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 Increased sedimentation rates at several of the study sites are most likely a result of increased hydrologic discharge and erosion rates on a progressively deforested landscape. These changes are evidenced in the northern parts of the lake prior to the 20th century, although a major acceleration of change dates to 1961 or thereabouts. That year is notable as one of exceptional rainfall and high lake levels throughout East and Central Africa, suggesting that, although anthropogenic activities may be ultimately responsible for much of the change in vegetation cover and erosion in the catchment, increasing the discharge of those sediments to Lake Tanganyika may require a climatic "trigger". Small, disturbed catchments or those with steep underwater slopes show much less evidence of such effects than do large catchments discharging onto more gentle lake floors. In these (post 1961) in some cases the deposition rate is 5 to 10 times greater than that found earlier in the cores (Fig. 3).
Under regimes where disturbance is very high and total sediment input is increasing invertebrate diversity is invariably low, and communities are dominated by species tolerant of heavy sediment loading. Again this is not strictly a 20th century phenomenon, particularly along the Burundian coastline, although the mid 20th century changes in accelerating catchment change are mirrored in some cores by correlative declines in diversity. Conversely, where or when disturbance levels are low, diversity is high and dominance is low. However at intermediate levels of disturbance the response is more complex and suggests thresholds of response. It seems likely that increased sedimentation will have a profound affect on the fauna of the lake and work still ongoing in Zambia is intended to measure the impact of a pulse of sediment deposits on an area of underwater habitat and its ability to recover. This work is not yet completed though early results indicate that the effect is very pronounced. The SedSS has worked with 14 national institutions and 10 international institutions and has involved the co-operation of over 50 scientists and technicians from the riparian countries. |
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