EZEKIEL, C.N. 1, Y.L. BUDEBA1 & D. MAC LEANAN

1Tanzania Fisheries Research Institute (TAFIRI), P.O. Box 475, Mwanza, Tanzania

Changes in limnological parameters in Lake Victoria waters, East Africa

Lake Victoria has undergone major ecological changes, mainly in the 1980s, due to a number of factors that include species introductions like Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus), and eutrophication. The increased hypoxic and anoxic conditions in the deeper parts of the lake have contributed to the rapid decline of some fish species. This is a result of changes in trophic relations in Lake Victoria that have reduced favourable habitable space.

Data on limnological parameters to cover all Lake Victoria waters were collected between February and August 2000 and 2001, respectively. Data was collected using the Seabird Profiler for dissolved oxygen, chlorophyll-a concentration, conductivity, and temperature. Water transparency was measured using the secchi disk.

The results have shown some ecological changes in the lake. For example: a decline in oxygen levels from 13 mg L-1 in 1960s to about 7 mg L-1 in 2000; a decline in water transparency in offshore waters from 15-20 m in 1960s and 13.5 m in 1990s to 8.2 m in 2000; increase in water conductivity from 97 µS C m-1 in 1960s to 199 µS C m-1 in 2000; also, chlorophyll-a concentration from 1.2-5.5 µg L-1 (offshore) and 10- 20 µg L-1 (inshore) in 1960s to 2.2 -10 µg L-1 (offshore) to 20-40 µg L-1 (inshore) in 2001.

There were seasonal (temporal) and spatial variations in the parameters where higher values for dissolved oxygen and water transparency were measured in August as compared to February. However, chlorophyll-a concentration was higher in February than in August. Conductivity showed no proper trend. Both temperature and oxygen stratification were greater in February than August.