Fabricius, 2003). High sedimentation rates (up to
100 mg dry weight cm
À2
) can kill exposed coral tissue
within a few days. Exposure to a few days of sedimentation can therefore cause long-term damage to coral
populations, by removing whole cohorts of small and sensitive corals. Previous studies have suggested
a sedimentation threshold of 10 mg cm
À2 day
À1
, as reefs
may be severely damaged at higher rates (Rogers, 1990).
Eutrophication may, however, not always increase the
rate of sedimentation, but may alter sediment properties.
Experiments have shown that when coral tissue is exposed
to sedimentation, the degree of damage increases with
bacterial activity (Hodgson, 1990) and with decreasing
grain sediment sizes (Weber et al., 2006). Nutrient-rich
sediments and sediments with high organic contents cause
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NUTRIENT POLLUTION/EUTROPHICATION
turbidity
Benthic irradiance is an important factor determining the
ability of organisms to conduct photosynthesis. At
a given water depth, it is strongly governed by sediment
resuspension, phytoplankton productivity, and other forms
of particulate and dissolved organic matter. Sediments
with large grain sizes settle near the source (e.g., in river
deltas), but the smallest sediment fractions (clay and silt
particles) remain suspended for days to weeks, during
which times they may be transported over tens to hundreds
of kilometers. Compared with larger grain sizes, these
small particles absorb more light (Moody et al., 1987)
and carry more nutrients and pesticides (Gibbs et al.,
1971). Historic data on water clarity in marine systems
are, however, sparse, and the conditions leading to longterm changes in water clarity in tropical coastal systems
are poorly understood. Exceptions are records of reduced
water clarity around a sewage outfall site in Kanehoe
Bay, Hawaii (Hunter and Evans, 1995), and around floating fish farms in the Northern Red Sea (Loya et al., 2004).
Benthic irradiance is an essential environmental factor
for corals, seagrasses, and algae. Corals can acclimatize
to changing irradiance by altering their pigment density.
As photoacclimation takes about 5–10 days, it is too slow
to compensate for energy losses at fluctuating turbidity
(Anthony and Hoegh-Guldberg, 2003). Shading from turbidity, therefore, leads to slower coral calcification and
thinner tissues. Coastal coral reefs in turbid water, therefore, tend to be restricted to the upper 4–10 m compared
to >40-m depths in clear oceanic waters (Yentsch et al.,
2002). Reef development tends to cease at around 4–8%
mean surface irradiance (Yentsch et al., 2002; Cooper
et al., 2007). Coral cover and species richness decline with
increasing depth at high turbidity, with the most
phototrophic species being the first to disappear. Importantly, macroalgae are also missing at deeper depths
despite the high availability of nutrients, as they are
quickly light limited (Hillebrand, 2005). On these dark
reef slopes, filter-feeder communities may be prolific. In
turbid but shallow water, both light and nutrient levels
are high and coral cover and coral growth rates may be
very high – but only if herbivores are effective in keeping
macroalgae in bay and if hydrodynamics prevent the accumulation of sediments. If herbivores are missing, the slowgrowing corals tend to be outcompeted by fast-growing
macroalgae.
Fishes play an essential role in the ecology of coral
reefs, with low abundances of herbivorous fish potentially
leading to a proliferation of macroalgae and declining
coral cover (Hawkins and Roberts, 2004). Importantly,
the abundance of herbivorous fish on coral reefs may
potentially decline not only from overfishing, but also
from nutrient pollution. Wolanski et al. (2004) reported
a negative relationship between water clarity and the
abundance of herbivores on the Great Barrier Reef, and
a similar result was found along a water-quality gradient
in the Caribbean (Mallela et al., 2007). On the Great Barrier Reef, herbivorous fish are not usually targeted by fishers suggesting minimal fishing pressure, yet macroalgae
cover extensive areas on many inshore reefs (De’ath and
Fabricius, 2010), suggesting that control by herbivores is
ineffective. The mechanisms for the apparent negative
relationship between abundances of herbivorous fishes
and poor water quality is still poorly understood; they
may include an avoidance of turbidity (Wolanski et al.,
2004) or a reduced food palatability, if algae trap large
amounts of sediments (Mallela et al., 2007). If such negative relationship was more widespread than presently realized, then water quality would not only directly but also
indirectly promote macroalgae, by releasing both nutrient
limitation and grazing pressure. A systematic investigation of the potential relationships between water quality
and the abundance herbivorous fishes is urgently needed.
Effects of altered sediment properties
Coastal nutrient pollution is often linked to altered sediment properties or sedimentation regimes. Coarse sediment grains usually settle within a few kilometers of the
discharge source, often contributing to forming river
deltas. However, fine particles including detritus, clay,
and silt particles may remain suspended for prolonged
periods of time. These fine particles carry a large proportion of the total river load of nutrients and pesticides,
which eventually settle onto the seafloor and onto the benthic organisms, sometimes tens to hundreds of kilometers
away from the source.
Previous studies have mostly focused on assessing the
effects of increased rates of sedimentation on coral reefs.
Sedimentation rate is a strong environmental driver for
coral reefs. It is known to reduce coral recruitment rates
and coral biodiversity, and many sensitive species are
missing or underrepresented in areas of high sedimentation (Rogers, 1990; Fabricius, 2005). Photophysiological
stress in corals increases linearly as the product of the
duration and amount of sediment exposure, i.e., two units
of sediment deposited on the coral for one time unit, exerts
a similar decline in photosynthetic yields as half the
amount deposited for twice the time (Philipp and
Fabricius, 2003). High sedimentation rates (up to
100 mg dry weight cm
À2
) can kill exposed coral tissue
within a few days. Exposure to a few days of sedimentation can therefore cause long-term damage to coral
populations, by removing whole cohorts of small and sensitive corals. Previous studies have suggested
a sedimentation threshold of 10 mg cm
À2 day
À1
, as reefs
may be severely damaged at higher rates (Rogers, 1990).
Eutrophication may, however, not always increase the
rate of sedimentation, but may alter sediment properties.
Experiments have shown that when coral tissue is exposed
to sedimentation, the degree of damage increases with
bacterial activity (Hodgson, 1990) and with decreasing
grain sediment sizes (Weber et al., 2006). Nutrient-rich
sediments and sediments with high organic contents cause
726
NUTRIENT POLLUTION/EUTROPHICATION