The power of the past: using sediments to track the effects of multiple stressors on lake ecosystems

1. One of the greatest challenges faced by limnologists, as well as most ecologists and environmental scientists, is finding data with time scales appropriate to their questions. Because of the general lack of reliable long-term monitoring data, it is often difficult to determine the nature and timing of ecosystem changes. In lieu of direct monitoring data, palaeolimnologists have developed a variety of physical, chemical and biological approaches to track past changes in aquatic ecosystems using proxy data archived in lake and river sediments. This article summarises a few of our recent palaeolimnological programs that have studied the effects of multiple stressors on lake ecosystems and demonstrates how palaeolimnological approaches can circumvent this common problem of data availability. 2. Lakewater calcium concentrations are declining in many softwater lake regions because logging and acid precipitation have lowered calcium levels in soils. In many cases, however, the onset of lakewater calcium decline predates direct observation, and so documenting the effects on freshwater ecosystems may be complex. By combining laboratory, field and palaeolimnological approaches, it is now evident that keystone taxa (e.g. Daphnia spp.) have been severely affected by these calcium declines. 3. Some of the most common complaints received by lake managers concern the smell and taste of water. Although the root causes of taste and odour problems vary, compounds released by certain species of algae are often responsible. In nutrient-poor or mesotrophic lakes, colonial chrysophytes are often the culprits, including scaled taxa of the genus Synura. Palaeolimnological approaches can be used to assess the various multiple stressors that influence the abundance of these phytoplankton. 4. Thematic implications: recent climatic warming is affecting a wide range of lake ecosystems in diverse and often complex ways across vast geographical regions, and this has added to the complexities of limnological responses to other stressors. As more palaeolimnological studies are completed, meta-analyses of sedimentary profiles can now be used to help disentangle the effects of climate warming from other environmental variables to determine how various components of lake ecosystems are responding to these multiple stressors.