Phosphorus and Oxygen Dynamics between Fall and Spring Turnover Events in a Small Canadian Shield Lake

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 and diffusion from the sediments. Fall turnover began once the entire water column become nearly isothermal (within 0.4 °C) at the deepest point of the lake and continued to the onset of winter stratification. Similarly, spring turnover began when the water column became nearly isothermal (within 0.4 °C) and continued until the average water column temperature reached 4.0 to 4.5 °C. Fall and spring turnover events, therefore, provide a crucial bi-annual link between surface and bottom waters. Long-term field observations were supplemented with output from a three-dimensional numerical model, to better understand the hydrodynamics of turnovers and associated dissolved oxygen (DO) and orthophosphate (PO4) dynamics within a small temperate lake during 2011 to 2020. Mid-basin convection contributed the most to turnover events. 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 days) compared to fall (~13 days). Photosynthetic production was modeled to contribute 92% of the total dissolved oxygen (DO) input to the lake, the remaining 8% was from atmospheric exchange. Of the DO production, 41% occurred under ice with a potential to contribute 17±11% of the hypolimnetic DO saturation deficit at spring turnover. The occurrence of spring turnover depended on the winter conditions. Warm winters caused a weak inverse stratification that shortened the deepening of the convective mixed layer (CML) under the ice and inhibited spring turnover induced mixing of DO into the hypolimnion. Conversely, cold winters caused an inverse stratification in which watercolumn became isothermal after the CML deepened and removed the stratification. Then, the incoming heat from the surface caused large vertical convections (spring turnover). Severe winters form a very strong inverse stratification that limits the effectiveness of spring turnover. Mineralization was the main PO4 source producing with 77-98% of the annual internal load. The remaining PO4 (2-23%) was from anoxic sediments. Fall turnover upwelled the trapped PO4 from hypolimnion into the photic zone where it could initiate the spring algal bloom. The bloom was postponed because phytoplankton was mixed to depth with fall turnover mixing events, where their growth was light limited until the seasonal increase in solar radiation in spring.

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