Quarterdeck Volume 6, Number 1, May 1998
NEGOM investigates variations in the gulf
As anyone who has visited the beach knows, the ocean is salty. But, just how salty is it in different places, at different depths, and at different times? How do other water properties such as temperature, dissolved oxygen, nutrients, particulate matter, and chlorophyll-a vary in time and space? What causes these variations? What can their spatial and temporal distributions tell us? What ocean processes cause the observed patterns?
As part of a new research project being conducted at Texas A&M University, my colleagues and I will investigate these questions over the next four years while we gather and study information on the waters over the U.S. continental shelf and slope in the northeast Gulf of Mexico.
Our project, "Northeast Gulf of Mexico (NEGOM): Chemical Oceanography and Hydrography," is one of five studies in a physical oceanography program sponsored by the Minerals Management Service (MMS) of the U.S. Department of the Interior. Scheduled to run from October 1997 through September 2001, it includes three years of field work and one year of data interpretation and synthesis. We will collect data on nine cruises to observe temporal and spatial distributions of water properties during spring, summer, and fall.
Our first cruise, November 16-26, 1997, aboard Texas A&M's R/V Gyre, was staged out of Pascagoula, Mississippi. Our cruise track covered 2,767 kilometers and an area approximately half the size of Texas. We took samples at 174 locations, or stations, across the gulf. The station depths ranged from eight meters to more than 1,000 meters.
At 80 stations, the ship continued steaming while we deployed an expendable bathythermograph (XBT). This probe continuously measured the temperature as it fell through the water and transmitted its data to the ship through a wire. The transmission stops when the wire breaks; the expendable probe rests on the sea floor.
At the other 94 stations, we stopped the ship to lower a conductivity-temperature-depth measuring instrument (CTD) for continuous profiling of water properties, and 10-liter Niskin bottles for collecting water samples at up to 12 discrete depths. From top to bottom through the water column, the CTD continuously recorded information about the water: temperature, conductivity, downwelling irradiance, percent light transmission, and fluorescence.
At all stations, we extracted water samples from the Niskin bottles and analyzed the samples for nutrients, oxygen, and salinity. At more than half the stations, we filtered the water samples and stored the filters for on-shore analysis of particulate material, particulate organic carbon, and phytoplankton pigments. Throughout the cruise, we operated an acoustic Doppler current profiler, which measures the current velocity in four-meter depth increments to a depth of 400 meters.
From the CTD measurements, we found that the water column at most locations had uniform temperature and salinity from the surface to a depth of at least 40 meters. This well mixed water was a result of pre-cruise cold fronts that mixed the layers of water that often have different temperatures and salinities.
In deeper water, we observed both this mixed layer, and, below it, variations in water properties indicative of other processes at work. For example, from our data about light transmission, we detected layers of water with high concentrations of particles extending off the sides of the DeSoto Canyon. These data suggest the presence of currents that may be influenced by the canyon.
Satellite imagery of sea-surface temperature and sea-surface height anomaly showed the presence of a clockwise-circulating eddy, shed from the Loop Current, at the southwest edge of the study area. We will analyze our CTD and bottle data to see if this eddy influenced the water properties or currents there. (See "Spin cycles" on page 10 for more about eddies.)
In addition to data collection for NEGOM scientists, we encouraged participation of scientists and students with complementary projects. High atop the flying bridge deck of the Gyre were two sets of "Big Eyes," telescopic binoculars with a range out to five miles. During daylight, the Big Eyes were managed by brave souls who fought off wind, weather, and mal de mer to search for marine mammals. White caps made observation of whale and dolphin blows difficult. Nevertheless, sperm whales and several dolphin varieties were observed. (For more about marine mammals in the gulf, read the GulfCet stories in this issue of Quarterdeck.)
The scientific crew deployed 23 drifters on behalf of other MMS researchers. With a very small area exposed to the air, these drifters follow the surface currents rather than being blown by the wind. Several times each day, they signal passing satellites. The satellites then relay the data to the shore-based researchers. At cruise end, all drifters were successfully reporting.
A major objective in bringing the NEGOM project to Texas A&M was to provide training and research opportunities for oceanography students. Of the 23 members of the scientific crew, eight were graduate students and one was an undergraduate.
The conditions experienced by these students were excellent for a November cruise. A front passed over us while the ship was steaming to the first sampling station, making that part of our ride rocky. But by the time we reached the station, the seas were calm and the crew members were getting their sea legs. After that, the weather was windy but fine, the data collection proceeded apace, and, unlike some cruises I have been on, the equipment worked without any breakdowns. We returned to shore a day earlier than scheduled-just in time to get home for Thanksgiving!
Conductivity is the electrical conductance of a seawater sample. By determining the ratio of the conductivity of the sample to that of a standard seawater of known salinity, the salinity of the field sample can be determined.
Downwelling irradiance refers to the sunlight that penetrates down into the water column. The amount that penetrates diminishes exponentially with depth.
Percent light transmission is the percent of a beam of light of known wavelength that is received by a sensor after the light passes through a known length of seawater. Particles in the water scatter and absorb the light, and thereby control how much light gets to the sensor. This gives a measure of the concentration of particles in the water.
Fluorescence is emission of radiation, especially visible light, resulting from exposure to external radiation. For our application it is the emission of light by phytoplankton as a reaction to a light source; measuring fluorescence gives us a relative measure of the chlorophyll-a content.
Salinity is the total grams of dissolved inorganic material in one kilogram of seawater, usually on the order of about 3.5% by weight.
Particulate organic carbon (POC) is the fraction of the organic carbon in a water sample that is in particle, rather than dissolved, form. Particles are those materials retained on a 0.7 micron filter after filtration of the water sample. These data are used to determine what portion of the particulate matter is organic and what is inorganic. The ultimate source of organic materials is living organisms, including their remains and excretions.
Phytoplankton pigments are the colored organic compounds made by plant species of plankton. In our study, the major classes of pigments are chlorophylls and carotenoids. These are the essential chemicals for photosynthesis, which transforms energy from sunlight into the energy that is used to synthesize the building blocks of life.
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Last updated May 1, 1998