Annual water residence time effects on thermal structure: A potential lake restoration measure?

Innovative methods to combat internal loading issues in eutrophic lakes are urgently needed to speed recovery and restore systems within legislative deadlines. In stratifying lakes, internal phosphorus loading is particularly problematic during the summer stratified period when anoxia persists in the hypolimnion, promoting phosphorus release from the sediment. A novel method to inhibit stratification by reducing residence times is proposed as a way of controlling the length of the hypolimnetic anoxic period, thus reducing the loading of nutrients from the sediments into the water column. However, residence time effects on stratification length in natural lakes are not well understood. We used a systematic modelling approach to investigate the viability of changes to annual water residence time in affecting lake stratification and thermal dynamics in Elterwater, a small stratifying eutrophic lake in the northwest of England. We found that reducing annual water residence times shortened and weakened summer stratification. Based on finer-scale dynamics of lake heat fluxes and water column stability we propose seasonal or sub-seasonal management of water residence time is needed for the method to be most effective at reducing stratification as a means of controlling internal nutrient loading.

Can reductions in water residence time be used to disrupt seasonal stratification and control internal loading in a eutrophic monomictic lake?

Anthropogenic eutrophication caused by excess loading of nutrients, especially phosphorus (P), from catchments is a major cause of lake water quality degradation. The release of P from bed sediments to the water column, termed internal loading, can exceed catchment P load in eutrophic lakes, especially those that stratify during warm summer periods. Managing internal P loading is challenging, and although a range of approaches have been implemented, long-term success is often limited, requiring lake-specific solutions. Here, we assess the manipulation of lake residence time to inhibit internal loading in Elterwater, a shallow stratifying lake in the English Lake District, UK. Since 2016, additional inflowing water has been diverted into the inner basin of Elterwater to reduce its water residence time, with the intention of limiting the length of the stratified period and reducing internal loading. Combining eight years of field data in a Before-After-Control-Impact study with process-based hydrodynamic modelling enabled the quantification of the residence time intervention effects on stratification length, water column stability, and concentrations of chlorophyll a and P. Annual water residence time was reduced during the study period by around 40% (4.9 days). Despite this change, the lake continued to stratify and developed hypolimnetic anoxia. As a result, there was little significant change in phosphorus (as total or soluble reactive phosphorus) or chlorophyll a concentrations. Summer stratification length was 2 days shorter and 7% less stable with the intervention. Our results suggest that the change to water residence time in Elterwater was insufficient to induce large enough physical changes to improve water quality. However, the minor physical changes suggest the management measure had some impact and that larger changes in water residence time may have the potential to induce reductions in internal loading. Future assessments of management requirements should combine multi-year observations and physical lake modelling to provide improved understanding of the intervention effect size required to alter the physical structure of the lake, leading to increased hypolimnetic oxygen and reduced potential for internal loading.

Interacting impacts of hydrological changes and air temperature warming on lake temperatures highlight the potential for adaptive management.

Globally, climate warming is increasing air temperatures and changing river flows, but few studies have explicitly considered the consequences for lake temperatures of these dual effects, or the potential to manage lake inflows to mitigate climate warming impacts. Using a one-dimensional model, we tested the sensitivity of lake temperatures to the separate and interacting effects of changes in air temperature and inflow on a small, short-residence time (annual average ≈ 20 days), temperate lake. Reducing inflow by 70% increased summer lake surface temperatures 1.0-1.2 °C and water column stability by 11-19%, equivalent to the effect of 1.2 °C air temperature warming. Conversely, similar increases in inflow could result in lake summer cooling, sufficient to mitigate 0.75 °C air temperature rise, increasing to more than 1.1 °C if inflow temperature does not rise. We discuss how altering lake inflow volume and temperature could be added to the suite of adaptation measures for lakes.