had a total abundance of
more than 1 percent of the
total catch (more than 54 to-
tal specimens) and only 14
of the invertebrate species
were represented by as
many as 308 specimens (1%
of the total)

Some 20,775 megafauna
were counted in the ana-
lyzed photographs of the
bottom and, for the most
part, these could only be

classified to major groups
(e.g., sponges, sea pens, brit-
tle stars, sea cucumbers, fish,
etc.) rather than species.
Nevertheless, no fewer than
190 species are believed to be
represented. Invertebrates
(20,287 specimens) were far
more numerous than fish (488
specimens). However, one
species each of a small sea
cucumber and a
chemosynthetic clam _ that
were photographed (neither

was taken in trawls) showed
exceptionally high density,
but only in a very small area.
These two instances
accounted for over half of
the camera observations. If
these are deleted from the
camera samples for compar-
ative purposes, 6,032 inver-
tebrates were counted along
with 488 fish. Invertebrates
outnumbered fish in the
photographs by a factor of
12. In the deep-sea Gulf, it

INVERTEBRATE DENSITY

CENTRAL REGION - FALL 1984

DENSITY (no/ba)

FISH DENSITY

CENTRAL REGION - FALL 1984

3

DENSITY (n0/ba)

Abundance of megafaunal invertebrates (upper) and fish (lower) caught in trawls is higher in
the shallower regions of the Continental Slope (generally above 3,300 feet) than in the deeper

depths.

18


A LOOK
AT THE DEEP

Let us now return to the
overwhelmingly larger and
massive soft-bottom matrix
in which the relatively few
and small chemosynthetic
communities are imbedded.
The photographs here were
taken by a camera suspended
on a wire from a ship and
maintained about 6 feet off
the bottom—dquite an
endeavor considering the
bottom was from over 1,000
feet to nearly 10,000 feet
below the ship. The camera
was equipped with a timer
and a strobe light set to take
a picture every eight seconds
as the ship drifted or
motored slowly along a
depth contour over a
distance of about one to
three miles. At a 6-foot
altitude above the bottom,
the size of the bottom area
photographed is a rectangle
with one side about 4 feet
and the other side a little
under 6 feet. The total area
depicted by the full
photograph is on the order of
25 square feet. A total of 46
different areas were photo-
graphed, some on multiple
occasions. Overall, 60 cam-
era drops were made using a
camera with a standard 28-
mm lens, taking 800 photo-
graphs per drop (48,000 total
images).

The Deep Ocean Bottom

At the time of this study,
17 years after the NGMCS
study was initiated, the
photographic images are still
available for all of the 46

The Benthic Underwater Camera System (BUCS) developed by LGL to

investigate the deep Gulf environment.

different locations inves-
tigated in the early to mid-
1980’s. The film rolls from
each station were cut into six
image-long strips of
consecutive frames and these
strips were placed in a sleeve
holding seven strips. After
removing some of the end
strips without information
(photographs taken as_ the
camera was being raised or
lowered), the photographic
record for each station was
reduced to slightly fewer
than 800 images. Those
images are now catalogued
by location in 17 to 19
numbered sheets, each
containing 42 images. We
randomly selected 100 six
image-long film strips to
characterize the bottom for
east, west, and central
regions of the Gulf that were
studied. The only
requirement was that each
station had to be represented
by at least one sample. The
600 images thus selected,
except for those with no
discernible subject, were
scanned onto the CD-ROM
that accompanies this report.

12

From these, we selected
representative images to
show the visual character of
the bottom habitat of the
continental slope of the Gulf
of Mexico.

Plate 1 shows repre-
sentative bottom photo-
graphs organized by depth
and by region across the
Gulf from east to west. The
series starts at the top with
samples taken at about
1,000-foot depths and
proceeds down to almost
10,000 feet. One word can
be used to describe the
visual character of the vast
majority of the Gulf of Mex-
ico Continental Slope
bottoms—monotonous. The
photographs reflect, that on
the whole, sediments are
fine-grained, and this was
confirmed by the grab
samples of the bottom taken
at these localities. Most of
the sediments occupying the
bottom are in the silty-clay
size range (that is, small or
fine-grained), with only
slight regional depth
variation in the proportions


Plate 2. Invertebrates (continued)

starfish

starfish brittle star

ee

sea cucumber crinoid



sediments. Extensive chem-
ical analyses were
conducted on water, mud,
and animal tissue samples.
Later in the study, additional
sampling was conducted by
using manned submersibles
to better investigate newly
discovered patches of large
tubeworms, mussel beds,
and clam beds scattered
across the slope in
proximity to hydrocarbons
seeping out of the bottom.

The results of the study
were presented in four
annual reports, with the
Year 3 and Year 4 reports
providing the final analyses.
These two final reports
included over 2,000 pages
describing the program and
its results. Known as the
"Northern Gulf of Mexico
Continental Slope
Study" (NGMCS), _ this
investigation provided the
first synoptic view of a

virtually pristine deep Gulf.

Selected overview data from
this study were published in
the formal scientific _lit-
erature, and the data and
specimens continue to be
used even today by
scientists and by graduate
students preparing dis-
sertations and theses.

Never, however, has a
nontechnical summary of
this investigative effort been
provided for managers and
laypersons. Benthic photo-
graphic sampling of the
bottom during the NGMCS
obtained about 48,000
images of the deep Gulf.
These images can _ be
used to provide visual
characterizations of the
seabed and the resident
biota of the deep Gulf.
These images have been
catalogued by sampling lo-
cation and depth, and were
again reviewed to extract

representative images of the
bottom and the resident
animals large enough to be
seen in photographs. Our
goal is to provide a clear
perception and under-
standing of what the deep
Gulf really looks like, and a
description of the major
environmental factors that
govern the shape and look
of the picture.

SETTING THE
STAGE—A PRIMER
ON THE DEEP

The Gulf of Mexico,
sometimes called America's
Sea, covers an area of more
than 1.5 million square
miles, and at its deepest it is
over 12,000 feet, or more
than 2 miles, to the bottom!
The Gulf is virtually
enclosed by the landmasses
of the United States,
Mexico, and Cuba. The
submerged margins of

The Northern Gulf of Mexico Continental Slope Study sampled 46 different areas at depths ranging
from slightly less than 1,000 feet (300 meters) to nearly 10,000 feet (about 3,000 meters).

3


seems clear that inverte-
brates are more numerous
than vertebrates (fish).

The trawl and camera
observations exhibited
similar patterns in
abundance and _ diversity.

The results of the benthic
photography, with one
exception, also showed
abundance to be higher at
depths above 3,300 feet than
in deeper zones, and higher
in the eastern Gulf than in
the western Gulf. Thus, the
two methods reflect overall
agreement in general
abundance trends.

As noted previously, the
most abundant animal on
the slope in the benthic
photography sampling was a
relatively small (about 1-
inch long) sea cucumber
called Peniagone. Over
11,000 of these small sea
cucumbers were counted in
the photographs. They were
all seen on the central tran-
sect and most were observed
in a very narrow depth
range. The peak abundance
occurred at a depth of about
4,000 feet. The very narrow
depth and _ station
distribution and their close
proximity to a
chemosynthetic community
suggest an association.
This, the most abundant of
any organism documented
by the photography, was not
taken by trawling. Many
animals in the deep sea are
small, fragile forms not
amenable to capture by
trawls.

Look closely and you will see that this bottom
is literally crawling with a small sea
cucumber called Peniagone.

For comparison, the most
abundant animal taken by
trawling (3,393 specimens)
was a small shrimp called the
thread-footed shrimp
(Nematocarcinus  rotundus).
This shrimp averaged about
three-fourths of an inch long
(maximum size was 1.2
inches) and was not identified
in the benthic photography. It
was taken in trawls on all
cruises, all transects, and at
nearly every station between
depths of 1,600 and 3,300
feet. This shrimp averaged
about the same size as the
small sea cucumber, yet,

despite being big enough to
see, was not documented in
the benthic photographs.
Thus, size alone does not
account for the differences
between what is seen with a
camera and what is taken in
a trawl. Patterns of aggre-
gation (clumped versus
scattered widely over a
large area), sample sizes,
and fragility of the animal
are also important factors.

The second-most abun-
dant animal seen by the
camera was a bivalve or
clam, now known to have
been a chemosynthetic
species. A very restricted
high-density zone was en-
countered by chance at a
single station (C7) on the
central transect. None was
collected by trawling
despite the large area
trawled.

The sea pens (Alcyo-
naria) were the third-most
photographed megafaunal
category, and species of sea
pen ranked seventh in the

A graceful, sedentary sea pen waves above the bottom, filtering the
water to obtain food in the energy-limited deep Gulf.


some bottom _ irregularity
like a rock or a hole. These
structures likely offered
cover or protection from
predation, as well as
represent sites where
organic matter tends to
accumulate. Such sites
might be food-rich com-
pared with surrounding soft
bottoms—a good spot to
feed if one eats detrital
matter or a good spot to
ambush those attracted to
organic matter accumula-
tions.

The physical appearance
of the deep-sea fauna
reflects adaptations for
living under conditions of
total or near total darkness,
crushing pressure, generally
weak currents, little food,
cold, and on a featureless

fine mud bottom. The
animals are usually small
and fragile, and are
anatomically adapted for
these severe conditions. The
bulk of the animals in the
deep Gulf eat by filtering the
water, sweeping or eating
the sediments, or scavenging
the occasional dead fish,
whale, or anything else
organic that falls to the
bottom.

DEEP-SEA OIL
AND GAS
DEVELOPMENT
OBSERVATIONS

Deep-sea oil and gas
development activities were
beginning about the same
time as the NGMCS study
was being completed in the
late 1980's.

Quantitative photography
taken with a large, unmanned
submersible remotely
operated vehicle (ROV) is
available from one of these
developments, the BP
Exploration, Inc. Pompano
Project. This development
proceeded with the drilling of
10 wells from 1992 to 1994
from a_ single-jacket
(Pompano Phase I) at a depth
of about 1,300 feet in MMS
Lease Block Viosca Knoll
989.

Phase II began with the
installation of a subsea
template in about 1,900 feet
of water four miles
downslope in MMS _ Block
Mississippi Canyon 28.
Seven wells were drilled at
this site between 1995 and

A deep-sea development showing drillship with ROV, subsea templates, and pipelines running up the slope to a
standard production platform in shallower water.

25


Plate 2. Invertebrates of the Deep

sponge

anemone



between about 500 and
1,500 feet); (2) the Ar-
chibenthal, Horizon A Zone
(median depth of 2,000 feet,
range from 1,500 to 2,500
feet); and (3) the
Archibenthal, Horizon B
Zone (median depth about
2,800 feet, range from 2,500
to about 3,100 feet).

Both schools of thought
agree that between roughly
3,300 and 6,500 to 7,500
feet there is a broad depth
range (called the Upper
Abyssal Zone) of low mega-
faunal abundance and
diversity containing a mix of
animals characteristic of
both deeper and shallower
depths. Below 6,500 to
7,500 feet (the Mesoabyssal
Zone), a true deep-sea
fauna emerges, charac-
terized by low abundance
and a _ predominance of
invertebrates. Whether there

are three or five biologically
meaningful zones repre-
sented on the slope is not of
importance for our discus-
sion, or perhaps even for
management purposes.
Nevertheless, the five-zone
naming scheme was used on
the CD-ROM to organize the
biota pictures, taken by the
deep-sea camera, by depth.
When reviewing these
images, the reader will see
that some species occur in
several depth zones.

Trawls and Cameras—
Different but Comple-
mentary

The bottom and near-
bottom megafauna_ were
sampled during the NGMCS
study by using two
fundamentally different but
complementary techniques,
bottom trawling and benthic
photography. The trawl was

about 30 feet wide and was
pulled along the bottom for
about one hour at each
location above 3,300 feet;
and for two hours, more or
less, at deeper sites. Overall,
given the trawling speed, the
trawl swept a bottom area of
some 5 to 20 acres, de-
pending on the depth being
trawled. In contrast, the
camera captured as many as
800 shots of the bottom at 8-
second intervals over several
miles, with each photograph
averaging about 25 square
feet. The total area photo-
graphed at a site would be on
the order of 20,000 square
feet, or less than one-half an
acre. The area trawled is thus
10 to 40 times larger than the
area photographed.

Because the area trawled
was much larger than the
area photographed, the trawl
sample typically yielded a

Opposing views of the zonation of faunal assemblages present at depth intervals down the Continental

Slope.

16


erosion of bottom sediments
can be a significant means of
energy transport from
shallow to deeper areas of
the bottom. Nevertheless,
the deep sea is a food-limited
system.

The recipient of this rain
(perhaps drizzle would be a
better term) is the bottom
zone of the deep ocean
referred to as the "benthic
boundary layer." Bio-
logically, this community
consists of the organisms
living in or on the sediments,
plus the assemblages of
animals in the overlying
water column immediately
above the bottom. Animals
living in this part of the
water column are said to be
"benthopelagic."
Benthopelagic animals are a
subset of the “nekton,”
swimming animals that live
in the water column. The
designation of benthopelagic
refers to the fact that,
although they live in the
water column (pelagic), they
mainly occupy the waters
just above the bottom (ben-
thos).

The sediment community
consists of bacteria and small
(often very small) inverte-
brates, above which, at the
bottom-water interface,
larger animals crawl,
ingesting both the sediments
and the biota living there as
food. Other types of animals
attach themselves into the
sediment or onto a small
rock and filter the water for
food particles as these pass
by in the current.

Denizens of the benthic boundary layer, a brittle star, and a colorful sea
cucumber. The sea cucumber eats surface sediments and the small animals

living in the sediments.

A benthopelagic rat tail fish cruises above the bottom, likely in search of
food.

In the water column
above the sediment water
interface, what we now know
as "benthopelagic" fish and
invertebrates feed on animals
in the water column, as well
as those on and in the
bottom. Many of these
benthopelagic species move
up and down the slope
bottoms, as well as higher

in the water column, to feed,
and then return to the bottom.
Sediment-dwelling animals
also make forays into the water
column. Thus, there is an
active exchange of food
throughout the water column,
as well as on and in the bottom
sediments. These migrations
result in a continuum of
communities from the top to
the bottom.


REFERENCES

The complete findings of the Northern Gulf of Mexico Continental Slope Study can
be found in four reports:

Gallaway, B.J., L.R. Martin, and R.L. Howard (Eds.). 1988. Northern Gulf of Mexico
Continental Slope Study, Annual Report: Year 3. Vols. I-III. Annual Report to the Minerals
Management Service, New Orleans, LA. Contract No. 14-120001-30212. OCS Study
MMS 87-0059 through 87-0061. 1,490 pp.

Gallaway, B.J. (Ed.). 1988. Northern Gulf of Mexico Continental Slope Study, Final Report:
Year 4. Vol. I: Executive Summary. Final report to the Minerals Management Service,
New Orleans, LA. Contract No. 14-120001-30212. OCS Study MMS 88-0052. 69 pp.

Gallaway, B.J. (Ed.). 1988. Northern Gulf of Mexico Continental Slope Study, Final Report:
Year 4. Vol. II: Synthesis Report. Final report to the Minerals Management Service, New
Orleans, LA. Contract No. 14-120001-30212. OCS Study MMS 88-0053. 318 pp.

Gallaway, B.J. (Ed.). 1988. Northern Gulf of Mexico Continental Slope Study, Final Report:

Year 4. Vol. III: Appendices. Final report to the Minerals Management Service, New
Orleans, LA. Contract No. 14-120001-30212. OCS Study MMS 88-0054. 378 pp.

28


Gulf to the west and south,
even into the Bay of Cam-
peche, before dissipating.

The Loop Current and its
branches drive the major
surface circulation of the
Gulf and account for some
of its biological features.
The currents bring pelagic
larvae and fishes, plant
material, and heat into the
eastern Gulf. However, the
currents also influence the
western Gulf to a lesser
degree by spinning off
eddies that drift slowly
westward. As can be seen,
the western Gulf is
somewhat isolated from the
Loop Current. This "iso-
lation" corresponds to the
observation that more
species in the western Gulf
are restricted to that region
alone (that is, they are not
distributed Gulfwide) than
is the case for eastern Gulf
species.

Mississippi River
Discharge Important

The second  environ-
mental factor that, like the
Loop Current, has effects on
a massive geographic scale
is the Mississippi River. Its
waters pour into the middle
of the northern Gulf (off the
United States mainland) and
then spread westward over
the continental shelf. Other
rivers drain into the Gulf,
but their influence is
dwarfed by that of the
Mississippi River System,
which is among a dozen of
the great rivers of the
world. It delivers about one

million cubic yards of sediment
and vast quantities of organic
material to the Gulf each day.
The huge volume of sediment
accounts for a building delta and
continental shelf to the west.

The mouth of the river is
located in a region where the
Continental Shelf is very narrow.
Consequently, accumulations of
sediments are deposited on the
shelf edge and may from time to
time slump down-slope, devas-
tating the bottom fauna. At the
same time, however, these events
deliver huge volumes of sedi-
ments and organic matter to the
lower reaches of the slope where
energy or food supply is limited.
The huge amount of sedimenta-
tion deposited on the slope in the
region near the mouth of the
river results in a submarine
blanket of soft sediments,
called the "Mississippi Fan."
This feature bisects the northern

continental slope of the Gulf,
with more typical slope fea-
tures found to the east and
west of this sediment
blanket.

Important Physical-
Chemical Factors
Vary by Depth

A third factor that in-
fluences the distribution of
slope animals is the presence
of vertical layers in the water
column. These layers
intersect the seabed at
various depths and create
changes in the physics and
chemistry of near-bottom
waters. These factors have
either direct or indirect
effects, or both, on the
bottom-dwelling animals.
These vertical layers can be

characterized by light,
temperature, pressure,
salinity, and dissolved

oxygen.

The Loop Current and its associated eddy features (cyclones and anticyclones).
Note that both the entrance and exit of the Gulf of Mexico are in the

southeastern part of the Gulf.

5


the Gulf are bounded by a
relatively shallow and flat
continental shelf (from the
shore to a depth of about 600
feet), which can be quite
broad in some places (for

example offshore west
Florida, Texas-Louisiana,
and Campeche) and

narrowly restricted in other
places (such as near the
mouth of the Mississippi
River and offshore eastern
Mexico). At the shelf break,
the seafloor begins to slope
steeply towards the abyssal
plain of the deep Gulf, often
terminating in a near-vertical
scarp or cliff-like formation
that extends on down to the
bottom. This region is called
the continental slope and
extends from the shelf edge
(600 feet deep) to a depth of

nearly 10,000 feet deep.
This is the region of interest
for this essay.

The Loop Current—
Dividing the Gulf

The configuration of the
landmasses_ enclosing the
Gulf restricts the size and
location of the openings that
allow water exchange with
adjacent seas. Both the inlet
and the outlet are located in
the southeast corner of the
Gulf basin. Water flows
into the Gulf from the
Caribbean Sea through a
narrow inlet (the Yucatan
Channel) situated between
the west end of the Yucatan
Peninsula and Cuba. The
deepest part of this inlet is
only 5,000 to 6,000 feet

compared to the 12,000 feet
in the deepest parts of the
Gulf. This flow continues
north towards the United
States mainland, then loops
to the east and bends south
along the Florida Peninsula.
Water exits the Gulf into
the Atlantic Ocean as it
flows easterly between the
south tip of Florida and
Cuba, the so-called Florida
Straits. This dominant
circulation feature of the
Gulf is called the "Loop
Current.". The northern
area of the "loop" often
pinches off to form cy-
clones (counterclockwise
circulation) and anti-
cyclones (clockwise
circulation). These eddies
wander along the Con-
tinental Slope region of the

aeyssat

rodent’
“T centrat) ouLF

GULF OF MEXICO

Physvographic Provinces

Physiography of submerged land forms in the Gulf of Mexico. The perimeter of the Gulf is ringed by a
shallow shelf, seaward of which the seafloor steeply slopes downward to the abyssal plain of the Gulf--the

Continental Slope.


A bottom trawl coming on board the research vessel as the deep-sea camera

system awaits its turn to photograph the same depth zone.

larger number of specimens
and species of large biota (for
example, over 5,000 fish
representing 126 species were
trawled) than the benthic
camera (488 fish were photo-
graphed, representing an un-
known number of species).
Comparisons of abundance
must be based on some stan-
dardized area (like number per
acre), but even then one must
make many qualifying
assumptions that are probably
not true. For example, one
must assume that the trawl

will catch everything en-
countered equally well as is
seen by the camera, which
will see everything caught
in trawls despite covering a
much smaller area. Both
methods have strong and
weak points, and these must
be taken into account in
making direct comparisons.
The methods are best
viewed as complementing
one another and this is the
view we have taken in this
essay.

The Megafauna
Observations

Some 59 trawl tows were
taken as part of the NGMCS
study, yielding 5,400
specimens of 126 species of
deep-sea fish, and over
30,689 invertebrates
representing 432 species.
The invertebrates in the
trawl catches were more
typically on the order of
four to five times more
numerous than fish, rather
than about the eight times
more numerous as_ is
indicated by the total
catches. Abundance of
trawled megafauna was
greatest on the upper slope
down to 3,300 feet than at
deeper depths, and higher in
the eastern Gulf than in the
western Gulf.

Both fish and invertebrate
collections taken by
trawling showed _ strong
species dominance patterns;
ie., the overall patterns
usually reflected the
abundance of only one or
two species. Only 22 of the
126 species of trawled fish

Observations taken by trawling and photography are complementary. The actual specimen (in this case, a sea robin)
taken by trawling enables the collection of detailed size, weight, food and habitat data and accurate identification to
species. The camera gives information about where these species live and their habitat.

17


oxygen minimum layer that
occurs at depths between
about 1,000 and 2,000 feet.
Where this layer bathes the
slope, there is a cor-
responding zone of high
biological standing stocks.
The depressed dissolved
oxygen concentration might
impact some species but not
others. Those species that
can deal with low oxygen
levels are successful. The
key point may be that the
oxygen levels in this
depth zone may not be
sufficient to oxidize organic
matter as rapidly as

elsewhere, making it more
accessible to biota as a
source of food or energy.

Sediment grain size and the
mix of these sizes greatly
influence the types of animals
that live in the sediments.
Sand-sized particles are the
largest, followed by silt- and
then clay-sized particles, which
are the smallest. On the conti-
nental slope of the Gulf, silty-
clays dominate the bottom,
especially at depths below about
1,600 feet. As will be seen later,
the visual character of the sedi-
ment blanket suggests a tranquil
environment in terms of
currents, with only a few
exceptions. Analysis of over 20
bottom sediment parameters
considered important to the
benthic fauna showed distinct
sediment zones by depth:

A conceptual model of the Gulf of Mexico Continental Slope. Food for the
deep sea comes from surface water production and from river outflow, but
only that part that is left over or degraded after being processed by the
shallower components of the ecosystem reaches the bottom. The deep Gulf
suffers from extreme energy limitation.

shallow (to 1,600 feet),
intermediate (1,600 to 6,500
feet), and deep (greater than
6,500 feet).

Limited Food or
Energy Sources

With one important
exception that will be
discussed below, the bottom
communities of the deep
Gulf live in a food- or
energy-limited environment
dependent upon production
from the surface layers of
the Gulf. The solar energy
that drives ocean systems is
fixed by phytoplankton into
chemical energy that is used
by animals. This process
occurs in the euphotic zone,
as we learned above. Much
of this plant production is
consumed by small grazing
animals (called
zooplankton) which, in turn,
are eaten by larger animals
like small fish. Even larger
animals feed on these, such
that a surface layer (the pe-
lagic zone) food web is
established. What is left
over, including waste and
undigested food material
passed through this food
web, ultimately sinks
through the water column to
the bottom. These organic
materials can be viewed as
an "organic rain" of food to
the bottom. Detrital rain
from the pelagic system is
supplemented by organic
detrital material from the
landmasses (terrestrial
carbon) in areas affected by
significant river discharge.
In addition to these rains of
detrital foods, slumping and


Plate 3. Fishes of the Deep

ie

on

a

rat fish

duckbill eel

hatchet fish viperfish



OCS Study
MMS 2001-065

The Deep Sea Gulf of Mexico:
An Overview and Guide

Inhabitants of the Cold, Dark, and Food-Limited Deep
Gulf Show Adaptations for Crushing Pressure
and the Ability to Subsist on the Gentle Rain of
Organic Particles from Above

U.S. Department of the Interior
Minerals Management Service
Gulf of Mexico OCS Region



occur in the total absence of
sunlight. Chemosynthetic
animals were defined as
those life forms that are able
to live on dissolved gasses
through a_ "symbiotic"
(living together for mutual
benefit) association with
chemosynthetic bacteria
living in their tissues. In re-
turn for their food, the larger
animals provide the bacteria
a secure home. Such
animals usually occur in
densely packed assemblages
of large animals over and
around the hydrocarbon
seeps or vent sites that
provide their energy. These
clusters of large animals, in
turn, attract other life forms
like snails, fishes, worms,
seastars, shrimp, and crabs,
much as a reef in shallow
water does. Taken all
together, these assemblages
form communities that do
not depend on the fall of
scarce foodstuffs from the
surface waters far above. By
virtue of the ample nutrient
source (the gasses and
symbiotic bacteria), chemo-
synthetic animals of the
deep sea have larger average

A chemosynthetic mussel bed (left) and a chemosynthetic assemblage of clams (right) living in the vicinity of
hydrocarbons seeping from the bottom. Note how sharply defined is the boundary between the mussel bed and the
surrounding soft bottom.

physical size than the
typically small deep-sea
animals. The great biomass
found at these deep sites is
usually hundreds to
thousands of times greater
than the biomass levels
found on the surrounding
seafloor. This exception
"proves the rule" that
nutrition is a limiting factor
to the abundance of most
animals in the deep sea.

Chemosynthetic commu-
nities were documented to
occur in the Gulf of Mexico
in 1984 around cold seeps,
places where hydrogen
sulfide and hydrocarbons
(mostly methane gas) seep

from the bottom. Such
communities have been
intensively studied since

their discovery and are the
subject of another MMS
nontechnical _ publication,
Gulf of Mexico
Chemosynthetic Com-
munities: A Teacher's
Companion. In summary,
Gulf of Mexico chemosyn-
thetic communities are of
four types: those dominated
by (1) tubeworms, (2) mus-

10

The

first

record of
tubeworms in the Central Gulf of Mexico,
a photograph from the NGMCS program
taken on 12 November 1984.

sels, (3) large clams living
on the surface, and (4) small
clams living under the
surface of the mud. Each of
these communities displays
distinctive characteristics in
terms of how they
aggregate, the size of the
aggregations, the geological
and chemical properties of
the habitats in which they
occur and, to some degree,
the associated "normal" bi-
ota that are attracted to and
live among them.

The number of known
chemosynthetic com-
munities in the Gulf of
Mexico now exceeds 50.

chemosynthetic


From the above de-
scriptions, one can see that
the bottom communities of
the deep sea occupy an
energy-poor environment,
with the energy constraints
becoming ever more severe
with increasing depth and
distance from land. This is
especially true in the Gulf
of Mexico. Primary pro-
duction in the euphotic
zone of the Gulf is not very
high, compared with other
oceanic areas. Therefore,
standing stocks of deep-sea
benthic communities in the
Gulf would be predicted to
be lower than in more
productive areas of the
world's oceans, and this
idea was not discounted by
the NGMCS results from
the mid- to late-1980’s.

The invertebrate com-
munities living in the
sediments of the deep Gulf
are not very numerous and
are very small compared
with their shelf
counterparts. This leads
to low biomass levels
and reflects the overall
food limitation of the
system. However, these
animals are very diverse,

A deep-sea crab crawls in an exotic forest of
chemosynthetic tubeworms, which can reach lengths of
up to 10 feet. Photo by J. Blair, National Geographic.

that is, many, many
different species are found.
The diversity of this com-
munity is as high or higher
than diversities char-
acteristic of tropical rain
forests, which are usually
considered to be the most
diverse of planet Earth
ecosystems.

The Exception — Chemo-
synthetic Communities

We have noted above that
there is an exception to the
food limitation character of
the deep sea. So far, we
have implied that
photosynthesis (conversion
of carbon dioxide to sugar
by plants using light energy)
is the only source of energy
to all animals, not counting
a few bacteria that can act
like plants in this regard.
Other than these forms
(plants and bacteria), we
have suggested that all other
animals eat living or dead
organic material originally
produced and/or derived
from plants at the base of
the food chain. In 1977, this
view of the world changed
with what has been
classified as one of the

major biological
discoveries of the century!

This discovery  origi-
nated in the deep waters of
the Pacific Ocean. Using
the research submersible
Alvin, scientists discovered
an oasis of densely packed
animals, where none was
expected, at a depth of
over 8,500 feet in the
vicinity of a hot, hy-
drothermal vent laden with
hydrogen sulfide. Bacteria
in the water around these
vents were using carbon
dioxide in the presence of
oxygen and _ hydrogen
sulfide (instead of light) to
fuel the development of
sugar. But what was
remarkable was that these
bacteria were not only
living in the water and on
the bottom in films or
mats, but were also living
in the tissues of numerous
newly discovered species
of giant tubeworms and
clams.

A new form of pro-
ductivity—called che-
mosynthesis—for animals
much larger than bacteria
had been discovered to

A starfish and crabs experience a bonanza of food
and shelter provided by a chemosynthetic mussel bed.
Photo by J. Blair, National Geographic.


The Department of the Interior Mission

As the Nation’s principal conservation agency, the Department of the Interior has
responsibility for most of our nationally owned public lands and natural resources.
This includes fostering sound use of our land and water resources; protecting our
fish, wildlife, and biological diversity; preserving the environmental and cultural
values of our national parks and historical places; and providing for the enjoyment
of life through outdoor recreation. The Department assesses our energy and
mineral resources and works to ensure that their development is in the best
interests of all our people by encouraging stewardship and citizen participation in
their care. The Department also has a major responsibility for American Indian
reservation communities and for people who live in island territories under U.S.
Administration.

The Minerals Management Service Mission

As a bureau of the Department of the Interior, the Minerals Management
Service’s (MMS) primary responsibilities are to manage the mineral resources
located on the Nation’s Outer Continental Shelf (OCS), collect revenue from the
Federal OCS and onshore Federal and Indian lands, and distribute those revenues.

Moreover, in working to meet its responsibilities, the Offshore Minerals
Management Program administers the OCS competitive leasing program and
oversees the safe and environmentally sound exploration and production of our
Nation’s offshore natural gas, oil and other mineral resources. The MMS
Minerals Revenue Management meets its responsibilities by ensuring the
efficient, timely and accurate collection and disbursement of revenue from
mineral leasing and production due to Indian tribes and allottees, States and the
U.S. Treasury.

The MMS strives to fulfill its responsibilities through the general guiding
principles of: (1) being responsive to the public’s concerns and interests by
maintaining a dialogue with all potentially affected parties and (2) carrying out its
programs with an emphasis on working to enhance the quality of life for all
Americans by lending MMS assistance and expertise to economic development
and environmental protection.


All photographs in this report and many more have
been compiled on an indexed CD-ROM.

Both this report and the CD ROM are
available from

U.S. Department of the Interior
Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)
1201 Elmwood Park Boulevard

New Orleans, Louisiana 70123-2394
Telephone Numbers: (504) 736-2519
1-800-200-GULF


Light is sufficient to
allow microscopic plants
(phytoplankton) and other
plants like seagrasses to
convert carbon dioxide to
sugars (a process called
photosynthesis) usable as
food by animals. However,
photosynthesis occurs to a
maximum depth of only
about 200 feet. Thus, the
entire slope is below the
depth where photosynthesis
can occur (the euphotic or
"true" light zone). Below
200 feet, twilight
conditions, ever darker with
depth, persist down to
depths of about 3,300 feet or
1,000 meters. Below this
depth, darkness _ prevails
year round. The zone
between 200 and 3,300 feet
is called the "disphotic"
zone (limited light). Deeper
depths are characterized as
“aphotic”; that is, without

light.

Surface water temper-
atures are influenced by
sunlight and air temperature.
Water temperature declines
rapidly with depth down to
about 3,300 feet (1,000
meters) where temperatures
average about 4 °C (about
39 °F) Gulfwide. From this
depth, water temperatures
remain uniformly cold down
to the bottom. Thus, a suc-
cinct characterization of the
continental slope is that it is
a dark and cold place to live.

Not only that, pressure
forces increase nearly
linearly with depth. At the
surface (one atmosphere)
we experience 14.7 pounds
of pressure per square inch.
There is roughly a one-
atmosphere increase in
pressure with each
additional 33 feet of water
depth. Thus, animals at
3,300 feet experience 100
atmospheres of pressure, or
1,470 pounds per square
inch on their body surface.
At 12,000 feet the pressure
is well over 5,000 pounds

per square inch! Thus, one
might expect that some
special physical adaptations
are in order, and that expecta-
tion is correct.

Salinity, like temperature,
is relatively high at the
surface (on average around 36
to 37 parts per thousand
[ppt]) and declines with
depth, but only by a few parts
per thousand (corresponding
bottom water salinity would
be about 35 ppt). With
dissolved oxygen content,
however, there is a discrete

26 30 34 38
348 350 352 354
4 6 8 0

356 358 360 362 364 366
12 14 16
ee eee

46 50

le 20 22
Deron n

=: separa io Laven ees
Lis
a Ze

‘OXYGEN MINIMUM LAYER

DEPTH (m)

a =:
8 8

8
io

ANTARCTIC I

MM O”H}OAAOO

WN

MEDIATE

Vertical water layers in the Gulf of Mexico. Note that below 3,300 feet (1,000
meters) the water is uniformly cold (about 4 °C). Further, it is at this level that

light totally disappears.


number of  trawl-caught
animals. Some 13 species of
alcyonarians were taken in
trawls. As a group, sea pens
were found over the full
range of depths sampled.

Deepwater shrimps and
crabs comprised over 45
percent of the invertebrates
captured by trawling and
about 25 percent of the
megafauna counted in the
photographs. While different
species individually showed

different depth ranges of
occurrence, overall abun-
dance was highest at around
1,700 feet and abundance
dropped dramatically below
3,300 feet as compared with
shallower depths. Interest-
ingly, densities (number per
unit area) reflected by the
trawl sampling were far
lower than densities esti-
mated by the photographs.
The same was true
for starfish and brittle-
stars (called asteroids and

a6 2378 8 4 ON S 12

Total Asteroid/Ophiuroid Density

© Photography
* Trawl

a6 23578 9 4 OH Ss 12

Total Demersal Fish Density

.
* Trew!

Comparative densities estimated by using cameras and trawls for shrimps and
crabs (Decapods, top), starfishes (Asteroids/Ophiuroids, middle) and fish

(bottom) from the top to the bottom of the Gulf Continental Slope.

20

ophuiroids by scientists) and
for fish.

Plate 2 shows photo-
graphs of some of the
characteristic deep-sea
invertebrates, including
shrimp and crabs, delicate
and not so delicate sponges,
large sea cucumbers,
anemones, squid, and other
invertebrates of the deep
Gulf. Likewise, Plate 3
shows characteristic forms
of deep-sea fish including
skates and rays, pancake
batfish, rat tail fish, hakes
and cod-like fish, gapers,
and others. Many of the
adaptations to the deep sea
look bizarre to those
familiar with sportsfish of
the shallow Gulf. More
examples of the deep-sea
fauna are shown on the CD-
ROM.

We reviewed all of the
48,000 photographs to
compile Plates 1 and 2,
selecting a representative
photograph of each different
animal we encountered. Not
all photographs of each
species were selected, just a
representative one. Never-
theless, one cannot miss the
observation that relatively
few photographs contain
any animal in view, and
even fewer have more than
one animal. With the few
exceptions of dense
aggregations, the animals
are widely scattered—few
and far between.

Another striking observa-
tion is that many of the fish
photographs and some of
the invertebrate photographs
show an association with


1997, and one additional well
was drilled in February-
March 1998. The wells were
drilled, in part, with nontoxic
synthetic-based fluids
(SBF’s), some of which were
discharged adhering to
pulverized cuttings. These
vegetable-based esters are
highly biodegradable and
may sus-tain bacterial activity
at levels intense enough to
cascade the effect upward in
the food chain.

Scorpion fish and crabs living in the structure provided by a subsea

Sediment core samples and template.

megafauna photographic
surveys were taken in July
1997 around the subsea
template, several months after
the completion of the original
seven wells drilled with the
subsea template, and in
February-March 1998,
immediately following the
drilling of the last well. The
core samples showed
remarkable densities of
infauna as compared with the
NGMCS study results. Even
more surprising was the
observation that high
densities of megafauna (fish
and invertebrates) were
present around the subsea
template. These organisms
were seen in association with 00 eeu
the structure of the template
and the pipelines, and
cruising over the rough
bottom marked by large
chunks of mud left by the
riserless drilling phase that

a8

occurs [ab the slaty of drilling oN 2a 4567 8 1234 1234 ‘1 ta tb te 2 2a 2b 2c 2d 2
the well. All indications were vg cruise v ay wanes MB CRUGEIV
that, at these depths, the FOMPANO PHASE

structure afforded by the Pfinniedh derail - Pah Based fn ed

template, pipelines, and rough stimated densities of demersal fish based upon photograpi ic transects
for the eastern and western Gulf of Mexico compared to estimates for

bottom has a reef effect, as has Pompano Phase II video transects. See map at bottom of page 2 for MMS sta-
tion locations.

26


The Deep Sea Gulf of Mexico:
An Overview and Guide

Benny J. Gallaway, John G. Cole, and Larry R. Martin

INTRODUCTION

The Gulf of Mexico—
sunbathing on sandy
beaches, swimming and
surfing, deep-sea fishing,
red snapper, reefs and oil
platforms, SCUBA diving,
shrimp dinners. These are
but a few of the word
associations and mental
images that come to mind
with the mention of this
natural resource. Many are
aware that this habitat
supports the Nation's most
valuable commercial
(shrimp) and_ recreational
(red snapper) fisheries, as
well as provides nearly 95
percent of the Nation's total
offshore petroleum _ pro-
duction. Nearshore Gulf
habitats and their creatures
have been featured in the
scientific literature, in
magazines, and on _ tele-
vision documentaries and
sporting shows for decades.
On a relative basis, the
nearshore Gulf habitat is
well understood and
appreciated by scientists and
laypersons alike.

However, it is only the
shallow margins of the Gulf
of Mexico that are really
understood and appreciated
by the lay public and,
indeed, many _ scientists.
Mention the Continental
Slope of the Gulf and few
mental images are invoked.
What it looks like, what

LGL Ecological Research Associates, Inc.

species live there, and why
and how they live there do
not readily come to mind.
When the Continental Slope
is discussed, "deep sea" takes
on a whole new meaning—
from 600 to 10,000 feet ver-
sus the 100 to 300 feet
normally thought of as deep-
water by most laypersons.
Yet, it is the Continental
Slope habitat of the Gulf
where vast new petroleum
reserves have been
discovered, and_ the
technology is in place to
develop these reserves. The
question becomes "Are we
ready for this develop-
ment?” That is, “Do we
know enough to estimate the
effects of the development on
the deep-sea environment?"
A common misconception is
that we know little or nothing
about the slope environment
and biota. This is far from the
truth.

The Bureau of Land
Management, whose offshore
responsibilities devolved
upon the Minerals
Management Service (MMS)
almost 20 years ago, recog-
nized the need for ecosystem
information about the
Continental Slope over two
decades ago. First, they com-
missioned a renowned deep-
sea ecologist, Dr. Willis
Pequegnat, who had
conducted extensive research
in this system, to compile all

2

the known information
about this Gulf habitat.
Drawing upon the results of
this study, published in
1983, MMS next designed
and contracted a multi-
million dollar research
program to LGL Ecological
Research Associates, Inc.
and Texas A&M University
to study this habitat during
the period 1983 to 1987.
The objective of this
multifaceted program was to
develop a basic knowledge
of the deep Gulf fauna and
their relationship with their
environment.

Geographically, the study
extended from the eastern to
the western Gulf and
focused on habitats and
biota living in waters on the
order of 1,000 to nearly
10,000 feet deep. The study
employed (1) standard
methodologies to measure
water temperature, salinity,
and dissolved oxygen from
the surface to the bottom;
(2) box corers to take mud
samples and biota living in
the mud from the bottom;
(3) trawls to capture larger
animals cruising along and
above the bottom; and (4)
deep-sea camera systems to
take photographs of the
bottom and the associated
animals and signs of animal
presence (tracks, trails,
burrows) recorded in the


Central Gulf
1,000 Feet

Western Gulf
2,000 Feet

Central Gulf
4,000 Feet

Central Gulf
8,000 Feet

Twilight
Zone

Eastern Gulf
2,000 Feet

Central Gulf
6,000 Feet

Eastern Gulf
8,000 Feet

Eastern Gulf
1,000 Feet

Central Gulf
3,000 Feet

Central Gulf
7,000 Feet

Central Gulf
9,000 Feet

Dark and Cold Zones
Plate 1 The Deep Gulf Bottom



about 3,300 feet, and then
declines as depth increases.
Above this level, between
1,000 and 3,300 feet, abun-
dance is higher than ob-
served at the deepest depths,
but lower than the mid-
slope, high abundance peak
seen at about 3,300 feet. We
have learned above that a
depth of about 3,300 feet is
where the temperature
reaches 4 °C and light dis-
appears. Benthopelagic fish
and invertebrates (the
megafauna or "large fauna")
will be shown to be much
more abundant between
1,000- and 3,300-foot
depths, than at deeper
depths. Predation pressure
may thus account for the
lower standing stocks of
infauna in the more shallow

zones of the Gulf slope
above 3,300 feet.

Large Animals
of the Deep Gulf

Deep-sea_ scientists
classify animals large
enough to be seen with the
naked eye as "mega-
fauna" (very large animals),
even though they may be as
small as one inch in length.
These include everything
from stalked, plant-like ani-
mals, to starfish, to shrimp
and crabs, to fish.
Characteristic assemblages
of these animals are thought
to be distributed in zones or
bands extending across
depth contours. However,
there is much debate over
how finely these bands can
be defined; and, even if
so, whether or not they are

biologically meaningful.
They may simply be arti-
facts of sampling at discrete
intervals along what is
actually a continuum. Most
all agree there is a
meaningful break some-
where at about 3,300 feet,
where light disappears and
water temperatures become
uniformly cold at about 4 °C.

On the slope at depths
above 3,300 feet, there is,
according to one school of
thought, a single biological
zone or continuum con-
taining a "distinct shelf"
fauna. Another school of
thought subdivides this depth
range into three parts with
some rather exotic names:
(1) the Shelf-Slope
Transition Zone (median
depth of about 1,000 feet
with a total depth range

MACROFAUNA DENSITY, TOTAL

CENTRAL REGION - FALL 1984

Macrofauna, food for larger animals, are most abundant at depths of about 3,300 feet (1,000 meters)
and decline thereafter in direct association with depth increase. Reduced abundance above 3,300 feet

may reflect predation pressure.

1S


DISCLAIMER

This report was prepared under a purchase order from the Minerals Management Service
(MMS) to LGL Ecological Research Associates, Inc. This report has been technically reviewed
by the MMS and approved for publication. Approval does not signify that contents necessarily
reflect the views and policies of the Service, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use. It is, however, exempt from
review and compliance with MMS editorial standards.

NOTE

Unless otherwise noted, all photographs in this document and the companion CD-ROM were
taken by Gregory S. Boland as part of the Northern Gulf of Mexico Slope Study, Contract 14-
12-0001-30212.

REPORT AVAILABILITY

Extra copies of this report and an indexed CD ROM containing all photographs in this report
may be obtained from the Public Information Office (Mail Stop 5034) at the following address:

U.S. Department of the Interior
Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)
1201 Elmwood Park Boulevard

New Orleans, Louisiana 70123-2394
Telephone Numbers: (504) 736-2519
1-800-200-GULF

CITATION
This study should be cited as:
Gallaway, B.J., J.G. Cole, and L.R. Martin. 2001. The deep sea Gulf of Mexico: an overview

and guide. U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico
OCS Region, New Orleans, LA. OCS Study MMS 2001-065. 27 pp.


Plate 3 Fishes (continued)

tripod fish

bat fish

scorpion fish flounder



OCS Study
MMS 2001-065

The Deep Sea Gulf of Mexico: An Overview And Guide

Authors

Benny J. Gallaway
John G. Cole
Larry R. Martin

Prepared Under Purchase Order #17037
By
LGL Ecological Research Associates, Inc.

1410 Cavitt Street
Bryan, Texas 77801

A deep sea crab, Rochinia crassa. Long appendages are
considered by some as a deep sea adaptation to prevent
sinking into soft oozes.

Published by

U.S. Department of the Interior
Minerals Management Service
Gulf of Mexico OCS Region


been noted for shallow- Research Platforms
for Deep-Sea Studies

water oil platforms.

THE PRESENT
AND FUTURE

The NGMCS._ investiga-
tion provided the first
synoptic view of an
undeveloped deep Gulf of
Mexico. Acceleration of
deepwater oil and gas
activities began in the mid-
1990’s. In 1997, MMS
sponsored a Deepwater
Workshop to determine if
new deep-sea studies were
needed to evaluate better the
effects of deepwater
developments. The
participants endorsed the
idea that new studies were
needed. The need for
updated information about
the continental slope was
echoed by attendees at
contemporary MMS
Information Transfer
Meetings, who stressed that
more also needed to be
known about the abyssal
plains of the deep Gulf basin
at the base of the slope. Manned Submersibles

Research Vessels

Thus, a new study, "The
Northern Gulf of Mexico
Continental Slope Habitats
and Benthic Ecology
Study," was funded by
MMS and began in 1999.
When it is completed, a
comprehensive data and
information base will be
available that can be
compared to the  pre-
development information
base gathered by the
Northern Gulf of Mexico
Continental Slope Study.

Platforms and Unmanned Submersibles
(ROV’s)

27


No two communities are
exactly the same. They
reflect locally distinct
geochemical microhabitats.
Communities can change
rapidly over distances of
only a few feet. These
communities range in depth
from less than 1,000 feet
to one found at nearly
10,000 feet. Scientists
believe that there are likely
to be many more, especially
in deep water. Natural oil
slicks seen at the water's
surface in the deep Gulf
support this view.

The level of natural oil
and gas seepage in the Gulf

is surprising. Estimates
obtained using photographs
taken from space indicate
that approximately 10 mil-
lion gallons of oil per year
seep naturally from the
bottom of the Gulf.
Although released slowly
over a very large area, this
natural seepage is the
equivalent of an Exxon
Valdez oil spill every year.

Many — surface slicks
coincide well with known
positions of chemo-
synthetic communities.

There are many surface
slick locations that have
yet to be

investigated

for the presence of
chemosynthetic communi-
ties, thus the belief that
chemosynthetic communities
are more numerous than
documented at present. In
summary, these rather un-
usual assemblages are given
preferential treatment in the
deep Gulf of Mexico. Before
operators can develop an
area, they are required to
submit geophysical survey
records that are used to
determine that the area is
clear of any chemosynthetic
communities before
development proceeds.

Attached to the bottom, a sea pen filters the overlying water for food.

11


Sand waves on the bottom show that, at
some times and places, strong currents
can occur in the deep Gulf.

of sand, silt, and
comprising the mud.

clay

The photographs also
reflect a tranquil current
environment, overall.
However, strong currents
can occur at some times and
places, as indicated by
cross-bedded ripple marks
in the sediments. Addition-
ally, some bottom pho-
tographs show small rocks
and pebbles appearing to sit
on pedestals above the
bottom, indicating the fine
sediments have been
scoured from around them
by currents. These instances,
however, occur in only a
few of the photographs. The
vast majority of the photo-
graphs document a tranquil
environment in which fine
sediments accumulate.

The animals living in the
bottom sediments of the
deep Gulf (called infauna)
are typically small, nu-
merous, and diverse. How
small are they? Well, one
component called the
"meiofauna" are those
animals that pass through a
0.300 mm (one-hundredth
of an inch) sieve but are
retained on a 0.063 (about
two-thousandths of an inch)
sieve. That is small. The

other component is called
the macrofauna ("large
infauna"). These are the
larger worms and other
small invertebrates that are
retained on the 0.300-mm
sieve. While they are larger
than one-hundredth of an
inch, a _ dissecting
microscope still has to be
used to be able to see and
pick them out of the
sediment samples.

Overall, the abundance
and biomass of these small
"food packages" for larger
animals decline with depth.
For the larger component
(the macrofauna), numbers
decline less rapidly than
biomass, suggesting that the
size of the animals
decreases with depth. Of
interest, the biomass of the

meiofaunal component of
the sediment-dwelling in-
fauna rivaled or exceeded
the biomass of the larger
component, the macrofauna.
This observation is not char-
acteristic of many other
deep-sea areas. The Gulf
macrofauna are thus quite
small as compared with
many other deep-sea
systems, especially at the
deepest depths. This
observation lends credence
to the idea that, in com-
parison with other slope
systems, food or energy
availability in the Gulf may
be especially low, thereby
limiting population density
levels.

The abundance of the
macrofauna in the deep Gulf
actually shows a peak in
abundance at a depth of

log Number / sq. m

Mecrofaune +
¥*352-o.000109%

Approximate Wet Weight
109 ug / sq. m

Depth (m)

Relative number of macrofauna and meiofauna by depth (upper) and
relative biomass by depth (lower). Macrofauna biomass declines more
rapidly than numbers, indicating a decrease in size.

14