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1
Physiographic, Oceanographic, and
Ecological Context of the Gulf of Mexico
Unique aspects of the Gulf of Mexico (GoM), its abundant hydrocarbon
resources and the exceptional habitat and ecosystems at risk, cannot be
understood without an initial consideration of the processes responsible for
creation and maintenance of the basin and its ecosystems. Thus, we begin
our report with an overview of the geographic, oceanographic, and ecologi-
cal setting of the GoM. It is only within this context that we can properly
identify appropriate approaches for delineating, quantifying, and valuing the
impact of the Deepwater Horizon Mississippi Canyon-252 (DWH) oil spill
on ecosystem services and hope to understand the complex and dynami-
cally changing baselines associated with the GoM. We also recognize that
the term “baseline” has a specific meaning in the context of the Natural
Resource Damage Assessment program (see Definitions in Introduction) and
will endeavor to incorporate that into our analysis and discussion.
GEOLOGIC AND PHYSIOGRAPHIC SETTING
The modern GoM originated approximately 200 million years ago (mya)
with rifting of the supercontinent of Pangaea. As this rifting continued, the
continental crust thinned and eventually shallow basins were flooded with
sea water through a connection to the Pacific Ocean. During this time (ap-
proximately 180-200 mya) thick deposits of salt and other evaporates ac-
cumulated in the shallow basin (Salvador, 1991). Today this salt plays a key
role in creating an environment that is conducive to the accumulation and
production of hydrocarbons. As rifting continued, the basin deepened, the
Yucatan Peninsula rotated from Florida (Pindell and Kennan, 2009), Atlantic
waters entered, and salt deposition stopped. The overall process resulted
in a shelf-rimmed basin approximately 3,500 m deep with steep carbonate
banks at the eastern (West Florida Escarpment) and southern (Campeche
Escarpment) margins (Figure 1.1).
31
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32 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
Desoto
Ca
Mississippi River Canyon
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Delta Region
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nt
e
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a
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Alaminos
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m
he
ec nt
mp me
Ca carp
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Salt Scuplted
an tform
Minibasins & Ridges
cat
Yu e Pla
nat
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FIGURE 1.1 ETOPO1 Global Relief Model of the Gulf of Mexico Large Marine Ecosystem with inset of the
Gulf-Caribbean complex (based on data from Amante and Eakins, 2009).
SOURCE: Based on data from Amante, C. andFigure 1-1ETOPO1 1 Arc-Minute Global Relief Model: Pro-
B. W. Eakins,
Text editable, map not
cedures, Data Sources and Analysis. NOAA Technical Memorandum NESDIS NGDC-24, 19 pp, March 2009.
Image constructed using Fledermaus visualization software (http://www.ivs3d.com/products/fledermaus/).
Since the time of this rifting and deepening of the basin (about 180
mya), the GoM has continuously received large amounts of sediment from
the surrounding continents with by far the greatest input coming from the
central portion of the North American continent. The modern Mississippi
and Atchafalaya rivers, with their extensive deltas and a deep-sea fan, are
the most recent manifestations of this sedimentation. The organic debris,
particulates, and dissolved nutrients introduced into the Gulf by these riv-
ers ensured high primary productivity, high carbon-content sediments, and
abundant hydrocarbon resource rocks. The subsidence of the basin along
with the massive sediment loads provided by river input created the appro-
priate burial conditions (pressure and heat) to form oil and gas from these
source rocks. Movement of the deeply buried salt created traps and paths
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33
PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
for the oil and gas as well as a unique morphology of basins and domes in
some areas of the slope (Figure 1.1). The GoM was, through its geologic past,
an ideal environment for the generation and accumulation of recoverable
hydrocarbon resources.
The entire continental margin in the northern GoM continues to be
shaped by high sediment loads and the movement of salt within the strata of
the margin. The salt tectonics (movement) generates hydrocarbon migration
paths from source to reservoir. The Macondo well targeted hydrocarbons
trapped in Miocene (~12 mya) sand strata that are bounded by several salt
dome features. Frequently, the migration paths lead to the sediment surface
(Roberts and Carney, 1997) resulting in the creation of cold seep commu-
nities dependent on chemically extreme conditions. Seepage from these
conduits results in the natural injection of gas, liquid hydrocarbons, and
brines into the deep water of the Gulf.
In addition to the impact of salt migration, typical margin-forming
processes like sea-level change, erosion, and currents have also shaped
the margin and impacted the creation of submarine canyons. The result is
a series of seafloor ridges, minibasins, canyons, and escarpments (Jackson
et al., 2010) (Figure 1.1). As topographically complex as the Gulf margin
is, however, the largest portion of this system is blanketed with sediments
built up from terrestrial runoff and from the remains of pelagic organisms
forming a vast soft-bottom habitat.
DELTA ENVIRONMENTS AND NEARSHORE HABITATS
The Mississippi River system has long dominated the geological and
biological landscape of the northern GoM. The watershed encompasses
41 percent of the lower 48 United States (~3.2 × 106 km2) surpassed in size
only by the Amazon and Zaire rivers (Milliman and Meade, 1983; Meade,
1996). The river’s length and discharge of freshwater and sediment rank it
among the world’s top ten rivers. The annual average freshwater discharge
of 580 km3 enters the northern GoM through two main distributaries: the
Mississippi River delta southeast of the city of New Orleans, Louisiana, and
the Atchafalaya River delta ~200 km to the west on the central Louisiana
coast (Meade, 1995).
Sediment deposition and accumulation are essential for maintaining the
delta, offsetting natural subsidence, and preventing drowning of wetlands.
Over tens of thousands of years, the flow of sediment-laden freshwater cre-
ated a series of delta lobes that prograded (moved seaward), subsided, and
switched across the northern Gulf coastal landscape, establishing a deltaic
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34 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
plain that eventually formed the current Mississippi River delta about 1,000
years ago (Penland et al., 1988). Alongshore flow of water and sandy sedi-
ments formed ridges and cheniers (historic barrier ridges) at different loca-
tions as sea level changed, which, in turn, provided forested canopy areas.
As the delta formation prograded, barrier island arcs formed. Over time the
barrier islands fragmented into smaller islands with coastal lagoons. Smaller
rivers created smaller deltas or drowned river basins that became bays and
estuaries. Multiple habitats were shaped over geologic time, which con-
tinue to experience natural evolution modified by human activities and the
persistent need for sediment input to counteract increasing sea level rise.
Wetlands across the coast were historically sustained by substantial
input of river sediments. Over two centuries, the transformation to a primar-
ily agricultural landscape, with water systems engineered for drainage of
agricultural lands, navigation, and flood control, has altered the river basin
landscape, changed flow regimes, and reduced the suspended sediment
load. These changes have lessened the ability of the watershed to buffer
the effect of excess nutrients and other pollutants and have contributed to
the loss of landforms in the watershed and at the coast (Boesch et al., 1994;
Turner and Rabalais, 2003). Watershed manipulations along with natural
deltaic processes and intense human development of the coastal zone have
resulted in the loss of over 5,000 km2 of wetlands since the 1930s (updated
from Barras, 2006).
Meade (1995) estimated that the sediment load of the Mississippi River
since the beginning of the twentieth century is roughly half of its contribution
in the early 1700s. During the twentieth century, the hydrology of the Missis-
sippi River system was greatly altered by locks, dams, reservoirs, earthwork
levees, channel straightening, and spillways for purposes of flood protection,
navigation, and water supply. The largest decrease in suspended sediments
occurred after 1950, when the natural sources of sediments in the drainage
basin were cut off from the Mississippi River mainstem by the construction
of large reservoirs on the Missouri and Arkansas rivers (Meade and Parker,
1985; NRC, 2008a; Blum and Roberts, 2009). For the period 1975-2006, the
mean suspended load for the combined Mississippi and Atchafalaya rivers
was 205 mt y-1 (Blum and Roberts, 2009), less than the time-average rates
for sediment storage that were necessary to construct the current delta plain.
Thus the modern delta plain is limited in sediment supply and will undergo
substantial drowning by 2100 because sea level is now rising at least three
times faster than during delta-plain construction (Blum and Roberts, 2009).
This change in sedimentation rates is just one part of the dynamic baseline
for the GoM that must be considered when assessing impacts on ecosystem
services in this region.
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35
PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
METEOROLOGIC AND OCEANOGRAPHIC SETTING
Meteorology
Wind plays an important role in shaping the physical environment of the
GoM. Yet, there is a surprising lack of literature providing an overview of the
region’s meteorology. One of the better analyses comes from Mueller and
Willis (1983), who examined a 30-year record from New Orleans and char-
acterized the weather into eight types, further categorized by three indices:
1. Continental Index (CI) characterized by northerly winds and dry
cooler air.
2. Tropical Index (TI) characterized by southerly warm and moist
winds.
3. Storminess Index (SI) characterized by strong winds driven by ex-
tratropical storms in the winter and tropical storms in the summer.
From October to January, the CI occurs approximately 60 percent of
the time, and then drops steadily to a minimum in July. Conversely the TI
peaks near 90 percent from June to August but is much less pronounced
during October to February, when it reaches a minimum near 20 percent.
The SI peaks at about 50 percent during the months of December to Febru-
ary when extratropical or “winter” storms pass on approximately a biweekly
basis; from April through October, the SI is approximately 25 percent. In
addition to the larger scale processes identified above, land-sea breezes
generated by cooler land temperatures at night are prominent in some parts
of the Gulf, especially coastal Texas (Yocke et al., 2000). They tend to be
weakest off Louisiana, probably due to the predominance of swamps and
a poorly defined coastline. Spatially, the land-sea breezes extend at most
about 50 km offshore.
A considerable number of meteorological measurements (wind velocity,
pressure, temperature, and humidity) are available from land-based coastal
sites as well as offshore buoys. The primary sources of data are archived at
the National Data Buoy Center (NDBC) although a substantial number of
measurements are collected by buoys operated by the Texas General Land
Office.1 Yocke et al. (2000) provide a summary of NDBC and coastal mea-
surements in the northeastern Gulf from 1996 to 1997. Figure 1.2 illustrates
wind roses for NDBC 42001 located roughly in the center of the Gulf at
26°N, 89.7°W. Plots near the northern coast look qualitatively similar. Since
See http://tabs-os.gerg.tamu.edu/tglo/.
1
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36 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
FIGURE 1.2 Seasonal wind roses for NDBC Buoy 42001 located roughly in the center of the Gulf. The dotted
circles indicate the percent time of occurrence while the colors indicate the speed bin.
SOURCE: Image created by committee usingFigure 1-2
data from NOAA’s National Data Buoy Center, station 42001
Bitmapped
(http://www.ndbc.noaa.gov/station_page.php?station=42001).
the Gulf tends to be relatively warm and humid, it is a primary source of
moisture for rain over approximately half of the continental United States
(Vachon et al., 2010).
Physical Oceanography
The GoM is the largest and northernmost series of basins forming the
Gulf-Caribbean complex (see Figure 1.1). Semi-isolated on the western
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PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
periphery of the Atlantic, the pelagic components of the complex are very
closely linked to the larger ocean by the intensified western boundary cur-
rent of the North Atlantic gyre. Surface waters of this current flow through
the shallow gaps between Caribbean islands and enter the Gulf through
the Yucatan Strait, exiting through the Florida Strait. Deep flow is less well
understood within the two deep channels connecting the Caribbean to the
Atlantic. The multiple types of ocean currents in the Gulf have been ana-
lyzed and summarized in the literature, including a Gulf-wide summary by
Wiseman and Sturges (1999); a thorough analysis and synthesis of historical
data in deepwater by Nowlin et al. (2000); and a more recent anthology
of papers on Gulf circulation, most focused on deepwater, by Sturgis and
Lugo-Fernandez (2005), with one paper in the anthology specifically sum-
marizing the state of knowledge (Schmitz et al., 2005).
Deepwater Circulation
In the east-central Gulf, ocean currents in the upper 1,000 m of the wa-
ter column are often dominated by the Loop Current, a warm ocean current
that eventually joins the Gulf Stream (Figure 1.3). The northward extent of
the Loop varies by 400 km over the span of roughly a year, at times extending
north to the outer shelf of Louisiana, Mississippi, Alabama, and the Florida
Panhandle. During this northward extension, the Loop “pinches” west of Key
West and forms an eddy (henceforth referred to as an “LCE,” Loop Current
Eddy) of 150-400 km in diameter. Eventually the LCE migrates to the west
at about 2-5 km d–1. After a journey of several months it collides with the
western Gulf shelf where it slowly decays over the course of a year. Hori-
zontally, the currents in an LCE vary nearly linearly from a peak of 1-2 m s–1
at the outer edge to near zero at the center. Below 75 m depth, the currents
decay exponentially and are minimal by 1,000 m (Cooper et al., 1990).
The Loop Current and LCE serve as important transport mechanisms
in the Gulf, bringing in approximately 28 × 106 m3 s–1 of relatively warm,
salty Caribbean waters. While attached to the Florida Current, most of this
water exits a week or so later through the Florida Strait, after the Loop has
entrained some indigenous Gulf water and diffused some of its own salty
warm water into the Gulf. The more significant transport occurs when an
LCE separates and later unravels in the western Gulf. Both mechanisms work
to transport pollutants and shelf waters, including freshwater runoff with its
constituents (sediment, nutrients, pollutants, and organic carbon) from the
major Gulf rivers.
In addition to the LCEs, the Gulf is teeming with mesoscale eddies
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38 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
FIGURE 1.3 Infrared satellite image showing major surface currents in the Gulf (LCE = Loop Current Eddy).
Red colors indicate warm water while blue colors indicate cold water.
SOURCE: Copyright © 2011 The Johns Hopkins University 1-3
Figure Applied Physics Laboratory. All Rights Reserved.
Bitmapped
(cyclones) on the order of 30 km in diameter with peak speeds of 50 cm s–1
(Schmitz et al., 2005; Figure 1.3). Some are generated by the strong shear
found along the periphery of the Loop Current and LCE but others form along
additional fronts, as demonstrated by the eddy separating colder and fresher
shelf and slope waters (Figure 1.3). As with the LCEs, mesoscale eddies have
the potential to transport freshwater constituents over distances, although
the amount of transport is limited by the smaller size (30 km) and shorter
life spans (on the order of one week). Both LCEs and mesoscale eddies are
important means of transporting water between the shelf and the deeper
Gulf (Schmitz et al., 2005).
Another persistent and energetic deepwater current in the Gulf can be
traced to planetary waves, also known as Rossby Waves, which were first
documented by Hamilton (1990). These ubiquitous waves have periods on
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39
PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
the order of 14 days and propagate across the deepwater Gulf generating
currents on the order of 15 cm s–1 through much of the water column. Within
a few kilometers of the Sigsbee Escarpment (Figure 1.1), the sharp topogra-
phy can trap and substantially intensify these planetary waves, generating
currents on the order of 100 cm s–1 near the bottom in 2,000 m of water
(Dukhovskoy et al., 2009). Because these waves have long wavelengths,
on the order of 100 km, they are also capable of transporting pollutants or
other materials some distance over a one-to-two-week period. The oscilla-
tory nature of waves, however, might also cause pollutants to be returned
when the current reverses direction. This phenomenon was thought to be
the mechanism of transport for the large deepwater plume of oil from the
Macondo wellhead (Camilli et al., 2010).
Currents driven by astronomical tides in the deepwater Gulf are weak
in amplitude with small spatial changes in phase (Reid and Whitaker, 1981).
Typical currents in deepwater are less than 2 cm s–1. Mean wind-driven cur-
rents in the deepwater Gulf are not readily discernible although a number of
authors conjecture the existence of weak anticyclonic gyres in the western
and central Gulf that could be partially driven by large-scale winds (see
Schmitz et al., 2005). Despite the weak mean winds, the Gulf is subjected
to strong storm winds. During hurricanes, currents on the order of 2 m s–1
can be generated over the mixed layer for a few hours. These storms can
also generate inertial currents with periods of approximately 24 hours and
oscillations of 50 cm s–1 that reach deep into the water column and persist for
several days after storm passage (Brooks, 1983). The storm-driven response
during winter storms is much weaker than during hurricanes, reflecting the
weaker winds and the deeper mixed layer typical of fall-winter months. Be-
cause the currents are primarily oscillatory, inertial currents cannot transport
pollutants more than a few tens of kilometers.
There have been few published investigations of the deeper layers of the
Gulf with the major exception of DeHaan and Sturges (2005) and Weatherly
et al. (2005); the currents they discuss are only a few cm s–1 in strength.
Shelf Circulation
Circulation on the shelf is closely tied to the local topography reflecting
the importance of friction in the current dynamics. In general, tidal currents
are on the order of 5 cm s–1. Regardless of the location, winds are a factor
especially during tropical cyclones or winter storms. The transfer of wind
energy through the water column is dramatically affected by stratification
induced by solar heating and the substantial freshwater discharge from the
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40 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
major rivers in the Gulf. When this stratification is strongest, near-bottom
currents become uncorrelated with the local winds, although if the winds
change rapidly some wind energy penetrates downward via inertial oscilla-
tions (Wiseman et al., 2004).
Other significant types of currents vary according to the region as
follows:
• The west Florida shelf is discussed in some detail by Weisberg et
al. (2005). Wind forcing is dominant over most of the shelf with
upwelling in the winter and downwelling in the summer. The Loop
Current sets up a large-scale sea-level gradient along the shelf that
models suggest can generate substantial southerly currents.
• On the Mississippi-Alabama shelf, the discharge from the Mobile-
Tombigbee, Pascagoula, Pearl, and Mississippi rivers are an impor-
tant influence on near-surface stratification, which in turn affects
the transfer of wind through the water column. A mean cyclonic
surface circulation has been suggested by Dinnel (1988), although
there is no evidence of this circulation in the more recent analysis
of DiMarco et al. (2005). On the outer shelf, major intrusions of
the Loop Current and LCE occur every few years and can persist
for one to two months causing large exchanges between the shelf
and slope as summarized by Schmitz et al. (2005).
• On the Louisiana-Texas shelf, the Mississippi and Atchafalaya riv-
ers play a large role in modifying the vertical stratification, which
in turn affects the transfer of wind through the water column. The
general circulation pattern is a large but weak (on the order of 5 cm
s–1 on average) cyclonic cell, though its direction can be reversed
during mid-summer in the presence of stronger westerly winds
(Cochrane and Kelly, 1986). Upwelling favorable conditions can
occur with winds from the north in the summer (Wisemann et al.,
2004).
Estuarine Circulation
Numerous estuaries break up the coastline along the northern Gulf. An
extensive system of bayous surrounds the Mississippi and Atchafalaya rivers.
Currents in these areas are dominated by river discharges and wind. Direct
transfer of wind momentum to the lower water column is often constrained
by freshwater stratification but, given the close proximity of land, the wind
can also set up larger scale pressure gradients that effectively drive flow
beneath the mixed layer.
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PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
ECOLOGICAL SETTING
Habitat Characterization
Within the context of ecosystem services, habitat provides one descrip-
tion of ecosystem structure. Stated simply, habitat is the place where an
organism or a recurrent suite of organisms lives. Given that biotic content
is the primary criteria for recognition of habitats, the geospatial task of map-
ping habitats should be based on the single criterion of a comprehensive
biotic inventory, at least conceptually. Unfortunately, such highly detailed
surveys are seldom feasible over management-relevant spatial scales and
time constraints. Habitat mapping, therefore, proceeds with the use of
alternative criteria. Among the numerous alternatives in use, one requires
narrowing actual biotic surveys to a few indicator species (e.g., mangrove
habitat, fish habitat, brown pelican habitat, red snapper habitat, cold coral
habitat, etc.). Habitats can also be delineated on the basis of abiotic factors
known to have a strong correlation with a particular suite of species (e.g.,
low salinity habitat, subtidal habitat, deepwater habitat, etc.). In an attempt
to include many types of information in habitat classifications various met-
rics have been proposed; Diaz et al. (2004) identified 64 separate metrics
that have been used or proposed. With such a multitude of criteria, it is easy
to understand that Franschetti et al. (2008) found 1,121 European marine
habitats, but was able to reduce them to a list of only 94.
Among the marine habitats, coastal ones are the best characterized
and delineated. The importance of emergent vegetation, where present, as
a structuring component of habitat has led to a long-standing tradition of
classification based on plant cover. The strong correlation between plant
community and abiotic factors such as inundation, salinity, and substrate
allow non-biotic mapping of habitats that are largely consistent with biotic
criteria. Beyond the beach and progressing into deeper water, characterizing
habitats becomes increasingly tentative. Data for strictly biotic criteria are
increasingly hard to gather, and the use of a few “indicator species” prob-
ably has minimal ecological relevance. Advancement in acoustic seafloor
mapping has resulted in considerable interest in habitat classification using
remote and rapid surveys of bathymetric and seafloor characteristics (Kenny
et al., 2003; Todd and Greene, 2008). The unfortunate limitation of these
approaches lies in the difficulty of determining if a substantial correlation
with seafloor biology exists (Diaz et al., 2004; Stevens and Connolly, 2004).
The approach may be best applied to bathymetric relief seafloors. It may
be far less useful when applied to the far more common mud bottoms like
those across much of the GoM (Zajac, 2008).
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44 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
Coastal habitats provide a wide range of ecosystem services, including
support for fishery production, water quality improvement, nutrient cycling,
wildlife habitat provision, recreational opportunities, storm surge protection,
carbon sequestration, and social support of coastal-based economies, such
as oyster harvesting, tourism (specifically ecotourism), resource extraction,
and water-borne transportation. These coastal habitats provide shelter and
food resources for fishes, crustaceans, and shellfish.
Wetlands are widely recognized for their capacity to remove nutrients
and pollutants from overlying waters, in effect improving water quality,
recycling reactive nitrogen to N2 gas and reducing the potential for eutro-
phication, defined as the increase in the rate of primary production and
accumulation of resulting organic carbon in an aquatic system (modified
Rabalais, 2004 and Nixon, 1995). Eutrophication is manifested as turbid
waters, growth of filamentous algae on seagrass blades, noxious and harmful
algal blooms, and oxygen depletion (as microbes decompose the accumulat-
ing carbon). The removal of nitrogen depends on the type of wetland, the
concentration of nitrogen entering the wetland, the water residence time,
and the acreage available for the denitrification process.
Offshore Habitats
Offshore habitats of the northern GoM start at the low-tide level on
coastal shores and extend to the Sigsbee abyssal plain with a maximum
depth > 3,800 m. About 40 percent of the area is covered by vertically mixed
shelf water that is influenced by freshwater inflows of the 20 large river sys-
tems draining into the coast. The break between continental shelf and slope
varies around the circumference of the Gulf. In general it begins between
100 and 150 m giving the shelf a width ranging from 90 km off southern
Texas to 220 km off Florida. Unusually narrow 12 km and 32 km shelves are
encountered off the mouth of the Mississippi River and at the head of Desoto
Canyon. The substrate is predominantly near-shore sands grading seaward
to silt and mud. There is a related transition in the species composition of
bottom fauna. Shelf populations of the shrimp Litopenaeus setiferus and Far-
fantepenaeus aztecus support a major commercial fishery. Fish such as red
snapper (Lutjanus campechanus) support both commercial and recreational
fisheries. The shelf contains essential habitat for estuarine-dependent species
with life histories that include an estuary-to-ocean migration.
The periphery of the Gulf of Mexico is characterized by a remarkable
diversity of hard-bottom features rising from the seafloor and forming a
series of elevated bathymetric features. Some of these features are ancient
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PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
shorelines, others former seabeds thrust up by salt movement. Still others
originate from ancient coral reef growth. These hard banks support diverse
communities of tropical and subtropical plants, invertebrates, and fishes and
are considered to be sensitive habitats. The structures that rise a few meters
above the surrounding seafloor, such as a series of shoals off the Atchafalaya
River delta, provide refugia from seasonal bottom-water hypoxia on the
northern Gulf of Mexico. Middle and outer shelf hard-bottom structures are
mostly drowned reefs, or are associated with salt domes and outcrops of
limestone, sandstone, claystone, and siltstone with a variety of soft coral,
sponge, and macroalgae. These banks are considered to be critical spawning
habitat for many commercially important species of groupers and snappers.
Banks reaching to the euphotic zone and supporting coral reefs are few in
the northernmost Gulf (East and West Flower Garden Banks) but become
increasingly abundant on the central and southern Florida coast (Pulley
Ridge and Tortugas Bank). The coral framework is dominated by Montastrea
annulari, Montastrea cavernosa, Diploria strigosa, and Porites astreoides.
Like coral reefs everywhere, the Gulf reefs are biodiversity hotspots with
strong aesthetic appeal that support fisheries, biological prospecting, and
recreational uses. Some of these high-diversity habitats can be found within
one of the designated areas of the National Marine Sanctuary Program, or are
listed as a Habitat of Particular Concern by BOEMRE for restricted activities.
Offshore from the edge of the shelf, two oceanographic features previ-
ously described impact the habitats of the deep basin: the Loop Current that
brings Caribbean waters into the Gulf from its southwestern boundary of the
Yucatan Strait and the anticyclonic cell circulation along the western side.
These two features are distinct because of seasonal differences in the depth
of their thermoclines (Cochrane, 1972) that create conditions for different
marine community composition. In the northern GoM, the continental slope
is atypically diverse due to the unusual combination of geological processes
described earlier. The complex geomorphologies of the sediment slope and
its basins and ridges have diverse fauna with typical depth-related declines
in biomass and species, replacement deposits, and an overall maximum
species richness that occurs at mid-slope depths. Fishes and crabs decrease
in diversity with depth, while echinoderms and sediment-dwelling worms
increase. The biological communities of the vast mud bottoms are the last
consumers of organic carbon before the residual carbon becomes buried.
Fisheries exploitation of deepwater is minimal, but specialized trawl fisheries
exist for royal red shrimp (Hymenoenaeus robustus), rock shrimp (Sicyonia
brevirostris), and calico scallop (Agropecten gibbus).
Surface-water habitats such as Sargassum mats (Wells and Rooker, 2004)
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46 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
and water column communities are influenced by a number of important
oceanographic variables, including
1. dissolved oxygen levels (Prince and Goodyear, 2006),
2. the location of the thermocline (Bigelow and Maunder, 2007),
3. light levels (Dewar et al., 2011), and
4. the presence of oceanic fronts in the edges of the Loop Current and
ocean eddies (Kleisner et al., 2007).
The thermocline can be a boundary for many epipelagic (upper water) or-
ganisms. Mesopelagic organisms occupy the waters below this boundary,
while photosynthetic organisms are absent in the bathypelagic zone which
is almost completely absent of light. Most organisms remain within their
respective zones, but some migrate between the epipelagic and mesopelagic
in search of prey, for example swordfish (Xiphias gladius) (Dewar et al.,
2011) and blue marlin (Makaira nigicans) (Kraus and Rooker, 2007). Other
organisms such as zooplankton and sperm whales (Physeter macrocephalus)
may migrate over the full depth of the water column. The vertical zones
extend over smaller or larger horizontal scales for epipelagic and mesope-
lagic fish (Kleisner et al., 2010). While the deep seafloor has traditionally
been considered an ecological sink receiving food from shallower water but
contributing little in the upward direction, this view is changing (Tenore et
al., 2006). Many deep-sea animals produce eggs and larvae that develop
among surface water food webs and participate in shallow water food
webs. Zooplankton serve as conveyors of carbon from the surface to greater
depths. Sperm whales forage at great depths (Amano and Yoshioka, 2003),
but must return to the surface to breath. Throughout the water column and
deep seafloor, microbes are contributing to the larger marine ecosystem by
returning nutrients through their metabolic functions.
The deep slope contains special habitats associated with the geochem-
istry of upward migrating salt, seeping hydrocarbons, and the precipitation
of calcium carbonate (Roberts and Carney, 1997). Active sites are associated
with chemosynthetic communities dominated by bathymodiolid mussels
and vestimentiferan tubeworms; species composition changes with depth in
a manner similar to mud-bottom species. Carbonate features with minimal to
no seepage support a diverse fauna of sessile and rock boring fauna. Unique
chemosynthetic and deep coral communities, such as Lophelia pertussa,
are biodiversity “hotspots” that receive regulatory protection from offshore
drilling. They are widely appreciated as natural systems and iconic species
of the deepwater GoM ecosystems.
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PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
Food Webs and Trophic Interactions
While the GoM’s physical habitats provide ecosystem boundaries, their
components link through complex trophic interactions. A food web is formed
by linkages between multiple organisms in a complex feeding hierarchy that
changes in time and in response to external factors (Pimm et al., 1991). The
number of links in the food chain that one organism is removed from another
describes the organism’s trophic level. There are rarely more than four or
five levels (primary producers, herbivores, omnivores, and carnivores) in a
food web, because only a fraction (10-20 percent) of the energy at each level
can be incorporated by the next level up. However, the organization within
each level and interactions among the levels may be more complex than
previously thought (Allen and Fulton, 2010; Montagnes et al., 2010). Each
species in an ecosystem is affected by other organisms, interactions between
trophic levels, and the environment. There are few single prey-single preda-
tor relationships, such that if one species is removed from an ecosystem,
several other species will be affected. Multi-level trophic interactions may
be more reflective of changes in trophic structure and potential ecosystem
services than the presence, absence, or relative abundance of a species.
Changes in trophic levels of global and regional catches are considered
by the Food and Agriculture Organization (2002) as a better reflection of
trends in fisheries than the proportion of fish stocks that are reported as de-
pleted, overexploited, fully exploited, and moderately exploited (Food and
Agriculture Organization, 2002). On the other hand, trophic level metrics,
such as mean trophic level, were not found to be useful indicators of ma-
rine biodiversity and ecosystem status, particularly fisheries status, because
the metrics are influenced by changes in economics, management, fishing
technology, and targeting patterns (Branch et al., 2010).
Biodiversity
Biodiversity is the degree of variation of life forms within an ecosystem
or biome. It is often a good measure of the health of an ecosystem. Bio-
diversity contributes to the stability of ecosystems, due to the diversity of
functional responses of community members to perturbation. From the point
of view of an individual organism, the ability to recover from a disturbance
can vary with different life history characteristics. For example, when an
organism can exploit a wide range of resources (as a generalist), a decrease
in biodiversity is often less likely to impact that organism. However, an
organism that can exploit only a limited range of resources (a specialist) is
more likely to be affected by a decrease in biodiversity. Biodiversity in the
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48 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
GoM, as elsewhere, is threatened by a number of factors including overex-
ploitation of living resources, reduced water quality, coastal development,
shipping, invasive species, factors associated with global climate change,
the expansion of hypoxic and anoxic zones, and increases in harmful algal
blooms (Fautin et al., 2010). Trophic structure and the complex dynamics of
trophic level interactions affect the rate at which a population will recover
from a negative impact.
Felder and Camp (2009) conducted the most recent, comprehensive
biodiversity survey of the Gulf. The high diversity of marine life that exists
in the Gulf makes it one of the most biodiverse oceanic water bodies on
the planet (Fautin et al., 2010). Unfortunately, detailed studies of biodiver-
sity in the GoM have been limited to a few well-studied regions including
the northwestern Gulf oil and gas region, the Florida Keys, and areas of
known high biodiversity like oyster reefs and seagrass beds. The effects
of preserving diversity can be broadly beneficial to a wide spectrum of
important ecosystem services, including fisheries, water quality, recreation,
and shoreline protection. Conserving diversity increases the likelihood that
ecosystems can adapt and recover following disturbances from natural or
anthropogenic causes (Palumbi et al., 2009). Plant and animal species of
interest—including keystone (organisms that play an influential role in main-
taining ecosystem structure), indicator (species that indicate the presence of
certain environmental conditions), commercially important, and endangered
species of the GoM—are highlighted in Table 1.1.
Microbial Diversity
Biodiversity takes on a different meaning when applied to the two
microbial domains of life, the Bacteria and the Archaea (single-celled or-
ganisms that lack a nucleus). Microbes dominate the global ocean, both
in terms of numerical abundance (averaging 106 per milliliter of seawater
for an estimated total of 1030 in the global water column) and of biomass,
up to 90 percent of the total (Fuhrman et al., 1989; Whitman et al., 1998).
Microbial biodiversity exceeds all plants and animals combined. Using state-
of-the-art, high-throughput DNA sequencing during the recently completed
Census of Marine Life, researchers discovered that the ocean contains, at
a minimum, many millions of species of Bacteria and Archaea (20,000 in
a single liter; Amaral-Zettler et al., 2010). Although the Census of Marine
Life was an extensive global endeavor, only a single location in the GoM
(Mississippi Canyon 118 on the Louisiana continental slope at 900 m depth)
was surveyed for microbes and results are not yet published. In general, the
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PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
TABLE 1.1 Species of Interest in the GoM Large Marine Ecosystem
Name Species Significance
Kemp’s Ridley sea turtle Lepidochelys kempii most endangered marine turtle in the
world; keystone species
Whooping crane Grus americana endangered
West Indian manatee Trichechus manatus ssp. latirostris endangered;
keystone species
Menhaden Brevoortia patronus largest commercial fishery by weight
Penaeid shrimp Litopenaeus setiferus (white), highest monetary value commercial fishery
Farfantepenaeus duorarum (pink),
Farfantepenaeus aztecus (brown)
Grouper and snapper various offshore commercially and recreationally
important
Atlantic croaker Micropogonias undulatus indicator species
American oyster Crassostrea virginica commercial coastal fishery;
indicator species
Blue crab Callinectes sapidus commercial coastal fishery
Spiny lobster Panulirus argus southern Gulf commercial fishery
Pink conch Eustrombus gigas regulated recreational fishery
Spotted sea trout Cynoscion nebulosus northern Gulf recreational fishery
Red drum Sciaenops ocellatus northern Gulf recreational fishery
Red snapper Lutjanus campecheanus northwestern Gulf recreational fishery
Bottlenose dolphin Tursiops truncatus well known by public; keystone species
American alligator Alligator mississippiensis keystone species
American crocodile Crocodylus acutus keystone species
Smooth cordgrass Spartina alterniflora keystone species
Saltmeadow cordgrass Spartina patens keystone species
Common reed Phragmites australis indicator species
Maidencane Panicum hemitomon indicator species
SOURCE: Derived from Fautin et al., 2010.
widespread occurrence of marine microbes, their ability to reproduce rap-
idly when conditions allow, and the functional redundancy built into their
communities mean that environmental changes and anthropogenic impacts
do not imperil the existence of specific groups, such as those that recycle nu-
trients, degrade hydrocarbons, or provide chemosynthetically derived food
for higher organisms at hydrocarbon seeps. Instead, altered environmental
conditions favor the reproduction and actions of those microbes best suited
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50 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
to prevailing conditions. A case in point is the DWH plume that stimulated
the growth of deep-sea indigenous γ-proteobacteria that are closely related
to known petroleum degraders (Hazen et al., 2010).
Human Interactions and Coastal Communities
There is a wide range of human communities affected by and interact-
ing with various components of the GoM ecosystem, depending on what
assumptions are made regarding boundaries and scope of interaction. For
example, consumers worldwide who eat seafood harvested from the GoM
are ultimately affected by the availability and quality of commercial fish and
shellfish stocks in the Gulf. Groups outside the region, however, often have
alternatives for purchasing fish from other U.S. regions and internationally.
Obtaining seafood from other sources threatens businesses around the Gulf
region. People living in or near the GoM system may rely on specific Gulf
resources which cannot be easily substituted.
Coastal U.S. counties in the GoM include 142 jurisdictions, as defined
by the Strategic Environmental Assessments Division of the National Oce-
anic and Atmospheric Administration (NOAA). Wilson and Fischetti (2010)
suggest an environmentally based approach for classifying coastal counties
to understand the interplay between human activities and water and habitat
quality along the coast, and include a greater area than simply the group of
counties that physically border the water. To be considered a coastal county
in the NOAA system, a county must meet at least one of the following crite-
ria: “1) at least 15 percent of a county’s total land area is located within the
Nation’s coastal watershed; or 2) a portion of or an entire county accounts
for at least 15 percent of a coastal cataloguing unit.”3
U.S. GoM coastal counties comprise 115,000 square miles of land area
(Bureau of the Census Statistical Abstract4), with 20.4 million residents, re-
flecting 7 percent of the overall U.S. population as of July 1, 2009. Residents
in these counties occupied 9,144,000 housing units; and in 2008, 463,000
non-farm business establishments employed 7,028,000 non-farm employ-
ees. Employment in coastal counties comprises about 18 percent of all
employment in the Gulf Coast states, although the proportion ranges as high
as 34 percent in Louisiana and 31 percent in Florida (Adams et al., 2004).
Major occupations in the northern Gulf of Mexico region as reported
in 2010 include oil and gas drilling, water transportation-related industries,
and leisure and hospitality, the latter particularly related to gaming and ca-
See http://www.census.gov/geo/landview/lv6help/coastal_cty.pdf.
3
See http://www.census.gov/compendia/statab/2011/tables/11s0026.pdf.
4
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PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
sinos but also including other types of recreation (Bureau of Labor Statistics,
U.S. Department of Labor5). Tourism continues to be a key driver of coastal
economic and property development (Yoskowitz, 2009).
Commercial fishing is prominent in the Gulf economy, with commer-
cial fisheries landings accounting for about 25 percent of all U.S. seafood
landings and about 21 percent of the total U.S. dockside fisheries value
(Adams et al., 2004). The Gulf fishery is characterized by a diverse fleet of
vessels harvesting from open water to near shore, including nearly 25,000
commercial fishing craft representing close to one-third of the entire U.S.
commercial fishing fleet (Adams et al., 2004). In 2008, 165 processing plants
and 229 wholesale plants employed nearly 10,000 workers that supported
the commercial fishery.6 In addition, the recreational fishing industry sup-
ports employment in coastal counties (Adams et al., 2004).
GoM Fisheries
The diversity of fishery species in the GoM is listed in the fishery man-
agement plans of the GoM Fishery Management Council (GoM FMC, 2010).
The species list includes 3 mackerels, 14 snappers, 15 groupers, 5 tilefish, 4
jacks, 2 sand perches, 1 gray triggerfish, 1 hogfish, 4 shrimp, 2 lobsters, and
2 stone crabs. There are other important species that occur in the GoM that
are managed under the highly migratory species fishery management plan
(NOAA, 2004). These include 8 species of tuna, 6 billfish, and 72 species
of sharks. This accounting does not include all harvested species because
many of lesser economic importance are not listed individually in the plans,
for example, dolphinfish, a species recognized to be part of the coastal
migratory pelagic fishery but not part of the managed units. There are also
important fishery species exclusively managed by coastal states, including
some commercially important estuarine species (e.g., sea trout and mullet)
and those reserved for recreational use, such as snook, tarpon, and bonefish.
Aquaculture
The GoM FMC manages offshore aquaculture operations in federal
waters of the Gulf. Although at present there are few aquaculture operations
in federal waters the potential use area is large and ranges throughout the
Gulf (Figure 1.4; GoM FMC, 2009).
See http://www.bls.gov/oes/highlight_gulf.htm.
5
See http://www.st.nmfs.noaa.gov/st1/fus/fus09/10_industrial2009.pdf.
6
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52 APPROACHES FOR ECOSYSTEM SERVICES VALUATION FOR THE GULF OF MEXICO
FIGURE 1.4 Potential for offshore aquaculture in the GoM. Pink represents all areas considered suitable
for aquaculture in the Gulf EEZ (28,719 nm2). Zones 1–13 (10,392 nm2) are preferred zones under the GoM
FMC aquaculture plan. Figure 1-4
SOURCE: GoM FMC, 2009.
Bitmapped
SUMMARY
This chapter presents an overview of the remarkably complex geologi-
cal, meteorological, oceanographic, and biological processes, a variety of
habitats, and complex ecological and human interactions at work in the
Gulf.
Geological processes working over millennia set the stage in the GoM,
creating a region of high productivity and accumulation of abundant hydro-
carbon resources. In the northern GoM, the Mississippi River shaped, and
continues to shape, the coastal areas with sediment-rich waters continuously
reforming the landscape into the Mississippi and Atchafalaya river deltas. The
Gulf’s oceanic environment is largely determined by wind and currents, which
respectively establish the regional weather patterns and serve as an important
transit system for pollutants and runoff throughout Gulf waters. The region is
also characterized by diverse ecological habitats, ranging from highly produc-
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PHYSIOGRAPHIC, OCEANOGRAPHIC, AND ECOLOGICAL CONTEXT OF THE GULF
tive, vegetated intertidal zones and wetlands along the shore to less productive
benthic and pelagic habitats of the open waters. These habitats provide the
setting for the Gulf’s prolific biodiversity—plants, animals, and microorgan-
isms—whose maintenance is essential to healthy and stable ecosystems.
The GoM is a rich environment with abundant natural resources, diverse
habitats, and biodiversity. Many human communities live in the region
and rely on the various ecosystem services for their economic livelihood.
In addition to the estimated 20.4 million residents, the Gulf of Mexico is
home to oil and gas production, commercial fisheries, transportation, and
recreational industries. The GoM is highly productive in both renewable and
non-renewable resources. Fisheries and tourism have long co-existed with
the petrochemical industry along much of the Gulf of Mexico shoreline,
with the exception of Florida.
Renewable and living resources and the food webs and habitats that
support them were exposed to varying levels of toxicity and exposure to oil
and natural gas from the Macondo well. The effects of the oiling are currently
being assessed in many habitats, across complex food webs, with regard
to short- and long-term impacts, as altered biogeochemical cycling, and in
relationship to ecosystem functioning. Some GoM ecosystems and processes
are well known, but knowledge in many cases may be sparse. This uneven
understanding of the ecosystem and processes is not uncommon but creates
uncertainty that has to be acknowledged as data and findings are synthe-
sized. The ability to detect impacts of the oil spill by way of the health of
organisms or the level of ecosystem functioning will require adequate data
and multiple lines of evidence of altered ecosystem processes. Failure to
recognize essential factors or processes within (and between) ecosystems of
the Gulf region may result in improper characterization or in misrepresenta-
tion of the full range of ecosystem functions that support ecosystem services.
Finding 1.1: The Gulf of Mexico comprises a large, complex ecosystem
that has been and continues to be subject to both natural and human
forces of change. Hence, the baselines against which the impact of
the spill can be assessed are both spatially and temporally dynamic.
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