anoxic basins include anoxic deep water fjord, such
as Saanich Inlet, the deeper basins of the Baltic, the
basin of the Black Sea, the Japanese Seto Inland Sea,
deep waters of the Sea of Cortez, Baja California,
and Santa Barbara Basin in the southern California
borderland.
Coastal Seas and Estuaries
Periodic hypoxia or anoxia also occurs on open
continental shelves, for example, the northern Gulf
of Mexico and the Namibian and Peruvian shelves
where upwelling occurs. More enclosed shelves such
as the northern Adriatic and the northwestern shelf
of the Black Sea also have periodic hypoxia or anoxia. In these instances, there is minimal exchange of
shelf-slope water and/or high oxygen demand on the
shallow shelf. Estuaries, embayments, and lagoons
are susceptible to the formation of hypoxia and anoxia if the water residence time is sufficiently long,
especially where the water column is stratified. Light
conditions are also important in these coastal habitats as a limiting factor on phytoplankton growth,
which, if excessive, contributes to high organic
loading within the confined waters.
Coastal ecosystems that have been substantially
changed as a result of eutrophication exhibit a series
of identifiable symptoms, such as reduced water
clarity, excessive, noxious, and, sometimes, harmful
algal blooms, loss of critical macroalgal or seagrass
habitat, development or intensification of oxygen
depletion in the form of hypoxia or anoxia, and, in
some cases, loss of fishery resources. More subtle
responses of coastal ecosystems to eutrophication
include shifts in phytoplankton and zooplankton
communities, shifts in the food webs that they support, loss of biodiversity, changes in trophic interactions, and changes in ecosystem functions and
biogeochemical processes.
In a review of anthropogenic hypoxic zones in
1995, Dı ´az and Rosenberg noted that no other environmental variable of such ecological importance
to estuarine and coastal marine ecosystems around
the world has changed so drastically, in such a short
period of time, as dissolved oxygen. For those reviewed, there was a consistent trend of increasing
severity (either in duration, intensity, or size) where
hypoxia occurred historically, or hypoxia existed
presently when it did not occur before. While hypoxic environments have existed through geologic
time and are common features of the deep ocean or
adjacent to areas of upwelling, their occurrence in
estuarine and coastal areas is increasing, and the
trend is consistent with the increase in human activities that result in nutrient over-enrichment.
The largest human-caused hypoxic zone is in the
aggregated coastal areas of the Baltic Sea, reaching
84 000 km
2
. Hypoxia existed on the northwestern
Black Sea shelf historically, but anoxic events became
more frequent and widespread in the 1970s and
1980s, reaching over areas of the seafloor up to
40 000 km
2 in depths of 8–40 m. There is also evidence that the suboxic zone of the open Black Sea
enlarged toward the surface by about 10 m since
1970. The condition of the northwestern shelf of the
Black Sea, in which hypoxia covered up to
40 000 km
2
, improved over the period 1990–2000
when nutrient loads from the Danube River decreased, but may be experiencing a worsening of
hypoxic conditions more recently.
Similar declines in bottom water dissolved oxygen
have occurred elsewhere as a result of increasing
nutrient loads and cultural eutrophication, for example, the northern Adriatic Sea, the Kattegat and
Skaggerak, Chesapeake Bay, Albemarle-Pamlico
Sound, Tampa Bay, Long Island Sound, New York
Bight, the German Bight, and the North Sea. In the
United States, over half of the estuaries experience
hypoxia at some time over an annual period and
many experience hypoxia over extensive areas for
extended periods on a perennial basis. The number
of estuaries with hypoxia or anoxia continues to rise.
Historic data on Secchi disk depth in the northern
Adriatic Sea in 1911 through the present, with few
interruptions of data collection, provide a measure of
water transparency that could be interpreted to depict surface water productivity. These data coupled
with surface and bottom water dissolved oxygen
content determined by Winkler titrations and nutrient loads outline the sequence of eutrophication in
the northern Adriatic Sea. Similar historical data
from other coastal areas around the world demonstrate a decrease in water clarity due to phytoplankton production in response to increased
nutrient loads that are paralleled by declines in water
column oxygen levels.
There are strong relationships between river flow
and nutrient flux into the Chesapeake Bay and
northern Gulf of Mexico and phytoplankton production and biomass and the subsequent fate of that
production in spring deposition of chlorophyll a.
Further there is a strong relationship between the
deposited chlorophyll a and the seasonal decline of
deep-water dissolved oxygen. Excess nutrients in
many watersheds are driven by agricultural activities
and atmospheric deposition from burning of fossil
fuels. The link with excess nutrients in more urban
areas, such as Long Island Sound, is with the flux of
nutrients associated from numerous wastewater
outfalls.
308 HYPOXIA
as Saanich Inlet, the deeper basins of the Baltic, the
basin of the Black Sea, the Japanese Seto Inland Sea,
deep waters of the Sea of Cortez, Baja California,
and Santa Barbara Basin in the southern California
borderland.
Coastal Seas and Estuaries
Periodic hypoxia or anoxia also occurs on open
continental shelves, for example, the northern Gulf
of Mexico and the Namibian and Peruvian shelves
where upwelling occurs. More enclosed shelves such
as the northern Adriatic and the northwestern shelf
of the Black Sea also have periodic hypoxia or anoxia. In these instances, there is minimal exchange of
shelf-slope water and/or high oxygen demand on the
shallow shelf. Estuaries, embayments, and lagoons
are susceptible to the formation of hypoxia and anoxia if the water residence time is sufficiently long,
especially where the water column is stratified. Light
conditions are also important in these coastal habitats as a limiting factor on phytoplankton growth,
which, if excessive, contributes to high organic
loading within the confined waters.
Coastal ecosystems that have been substantially
changed as a result of eutrophication exhibit a series
of identifiable symptoms, such as reduced water
clarity, excessive, noxious, and, sometimes, harmful
algal blooms, loss of critical macroalgal or seagrass
habitat, development or intensification of oxygen
depletion in the form of hypoxia or anoxia, and, in
some cases, loss of fishery resources. More subtle
responses of coastal ecosystems to eutrophication
include shifts in phytoplankton and zooplankton
communities, shifts in the food webs that they support, loss of biodiversity, changes in trophic interactions, and changes in ecosystem functions and
biogeochemical processes.
In a review of anthropogenic hypoxic zones in
1995, Dı ´az and Rosenberg noted that no other environmental variable of such ecological importance
to estuarine and coastal marine ecosystems around
the world has changed so drastically, in such a short
period of time, as dissolved oxygen. For those reviewed, there was a consistent trend of increasing
severity (either in duration, intensity, or size) where
hypoxia occurred historically, or hypoxia existed
presently when it did not occur before. While hypoxic environments have existed through geologic
time and are common features of the deep ocean or
adjacent to areas of upwelling, their occurrence in
estuarine and coastal areas is increasing, and the
trend is consistent with the increase in human activities that result in nutrient over-enrichment.
The largest human-caused hypoxic zone is in the
aggregated coastal areas of the Baltic Sea, reaching
84 000 km
2
. Hypoxia existed on the northwestern
Black Sea shelf historically, but anoxic events became
more frequent and widespread in the 1970s and
1980s, reaching over areas of the seafloor up to
40 000 km
2 in depths of 8–40 m. There is also evidence that the suboxic zone of the open Black Sea
enlarged toward the surface by about 10 m since
1970. The condition of the northwestern shelf of the
Black Sea, in which hypoxia covered up to
40 000 km
2
, improved over the period 1990–2000
when nutrient loads from the Danube River decreased, but may be experiencing a worsening of
hypoxic conditions more recently.
Similar declines in bottom water dissolved oxygen
have occurred elsewhere as a result of increasing
nutrient loads and cultural eutrophication, for example, the northern Adriatic Sea, the Kattegat and
Skaggerak, Chesapeake Bay, Albemarle-Pamlico
Sound, Tampa Bay, Long Island Sound, New York
Bight, the German Bight, and the North Sea. In the
United States, over half of the estuaries experience
hypoxia at some time over an annual period and
many experience hypoxia over extensive areas for
extended periods on a perennial basis. The number
of estuaries with hypoxia or anoxia continues to rise.
Historic data on Secchi disk depth in the northern
Adriatic Sea in 1911 through the present, with few
interruptions of data collection, provide a measure of
water transparency that could be interpreted to depict surface water productivity. These data coupled
with surface and bottom water dissolved oxygen
content determined by Winkler titrations and nutrient loads outline the sequence of eutrophication in
the northern Adriatic Sea. Similar historical data
from other coastal areas around the world demonstrate a decrease in water clarity due to phytoplankton production in response to increased
nutrient loads that are paralleled by declines in water
column oxygen levels.
There are strong relationships between river flow
and nutrient flux into the Chesapeake Bay and
northern Gulf of Mexico and phytoplankton production and biomass and the subsequent fate of that
production in spring deposition of chlorophyll a.
Further there is a strong relationship between the
deposited chlorophyll a and the seasonal decline of
deep-water dissolved oxygen. Excess nutrients in
many watersheds are driven by agricultural activities
and atmospheric deposition from burning of fossil
fuels. The link with excess nutrients in more urban
areas, such as Long Island Sound, is with the flux of
nutrients associated from numerous wastewater
outfalls.
308 HYPOXIA