Weather-related drivers of thermal mixing and the dissolved oxygen regime of a polymictic bay

Session: Oxygen Cycling and Hypoxia: Processes, Impacts, and Management

Robyn Jones, Nipissing University, [email protected]
April James, Nipissing University, [email protected]
Dan Walters, Nipissing University, [email protected]
Andrew Paterson, Ontario Ministry of the Environment and Climate Change, [email protected]

Abstract

Challenges exist in identifying the influence of weather on dissolved oxygen (DO) regimes for dynamic lakes. Callander Bay, Lake Nipissing is classified as polymictic. Concerns exist over water quality including cyanobacterial blooms and nutrient loading. Periods of anoxia (<1mg/L) can induce internal nutrient loading and may contribute to the occurrence of harmful algal blooms that threaten the drinking water supply.  Five summers of high-frequency data (2014-18) (ten minute weather, water temperature and DO profiles) are analyzed to understand the influence of weather on anoxia within Callander Bay. Over this period, significant differences in thermal stratification were observed. The number of mixing events ranged from 2 to 11 per field season.  Furthermore, in 2018 the bay was dimictic. Hot summer air temperatures parallel strong stratification. The anoxic factor ranged from 0 to 130 days (2018).  DO near lake sediments increased post-mixing events, but remained lower than surface readings, even under isothermal conditions. DO depletion occurred linearly as the summers progressed but additionally could be interrupted by non-lake-mixing events. In 2018 and 2017 non-mixing events, when DO increased, occurred when wind speeds were > 1 SD above seasonal average. Further investigation into other factors (e.g. flow and wind direction) are of consideration.