The sensitivity of a Tanzanian crater lake to catastrophic tephra input and four millennia of climate change

Diatom genera in many large East African lakes change little throughout the Holocene period suggesting relatively stable ecological conditions and som e resilience to environmental change. Ecosystem stability is less common in smaller, more sensitive lakes, such as those within volcanic craters, wher e external impacts can cause abrupt and rapid fluctuations. A 4100-year dia tom and cyanobacteria pigment record from Lake Massoko, a volcanic crater l ake in southern Tanzania, is used to illustrate important switches in resou rce ratios following tephra deposition 1190 years ago. It is hypothesized t hat the tephra reduced the rate of P diffusion from the sediments and incre ased the Si:P ratio in the lake. A period of acute change in planktonic dia tom communities resulted from the tephra impact and lasted c. 110 years. Th e magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall. The statistical significance of the teph ra impact relative to that of catchment and climate change has been tested using variance partitioning and rate-of-change analysis. Multiproxy indicat ors show an important period of positive water balance 1700 ago and a relat ively dry episode persisting between 1000 and 400 years ago. The lake ecosy stem is shown to be highly sensitive to both climate change and tephra depo sition.