African Climate Rise Taking Toll on Fish, Nature Study Finds

POUGHKEEPSIE, N.Y. — Researchers studying east Africa's Lake Tanganyika, which holds 18% of the world's liquid freshwater and is suffering a steadily lower fish catch, have linked the region's accelerating heat and the lake's rising surface water temperature to the decline of a lake water mixing process essential to fish nutrition. These findings will be reported in the August 14, 2003 issue of the journal Nature, where the researchers contend that climate change in the region — which has been especially pronounced since the 1970's — is the primary cause of Lake Tanganyika's sizeable fish catch drop, exceeding the impact of local human activity such as over fishing and development.
"The mixing of surface and deep water layers in a lake stirs up nutrients from the lower depths that fish depend upon," explains biologist Catherine M. O'Reilly, a visiting assistant professor of environmental science at Vassar College, and the lead author of the Nature study. "But the wider the temperature gap becomes between surface and deep water layers, the more difficult it is for these layers to mix and for the nutrients to reach fish. The consequences at Lake Tanganyika are very serious, because it provides 41% of the animal protein to four surrounding countries [Burundi, Democratic Republic of Congo, Tanzania, and Zambia]."
The Nature study, "Climate change decreases aquatic ecosystem productivity of Lake Tanganyika," has added global implications because it originates near the equator, where far less research has been conducted that would indicate the ecological effects of climate warming, according to O'Reilly. By contrast, she points out, "Numerous effects have been demonstrated through research in the higher latitudes, which are in closer proximity to the industrialized world." O'Reilly and her colleagues report that Lake Tanganyika's surface water temperature increased 0.1°C each decade from 1913-2000, at a rate proportional to the 0.6°C average worldwide climate warming trend during that period. By comparison, over the entire nine decades Tanganyika's deep water layer increased by only 0.31°C
To substantiate the changes caused by this growing water temperature gap, the researchers sampled sediment layers from four Tanganyika shore areas with distinct land uses, and compared them to samples taken in the 1930's. All the comparisons showed a series of nutrient patterns consistent with decades of reduced water mixing.
The study goes on to address east Africa's steep warming since the late 1970's, which has coincided with a 30-50% decline in Lake Tanganyika's sardine catch, and an even bigger decline in the lake's large fish population.

Average temperatures in the region have increased every year since 1977, but were quite variable in the preceding six decades. Moreover, Vassar's O'Reilly explains, climatologists project a 1.3-1.7°C increase in east Africa's average air temperature for the next 80 years; this temperature climb will only further stunt water mixing and fish health, O'Reilly says, unless unusually high wind trends were to emerge.
Tanganyika is the world's second largest lake, measuring one mile deep, 406 miles long, and 31 miles wide.
Catherine O'Reilly and her international co-authors on the Nature article collaborate through the University of Arizona's Nyanza Project, an ongoing interdisciplinary research and training program on tropical lakes that receives major support from the National Science Foundation.
Vassar College is a highly selective, coeducational, independent, residential liberal arts college founded in 1861.

Individuals with disabilities requiring accommodations or information on accessibility should contact Campus Activities Office at (845) 437-5370. Without sufficient notice, appropriate space and/or assistance may not be available.

Vassar College is a highly selective, coeducational, independent, residential liberal arts college founded in 1861.

This article was posted on Friday, August 15th, 2003.