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USGS Heads to Lake Tanganyika to Study Delivery of River-Borne Contaminants

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eastern shoreline of Lake Tanganyika
Lake Tanganyika: Eastern shoreline of Lake Tanganyika just north of Kigoma. Note the contrast in vegetation. The front range is part of Gombe National Park and is thus protected. The highlands are unprotected and have consequently been heavily deforested.

Lake Tanganyika study area
USGS-NGS Lake Tanganyika Project, July 8-27, 1999: Basemap of our study site and sampling locations on Lake Tanganyika. Our operation's headquarters were based out of the Tanzanian Fisheries Laboratory in Kigoma (TAFIRI). Our fieldwork was also closely associated to a tropical paleolimnological REU program coordinated by Andrew Cohen (University of Arizona). [larger version 56KB]
This summer a research cruise directed by scientists from the U.S. Geological Survey (Peter Swarzenski, St. Pete, and Bill Orem, Reston) investigated the effects of heightened population pressure on the water quality of select rivers entering Lake Tanganyika, Tanzania. The cruise was conducted in collaboration with the University of South Carolina (Jim Krest) and University of South Florida (Bob Bryne and Greta Klungness).

Lake Tanganyika is the largest (~34,000 km2) and oldest (~12 Ma) of the East African Rift Valley Lakes. It is also the second deepest lake in the world (after Lake Baikal) and has become a "hot spot" of biological evolution during its lengthy and complex geological past. Roughly 600 species of vertebrates and invertebrates are endemic to the lake. Hundreds of species of fish and invertebrates may be found along the lake's rocky shorelines, often at a single submarine boulder or outcrop. More than 1,500 species of animals and plants have been described from the lake, making it biologically one of the richest lakes on earth.

In recent years, Lake Tanganyika, like many other biologically sensitive areas, has begun to feel the effects of increased population pressure. Fishing practices, for example, have become much more efficient, and consequently, more destructive. However, the most damaging threat to the lake's biodiversity appears to be due to a heightened rate of sediment influx, especially from the heavily impacted smaller watersheds of northern Lake Tanganyika, where large-scale deforestation and farming practices have led to a dramatic increase in soil erosion rates. (See shoreline photo below.) A progressive increase in population pressure (this region has among the world's greatest rural population densities) has forced a change in land use from pristine tropical forests (e.g., Gombe National Park) to small agricultural plots located on steep, denuded slopes bordering the lake.

Manual piston coring
Collecting core samples: Manual piston coring in the Luichi River delta. We collected two 1-m cores at each river site for sediment and pore-water samples.
As a result, accelerated erosion rates supply streams and rivers with an increased suspended particulate load, which is deposited as fine-grained silts and clays in the rocky deltas. Record sediment accumulation rates in highly impacted river systems can reach up to 100 cm yr-1.

Analogous to the destructive impact of sediment pollution on coral reefs around the world, the freshly eroded sediments entering the lake adversely affect biodiversity not only by decreasing species habitat, but also by efficiently complexing certain essential nutrients/trace elements. Lateritic particles, such as Fe or Mn oxides, are strong complexing agents for nutrients and trace metals. We are exploring the relation between the scavenging capability of freshly weathered oxide surfaces and the availability of nutrients/trace metals as a control on nearshore biodiversity.

Field work was closely interwoven with Andrew Cohen's (University of Arizona) Lake Tanganyika paleolimnology REU (Research Experience for Undergraduates) program. This collaboration not only greatly simplified logistics in Tanzania but also extended the scientific capability of our team because we were able to combine resources.

Congolese freighter R/V Maman Bonita
Above: We used the Congolese freighter R/V Maman Bonita as our research platform for gravity coring and delta work.
To meet our objectives, sediment piston cores were collected from contrasting river deltas and river channels. Pore waters were routinely extracted from the sediments to examine diagenetic signatures indicative of anthropogenic impact. Whole sediment cores were sectioned to examine historic contaminant inventories.

Water-column samples at an offshore site were collected to examine contaminant-transport processes and scavenging rates. We are now in the process of analyzing samples for such constituents as 210Pb, 137Cs, nutrients, metals, PAHs (polycyclic aromatic hydrocarbons), stable isotopes, total sulfur, total nitrogen, and total phosphorus.

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