Impacts of water residence time on lake thermal structure:Implications for management and climate change

Lakes provide globally important ecosystem services. However, the dual threats of eutrophication and climate change threaten lakes’ ability to provide these services. Water residence time (WRT), the ratio between lake volume and discharge, is important for lake functioning, affecting nutrient loading, time available for biogeochemical processes, and flushing of biota. WRT manipulations have been proposed as a novel management intervention to restore eutrophic lakes by inhibiting seasonal stratification and thereby preventing hypolimnetic anoxia and associated internal loading. However, the impact of WRT on lake thermal structure is not well understood, and the contribution of inflows to lake heat budgets are often overlooked. Changes to WRT are also relevant in a climate change context. River flow changes, driven by evaporation and precipitation changes, are projected for more than three-quarters of the landmass. This thesis uses long-term and high-frequency data from a small UK lake, alongside hydrodynamic modelling to examine the extent to which management and climate-induced WRT changes could affect lake thermal structure and subsequent impacts on lake function. Results reveal that annual WRT changes impact lake temperatures year-round, with the direction of change seasonal. Shorter WRT caused cooling in the summer and warming in the winter. Annual WRT reductions failed to inhibit stratification, only weakening stability, and hypolimnetic anoxia persisted. However, results showed that reductions in WRT were enhancing deep-water oxygenation and that strategic sub-seasonal management could potentially control internal loading. WRT changes during the summer stratified period were associated with short-term reductions in water column stability, increased vertical mixing, and replenishment of deep-water oxygen. Results also highlighted that in the northwest of England, river flow changes are likely to be exacerbating air temperature warming impacts on lakes. Failing to account for future WRT changes could be underestimating the impact of climate change, in a multitude of short-residence time lakes