Hypolimnetic oxygen depletion in a deep oligomictic lake under climate change

Dissolved oxygen (DO) concentration is a fundamental metric to describe climate-related alterations in deep lakes. Increasing water temperatures enhance thermal stratifcation, leading in temperate basins to a growing isolation of deep waters. This leads to the depletion of hypolimnetic DO, which adds up to limited nutrient circulation and restricted replenishment of the trophogenic layers. With vanishing convective mixing, it is commonly believed that the only source of hypolimnetic DO replenishment will be represented by deep intrusions of cold oxygenated waters from the tributaries. In this study, we frst analyse the 1993-2020 long-term observed trends of DO concentrations in the subalpine deep oligomictic Lake Maggiore (Italy/Switzerland). Then, through an algorithm calculating daily intrusion depths and mass discharges of DO for the major tributaries, we show that deep insertions are suppressed for increasing winter water temperatures and residual thermal stratifcation. Turbulent entrainment is proved fundamental for DO replenishment, leading to mass discharges of DO released into the deep hypolimnion up to more than two orders of magnitude larger than the original ones from the tributaries. Last, we discuss the results of simulations made through a one-dimensional coupled ecological-hydrodynamic model about the possible efects of a full turnover on DO concentrations in the deep hypolimnion. Two cases are displayed, with the turnover taking place either now or with an anoxic hypolimnion deriving from decades of isolation due to severe climate warming. Through this study, climate warming is shown to be a fundamental driver of DO in Lake Maggiore, its depletion harming both water quality and the ecosystem.