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

Abstract

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 2 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.

Lake management under severe drought: Lake Mead, Nevada/Arizona

“…There is no significant correlation between historically observed residence time and phosphorus concentration from 2004 to 2020, with a Spearman's rho (Gibbons, 1985) correlation value of 7.18% in summer ( p ‐value near 1). A recent study (Olsson et al, 2022) corroborates the phosphorus/residence time link found in Lake Mead and concludes that a 40% reduction in residence time does not significantly affect phosphorus or chlorophyll concentrations.…”

Conceptual models of dissolved carbon fluxes in a two-layer stratified lake: interannual typhoon responses under extreme climates

“…The results of these studies suggest that river intrusion and thermal stratification are key factors shaping the seasonal and interannual patterns of C fluxes during typhoon disturbances. River intrusion controls not only the C fluxes, algal biomass, and nutrient loading but also influences the length of stratification and hydraulic retention times (Lin et al, 2021(Lin et al, , 2022Maranger et al, 2018;Nakayama et al, 2020;Olsson et al, 2022a, b;Zwart et al, 2017;Vachon and Del Giorgio, 2014). Therefore, we hypothesized that allochthonous C loading and river inflow intrusion might affect DIC and DOC distributions.…”

Analysis of Three Dimethyl Sulfides in Freshwater Lakes Using Headspace Solid-Phase Microextraction-Gas Chromatography with Flame Photometric Detection

“…Recent studies have shown that the DMS concentration in the water column of Yangcheng Lake, a shallow eutrophic lake, is significantly higher than that in deep-water lakes [11]. Therefore, DMSs have become a prevalent olfactory substance in freshwater bodies, including eutrophic lakes, reservoirs, and rivers [12][13][14].…”