to estuaries have been increasing through time, resulting
in escalating eutrophication of estuarine water bodies
(Howarth et al., 2002a; Rabalais, 2002; Bricker et al.,
2007; Burkholder et al., 2007; Howarth, 2008; Anderson
et al., 2010). Of the 138 estuaries in the USA examined
by Bricker et al. (2007), 84 were determined to be highly
eutrophic. Greater nitrogen and phosphorus loads to estuaries and coastal marine waters are attributed to accelerating coastal development and intensification of agriculture
(Howarth et al., 2002b; Galloway et al., 2008; Howarth,
2008; Anderson et al., 2010). Aside from agriculture
sources, these nutrients derive from lawn fertilization,
stormwater runoff, municipal and industrial wastewaters,
malfunctioning septic systems, groundwater seepage,
and atmospheric deposition. The eutrophication of estuaries is manifested by an array of damaging cascading
changes in ecosystem structure and function such as
decreased dissolved oxygen levels, increased microalgal
and macroalgal abundance, occurrence of harmful algal
blooms (HABs), loss of seagrass habitat, reducedbiodiversity, declining fisheries, imbalanced food webs,
altered biogeochemical cycling, and diminished ecosystem
services (Kennish and de Jonge, 2011).
Sewage and organic wastes
Sewage and organic wastes also increase nutrient and
organic carbon loading that can exacerbate estuarine
eutrophic conditions (Kennish, 2001a). Organic carbon
enrichment is coupled to elevated biochemical oxygen
demand and depleted dissolved oxygen levels in some
water bodies. These wastes derive from domestic and
industrial sources, livestock and fish processing facilities,
wildlife inputs, aquaculture operations, and other sources.
Hypoxia of estuarine and coastal marine environments has
increased worldwide over the past several decades due to
eutrophication driven by nutrient enrichment and organic
carbon loading (Diaz and Rosenberg, 1995, 2008).
The calefaction of estuarine waters by the release of
heated effluent from electric generating stations and other
industrial facilities can also deplete dissolved oxygen
levels. Thermal discharges from electric generating stations have commonly increased mortality of susceptible
estuarine organisms in near-field regions of outfall sites.
Aside from increased mortality due to reduced dissolved
oxygen, heat-shock and cold-shock mortality at electric
generating stations has occasionally caused mass mortality of finfish populations which cannot adapt to the rapid
changes in water temperature associated with abrupt
changes in station operation (Kennish, 1992).
Habitat loss and alteration
Coastal population growth and development have led to
substantial estuarine habitat loss and alteration, impacting
biotic communities. Physical alteration is associated with
the dredging of channels and other subtidal areas, the construction of hardened shorelines, and the removal of vegetation and soils during construction of buildings,
roadways, and other infrastructure elements in watersheds
that convert natural habitats to impervious cover and
increase erosion, runoff, and nonpoint pollution to estuarine water bodies. Water quality is often degraded as
a result. Other adverse effects include habitat fragmentation, ecosystem isolation, and functional degradation of
upland and wetland complexes (Kennish, 2001a). Some
of these changes can be intractable.
Historically, salt marshes and other wetland habitat bordering estuarine basins have been altered extensively by
grid ditching, marsh diking, draining and filling (for agriculture), impoundments for wildlife, and reclamation for
domestic and industrial development. The hydrology of
wetland systems has been invariably changed by construction of impoundment dikes, water-control embankments,
levees, dams for flood control, as well as canals and spoil
banks. Tidal flooding, water flow, and drainage are often
modified, reducing sediment loading to the marsh surface
and even arresting vertical accretion and hastening marsh
submergence (Kennish, 2001b). Concomitantly, the loss
of marsh in many regions is accelerating due to climate
change-driven sea-level rise. Human activities have eliminated more than 50 % of the original tidal marsh habitat in
the USA and more than 70 % of the mangroves fringing
the coast of Puerto Rico (Kennish, 1997; Alongi, 1998).
The destruction of wetland habitat decreased substantially
in the USA after enactment of the 1972 Clean Water Act.
Localized areas along estuarine shorelines have been
affected by construction and use of docks, piers, boat
ramps, and marinas. Other shorelines have been modified
by marine engineering structures such as bulkheads and
revetments for bank stabilization, jetties at inlets,
stormwalls, and other protective features. Constructing
physically static structures in dynamic estuarine environments has impacted the function of sensitive habitats often
diminishing their ecological value.
Chemical contaminants
Many estuaries receive a wide range of chemical contaminants because they are located in close proximity to
heavily populated metropolitan centers and other developed coastal areas. Urbanized estuaries are often the most
heavily impacted. Major sources and delivery systems
include agricultural and urban runoff, municipal and
industrial discharges, groundwater inputs, riverine inflow,
and atmospheric deposition. Chemical contaminants
entering estuaries may concentrate in the water column,
accumulate in bottom sediments and organisms, or exit
to coastal waters. Bottom sediments of estuaries are typically repositories of chemical contaminants because many
of these substances are particle reactive, sorbing to grain
surfaces, and ultimately settling to the estuarine floor.
Among the most important groups of chemical contaminants found in estuarine environments are halogenated
hydrocarbons, polycyclic aromatic hydrocarbons
(PAHs), and metals. These contaminants are potentially
damaging to estuarine habitats and hazardous to estuarine
30
ANTHROPOGENIC IMPACTS
in escalating eutrophication of estuarine water bodies
(Howarth et al., 2002a; Rabalais, 2002; Bricker et al.,
2007; Burkholder et al., 2007; Howarth, 2008; Anderson
et al., 2010). Of the 138 estuaries in the USA examined
by Bricker et al. (2007), 84 were determined to be highly
eutrophic. Greater nitrogen and phosphorus loads to estuaries and coastal marine waters are attributed to accelerating coastal development and intensification of agriculture
(Howarth et al., 2002b; Galloway et al., 2008; Howarth,
2008; Anderson et al., 2010). Aside from agriculture
sources, these nutrients derive from lawn fertilization,
stormwater runoff, municipal and industrial wastewaters,
malfunctioning septic systems, groundwater seepage,
and atmospheric deposition. The eutrophication of estuaries is manifested by an array of damaging cascading
changes in ecosystem structure and function such as
decreased dissolved oxygen levels, increased microalgal
and macroalgal abundance, occurrence of harmful algal
blooms (HABs), loss of seagrass habitat, reducedbiodiversity, declining fisheries, imbalanced food webs,
altered biogeochemical cycling, and diminished ecosystem
services (Kennish and de Jonge, 2011).
Sewage and organic wastes
Sewage and organic wastes also increase nutrient and
organic carbon loading that can exacerbate estuarine
eutrophic conditions (Kennish, 2001a). Organic carbon
enrichment is coupled to elevated biochemical oxygen
demand and depleted dissolved oxygen levels in some
water bodies. These wastes derive from domestic and
industrial sources, livestock and fish processing facilities,
wildlife inputs, aquaculture operations, and other sources.
Hypoxia of estuarine and coastal marine environments has
increased worldwide over the past several decades due to
eutrophication driven by nutrient enrichment and organic
carbon loading (Diaz and Rosenberg, 1995, 2008).
The calefaction of estuarine waters by the release of
heated effluent from electric generating stations and other
industrial facilities can also deplete dissolved oxygen
levels. Thermal discharges from electric generating stations have commonly increased mortality of susceptible
estuarine organisms in near-field regions of outfall sites.
Aside from increased mortality due to reduced dissolved
oxygen, heat-shock and cold-shock mortality at electric
generating stations has occasionally caused mass mortality of finfish populations which cannot adapt to the rapid
changes in water temperature associated with abrupt
changes in station operation (Kennish, 1992).
Habitat loss and alteration
Coastal population growth and development have led to
substantial estuarine habitat loss and alteration, impacting
biotic communities. Physical alteration is associated with
the dredging of channels and other subtidal areas, the construction of hardened shorelines, and the removal of vegetation and soils during construction of buildings,
roadways, and other infrastructure elements in watersheds
that convert natural habitats to impervious cover and
increase erosion, runoff, and nonpoint pollution to estuarine water bodies. Water quality is often degraded as
a result. Other adverse effects include habitat fragmentation, ecosystem isolation, and functional degradation of
upland and wetland complexes (Kennish, 2001a). Some
of these changes can be intractable.
Historically, salt marshes and other wetland habitat bordering estuarine basins have been altered extensively by
grid ditching, marsh diking, draining and filling (for agriculture), impoundments for wildlife, and reclamation for
domestic and industrial development. The hydrology of
wetland systems has been invariably changed by construction of impoundment dikes, water-control embankments,
levees, dams for flood control, as well as canals and spoil
banks. Tidal flooding, water flow, and drainage are often
modified, reducing sediment loading to the marsh surface
and even arresting vertical accretion and hastening marsh
submergence (Kennish, 2001b). Concomitantly, the loss
of marsh in many regions is accelerating due to climate
change-driven sea-level rise. Human activities have eliminated more than 50 % of the original tidal marsh habitat in
the USA and more than 70 % of the mangroves fringing
the coast of Puerto Rico (Kennish, 1997; Alongi, 1998).
The destruction of wetland habitat decreased substantially
in the USA after enactment of the 1972 Clean Water Act.
Localized areas along estuarine shorelines have been
affected by construction and use of docks, piers, boat
ramps, and marinas. Other shorelines have been modified
by marine engineering structures such as bulkheads and
revetments for bank stabilization, jetties at inlets,
stormwalls, and other protective features. Constructing
physically static structures in dynamic estuarine environments has impacted the function of sensitive habitats often
diminishing their ecological value.
Chemical contaminants
Many estuaries receive a wide range of chemical contaminants because they are located in close proximity to
heavily populated metropolitan centers and other developed coastal areas. Urbanized estuaries are often the most
heavily impacted. Major sources and delivery systems
include agricultural and urban runoff, municipal and
industrial discharges, groundwater inputs, riverine inflow,
and atmospheric deposition. Chemical contaminants
entering estuaries may concentrate in the water column,
accumulate in bottom sediments and organisms, or exit
to coastal waters. Bottom sediments of estuaries are typically repositories of chemical contaminants because many
of these substances are particle reactive, sorbing to grain
surfaces, and ultimately settling to the estuarine floor.
Among the most important groups of chemical contaminants found in estuarine environments are halogenated
hydrocarbons, polycyclic aromatic hydrocarbons
(PAHs), and metals. These contaminants are potentially
damaging to estuarine habitats and hazardous to estuarine
30
ANTHROPOGENIC IMPACTS