conditions of hypoxia preserved in sediments, where
sediments accumulate, provide clues to prior hydrographic and biological conditions.
Data from sediment cores taken from the Louisiana Bight adjacent to the Mississippi River where
sediments accumulate with their biological and
chemical indicators document increased recent eutrophication and increased organic sedimentation in
bottom waters, with the changes being more apparent in areas of chronic hypoxia and coincident with
the increasing nitrogen loads from the Mississippi
River system beginning in the 1950s. This evidence
comes as an increased accumulation of diatom remains and marine-origin carbon accumulation in the
sediments.
Benthic microfauna and chemical conditions provide several surrogates for oxygen conditions. The
mineral glauconite forms under reducing conditions
in sediments, and its abundance is an indication of
low-oxygen conditions. (Note that glauconite also
forms in reducing sediments whose overlying waters
are 42 mg l
À 1 dissolved oxygen.) The average
glauconite abundance in the coarse fraction of sediments in the Louisiana Bight was B5.8% from 1900
to a transition period between 1940 and 1950, when
it increased to B13.4%, suggesting that hypoxia
‘may’ have existed at some level before the 1940–50
time period, but that it worsened since then.
Benthic foraminiferans and ostracods are also
useful indicators of reduced oxygen levels because
oxygen stress decreases their overall diversity as
measured by the Shannon–Wiener diversity index
(SWDI) and shifts community composition. Foraminiferan and ostracod diversity decreased since the
1940s and early 1950s, respectively. While presentday foraminiferan diversity is generally low in the
Louisiana Bight, comparisons among assemblages
from areas of different oxygen depletion indicate
that the dominance of Ammonia parkinsoniana
over Elphidium spp. (A–E index) was much more
pronounced in oxygen-depleted compared to welloxygenated waters. The A–E index has also proven
to be a strong, consistent oxygen-stress signal in
other coastal areas, for example, Chesapeake Bay
and Long Island Sound. The A–E index from sediment cores increased significantly after the 1950s,
suggesting increased oxygen stress (in intensity or
duration) in the last half century. Buliminella
morgani, a hypoxia-tolerant species, known only
from the Gulf of Mexico, dominates the present-day
population (450%) within areas of chronic seasonal
hypoxia, and has also increased markedly in recent
decades. Quinqueloculina sp., a hypoxia-intolerant
foraminiferan, was a conspicuous member of the
fauna from 1700 to 1900, indicating that oxygen
stress was not a problem prior to 1900, but this
species is no longer present on northern Gulf of
Mexico shelf in the Louisiana Bight.
Multiple lines of evidence from sediment cores
indicate an overall increase in phytoplankton productivity and continental shelf oxygen stress (in intensity or duration) in the northern Gulf of Mexico
adjacent to the plume of the Mississippi River, especially in the last half of the twentieth century. The
changes in these indicators are consistent with the
increases in river nitrate-N loading during that same
period.
OMZ intensity and distribution vary over geological timescales as a result of shifts in productivity
or circulation over a few thousands to 10 ky. These
changes affect expansions and contractions of the
oxygen-depleted waters both vertically and horizontally. Paleoindicators, including foraminiferans,
organic carbon preservation, carbonate dissolution,
nitrogen isotopes, and Cd:Ca ratios that reflect
productivity maxima and shallow winter mixing of
the water column, are used to trace longer-term
changes in OMZs, similar to studies of continental
shelf sediment indicators.
Consequences
Direct Effects
The obvious effects of hypoxia/anoxia are displacement of pelagic organisms and selective loss of demersal and benthic organisms. These impacts may be
aperiodic so that recovery occurs; may occur on a
seasonal basis with differing rates of recovery; or
may be permanent so that a shift occurs in long-term
ecosystem structure and function. As the oxygen
concentration falls from saturated or optimal levels
toward depletion, a variety of behavioral and
physiological impairments affect the animals that
reside in the water column or in the sediments or that
are attached to hard substrates (Figure 4). Hypoxia
acts as an endocrine disruptor with adverse effects on
reproductive performance of fishes, and loss of secondary production may therefore be a widespread
environmental consequence of hypoxia. Mobile animals, such as shrimp, fish, and some crabs, flee
waters where the oxygen concentration falls below
3–2 mg l
À 1 .
As dissolved oxygen concentrations continue to
fall, less mobile organisms become stressed and move
up out of the sediments, attempt to leave the seabed,
and often die (Figure 5). As oxygen levels fall from
0.5 toward 0 mg l
À 1
, there is a fairly linear decrease
in benthic infaunal diversity, abundance, and biomass. Losses of entire higher taxa are features of the
HYPOXIA 311
sediments accumulate, provide clues to prior hydrographic and biological conditions.
Data from sediment cores taken from the Louisiana Bight adjacent to the Mississippi River where
sediments accumulate with their biological and
chemical indicators document increased recent eutrophication and increased organic sedimentation in
bottom waters, with the changes being more apparent in areas of chronic hypoxia and coincident with
the increasing nitrogen loads from the Mississippi
River system beginning in the 1950s. This evidence
comes as an increased accumulation of diatom remains and marine-origin carbon accumulation in the
sediments.
Benthic microfauna and chemical conditions provide several surrogates for oxygen conditions. The
mineral glauconite forms under reducing conditions
in sediments, and its abundance is an indication of
low-oxygen conditions. (Note that glauconite also
forms in reducing sediments whose overlying waters
are 42 mg l
À 1 dissolved oxygen.) The average
glauconite abundance in the coarse fraction of sediments in the Louisiana Bight was B5.8% from 1900
to a transition period between 1940 and 1950, when
it increased to B13.4%, suggesting that hypoxia
‘may’ have existed at some level before the 1940–50
time period, but that it worsened since then.
Benthic foraminiferans and ostracods are also
useful indicators of reduced oxygen levels because
oxygen stress decreases their overall diversity as
measured by the Shannon–Wiener diversity index
(SWDI) and shifts community composition. Foraminiferan and ostracod diversity decreased since the
1940s and early 1950s, respectively. While presentday foraminiferan diversity is generally low in the
Louisiana Bight, comparisons among assemblages
from areas of different oxygen depletion indicate
that the dominance of Ammonia parkinsoniana
over Elphidium spp. (A–E index) was much more
pronounced in oxygen-depleted compared to welloxygenated waters. The A–E index has also proven
to be a strong, consistent oxygen-stress signal in
other coastal areas, for example, Chesapeake Bay
and Long Island Sound. The A–E index from sediment cores increased significantly after the 1950s,
suggesting increased oxygen stress (in intensity or
duration) in the last half century. Buliminella
morgani, a hypoxia-tolerant species, known only
from the Gulf of Mexico, dominates the present-day
population (450%) within areas of chronic seasonal
hypoxia, and has also increased markedly in recent
decades. Quinqueloculina sp., a hypoxia-intolerant
foraminiferan, was a conspicuous member of the
fauna from 1700 to 1900, indicating that oxygen
stress was not a problem prior to 1900, but this
species is no longer present on northern Gulf of
Mexico shelf in the Louisiana Bight.
Multiple lines of evidence from sediment cores
indicate an overall increase in phytoplankton productivity and continental shelf oxygen stress (in intensity or duration) in the northern Gulf of Mexico
adjacent to the plume of the Mississippi River, especially in the last half of the twentieth century. The
changes in these indicators are consistent with the
increases in river nitrate-N loading during that same
period.
OMZ intensity and distribution vary over geological timescales as a result of shifts in productivity
or circulation over a few thousands to 10 ky. These
changes affect expansions and contractions of the
oxygen-depleted waters both vertically and horizontally. Paleoindicators, including foraminiferans,
organic carbon preservation, carbonate dissolution,
nitrogen isotopes, and Cd:Ca ratios that reflect
productivity maxima and shallow winter mixing of
the water column, are used to trace longer-term
changes in OMZs, similar to studies of continental
shelf sediment indicators.
Consequences
Direct Effects
The obvious effects of hypoxia/anoxia are displacement of pelagic organisms and selective loss of demersal and benthic organisms. These impacts may be
aperiodic so that recovery occurs; may occur on a
seasonal basis with differing rates of recovery; or
may be permanent so that a shift occurs in long-term
ecosystem structure and function. As the oxygen
concentration falls from saturated or optimal levels
toward depletion, a variety of behavioral and
physiological impairments affect the animals that
reside in the water column or in the sediments or that
are attached to hard substrates (Figure 4). Hypoxia
acts as an endocrine disruptor with adverse effects on
reproductive performance of fishes, and loss of secondary production may therefore be a widespread
environmental consequence of hypoxia. Mobile animals, such as shrimp, fish, and some crabs, flee
waters where the oxygen concentration falls below
3–2 mg l
À 1 .
As dissolved oxygen concentrations continue to
fall, less mobile organisms become stressed and move
up out of the sediments, attempt to leave the seabed,
and often die (Figure 5). As oxygen levels fall from
0.5 toward 0 mg l
À 1
, there is a fairly linear decrease
in benthic infaunal diversity, abundance, and biomass. Losses of entire higher taxa are features of the
HYPOXIA 311