By Peter H. Santschi and Gary A. Gill
LOER is equipped with laboratory instrumentation and field sampling equipment for trace-element and radiochemical studies. Research topics that have been addressed in LOER facilities include study of fate and effects of toxic trace metals in natural waters, chemical speciation of metals and metalloids in water and sediments, bioaccumulation and food chain transfer of toxic elements, atmospheric transport processes and fluxes of trace elements and radionuclides into coastal waters, the role of colloids in mediating the uptake of trace metals and radionuclides by organisms and suspended matter, dynamics of particles in water and in sediments, determination of chemical fluxes from sediments, sediment transport and erosion processes, geochronology of sediments, carbonates and other deposits and the role of continental margins in exporting organic carbon, radionuclides, and trace metals to the open ocean via colloidal transport.
Principal investigators, funded by local, state, and federal agencies, oversaw a combined 1993-1994 budget of $1.3 million. Since LOER's inception it has obtained over $3.3 million of extramural funding for multi-year research in coastal areas of the Gulf of Mexico and the bays and estuaries of Texas. These programs have resulted in over 30 publications and funded approximately 60 days at sea aboard the R/V Gyre since 1990.
Currently, ten graduate students in the Department of Oceanography use LOER facilities to conduct research in Galveston for their masters or doctoral research projects. Thanks to matching funds from the Texas Institute of Oceanography for some of our research, LOER has been able to train undergraduate students for careers in ocean and environmental science. Since 1989, LOER principal investigators employed more than 20 Marine Sciences and Marine Biology undergraduates at Texas A & M University in Galveston. Many of them also conducted independent studies on related topics. LOER investigators currently employ nine undergraduate students on various research projects. Graduate and undergraduate students conduct much of the routine sample collection in the field and sample analysis in the lab. Over the past year, they carried out considerable field work by participating in three research cruises to the Gulf of Mexico aboard the R/V Gyre . The work they conduct often provides necessary experience for a graduate career or job in an environmental consulting company. [201 K]
Interaction of trace elements with natural organic matter in aqueous systems plays an important role in geochemical and radio-chemical research of Galveston researchers, Drs. Peter Santschi and Gary Gill. Trace element-organic associations are highly important in aqueous systems. Chemical speciation can affect important biogeochemical processes including growth response of organisms, bioavailibility to organisms, food chain transfer (bioconcentration), and the transport and fate of trace elements in aquatic environments. Currently, interrelated research projects underway at LOER investigate the importance of trace element-organic matter interactions in marine and aquatic systems to the chemistry, fate and effects of trace metals and radioactive analogues in aquatic systems. Many of these projects are conducted in collaboration with researchers from other universities. Galveston's researchers concentrate on developing new tools to assess the rates, transformations and effects of metal ions, radionuclides and organic carbon species in marine and freshwater environments. This includes age determination of geological materials combined with accurate assessment of the concentrations of corresponding chemical species.
[9K] Emerging evidence shows the importance of colloidal particles in mediating particle adsorption and transport processes of trace elements in aquatic systems. Colloidal particles are defined by size, typically ranging between 1 nanometer to about 1 micrometer, which includes microparticles and macromolecules. Colloids are isolated from natural waters using tangential flow ultra-filtration techniques. Natural colloidal material includes macromolecules of different structures and properties, so molecular weight "cut-offs" are not highly precise with respect to molecular weight or size. Colloidal macromolecular organic molecules can greatly affect the removal pathways of heavy metals from the water, and their bioavailability and toxicity to aquatic organisms. Although the fate of many trace metals in estuarine and coastal waters is intimately tied to that of dissolved and colloidal organic matter and inorganic colloids, little work has been done to establish the association of trace elements with colloidal organic matter. Colloids often appear to be reactive intermediaries in the sorption process, leading to slower uptake by suspended particles because a substantial fraction of trace metals is first adsorbed by colloids, which coagulate more slowly with suspended matter. [17K]
Many natural colloids contain substantial amounts of trace elements, including potentially toxic heavy metals such as lead, mercury, silver, and cadmium. Heavy metals are among the most toxic, ubiquitous, and persistent contaminants in estuarine habitats. Toxicity to aquatic organisms, bioavailability and the potential for bioaccumulation and food chain transfer depends critically on the specific physico-chemical forms or species present in the substrate. Metal ions can bind to inorganic or organic ligands, including organic macromolecules. Some of these organic complexes have very different properties and chemical reactivities and, therefore, biological availabilities.
Several studies of estuarine, shelf and slope regions of the Gulf of Mexico have been conducted aboard the R/V Gyre. Measurements of ultra-low concentrations of radioisotopes and trace metals, present in the water at 10(-8) to 10(-20) molar concentrations, necessitated the development and adaptation of new analytical techniques and oceanographic field equipment for sampling, in situ extraction, filtration or ultrafiltration of large volumes of water, and for determination of their small fluxes. Since it is paramount for this kind of work to avoid any sources of contamination, special clean techniques needed to be established in the field and lab, and special equipment was needed for analysis in the laboratory.
Results obtained by graduate student Liang-Saw Wen from the Gulf of Mexico suggest that in waters off the shelf/slope break (500-meter depth), concentrations of lead, zinc, copper, and silver are only slightly higher than in the open ocean. He also found that a major fraction of these metals is associated with colloids in slope waters and in Galveston Bay. Despite large concentrations of industrial and urban complexes around Galveston Bay (where 30-50% of chemicals in the United States are processed), trace-metal concentrations in the water of its main sections remain low. This self-cleansing capacity of Texas estuaries is particularly interesting and provides a focus for research efforts.
While metal concentrations in Galveston Bay are higher than nearby gulf waters, they are orders of magnitude lower than those reported by various government agencies for the same waters. A summary of data from the Texas Water Quality Board and Water Commission shows that from 1980 to 1987 concentrations of 10-100 micrograms per liter of lead, 1-200 micrograms per liter of copper and 0.1-4.0 micrograms per liter of mercury were reported. Our own measurements for 1989 to 1994, funded by the Texas Chemical Council and Sea Grant, showed 0.01-0.1 micrograms per liter of lead, 0.5-1.5 micrograms per liter of copper and 0.1-1.0 nanograms per liter of mercury. This discrepancy testifies to the necessity of stringent precautions in collecting unbiased water samples for trace element analysis. Most previous efforts did not apply ultra-clean sample collection and handling techniques, and seriously compromised the sample collections. Concentrations of some trace metals (e.g. copper and silver) and relationships between sediment-water partition coefficients and particle concentrations that we established for Texas estuaries and rivers are currently being used by state agencies for regulatory purposes.
A challenge in oceanography in this decade is to understand the role of dissolved organic carbon (DOC) cycling in oceanographic processes. Our knowledge of DOC cycling in aquatic systems is still in its infancy, despite decades of intense research. The reservoir of DOC in the ocean is much larger than that of particulate organic carbon and a significant fraction of DOC is in the form of colloidal organic carbon (COC). The quantity of oceanic DOC is comparable in size to the atmospheric CO2 reservoir. [9K] Perplexing yet interesting aspects of dissolved organic matter (DOM), concern its apparent old age and fast turnover times within some of its fractions. Apparent 14-C ages for DOC have been reported by others to be 1,000 to 6,000 years. In contrast, reported rapid microbial carbon uptake and release rates and short coagulation times of colloidal organic matter (COM) indicate that a significant portion of the DOM pool turns over in weeks to months. Our radioisotopic analyses of COM provide further constraints on turnover times of COM along transects from near-shore to offshore stations in the Gulf of Mexico, including the open-ocean conditions provided by warm-core rings located at the offshore stations. By ultrafiltering 200-1000 liters of sea water, graduate student Laodong Guo isolated enough COM to have it analyzed by accelerator mass spectrometry. The results showed that macromolecular material larger than 10,000 Daltons (atomic mass units) is contemporary in age, while material isolated with a lower molecular weight cut-off ultrafilter had an apparent 14-C age of 400-4000 years, similar to those reported by other researchers for whole-DOC. It appears therefore that the higher the molecular weight the younger the COM.
In the paradox presented by this data, COC appears to be old and unreactive according to radiocarbon dates, which can only provide an average age for all the compounds that constitute COC. As stated above, however, certain fractions of COC are very reactive and have fast turnover times. To investigate we turned to naturally-occuring thorium isotopes, powerful tracers of the kinetics of particle cycling in marine systems. Due to the high particle reactivity of thorium ions in seawater, they can be used to derive particle and colloidal residence times and trace-metal scavenging rates for isolated compounds in COC. [9K] Our research attempts to reconcile biological, biochemical and physical views of organic carbon cycling. In order to reliably measure DOC and its colloidal fractions, we must obtain uncontaminated measurements of DOC and COC, and apply large-volume cross-flow ultrafiltration techniques.
Mercury contamination in aquatic systems has been recognized as a serious environmental problem for more than four decades. The concern driving current research of mercury in aquatic systems is the potential human health risk associated with consumption of fish with elevated mercury levels. In many cases mercury in contaminated systems is related to known point sources, but fish in regions with no known local anthropogenic mercury sources have also been found to contain levels of mercury above safe consumption guidelines.
Mercury, like many other heavy metals, interacts strongly with dissolved and colloidal organic matter in aquatic systems. That interaction can consist of complex formation with organic ligands, adsorption onto colloidal and particulate organic material and production of organo-mercurials, which involves biotic formation of a covalent bond between mercury and an organic group. Gary Gill and a graduate student, Mary Stordal, found that a major fraction of the mercury present in estuarine systems is associated with macromolecular (colloidal) organic material, and that a small portion of this mercury exists as the highly toxic and bioaccumulated methylmercury form. Mercury's chemistry in aquatic systems is complex because no thermodynamically predictable interaction exists between solution complexes of inorganic mercury (Hg [II]) and organo-mercury compounds, such as methylmercury. Bacteria produce methylmercury in natural waters and sediments from inorganic mercury that naturally resides in the ecosystem and from inorganic mercury added by exogenous sources. The partitioning of mercury between these two pools is kinetically controlled by the net effect of the production (methylation) and destruction (de-methylation) of methylmercury by biologically mediated processes. The relative abundance of methylated mercury species in aquatic systems is of particular concern because these compounds are highly toxic, they constitute the major form of mercury accumulated in fish tissues, and can enter the food chain by direct uptake from solution.
Dr. Gary Gill and the graduate and undergraduate students working with him currently conduct interrelated research programs on biogeochemical cycling and transport of mercury in the environment. These investigations include a project to identify sources of mercury pollution in Lavaca Bay, Texas, a study of the atmospheric deposition of mercury and other trace elements into the Everglades in Florida, a study of mercury transport in the Sacramento Valley (California) and Carson River (Nevada) watersheds from relic mining activities in the Sierras, investigations of mercury cycling in San Francisco Bay estuary, an ecological assessment of mercury contamination in Clear Lake, California (an EPA superfund site), and a laboratory and field study to develop and test methods to determine the rate of bacterial mercury methylation in the water column and sediments using radioisotopic techniques.
[9K] Investigations into the production of methylmercury in natural environments using radioisotopic methods yielded significant new findings regarding rates of methylmercury production. It was not previously possible to directly and quantitatively measure in situ production rates of methylmercury in the environment. While radioisotopic methods had been previously applied to methylmercury production rate studies, they were conducted using radioisotopic mercury additions several orders of magnitude above ambient environmental mercury levels. Hence there was great concern about the reliability of the production rates obtained. When we developed the method to work at more realistic mercury-addition levels, we observed dramatically increased methylmercury production rates. These findings suggest that production rates at natural mercury levels in aquatic systems are 10 to 100 times greater than previously believed possible. Therefore, mercury entering aquatic systems might convert quite quickly to bioaccumulated, toxic methylmercury and readily move up the aquatic food chain if environmental conditions favor the micro-organisms responsible.
Oceanography, Texas A&M University
Updated July 24, 1995