to the eutrophication-related seasonal hypoxia that has
occurred on the shallow northwestern continental shelf
(Tolmazin, 1985; Zaitsev, 1992; Mee, 2001, 2006). The
Cariaco Basin also supports a large persistent anoxic area
below 250 m. Restricted circulation and high primary
production within the basin support this anoxia
(Müller-Karger et al., 2001; Müller-Karger et al., 2004).
This naturally occurring anoxic basin allows for sediments
to be deposited without bioturbation, forming varves of
alternating light and dark color, which correspond to the
dry or rainy season (Haug et al., 2001). Because of water
column anoxia, anoxic basins have a unique biogeochemistry that resembles that of anaerobic sediments (Madrid
et al., 2001; Stoeck et al., 2003). Bacteria inhabit both
the oxic and anoxic portions of the water column, with
a maximum in the suboxic interface (Taylor et al., 2001).
The suboxic layer oscillates between 200 and 300 m in
the Cariaco Basin and 150 and 200 m in the Black Sea
(Oguz, 2005).
Recently, a troubling decline in the oxygen content in
the open ocean is becoming apparent. Much of the decline
is related to global climate change but declining trends
near the coastline may be stronger and related to human
activities (Stramma et al., 2008; Stramma et al., 2012;
Gilbert et al., 2009; Keeling et al., 2010). The upper depth
limit of OMZs has major implications for fisheries.
Expansion of OMZs toward the surface in the eastern tropical Pacific has limited the depth distribution of tropical
pelagic marlins, sailfish, and tunas into a narrower surface
layer of oxic water about 50-100 m thick (Prince and
Goodyear, 2006). The high-performance physiology of
these fishes leads to a relatively high hypoxic threshold
(Brill, 1996), making any reduction in oxygen
problematic. Declining oxygen and expansion of the
OMZ in the tropical northeast Atlantic Ocean toward the
surface are also restricting usable habitat of billfishes
and tunas. From 1960 to 2010, Stramma et al. (2012)
found hypoxia-based habitat compression to decreasing
their suitable habitat by 15 %. The combination of
shallowing of OMZs encroaching onto outer continental
shelves and increased coastal eutrophication-induced
hypoxia will eventually reduce suitable habitat space for
both pelagic and demersal fishes.
Anthropogenic influence on dissolved oxygen
In contrast to OMZs, upwelling zones, and deep basins,
much of the hypoxia and anoxia in shallow coastal marine
areas have developed within the last 50 years and are
closely associated with anthropogenic activities. Diaz
and Rosenberg (1995) noted that no other environmental
variable of such ecological importance to estuarine and
coastal marine ecosystems has changed so drastically in
such a short period of time. Through time, there have been
consistent trends of increasing severity in duration, intensity, or extent of hypoxia in areas with long-term data, for
example, the northern Adriatic Sea (Justić et al., 1987).
Once a system develops hypoxia, it can quickly become
an annual event and a prominent feature affecting energy
flow (Elmgren, 1989; Baird et al., 2004). Currently, there
are over 600 hypoxia areas associated with anthropogenic
activities in the world’s coastal areas covering more than
245,000 km
2 of sea bottom (Diaz and Rosenberg, 2008;
Diaz et al., 2010; Conley et al., 2011; Figure 3).
The worldwide distribution of coastal hypoxia is
related to major population centers or is closely associated
with developed watersheds that export large quantities of
nutrients, specifically nitrogen and phosphorus. Up to
1970, there were scattered reports of hypoxia in North
America and northern Europe. By the 1990s, coastal
hypoxia was prevalent in North America, northern
Europe, and Japan. By the 2000s, there were increased
reports of hypoxia in South America, southern Europe,
and Australia (Figure 3). Considering the close association
of human population and hypoxia, it is likely that Asia and
the Indo-Pacific have many unreported hypoxic areas.
Eutrophication can be defined as the increase in the rate
of production of organic matter and accumulation of that
organic carbon within an ecosystem (Nixon, 1995;
Rabalais, 2004). This is typically in excess of what an
ecosystem is normally adapted to processing and is only
part of a complex web of stressors that interact to shape
and direct ecosystem level responses to stressors (Cloern,
2001). The primary driver of eutrophication in both freshwater and marine systems is excess nutrient enrichment,
but physical conditions that limit reaeration of bottom water
are also necessary for the development of hypoxia. Phosphorus is generally the limiting nutrient in freshwater
(Schindler, 1977), and increases in anthropogenic phosphorus have caused increased algal production and eutrophication in freshwater ecosystems worldwide (Carpenter et al.,
1999; Smith et al., 2006). For marine systems the limiting
nutrient tends to be nitrogen (Howarth and Marino, 2006).
This basic difference is related to the physical properties
of phosphorus and nitrogen compounds and their biogeochemical cycling through the freshwater and marine
environments. Basically, phosphorus tends to be more limiting in fresher, brackish waters and river plumes, and nitrogen is more likely to be limiting in the marine end member
of an estuary or a freshwater-dominated coastal system.
Eutrophication and associated hypoxia in freshwater
systems became widespread in the twentieth century, but
effective nutrient management has reversed this trend
where it has been rigorously implemented (Jeppesen
et al., 2005). In tidal portions of rivers and other water
bodies near dense population centers, severe hypoxia
and anoxia have been caused by discharge of raw sewage,
which is high in both nutrients and organic matter. Areas
devoid of fishes were reported as early as the late 1800s,
for example, the Mersey Estuary, UK, and persisted until
improvements in sewage treatment were implemented
(Jones, 2006). Much of the hypoxia and anoxia in shallow
coastal marine and estuarine areas are recent in origin and
related to a combination of nitrogen and phosphorus from
agriculture and human waste and atmospheric deposition
of nitrogen.
ANOXIA, HYPOXIA, AND DEAD ZONES
23
occurred on the shallow northwestern continental shelf
(Tolmazin, 1985; Zaitsev, 1992; Mee, 2001, 2006). The
Cariaco Basin also supports a large persistent anoxic area
below 250 m. Restricted circulation and high primary
production within the basin support this anoxia
(Müller-Karger et al., 2001; Müller-Karger et al., 2004).
This naturally occurring anoxic basin allows for sediments
to be deposited without bioturbation, forming varves of
alternating light and dark color, which correspond to the
dry or rainy season (Haug et al., 2001). Because of water
column anoxia, anoxic basins have a unique biogeochemistry that resembles that of anaerobic sediments (Madrid
et al., 2001; Stoeck et al., 2003). Bacteria inhabit both
the oxic and anoxic portions of the water column, with
a maximum in the suboxic interface (Taylor et al., 2001).
The suboxic layer oscillates between 200 and 300 m in
the Cariaco Basin and 150 and 200 m in the Black Sea
(Oguz, 2005).
Recently, a troubling decline in the oxygen content in
the open ocean is becoming apparent. Much of the decline
is related to global climate change but declining trends
near the coastline may be stronger and related to human
activities (Stramma et al., 2008; Stramma et al., 2012;
Gilbert et al., 2009; Keeling et al., 2010). The upper depth
limit of OMZs has major implications for fisheries.
Expansion of OMZs toward the surface in the eastern tropical Pacific has limited the depth distribution of tropical
pelagic marlins, sailfish, and tunas into a narrower surface
layer of oxic water about 50-100 m thick (Prince and
Goodyear, 2006). The high-performance physiology of
these fishes leads to a relatively high hypoxic threshold
(Brill, 1996), making any reduction in oxygen
problematic. Declining oxygen and expansion of the
OMZ in the tropical northeast Atlantic Ocean toward the
surface are also restricting usable habitat of billfishes
and tunas. From 1960 to 2010, Stramma et al. (2012)
found hypoxia-based habitat compression to decreasing
their suitable habitat by 15 %. The combination of
shallowing of OMZs encroaching onto outer continental
shelves and increased coastal eutrophication-induced
hypoxia will eventually reduce suitable habitat space for
both pelagic and demersal fishes.
Anthropogenic influence on dissolved oxygen
In contrast to OMZs, upwelling zones, and deep basins,
much of the hypoxia and anoxia in shallow coastal marine
areas have developed within the last 50 years and are
closely associated with anthropogenic activities. Diaz
and Rosenberg (1995) noted that no other environmental
variable of such ecological importance to estuarine and
coastal marine ecosystems has changed so drastically in
such a short period of time. Through time, there have been
consistent trends of increasing severity in duration, intensity, or extent of hypoxia in areas with long-term data, for
example, the northern Adriatic Sea (Justić et al., 1987).
Once a system develops hypoxia, it can quickly become
an annual event and a prominent feature affecting energy
flow (Elmgren, 1989; Baird et al., 2004). Currently, there
are over 600 hypoxia areas associated with anthropogenic
activities in the world’s coastal areas covering more than
245,000 km
2 of sea bottom (Diaz and Rosenberg, 2008;
Diaz et al., 2010; Conley et al., 2011; Figure 3).
The worldwide distribution of coastal hypoxia is
related to major population centers or is closely associated
with developed watersheds that export large quantities of
nutrients, specifically nitrogen and phosphorus. Up to
1970, there were scattered reports of hypoxia in North
America and northern Europe. By the 1990s, coastal
hypoxia was prevalent in North America, northern
Europe, and Japan. By the 2000s, there were increased
reports of hypoxia in South America, southern Europe,
and Australia (Figure 3). Considering the close association
of human population and hypoxia, it is likely that Asia and
the Indo-Pacific have many unreported hypoxic areas.
Eutrophication can be defined as the increase in the rate
of production of organic matter and accumulation of that
organic carbon within an ecosystem (Nixon, 1995;
Rabalais, 2004). This is typically in excess of what an
ecosystem is normally adapted to processing and is only
part of a complex web of stressors that interact to shape
and direct ecosystem level responses to stressors (Cloern,
2001). The primary driver of eutrophication in both freshwater and marine systems is excess nutrient enrichment,
but physical conditions that limit reaeration of bottom water
are also necessary for the development of hypoxia. Phosphorus is generally the limiting nutrient in freshwater
(Schindler, 1977), and increases in anthropogenic phosphorus have caused increased algal production and eutrophication in freshwater ecosystems worldwide (Carpenter et al.,
1999; Smith et al., 2006). For marine systems the limiting
nutrient tends to be nitrogen (Howarth and Marino, 2006).
This basic difference is related to the physical properties
of phosphorus and nitrogen compounds and their biogeochemical cycling through the freshwater and marine
environments. Basically, phosphorus tends to be more limiting in fresher, brackish waters and river plumes, and nitrogen is more likely to be limiting in the marine end member
of an estuary or a freshwater-dominated coastal system.
Eutrophication and associated hypoxia in freshwater
systems became widespread in the twentieth century, but
effective nutrient management has reversed this trend
where it has been rigorously implemented (Jeppesen
et al., 2005). In tidal portions of rivers and other water
bodies near dense population centers, severe hypoxia
and anoxia have been caused by discharge of raw sewage,
which is high in both nutrients and organic matter. Areas
devoid of fishes were reported as early as the late 1800s,
for example, the Mersey Estuary, UK, and persisted until
improvements in sewage treatment were implemented
(Jones, 2006). Much of the hypoxia and anoxia in shallow
coastal marine and estuarine areas are recent in origin and
related to a combination of nitrogen and phosphorus from
agriculture and human waste and atmospheric deposition
of nitrogen.
ANOXIA, HYPOXIA, AND DEAD ZONES
23