Nutrients exert a stronger control than climate on recent diatom communities in Esthwaite Water: evidence from monitoring and palaeolimnological records

1. A long-term monitoring programme on phytoplankton and physicochemical characteristics of Esthwaite Water (England) that started in 1945 provides a rare opportunity to understand the effects of climate and nutrients on a lake ecosystem. 2. Monitoring records show that the lake experienced nutrient enrichment from the early 1970s, particularly after 1975, associated with inputs from a local sewage treatment plant, resulting in marked increases in concentration of soluble reactive phosphorus (SRP). Climatic variables, such as air temperature (AirT) and rainfall, exhibit high variability with increasing trends after 1975. 3. Diatom analyses of an integrated 210Pb-dated lake sediment core from Esthwaite Water, covering the period from 1945 to 2004, showed that fossil diatoms exhibited distinct compositional change in response to nutrient enrichment. 4. Redundancy analysis (RDA) based on diatom and environmental data sets over the past 60 years showed that the most important variables explaining diatom species composition were winter concentrations of SRP, followed by AirT, independently explaining 22% and 8% of the diatom variance, respectively. 5. Additive models showed that winter SRP was the most important factor controlling the diatom assemblages for the whole monitoring period. AirT had little effect on the diatom assemblages when nutrient levels were low prior to 1975. With the increase in nutrient availability during the eutrophication phase after 1975, climate became more important in regulating the diatom community, although SRP was still the major controlling factor. 6. The relative effects of climate and nutrients on diatom communities vary depending on the timescale. RDA and additive model revealed that climate contributed little to diatom dynamics at an annual or decadal scale. 7. The combination of monitoring and palaeolimnological records employed here offers the opportunity to explore how nutrients and climate have affected a lake ecosystem over a range of timescales. This dual approach can potentially be extended to much longer timescales (e.g. centuries), where long-term, reliable observational records exist.