Gulf of Mexico
The Gulf of Mexico is located at the south eastern corner of North America
bordered by Mexico, Texas, Louisiana, Mississippi, Alabama, and Florida. It has a surface area
of 1.5 million km2 and marine shoreline of ~5,700 km from the tip of the Yucatan to Cape Sable
in Florida. Water enters the Gulf through the Yucatan Strait, circulate as the Loop Current,
and then exits through the Florida Strait eventually forming the Gulf Stream. Portions of the
Loop Current often break away forming eddies or 'gyres' which affect regional current patterns.
Drainage into the Gulf of Mexico is extensive and includes 20 major river systems. Annual
freshwater inflow to the Gulf is approximately 10.6x1011 m3 per year. 85 % of this flow comes
from the United States, with 64 % originating from the Mississippi River alone.
The most infamous "dead zone" develops in the Gulf of Mexico each
summer, threatening fisheries. This dead zone is an area of anoxic and hypoxic water
thought to develop because an excess of nitrogen from farm fertilizers, sewage and emissions
from vehicles and factories enters the gulf predominately via the Mississippi River. A
"nitrogen cascade" results as nitrogen flows untreated into the gulf and triggers the
proliferation of plankton, which in turn depletes oxygen in the water. While fish might
flee this suffocation, slow moving, bottom-dwelling creatures like clams, lobsters and
oysters are less able to escape. Hypoxia in the Gulf of Mexico coincides with water
column stratification, high salinities, and poor mixing. These physical processes
interact with the biological processes responding to eutrophication across large
spatial scales (4000 to 24,000 km2).
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Current and Recent Projects
Mapping Primary Productivity in the Gulf of Mexico
The Phytoplankton Dynamics Groups was involved in three research programs between 2004
and 2006, in all cases, primarily with the responsibility of mapping primary
productivity across the Gulf of Mexico. Surveys were conducted aboard research cruises
ranging from 5 to 14 days; Dr. Quigg and her students were involved in a total of 12
cruises. While traditional C-14 techniques were used to measure primary production on
all cruises, in 2005, a Fast Repetition Rate Fluorometer (FRRF) was also used to
continuously map primary productivity on several cruises. In 2006, two new instrument
packages were tested: A Fluorescence Induction and Relaxation (FIRe) system for
continuous mapping of primary production and Phytoplankton Pulse Amplitude-Modulated
fluorometer (Phyto-PAM) for analysis of phytoplankton community composition and generation
of Rapid Light Curves (analogous in principle to photosynthesis versus irradiance curves).
Sources:
Gulfbase.org,
MSNBC.
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Seasonal Mapping of Productivity and Nutrient Limitation.
PI: Dr. Jim Ammerman of the New York Sea Grant and Dr. Jason Sylvan of the University of Southern California.
The objective of this study was to map surface primary productivity and nutrient
limitation in the Mississippi Plume and the Louisiana shelf several times during the
high flow period in the spring and early summer. Primary productivity will be mapped
using a Fast Repetition Rate (FRR) Fluorometer and nutrient limitation will be mapped
by measurement of cell-surface enzyme activities and nutrients. These productivity
measurements will be compared with satellite remote sensing of phytoplankton pigments
as well as shipboard bicarbonate uptake measurements. We expect predominantly phosphorus
limitation during this time period so the major enzyme activity to be measured will be
alkaline phosphatase, though enzymes responsive to nitrogen limitation will also be
measured. Many measurements will be made continuously underway with automated instruments
in real time, though discrete samples will also be analyzed.
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Gulf of Mexico Ecosystems and Hypoxia Assessment.
PI: Dr. Steve DiMarco with Oceanography, Texas A&M University, College Station.
NGOMEX - NOAA Coastal Ocean Program.
The processes that control and maintain the hypoxic zone in this region are complex and
their relative strengths are known to vary temporally and spatially. Although close to
the Mississippi River Delta, hypoxia is thought to be driven by biological processes,
further downstream, the dominant controlling processes are mostly physical. Currents and
winds combine to break down the vertical stratification necessary to sustain the low dissolved
oxygen. Because the eastern region of the shelf (between 91°W and 89°W) is almost always
hypoxic in mid-summer, it is likely that variability of the western region (between 91°W and the
TX border), that largely controls the total size of the hypoxic area in a given year. Therefore,
understanding the interactions of the physical, biological, and geochemical processes on the western
shelf is critical for a comprehensive description of the mechanisms that control hypoxia. An
integrated, multidisciplinary study of the TX-LA Shelf was conducted which targeted process-oriented
high spatial resolution hydrographic surveys, multidisciplinary moored observations, and a coupled
model to examine small (10 km) temporal and spatial scales of variability of hypoxia.
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Th(IV) and Pa(IV,V) Binding to exopolymeric acid and polysaccharides in the marine environment.
PI: Dr. Peter Santschi with Marine Science, Texas A&M University at Galveston
NSF - Chemical Oceanography Program.
Th(IV) and Pa(IV,V) isotopes are important proxies in oceanographic investigations.
Even though almost routine, these approaches rely on often poorly constrained, empirically
determined and variable isotope ratios or ratios to POC. Previously conducted laboratory and
field investigations suggest that Th(IV) removal could be controlled through binding by
exopolymeric acid polysaccharide (APS) rich biomolecules, potentially produced by both
phytoplankton and bacteria. We hypothesize that Pa(V) present in ocean water must first be
reduced to Pa(IV) by organic biomolecules before efficient binding to solid phases can occur.
We further propose that the most efficient binding would occur to APS-rich biomolecules
produced by phytoplankton species such as diatoms, prymnesiophytes and cyanobacteria.
The program investigated possible fractionation mechanisms between Pa(IV.V) and Th(IV)
in the ocean. Laboratory studies consist of uptake experiments to a number of substrates,
including purified APS harvested from phytoplankton and bacterial cultures to be used in
Th(IV) and Pa(IV,V) binding assessments. The most important analytical task will be to
better characterize, both chemically, in terms of molecular composition, and physically,
in terms of surface activity, the newly discovered strongly Th(IV) complexing APS of ~13
kDa molecular weight, found in particulate and colloidal material collected from the
Gulf of Mexico, Atlantic and Pacific Ocean and the South China Sea. The proposed field
program included collection and extraction of diverse types of organic matter for use
in laboratory studies, as well as the determination of temporal and spatial variations of
radiochemical and biochemical parameters.
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