Introduction
Hypoxia, or low oxygen, was first documented in the northern
Gulf of Mexico off the Louisiana coast in 1972.
Sporadic occurrences were observed in subsequent
years. In 1975 and 1976 two cruises were conducted
specifically to map a suspected area of low oxygen along the
Louisiana coast. These maps indicated small, disjunct
areas of hypoxia. With an increase in oceanographic
research in the Gulf, more reports of hypoxia
occurred. The first concerted, continuous and
consistent documentation of temporal and spatial extent of
hypoxia on the Louisiana and Texas continental shelf began
in 1985 with funding from the National Oceanic and
Atmospheric Administration, National Ocean Service.
Dr. Don Boesch, then Director of Louisiana Universities
Marine Consortium, initiated the study, which was led by Dr.
Nancy N. Rabalais of LUMCON and Drs. R. Eugene Turner and
William J. Wiseman, Jr. of Louisiana State
University.
Over the subsequent two decades the research team expanded
their studies, included more and more research components
and collaborators, and began unraveling the dynamics of
coastal hypoxia adjacent to the Mississippi River system and
the relationships with physical, biological and chemical
process in a river-dominated coastal ecosystem. The
results of these research programs and many more are
presented here. More details are found in the other pages of this website, particularly Results, Library
and Education sections.
(Back
to top)
What is Hypoxia?
Hypoxia for the Gulf of Mexico is defined as dissolved
oxygen concentration less than 2 mg/L, or 2 ppm, based on
the observational data that fish and shrimp that normally
live on the bottom cannot be captured in a bottom-dragging
trawl below the 2 mg/L level. In other coastal waters
the limit for hypoxia may be up to 3 or 5 mg/L.
Hypoxia occurs
naturally in many parts of the world’s marine
environments, such as fjords, deep basins, open ocean oxygen
minimum zones, and oxygen minimum zones associated with
western boundary upwelling systems. Hypoxic and anoxic
(no oxygen) waters have existed throughout geologic time,
but their occurrence in shallow coastal and estuarine areas
appears to be increasing. The second largest zone of
oxygen-depleted coastal waters in the global ocean is in the
northern Gulf of Mexico on the Louisiana/Texas continental
shelf at the terminus of the Mississippi River system. The
size of the Gulf of Mexico hypoxic zone reaches up to 22,000
km2 in mid-summer.
The hypoxic zone
in the northern Gulf of Mexico (average for 1993-2001) is
about the size of the state of New Jersey or the states of
Rhode Island and Connecticut combined. The largest size to
date equals the size of Massachusetts. Its
extent on the bottom is twice the total surface area of the
whole Chesapeake Bay, and its volume is several orders of
magnitude greater than the hypoxic water mass of Chesapeake
Bay.
Bacteria consume oxygen during decomposition of the excess
carbon that sinks from the upper water column to the
seabed. There will be a net loss of oxygen in the
lower water column, if the consumption rate is faster than
the diffusion of oxygen from surface waters to bottom
waters. Hypoxia is more likely when stratification of
the water column occurs and will persist as long as oxygen
consumption rates exceed those of resupply. Oxygen
depletion occurs more frequently in coastal areas with
longer water residence times, with higher nutrient loads,
and with stratified water columns.
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. No other environmental
variable of such ecological importance to estuarine and
coastal marine ecosystems around the world has changed so
drastically, and in such a short period of time, as
dissolved oxygen. The severity of hypoxia (either
duration, intensity, or size) increased where hypoxia
occurred historically or hypoxia exists now when it did not
occur previously. The severity of hypoxia has
increased in the northern Gulf of Mexico according to
indicators identified in sediment samples from the affected
area, and the size and frequency of occurrence have
increased as the flux of nitrate increased during the last
half of the 20th century.
(Back
to top)
Eutrophication is the increase in the rate of carbon
production and carbon accumulation in an aquatic ecosystem
(modified from Nixon, 1995). The definition was
developed initially as a description for the natural aging
process of freshwater systems, and has been more recently
applied to estuarine and coastal systems. The source
of the increased organic carbon may come from within the
system (autochthonous) or from outside the system
(allochthonous). This distinction is relevant when
management strategies are developed to reverse
eutrophication and to identify the sources and mechanisms of
carbon accumulation. For example, a coastal system
could become eutrophic from an increased delivery of organic
carbon from terrestrial sources or from nutrient-enhanced
primary production resulting from increased nutrient
loads. Reducing organic loading from riverine sources
would require different management strategies than that
required to reduce nutrient loads.
There is little doubt that human population growth and its
associated activities have altered the landscape, hydrologic
cycles, and the flux of nutrients essential to plant growth
at accelerating rates over the last several centuries
(Vitousek et al., 1997; Galloway and Cowling, 2002; Galloway
et al., 2003). In an effort to support human
population and to address the need for economic growth,
humans have increased significantly the flux of nitrogen and
phosphorus to aquatic and terrestrial ecosystems through
alterations of global cycles of those nutrients.
Excess nutrients are finding their way to the coastal ocean
in increasing amounts especially during the last half of the
20th century. There are thresholds of nutrient loading
above which the nutrient inputs no longer stimulate entirely
positive responses from the ecosystem such as increased
fisheries production. Instead, land-based sources of
nutrients are causing problems, for example, poor water
quality, noxious algal blooms, oxygen depletion and in some
cases, loss of fisheries production. Over the last
four decades it has become increasingly apparent that the
effects of excess nutrients that lead to eutrophication are
not minor and localized, but have large-scale implications
and are spreading rapidly.
The definition of eutrophication given above
recognizes that eutrophication is not a trophic state, but
a process involving changes leading to higher ecosystem
production. The causes of eutrophication should not
be confused with the process itself. The causes may
include changes in physical characteristics of the system
such as changes in hydrology, changes in biological
interactions such as reduced grazing, or an increase in
the input of organic and inorganic nutrients. For
example, upwelling systems cycle through phases of
increased nutrient availability, high primary and
secondary productivity, and often oxygen depletion in the
lower water column. The trophic status of upwelling
systems would be considered
‘eutrophic’—an organic carbon supply of
300-500 g C m-2y-1, as defined by Nixon (1995).
Upwelling systems, however, are not undergoing
eutrophication any more than mid-ocean oxygen minimum
zones, which follow a similar process of organic matter
accumulation and subsequent organic decomposition
resulting in oxygen depletion that affects mid-water
plankton and benthos where the zones impinge on
continental shelves and slopes and sea mounts (Levin et
al., 1991, 2000; Levin and Gage, 1998). There is
sufficient evidence that coastal ecosystems are
experiencing eutrophication, i.e., changes in the rate of
primary production over long periods, with subsequent
effects on multiple trophic levels. While the causes
may be multiple and interactive, eutrophication in the
coastal ocean and in the 20th and 21st centuries is more
often caused by increased loads of nutrients that would
otherwise limit the growth of phytoplankton.
A
variety of responses, such as noxious algal blooms, fish
kills, oxygen depletion, or seagrass losses, should also not
be confused with the process of eutrophication. The
responses are multiple and may often result in
‘increases’ or ‘decreases’ of
components of coastal ecosystems, to which humans often
ascribe beneficial or detrimental values. There is
little doubt that there have been ecosystem-level changes in
coastal systems as a result of eutrophication.
The accelerated time course of coastal
eutrophication in the northern Gulf of Mexico since the
1950s was typical for most temperate coastal regions at
the terminus of modified rivers flowing through developed
countries. In the northern Gulf of Mexico, the time
course of eutrophication and hypoxia followed most closely
the exponential growth of fertilizer use beginning in the
1950s. Elsewhere in the world, the relative
proportion of agriculture-source nutrients may not be as
high as in the Mississippi River basin, but other sources
of nutrients, municipal and industrial wastewater and
atmospheric deposition of oxidized and reduced forms of
nitrogen, also increased substantially since the
1950s. The consumption of fertilizers has plateaued
in many developed countries, but continues to increase in
developing countries. There is no indication that
fertilizer use will decrease, and controlling the nonpoint
sources of nutrient pollution has proven much more
difficult than controls emplaced for point sources.
Without the curtailing of nutrient loads, the trajectory
of coastal water quality degradation in the northern Gulf
of Mexico will likely continue, or perhaps worsen under
scenarios of increased precipitation in climate change
models. Elsewhere in the world, eutrophication, with
sometimes accompanying hypoxia, will continue without a
reduction in nutrient loads and will certainly accelerate
in areas where nutrient loads are on the rise.
( Back to
top)
|
