Turnover in a small Canadian shield lake

Abstract

In dimictic lakes, the stable density stratification during summer and winter inhibits vertical mixing of nutrients and oxygen. This favors the development of hypolimnetic hypoxia, which degrades cool‐water fish habitat and enhances nutrient mineralization from the sediments. Fall and spring turnover events, therefore, provide a crucial biannual link between surface and bottom waters. However, the physical processes occurring to mix lakes during turnover events remain comparatively uninvestigated. In this study, long‐term field observations were supplemented with output from a three‐dimensional numerical model, to better understand turnover events within a small temperate lake during 2011–2017. Mid‐basin penetrative convection, sidearm convection, and wind‐induced mixing were identified, with mid‐basin convection likely contributing the most to turnover events; a typical side‐arm convective plume had 2% (spring) to 4% (fall) of the mass flux compared to those at mid‐basin. During fall turnover, wind shear only mixed the upper 35% of the surface mixed layer, with convection acting to deepen below. During spring turnover, ice‐cover sheltered the lake from wind, causing convection to be the only process occurring and lengthening the turnover duration (~ 51 d) compared to fall (~ 13 d).

The dissolved oxygen budget of a small Canadian Shield lake during winter

“…Solar radiation penetrates the watercolumn based on the Beer–Lambert law with the light extinction coefficient adjusted locally according to the concentration of inorganic and detrital particulates, dissolved organic carbon and algae. Low winter snowfall at Eagle Lake (average < 6 cm; Ghane and Boegman 2021) and reduced wind sheltering after fall leaf drop result in minimal snow accumulation (< ~10 cm yr −1 ; Oveisy and Boegman 2014). As a result, ice cover (as opposed to snow) primarily limits penetration of shortwave solar radiation during winter.…”

“…Inter‐annual differences in summer DO result from variability in climate (Nürnberg et al 2019), nutrient loads (Scavia et al 2014), and primary production (Nelligan et al 2019). For example, warming can shorten the winter ice‐on period (Pilla and Williamson 2022), cause incomplete turnover (Boegman et al 2012; Woolway et al 2014), decrease the DO concentration at the end of spring turnover (Yang et al 2020; Ghane and Boegman 2021) and prolong summer stratification (Stainsby et al 2011), leading to more severe late summer hypoxia (DO < 5 mg L −1 ; Molot et al 1992; Nakhaei et al 2021 a ).…”

“…Three‐dimensional hydrodynamic‐biogeochemical models are used to temporally and spatially supplement field data between turnover events and moorings, respectively, particularly during ice‐cover when observations are hard to acquire. Unlike one‐dimensional models, three‐dimensional models directly simulate, as opposed to parameterizing, complex sub basin‐scale processes such as the formation of cold oxygen‐rich plumes resulting from penetrative and sidearm convection (Fer et al 2002; Ghane and Boegman 2021). The three‐dimensional Estuary and Lake Computer Model (ELCOM) coupled to the Computational Aquatic Ecosystem Dynamics Model (CAEDYM) has been successfully applied to a range of lakes from large (~ 100 s of km) to small (~ 10 s of m) for coupled hydrodynamic‐biogeochemical studies, for example, Lake Erie (Leon et al 2011; Oveisy et al 2014; Bocaniov et al 2016), Lake Simcoe (Schwalb et al 2015), Lake Minnetonka (Missaghi et al 2017), and Edmonton stormwater ponds (Nakhaei et al 2021 b ).…”

“…The end‐of‐summer DO profile is, therefore, largely dependent upon the spring turnover DO concentration, the morphology of the lake basin, and the sediment oxygen demand (Molot et al 1992; Livingstone and Imboden 1996; Nakhaei et al 2021 a ). Consequently, the basin‐scale vertical biogeochemical fluxes, that sustain lake biogeochemistry, primarily occur during/between the fall and the subsequent spring turnover events (Bouffard and Wüest 2019; Boegman 2020; Ghane and Boegman 2021). Present research must focus on understanding the physical‐biogeochemical cycling during these time periods.…”

“…At temperate latitudes, five phases occur between turnover events: fall turnover, pre‐winter, winter (winter I + winter II), spring turnover, and a transient period prior to summer stratification (Yang et al 2020). Here, we define the onset of fall turnover to be when the water column is nearly isothermal, T surface − T bottom < 0.4°C (gradient of < 0.01°C m −1 in Eagle Lake; Foley et al 2012; Ghane and Boegman 2021). Fall turnover then ends when | T surface − T bottom | > 1°C (gradient of − 0.03°C m −1 in Eagle Lake; Woolway et al 2014).…”