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Keywords:

  • extreme events;
  • modelling;
  • oxygen;
  • temperature;
  • vendace

Summary

1. Bassenthwaite Lake in Cumbria is one of only two English lakes containing a population of vendace (Coregonus albula). The spatial distribution and survival of this fish is strongly influenced by the temperature and oxygen content of the water. In summer, this fish moves into deeper, colder water but avoids areas where the oxygen content is low.

2. In recent years, there has been a dramatic decline in the number of vendace found in the lake, a trend that may have been exacerbated by a succession of warm summers. Bassenthwaite only becomes stably stratified during calm, warm periods when a significant proportion of the deep water becomes anoxic.

3. Here, a one dimensional (1-D) process-based temperature-oxygen model is used to simulate the year-to-year variations in the severity of these ‘extreme events’. The model is validated using field measurements acquired in the 1990s and used to predict the range of depths accessible to the vendace.

4. An empirical, weather-driven model is then used to ‘hindcast’ the mixing characteristics of the lake in the 1980s and estimate the proportion of the habitat lost during warm, calm summers. These simulations show that periods of stable thermal stratification have become increasingly common in recent years. In the 1980s, only one ‘extreme event’ was identified but four such events were recorded in the 1990s.

5. The results are discussed in relation to the conservation status of the species and the potential effect of climate change on its survival in the English Lake District.