The Deepwater Program: Northern Gulf of Mexico Continental Slope
Habitat and Benthic Ecology
Increasing exploration and exploitation of fossil hydrocarbon
resources in the deep-sea prompted the Minerals Management Service
of the U.S. Department of the Interior to support an investigation
of the structure and function of the assemblages of organisms
that live in association with the sea floor in the deep-sea. The
program, Deep Gulf of Mexico Benthos or DGoMB, is studying the
northern Gulf of Mexico (GOM) continental slope from water depths
of 300 meters on the upper continental slope out to greater than
3,000 meters water depth seaward of the base of the Sigsbee and
Florida Escarpments. The study is focused on areas that are the
most likely targets of future resource exploration and exploitation.
However, to develop a Gulf-wide perspective of deep-sea communities,
sampling in areas beyond those thought to be potential areas for
exploration has been included in the study design.
The program
is designed to gain a better ability to predict variations in
the structure and function of animal assemblages in relation to
water depth, geographic location, time and overlying water mass.
Biological studies are integrated with measurements of physical
and chemical hydrographic parameters, sediment geochemical properties
and geological characteristics that are known to influence benthic
community distributions and dynamics. A set of eight (8) hypotheses
are being tested on the basis of measures of benthic community
structure. It is hypothesized that community structure varies
as a function of:
1) water depth,
2) geographic location (east vs. west),
3) association with canyons,
4) association with mid-slope basins,
5) sea surface primary productivity,
6) proximity to hydrocarbon seeps,
7) time (seasonal and interannual scales), and
8) association with the base of escarpments.
Measures
of community structure used to test the hypotheses are variations
in diversity, similarities in assemblage composition (at the species
level), variations in biomass and abundance, and the mean size
of individuals within specific size categories.
The underlying
premise of the hypotheses to be tested is that deep-sea communities
are food limited. This premise leads to the hypothesis that variations
in community structure in time and space are a function of the
input of food to the seafloor. In other words, community dynamics
and structure are dependent on the availability and quality of
food resources. Corollary hypotheses test the possibility that
each independent variable is related in some way to how organic
matter from a variety of potential sources is utilized by the
benthic community.
After defining
community structure, the next set of objectives use this information
to infer the flux of organic carbon into and through the ecosystem.
The conceptual model assumes that community structure and function
are tightly coupled. Presently there is little reason to reject
this generalization, but direct evidence for it in the deep-sea
is at best fragmentary.
The conceptual
model represents each of the principal size categories of the
living components as standing stocks at each study site in the
survey. The model includes demersal fishes, megafauna, scavengers,
macrofauna, meiofauna, and heterotrophic bacteria. This model
(Figure 1), of a sediment-associated food web, can be coupled
with a model of fossil hydrocarbon utilization by chemoautotrophic
organisms including large invertebrates that house endosymbionts.
This linkage is yet to be explicitly established and is the basis
for one of the hypotheses being tested. The boxes in the model
represent standing stocks which have units of biomass (organic
carbon per unit area) whereas the arrows represent flux between
boxes and hence have units of organic carbon per unit area per
unit time. For consistency, the units are mg C m-2 and mg C m-2
day-1. Data from the survey portion of the program quantifies
standing stocks across the survey area. Respiration rates are
estimated on the basis of organism size and temperature from established
relationships in the published literature. The fluxes represent
transfers between components and are calculated by difference
to balance respiratory losses at steady state. Burial loss of
carbon is organic carbon (detrital) concentration times sediment
accumulation rate. Input to the bottom is solved for by assuming
that it is equal to the sum of the respiration and burial losses
at steady state.
The second
phase of the project is designed to test the model. Direct measurements
will be made of fluxes. This will be carried out by two field
programs in June of 2001 and 2002. Total sediment community respiration
will be determined using a benthic lander and incubation chambers.
Total respiration will be partitioned by measuring bacterial activity
in pressure chamber incubations at in situ temperatures. Uptake
and respiration will be determined using mixed amino acids labeled
with radiocarbon. Sulfate reduction will be measured using radio-labeled
sulfate incubation of samples from sediment cores. Lander/chamber
fluxes that are to be measured include oxygen, dissolved inorganic
carbon, inorganic nitrogen, phosphate, and silicate. Scavenger
domains of occupation will be estimated using baited traps, time-lapse
cameras and an ADCP to estimate vertical and horizontal eddy mixing
and mean current direction. Stable isotopes of carbon and nitrogen
will be used to determine the food chain's structure and linkages.
Physical and biological mixing will be estimated using a suite
of natural radionuclides characterized by an appropriate range
of decay rates. Data from the second field year will be used to
adjust model parameters. The location of the experimental sites
will be evaluated according to the model, sampling results, and
on-going testing of programmatic hypotheses. Experiments during
the third field year will be designed to further validate the
revised model rates and parameters. Sampling sites will be selected
as needed to improve the resolution of the models and advance
the testing of the hypotheses.
Figure 1.
