in the rate of supply of organic matter to an ecosystem.
The National Academy of Sciences (1969) defined the
term as the natural or artificial addition of nutrients to
a waterbody and the effects of the added nutrients on the
system. Similarly, the European Union Directives has
focused on nutrient inputs to a waterbody and its consequences (Anonymous, 2000). In this case, eutrophication
has been defined as the process of nutrient enrichment
and increase in the rate of organic matter input in
a waterbody leading to an array of cascading changes in
ecosystem structure and function such as decreased
dissolved oxygen levels (hypoxia/anoxia), increased
microalgal and macroalgal abundance, elevated epiphytic
growth, occurrence of harmful algal blooms (HABs), loss
of essential habitat (e.g., seagrass and shellfish beds),
reduced biodiversity, declining fisheries, imbalanced food
webs, altered biogeochemical cycling, and diminished
ecosystem services (Kennish and de Jonge, 2011).
Eutrophication of estuarine ecosystems
Eutrophic conditions have developed in many estuarine
systems bordered by watersheds with increasing agricultural and urban land use, and the effects are most acute
in shallow coastal bays and coastal lagoons with restricted
circulation and protracted water residence times (Nixon
et al., 2001; Burkholder et al., 2007; McGlathery et al.,
2007; Anderson et al., 2010; Kennish and Paerl, 2010;
Giordano et al., 2011; Howarth et al., 2011). For example,
moderate to high levels of eutrophication have been
documented in an array of mid-Atlantic coastal lagoons
(USA). Among these impacted systems are Great South
Bay (NY); Barnegat Bay-Little Egg Harbor Estuary
(NJ); Rehoboth, Indian, and Little Assawoman Bays
(Delaware Inland Bays); Assawoman Bay, Isle of Wight
Bay, and St Martin River (Northern Maryland Coastal
Bays); and Newport, Sinepuxent, and Chincoteague Bays
(Southern Maryland Coastal Bays). Similar impacts are
apparent in other countries as well, for example,
(1) Wadden Sea and Ems Estuary (Netherlands and
Denmark), (2) Peel-Harvey Estuary (Australia),
(3) Shenzhen Bay (China), and (4) Ghana coastal lagoons
(Africa).
Sources of nutrient enrichment
Human population growth and development continue to
escalate in the coastal zone. More than 75 % of the world’s
human population inhabits coastal river basins. Hence, it
is understandable why nutrient enrichment has become
a major problem in estuaries worldwide. Major sources
of nutrients to estuarine systems include farmlands,
stormwater runoff, wastewater discharges, groundwater
seepage, and atmospheric deposition. Anderson et al.
(2010) stressed that nutrient enrichment by reactive nitrogen in coastal waters is accelerating due to development
and intensification of agriculture. Nonpoint sources of
nutrient pollution are significant not only due to increased
development, greater impervious surfaces, and runoff in
coastal watersheds but also due to atmospheric deposition.
According to Paerl et al. (2002), for example, atmospheric
deposition accounts for $10–40 % of the new nitrogen
loading to estuaries investigated along the East Coast
and eastern Gulf of Mexico (USA). Complicating matters,
estuaries are important repositories of nutrients delivered
from watersheds and the atmosphere. For example, bottom sediments typically serve as a pool and a secondary
source of nutrients to the water column in these susceptible ecosystems. Studies have shown that nutrient concentrations in estuarine bottom sediments may be 10- to
100-fold higher than in the water column (Sand-Jensen
and Borum, 1991; Burkholder et al., 2007).
Ecosystem impacts
Nutrient enrichment (most notably nitrogen and phosphorus) from coastal watersheds and the atmosphere (via dry
and wet deposition) is an important driver of biotic change
in estuaries. It can cause significant shifts in primary production and plant biomass, as well as changes in the composition of autotrophs – microalgae, macroalgae, and
rooted macrophyte assemblages that modulate higher
trophic-level dynamics. Thus, the effects of altered
bottom-up controls on the biotic structure and function
of a system can be far reaching. Nutrient enrichment and
the resulting eutrophic impacts often pose serious threats
to an estuary because they can lead to long-term,
ecosystem-wide decline, adversely affecting biotic
resources and human uses.
Nitrogen concentrations in estuarine waters typically
range from <1 to 60 mmol l
À1
, while phosphorus concentrations generally range from <1 to 10 mmol l
À1 (Kennish,
2001a). Much higher nutrient levels may occur in eutrophic systems. Phosphate concentrations are usually much
lower than nitrate concentrations in estuarine waters in
large part because phosphate readily sorbs to particulate
matter or forms insoluble precipitates that accumulate in
bottom sediments (Kennish and de Jonge, 2011). This process affects the biologically available fraction of phosphorus. Both inorganic and organic nutrient forms must be
considered in assessment of estuarine eutrophication.
Depending on the physicochemical and biotic conditions, the consequences of nutrient enrichment and
increase in the rate of organic matter supply in an estuary
are numerous and varied which can cause declining system stability and resilience. The long-term effect of eutrophication is potentially the permanent alteration or loss of
biotic communities and habitats and great ecosystem-level
degradation. This is so because the eutrophication process
disrupts the ecological interrelationships and functioning
of coastal water bodies. Nutrient enrichment stimulates
algal production and sets into motion changes in the ecosystem from the bottom-up, literally altering the foundations of the way the estuary functions. As algal
populations bloom, die off, and then sink to the floor of
EUTROPHICATION
305
The National Academy of Sciences (1969) defined the
term as the natural or artificial addition of nutrients to
a waterbody and the effects of the added nutrients on the
system. Similarly, the European Union Directives has
focused on nutrient inputs to a waterbody and its consequences (Anonymous, 2000). In this case, eutrophication
has been defined as the process of nutrient enrichment
and increase in the rate of organic matter input in
a waterbody leading to an array of cascading changes in
ecosystem structure and function such as decreased
dissolved oxygen levels (hypoxia/anoxia), increased
microalgal and macroalgal abundance, elevated epiphytic
growth, occurrence of harmful algal blooms (HABs), loss
of essential habitat (e.g., seagrass and shellfish beds),
reduced biodiversity, declining fisheries, imbalanced food
webs, altered biogeochemical cycling, and diminished
ecosystem services (Kennish and de Jonge, 2011).
Eutrophication of estuarine ecosystems
Eutrophic conditions have developed in many estuarine
systems bordered by watersheds with increasing agricultural and urban land use, and the effects are most acute
in shallow coastal bays and coastal lagoons with restricted
circulation and protracted water residence times (Nixon
et al., 2001; Burkholder et al., 2007; McGlathery et al.,
2007; Anderson et al., 2010; Kennish and Paerl, 2010;
Giordano et al., 2011; Howarth et al., 2011). For example,
moderate to high levels of eutrophication have been
documented in an array of mid-Atlantic coastal lagoons
(USA). Among these impacted systems are Great South
Bay (NY); Barnegat Bay-Little Egg Harbor Estuary
(NJ); Rehoboth, Indian, and Little Assawoman Bays
(Delaware Inland Bays); Assawoman Bay, Isle of Wight
Bay, and St Martin River (Northern Maryland Coastal
Bays); and Newport, Sinepuxent, and Chincoteague Bays
(Southern Maryland Coastal Bays). Similar impacts are
apparent in other countries as well, for example,
(1) Wadden Sea and Ems Estuary (Netherlands and
Denmark), (2) Peel-Harvey Estuary (Australia),
(3) Shenzhen Bay (China), and (4) Ghana coastal lagoons
(Africa).
Sources of nutrient enrichment
Human population growth and development continue to
escalate in the coastal zone. More than 75 % of the world’s
human population inhabits coastal river basins. Hence, it
is understandable why nutrient enrichment has become
a major problem in estuaries worldwide. Major sources
of nutrients to estuarine systems include farmlands,
stormwater runoff, wastewater discharges, groundwater
seepage, and atmospheric deposition. Anderson et al.
(2010) stressed that nutrient enrichment by reactive nitrogen in coastal waters is accelerating due to development
and intensification of agriculture. Nonpoint sources of
nutrient pollution are significant not only due to increased
development, greater impervious surfaces, and runoff in
coastal watersheds but also due to atmospheric deposition.
According to Paerl et al. (2002), for example, atmospheric
deposition accounts for $10–40 % of the new nitrogen
loading to estuaries investigated along the East Coast
and eastern Gulf of Mexico (USA). Complicating matters,
estuaries are important repositories of nutrients delivered
from watersheds and the atmosphere. For example, bottom sediments typically serve as a pool and a secondary
source of nutrients to the water column in these susceptible ecosystems. Studies have shown that nutrient concentrations in estuarine bottom sediments may be 10- to
100-fold higher than in the water column (Sand-Jensen
and Borum, 1991; Burkholder et al., 2007).
Ecosystem impacts
Nutrient enrichment (most notably nitrogen and phosphorus) from coastal watersheds and the atmosphere (via dry
and wet deposition) is an important driver of biotic change
in estuaries. It can cause significant shifts in primary production and plant biomass, as well as changes in the composition of autotrophs – microalgae, macroalgae, and
rooted macrophyte assemblages that modulate higher
trophic-level dynamics. Thus, the effects of altered
bottom-up controls on the biotic structure and function
of a system can be far reaching. Nutrient enrichment and
the resulting eutrophic impacts often pose serious threats
to an estuary because they can lead to long-term,
ecosystem-wide decline, adversely affecting biotic
resources and human uses.
Nitrogen concentrations in estuarine waters typically
range from <1 to 60 mmol l
À1
, while phosphorus concentrations generally range from <1 to 10 mmol l
À1 (Kennish,
2001a). Much higher nutrient levels may occur in eutrophic systems. Phosphate concentrations are usually much
lower than nitrate concentrations in estuarine waters in
large part because phosphate readily sorbs to particulate
matter or forms insoluble precipitates that accumulate in
bottom sediments (Kennish and de Jonge, 2011). This process affects the biologically available fraction of phosphorus. Both inorganic and organic nutrient forms must be
considered in assessment of estuarine eutrophication.
Depending on the physicochemical and biotic conditions, the consequences of nutrient enrichment and
increase in the rate of organic matter supply in an estuary
are numerous and varied which can cause declining system stability and resilience. The long-term effect of eutrophication is potentially the permanent alteration or loss of
biotic communities and habitats and great ecosystem-level
degradation. This is so because the eutrophication process
disrupts the ecological interrelationships and functioning
of coastal water bodies. Nutrient enrichment stimulates
algal production and sets into motion changes in the ecosystem from the bottom-up, literally altering the foundations of the way the estuary functions. As algal
populations bloom, die off, and then sink to the floor of
EUTROPHICATION
305