Insights from long-term studies in the Windermere catchment: external stressors, internal interactions and the structure and function of lake ecosystems

1. Lakes provide essential ecosystem goods and services on which humans depend, and are integral to many global biogeochemical cycles, yet are sensitive to environmental perturbation operating at global, regional and local scales, many resulting from human influence.2. The complex web of external stressors and internal interactions that control the biological structure and ecological function of lakes requires a ` systems approach', where different trophic levels are studied and different approaches including long-term monitoring are taken. 3. This Special Issue celebrates the ecological insights that have been gained from long-term monitoring and research on the lakes in the catchment of Windermere in the English Lake District. The research was started by the Freshwater Biological Association in the 1930s and has been continued by the Centre for Ecology & Hydrology since 1989 to produce a valuable record of lake response to changing conditions extending over 65 years. 4. We introduce a conceptual diagram of lake response and feedbacks to a hierarchy of conditions at different scales and interactions within the lake and illustrate it using the contributions of papers in the literature and this Special Issue. 5. Long-term monitoring is often threatened at times of economic austerity, yet its continuation as part of a programme of research is a key activity that can produce mechanistic understanding and insights, and data to produce and validate models, and hence forecast future responses. These scientific disciplines interact synergistically to help us understand the whole system and safeguard the ecosystem services provided by lakes.