One of the major problems with palaeoclimate investigations in volcanic regions is that tephra inputs to lakes can cause changes in proxies analogous to those of climate forcing. We review the range of impacts thought to be associated with tephra deposition, distinguishing between direct effects on lake ecosystems and indirect changes to catchment nutrient cycles. To achieve better understanding of these complex responses, we have used high-resolution diatom analysis from around 17 tephra layers, in three Mexican lakes. A positive response to the tephra inputs has been identified for over half of these layers. The most common response was for diatom concentrations to increase after tephra deposition; also, in plankton dominated systems, Fragilaria spp. replaced Stephanodiscus spp. and Aulacoseira spp. An increase in the supply of silica to the lakes through their catchments is probably the cause of the diatom changes in the lakes studied. Direct effects of the tephra inputs to these lakes can be excluded as they would not generate changes of sufficient longevity. The impact of these tephras, which lasted for several decades, was insufficient to perturb the underlying long-term climate forcing of these lake systems. (C) 2004 Elsevier Ltd. All rights reserved.