intensity of tropical storms (Nicholls et al., 2007), which
can severely impact estuaries. In one of the first intensive
studies of hurricane impacts, the June 1972 storm Agnes in
the eastern United States, there were widespread, longlasting effects on Chesapeake Bay circulation, salinity,
water quality, and ecosystems (Bailey et al., 1975; Davis
et al., 1977). Similarly, three hurricanes that hit coastal
North Carolina in 1999 caused 50- to 500-year floods,
lowered salinity, and enhanced nitrogen loading to Pamlico Sound, which together had multiyear effects on
coastal ecosystems (Paerl et al., 2001). Large storms also
affect coastal wetlands notably through the impacts of
storm surge, wind, and freshwater flushing on wetland soil
dynamics and elevation (Cahoon, 2006).
Modeling precipitation changes and impacts
One challenge in estuarine research is predicting future
precipitation/streamflow changes due to higher CO 2 concentrations. In the mid-Atlantic region of the eastern
United States (Chesapeake Bay, the Delaware Bay, and
Hudson River Estuary), impacts on streamflow ranged
from a decrease of 40 % to an increase of 30 %, although
results varied by season (Najjar et al., 2009). In a study of
San Francisco Bay, Knowles and Cayan (2002) found that
changes in winter snowpack and reduction in spring runoff would lead to elevated salinity (see Cloern et al.,
2011). Future progress in this emerging field will come
from linking downscaled climate models with watershed
and estuarine hydrodynamic models, especially as
improvements are made in predicted precipitation
response to future climate change.
Global and regional temperature
Compared to precipitation-driven changes, the impacts of
changing temperature may be less obvious in the short
term, but nonetheless aquatic temperatures are important
in estuarine functioning and ecosystems. Global mean
annual and regional ocean temperatures are expected to
rise over future decades to centuries due to elevated atmospheric CO 2 concentrations (Najjar et al., 2009), and in
theory, this warming might lead to poleward range shifts
in temperature-sensitive species (Helmuth et al., 2002;
Przeslawski et al., 2012). Moreover, there is indisputable
evidence that the world’s oceans have been warming for
at least the last 50 years (Levitus et al., 2012), and
paleoclimate records show that marine species experienced large climate-driven biogeographic range shifts
over 10
4
–10
7 year timescales. These shifts are best
documented in marine sediment records of major microfossil groups (diatoms, dinoflagellates, foraminifera, radiolarian, ostracodes) during glacial-interglacial cycles of
the 500,000 years when Earth’s mean annual temperature
fell $5
C during glacial periods (Kucera et al., 2005).
In addition to open-ocean sea faunal and floral biogeographic shifts, paleo-records from estuaries and coasts
also show Holocene temperature-induced biogeochemical
and productivity changes such as the sedimentary
record of LIA cooling in Kagoshima Bay, Japan (Kuwae
et al., 2007).
In addition to large-scale range shifts, several indirect
impacts of rising temperatures deserve mention: reduced
sea ice, especially in marginal subarctic seas; coal
bleaching; expanded geographic ranges of harmful algal
bloom species; and mangrove species expansion among
others (Nicholls et al., 2007). Case studies include the
Bering-Chukchi Seas (Grebmeier, 2012), the Changjiang
River Estuary (Ma et al., 2009), Mediterranean coastal
systems (Bensoussan et al., 2010), Narragansett Bay,
Rhode Island (Nixon et al., 2009), and the Gulf of Mexico
(Bianchi et al., 2013).
Sediment processes
Coastal sedimentary processes influenced by climate
include erosion (in the watershed and estuary), transport
(in suspension and along river and estuarine bottoms),
and deposition in an estuary, bay, or fjord. However,
deciphering climate impacts on sedimentation is difficult
due to large-scale anthropogenic activities. On the global
scale, Syvitski et al. (2005) estimate that humans account
for 2.3 Æ 0.6 billion metric tons per year but that sediment
retention in reservoirs, totaling 100 billion metric tons
(bmt) in recent decades, reduces the sediment reaching
the world’s coasts by 1.4 Æ 0.3 bmt per year (see Milliman
and Farnsworth, 2011). On a regional scale, Saenger
et al. (2008) found that postcolonial agricultural land
clearance in the Chesapeake Bay watershed increased sediment accumulation rates by several times, but there were
complex leads and lags related to climatic factors.
Nonetheless, preindustrial climate changes are known
to affect sediment flux to coastal systems. For example,
in subpolar fjords in Svalbard, Szczucinski et al. (2009)
found that post-Little Ice Age temperature increase and
glacier retreat had large impacts on sediment
accumulation.
Within an estuary or bay, sediment affects a variety of
factors including turbidity, light penetration, and the distribution of submerged aquatic vegetation (including sea
grasses). This applies both to clastic sediment, often
referred to as mineral matter, and particulate organic material, much of which is produced by algal productivity
fueled by high nutrient concentrations. Sediment also
plays an important role in the development of estuarine
turbidity maximum zones (ETM, also called turbidity
maximum zones, TMZ), a characteristic feature of many
estuaries. It has long been known that trapping of
suspended material in ETMs can be enhanced by
increased vertical stratification due to large freshwater
influx (Geyer, 1993). The physics of circulation near these
salinity gradients are such that they trap clastic sediment
and phytoplankton-derived organic material that has been
transported to or resuspended within the ETM, resulting
in high nutrient concentrations (Uncles et al., 2006;
124
CLIMATE CHANGE
can severely impact estuaries. In one of the first intensive
studies of hurricane impacts, the June 1972 storm Agnes in
the eastern United States, there were widespread, longlasting effects on Chesapeake Bay circulation, salinity,
water quality, and ecosystems (Bailey et al., 1975; Davis
et al., 1977). Similarly, three hurricanes that hit coastal
North Carolina in 1999 caused 50- to 500-year floods,
lowered salinity, and enhanced nitrogen loading to Pamlico Sound, which together had multiyear effects on
coastal ecosystems (Paerl et al., 2001). Large storms also
affect coastal wetlands notably through the impacts of
storm surge, wind, and freshwater flushing on wetland soil
dynamics and elevation (Cahoon, 2006).
Modeling precipitation changes and impacts
One challenge in estuarine research is predicting future
precipitation/streamflow changes due to higher CO 2 concentrations. In the mid-Atlantic region of the eastern
United States (Chesapeake Bay, the Delaware Bay, and
Hudson River Estuary), impacts on streamflow ranged
from a decrease of 40 % to an increase of 30 %, although
results varied by season (Najjar et al., 2009). In a study of
San Francisco Bay, Knowles and Cayan (2002) found that
changes in winter snowpack and reduction in spring runoff would lead to elevated salinity (see Cloern et al.,
2011). Future progress in this emerging field will come
from linking downscaled climate models with watershed
and estuarine hydrodynamic models, especially as
improvements are made in predicted precipitation
response to future climate change.
Global and regional temperature
Compared to precipitation-driven changes, the impacts of
changing temperature may be less obvious in the short
term, but nonetheless aquatic temperatures are important
in estuarine functioning and ecosystems. Global mean
annual and regional ocean temperatures are expected to
rise over future decades to centuries due to elevated atmospheric CO 2 concentrations (Najjar et al., 2009), and in
theory, this warming might lead to poleward range shifts
in temperature-sensitive species (Helmuth et al., 2002;
Przeslawski et al., 2012). Moreover, there is indisputable
evidence that the world’s oceans have been warming for
at least the last 50 years (Levitus et al., 2012), and
paleoclimate records show that marine species experienced large climate-driven biogeographic range shifts
over 10
4
–10
7 year timescales. These shifts are best
documented in marine sediment records of major microfossil groups (diatoms, dinoflagellates, foraminifera, radiolarian, ostracodes) during glacial-interglacial cycles of
the 500,000 years when Earth’s mean annual temperature
fell $5
C during glacial periods (Kucera et al., 2005).
In addition to open-ocean sea faunal and floral biogeographic shifts, paleo-records from estuaries and coasts
also show Holocene temperature-induced biogeochemical
and productivity changes such as the sedimentary
record of LIA cooling in Kagoshima Bay, Japan (Kuwae
et al., 2007).
In addition to large-scale range shifts, several indirect
impacts of rising temperatures deserve mention: reduced
sea ice, especially in marginal subarctic seas; coal
bleaching; expanded geographic ranges of harmful algal
bloom species; and mangrove species expansion among
others (Nicholls et al., 2007). Case studies include the
Bering-Chukchi Seas (Grebmeier, 2012), the Changjiang
River Estuary (Ma et al., 2009), Mediterranean coastal
systems (Bensoussan et al., 2010), Narragansett Bay,
Rhode Island (Nixon et al., 2009), and the Gulf of Mexico
(Bianchi et al., 2013).
Sediment processes
Coastal sedimentary processes influenced by climate
include erosion (in the watershed and estuary), transport
(in suspension and along river and estuarine bottoms),
and deposition in an estuary, bay, or fjord. However,
deciphering climate impacts on sedimentation is difficult
due to large-scale anthropogenic activities. On the global
scale, Syvitski et al. (2005) estimate that humans account
for 2.3 Æ 0.6 billion metric tons per year but that sediment
retention in reservoirs, totaling 100 billion metric tons
(bmt) in recent decades, reduces the sediment reaching
the world’s coasts by 1.4 Æ 0.3 bmt per year (see Milliman
and Farnsworth, 2011). On a regional scale, Saenger
et al. (2008) found that postcolonial agricultural land
clearance in the Chesapeake Bay watershed increased sediment accumulation rates by several times, but there were
complex leads and lags related to climatic factors.
Nonetheless, preindustrial climate changes are known
to affect sediment flux to coastal systems. For example,
in subpolar fjords in Svalbard, Szczucinski et al. (2009)
found that post-Little Ice Age temperature increase and
glacier retreat had large impacts on sediment
accumulation.
Within an estuary or bay, sediment affects a variety of
factors including turbidity, light penetration, and the distribution of submerged aquatic vegetation (including sea
grasses). This applies both to clastic sediment, often
referred to as mineral matter, and particulate organic material, much of which is produced by algal productivity
fueled by high nutrient concentrations. Sediment also
plays an important role in the development of estuarine
turbidity maximum zones (ETM, also called turbidity
maximum zones, TMZ), a characteristic feature of many
estuaries. It has long been known that trapping of
suspended material in ETMs can be enhanced by
increased vertical stratification due to large freshwater
influx (Geyer, 1993). The physics of circulation near these
salinity gradients are such that they trap clastic sediment
and phytoplankton-derived organic material that has been
transported to or resuspended within the ETM, resulting
in high nutrient concentrations (Uncles et al., 2006;
124
CLIMATE CHANGE