River drains almost half of the conterminous United States as it tracks an
ancient midcontinent rift valley on its way from northern Minnesota to the
Gulf of Mexico. This river system ranks sixth and seventh worldwide, in terms
of annual suspended sediment loads and water discharge, respectively. Combined,
the Mississippi River and its last major downstream distributary, the Atchafalaya
River, provide approximately 90 percent of the freshwater input to the Gulf
of Mexico. In addition to its sheer magnitude, the Mississippi River also
provides both an abundant source of water and an efficient mode of transportation
for diverse industries.
This SeaWiFS image of the Mississippi River plume on March 22, 2001, shows site locations of coring stations in the Mississippi Bight for studying sediment-associated contaminants. A sites represent variable degrees of oxygen depletion in the water column and variable influences from the river discharge; B sites indicate where researchers are studying short-term deposits. Image courtesy of Peter Swarzenski.
Because this river is also a direct source of drinking water to many downstream municipalities, including New Orleans, an accurate assessment of the environmental state of the Mississippi River and its discharge to the Gulf is both warranted and essential. Thus, the U.S. Geological Survey (USGS), working with scientists from various universities and state agencies, is investigating the historic downstream delivery of sediment-associated contaminants into the Gulf of Mexico.
We propose that basic biogeochemical processes in the lower Atchafalaya River, which empties into a broad, shallow receiving basin, are fundamentally different from those in the lower Mississippi River, which discharges into the Gulf of Mexico very close to the shelf break. Secondly, we think that river discharge, as well as complex physical processes, affect the flux and fate of materials moving off the continent into the Gulf of Mexico.
Previous studies confirm that dissolved constituents carried in the lower Mississippi River are generally not significantly elevated relative to other large rivers globally, despite the implied industrial influences. However, most river trace metals reside in a particulate (greater than 0.45 micrometers) pool that often includes reactive colloidal-sized particles.
The fate of sediment-hosted contaminants is linked dynamically to particles within a river. Biogeochemical reactions can remove a contaminant from the dissolved pool, which leads to its deposition and accumulation in riverbed sediments. During times of increased discharge, such as flooding and spring melt events, resuspension of the contaminants from the sediments into the rivers water column may occur. These processes of short-term storage and resuspension of sediments are closely tied to the hydrologic cycle and are significant in determining the subsequent flux of materials off the continent.
While the Mississippi River receives most of its water from the Ohio River basin, most of the suspended sediments originate from the Missouri River watershed. At the Old River control structure upstream of Tarbert Landing, Miss., two human-made channels connect the main stem Mississippi with the Red and Atchafalaya rivers. This regulated diversion of composite Mississippi River water into the more recently formed Atchafalaya River (a branch since the 1500s) allows for a direct comparison of the effect of changing basins and shelf conditions on the off-continent material flux.
Alterations to the natural flow of the river can affect both suspended sediment and dissolved loads. As the flow of the Mississippi is largely regulated, the timing, location and energy of river water being discharged to the Gulf plays a critical role in the development of offshore water column stratification and the availability of select river-borne constituents to biological processes (bioavailability).
Suspended sediment concentrations in the Mississippi River have declined dramatically during the last half of the 20th century. For example, long-term suspended sediment loads in the river have fluctuated from about 1,576,000 tons per day in 1951 to about 350,000 tons per day in 2002. Researchers have also observed declines in the concentrations and loadings of organic carbon. Large flood events may both affect future downstream suspended sediment concentrations and inject elevated river-borne constituents directly into offshore waters.
The concentration of some nutrients transported downstream has increased dramatically during the last decades. For example, the current (1980-1999) average nitrate flux has increased almost threefold from what it was from 1955 to 1970. The heightened flux of bioavailable nitrogen to the Gulf is partially responsible in the formation and intensity of a large hypoxic (low oxygen concentrations in the water column, defined as less than 2 milligrams per liter) zone that occurs regularly in portions of the Mississippi Bight during summer months. The largest zone of hypoxia in the western Atlantic occurs in this region, and has been attributed in part to recent increased fertilizer use upstream.
To address linkages between the short-term flux of material off the continent and coastal hypoxia, we are currently comparing marine sediments at sites where the overlying water column has been chronically hypoxic to sites that are typically oxygen-rich. We are also developing a suite of geochemical tracers and biomarkers to provide information about historic hypoxic conditions, as preserved in the recent sedimentary record. Presumably, we can distinguish modern hypoxia from pre-anthropogenic hypoxia by evaluating historic off-continent material fluxes in age-dated sediments, as well as in individual coral bands.
Bob Rosenbauer of the U.S. Geological Survey in Menlo Park, Calif., and Brian Kindinger of ETI Professionals in St. Petersburg, Fla., sample a sediment box core collected on the R/V Longhorn during a summer 2003 cruise off the Mississippi River. Photo by Marci Marot, USGS.
At the Flower Garden Banks coral reefs, which are located in the direct pathway of the eastward-flowing Mississippi River plume, a suite of trace elements analyzed from coral growths already are revealing a continuous record of Mississippi River discharge. We are seeing a possible linkage between past fluctuations in Mississippi River discharge and offshore hypoxic events prior to the widespread use of fertilizers upstream.
Analyses of sediment cores using organic and inorganic tracers as well as benthic foraminifera appear to provide a reliable record of the historic variability of hypoxia in the northern Gulf of Mexico over the past few centuries. Natural variability in hypoxic events may be driven largely by flooding cycles of El Niño/La Niña prior to recent increases in nutrient loading. Specifically, large floods in 1979, 1983, 1993 and 1998, compounded with the widespread use of fertilizers, also appear at least partially responsible for the recent (post-1980) dramatic increase of hypoxic events in the Mississippi Bight.
Planned research will continue to explore and strengthen relationships between off-continent material flux, the rivers delivery of contaminants in sediment, and the occurrence and scale of hypoxic events. These studies will take an integrated scientific approach that incorporates the fields of geochemistry, geology, hydrology and biology.