water body and reef areas with abundant herbivores are
relatively resistant to exposure to nutrient pollution.
What is eutrophication?
Eutrophication, or nutrient pollution, is defined as the process of enrichment of waters with plant nutrients and
organic matter, often resulting in a chain of trophic
changes. Eutrophication is typically caused by the elevated supply and availability of the macronutrients nitrogen (N) and/or phosphorus (P), which stimulate the
growth of microbes, micro- and macroalgae, and lead to
an increased supply of organic matter. This in turn alters
the physical and chemical environment by reducing water
clarity and organically enriching sediments and eventually
results in potentially profound changes in biogeochemical, physiological, and ecological conditions. If the nutrients and sediments leading to eutrophication occur at
a concentration causing such environmental harm, they
fit the definition of “pollutants.” Minor cases of eutrophication, i.e., if they occur at above “natural” concentrations
without causing environmental harm, they fit the definition of “contaminants.” Nutrient pollution is only one of
the many forms of aquatic pollution, others being pollution by pesticides, industrial effluents, pharmaceuticals,
oils spills, fecal bacteria, etc., which are not being covered
here. Nutrient levels can also be quite high in some
undisturbed ecosystems, yet when human activity elevates
nutrients in environments with naturally low nutrient
availability, the resulting conditions may be far removed
from natural equilibria. The relative trophic state of
aquatic ecosystems is sometimes classified as “oligotrophic” (low nutrient availability), “mesotrophic,” or “eutrophic” (relatively high nutrient availability, either naturally
or from nutrient pollution). This classification is only of
limited use and should only be applied when comparing
within ecosystems’ types, because a eutrophic lake
typically has far higher nutrient concentrations than
a eutrophic coastal marine water body. However, it is
important to understand that all of these ecosystems may
be subject to eutrophication, i.e., an anthropogenic enrichment with nutrients compared to their previous state. Due
to its cascading effects on ecosystems and the incremental
changes from natural overcontaminated to polluted conditions, the question of nutrient pollution is very complex
and broad in scope.
Well-described symptoms of eutrophication are
increased phytoplankton production, bacterial and fungal
growth, an increased frequency of visible algal blooms
(such as red tides) or marine snow aggregates, and/or the
massive development of attached and/or floating algal
mats (Vollenweider, 1992). Further, water transparency
is often reduced from suspended detritus, mineral particles, and plankton, and dissolved oxygen levels in the bottom waters may be depleted and can cause fish kills.
Economic consequences attributed to severe eutrophication include a reduction in the health of shellfish, fish,
and benthos; reduced tourism; ciguatera shellfish
poisoning; and potential effects on human health from dermatitis or conjunctivitis (Vollenweider, 1992). However,
the symptoms of eutrophication are not always consistent
and predictable across the World’s aquatic ecosystems.
Eutrophication is usually caused by the excessive runoff of fertilizers from farms, soil-bound nutrients from
cleared lands, sewage effluents, or discharges from aquaculture into a poorly mixed or partly enclosed water body.
The amount of nutrients and sediments lost from
a catchment is well predicted by its geographic features
including soil type, slope angle, and rainfall. Additionally,
soils and fertilizers flow from cleared and overgrazed
lands, sewage, or aquaculture farms into coastal marine
ecosystems, including coastal coral reefs (Vitousek et al.,
1997; Tilman et al., 2001; Smith et al., 2003). Globally,
the use of nitrogen fertilizer has increased more than
6-fold since 1960 (Matson et al., 1997), and an additional
1% of the earth’s surface is being cleared every year.
About 45% of the global human population ($100 people
km
À2
) are now living in coastal areas, and human population is growing even faster along the coast than elsewhere:
the number of people living in the coastal area has
increased by 765 million to $3.1 billion people since
1990 alone (Crossland et al., 2005). All this contributes
to increasing losses of nutrients from the land and
a rapidly intensifying exploitation of coastal resources
(Crossland et al., 2005). Oxygen-depleted seafloor zones,
attributable to runoff of agricultural nitrogen and phosphorus, have doubled in area in the last few decades –
indicating that many marine water bodies are becoming
more eutrophic (Diaz and Rosenberg, 2008; VaquerSunye and Duarte, 2008).
Effects of nutrient pollution and eutrophication
on coral reefs
Coral reefs can be found in nutrient, light, and sedimentation conditions that may vary, both spatially and temporally, by 1–2 orders of magnitude even in undisturbed
settings. New dissolved and particulate inorganic and
organic nutrients and sediments may originate from rivers,
the upwelling of nutrient-rich deeper water bodies, nitrogen fixation by benthic and pelagic cyanobacteria and
algae, diffuse coastal runoff, groundwater seepage, and
rain and settling dust from the atmosphere. Reef communities naturally change with nutrient, light, and sedimentation conditions: from terrestrially influenced conditions
(characterized by fluctuating salinity, more variable or
higher nutrient load, siltation, and turbidity) to oceanic
conditions where nutrients, siltation, and turbidity are typically low. Reef environments are not typically classified
by their trophic state (i.e., oligo-, meso- or eutrophic),
but reefs in all trophic states may be affected by
eutrophication.
Coral reefs are built by the calcium carbonate skeletons
of hard corals (order Scleractinia), which live in symbiosis
with endosymbiotic unicellular algae (zooxanthellae). The
tens of thousands of species associated with coral reefs
NUTRIENT POLLUTION/EUTROPHICATION
723
relatively resistant to exposure to nutrient pollution.
What is eutrophication?
Eutrophication, or nutrient pollution, is defined as the process of enrichment of waters with plant nutrients and
organic matter, often resulting in a chain of trophic
changes. Eutrophication is typically caused by the elevated supply and availability of the macronutrients nitrogen (N) and/or phosphorus (P), which stimulate the
growth of microbes, micro- and macroalgae, and lead to
an increased supply of organic matter. This in turn alters
the physical and chemical environment by reducing water
clarity and organically enriching sediments and eventually
results in potentially profound changes in biogeochemical, physiological, and ecological conditions. If the nutrients and sediments leading to eutrophication occur at
a concentration causing such environmental harm, they
fit the definition of “pollutants.” Minor cases of eutrophication, i.e., if they occur at above “natural” concentrations
without causing environmental harm, they fit the definition of “contaminants.” Nutrient pollution is only one of
the many forms of aquatic pollution, others being pollution by pesticides, industrial effluents, pharmaceuticals,
oils spills, fecal bacteria, etc., which are not being covered
here. Nutrient levels can also be quite high in some
undisturbed ecosystems, yet when human activity elevates
nutrients in environments with naturally low nutrient
availability, the resulting conditions may be far removed
from natural equilibria. The relative trophic state of
aquatic ecosystems is sometimes classified as “oligotrophic” (low nutrient availability), “mesotrophic,” or “eutrophic” (relatively high nutrient availability, either naturally
or from nutrient pollution). This classification is only of
limited use and should only be applied when comparing
within ecosystems’ types, because a eutrophic lake
typically has far higher nutrient concentrations than
a eutrophic coastal marine water body. However, it is
important to understand that all of these ecosystems may
be subject to eutrophication, i.e., an anthropogenic enrichment with nutrients compared to their previous state. Due
to its cascading effects on ecosystems and the incremental
changes from natural overcontaminated to polluted conditions, the question of nutrient pollution is very complex
and broad in scope.
Well-described symptoms of eutrophication are
increased phytoplankton production, bacterial and fungal
growth, an increased frequency of visible algal blooms
(such as red tides) or marine snow aggregates, and/or the
massive development of attached and/or floating algal
mats (Vollenweider, 1992). Further, water transparency
is often reduced from suspended detritus, mineral particles, and plankton, and dissolved oxygen levels in the bottom waters may be depleted and can cause fish kills.
Economic consequences attributed to severe eutrophication include a reduction in the health of shellfish, fish,
and benthos; reduced tourism; ciguatera shellfish
poisoning; and potential effects on human health from dermatitis or conjunctivitis (Vollenweider, 1992). However,
the symptoms of eutrophication are not always consistent
and predictable across the World’s aquatic ecosystems.
Eutrophication is usually caused by the excessive runoff of fertilizers from farms, soil-bound nutrients from
cleared lands, sewage effluents, or discharges from aquaculture into a poorly mixed or partly enclosed water body.
The amount of nutrients and sediments lost from
a catchment is well predicted by its geographic features
including soil type, slope angle, and rainfall. Additionally,
soils and fertilizers flow from cleared and overgrazed
lands, sewage, or aquaculture farms into coastal marine
ecosystems, including coastal coral reefs (Vitousek et al.,
1997; Tilman et al., 2001; Smith et al., 2003). Globally,
the use of nitrogen fertilizer has increased more than
6-fold since 1960 (Matson et al., 1997), and an additional
1% of the earth’s surface is being cleared every year.
About 45% of the global human population ($100 people
km
À2
) are now living in coastal areas, and human population is growing even faster along the coast than elsewhere:
the number of people living in the coastal area has
increased by 765 million to $3.1 billion people since
1990 alone (Crossland et al., 2005). All this contributes
to increasing losses of nutrients from the land and
a rapidly intensifying exploitation of coastal resources
(Crossland et al., 2005). Oxygen-depleted seafloor zones,
attributable to runoff of agricultural nitrogen and phosphorus, have doubled in area in the last few decades –
indicating that many marine water bodies are becoming
more eutrophic (Diaz and Rosenberg, 2008; VaquerSunye and Duarte, 2008).
Effects of nutrient pollution and eutrophication
on coral reefs
Coral reefs can be found in nutrient, light, and sedimentation conditions that may vary, both spatially and temporally, by 1–2 orders of magnitude even in undisturbed
settings. New dissolved and particulate inorganic and
organic nutrients and sediments may originate from rivers,
the upwelling of nutrient-rich deeper water bodies, nitrogen fixation by benthic and pelagic cyanobacteria and
algae, diffuse coastal runoff, groundwater seepage, and
rain and settling dust from the atmosphere. Reef communities naturally change with nutrient, light, and sedimentation conditions: from terrestrially influenced conditions
(characterized by fluctuating salinity, more variable or
higher nutrient load, siltation, and turbidity) to oceanic
conditions where nutrients, siltation, and turbidity are typically low. Reef environments are not typically classified
by their trophic state (i.e., oligo-, meso- or eutrophic),
but reefs in all trophic states may be affected by
eutrophication.
Coral reefs are built by the calcium carbonate skeletons
of hard corals (order Scleractinia), which live in symbiosis
with endosymbiotic unicellular algae (zooxanthellae). The
tens of thousands of species associated with coral reefs
NUTRIENT POLLUTION/EUTROPHICATION
723