Lake remediation by top-down and bottom-up management : an ecosystem-scale experiment in the English Lake District

Anderson, Amy and Grey, Jonathan (2018) Lake remediation by top-down and bottom-up management : an ecosystem-scale experiment in the English Lake District. PhD thesis, Lancaster University.

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Lake ecosystems face a variety of pressures arising from catchment-based anthropogenic activities and long-term changes in background environmental conditions, which greatly threaten their structure, function and the sustainable provision of essential ecosystem services. The response of lakes to external perturbations is controlled by a complex interaction of top-down (predation) and bottom-up (resource availability) food web processes. A better understanding of how these processes interact is vital if they are to be effectively manipulated in the remediation of degraded lakes. In this study, Esthwaite Water, a eutrophic lake in the English Lake District, was used as the model ecosystem to investigate the relative importance of these two opposing forces in determining water quality changes in response to anthropogenic pressures, specifically eutrophication and climate change. The research is presented as three studies which address the historical, contemporary and future impact of stressors on the ecological structure and function of Esthwaite Water. Analysis of long-term monitoring data determined that phosphorus availability was the main driver of water quality degradation throughout the study period (1970 to 2014). Despite substantial reductions to the external phosphorus load in later decades, water quality improvements were not observed. Phosphorus release from lake sediments and the effects of climate change on phytoplankton community structure appear to have buffered against the desired impact of reduced external nutrient loads. Analysis of contemporary surface sediments revealed a legacy of phosphorus enrichment from years of high external loading, including from aquaculture cages previously installed on the lake. Much of the sediment phosphorus was determined to be stored in release-sensitive forms. The PROTECH lake model was used to test the response of the phytoplankton community to variations in both nutrient availability and zooplankton grazing rate. The dominant controlling factor on total phytoplankton biomass was nutrient availability. When nutrient loads were high, cyanobacteria dominated the phytoplankton community. Such conditions limited the potential for top-down regulation by herbivory owing to food quality constraints upon zooplankton. The inclusion of climate warming in modelling scenarios resulted in the exacerbation of the symptoms of eutrophication. As such, it is recommended that future attempts at water quality improvement take a bottom-up approach, with a focus on controlling the internal phosphorus load.

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Thesis (PhD)
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05 Apr 2019 08:55
Last Modified:
16 Jul 2024 05:44