Abstract
Abstract. In deep stratified lakes, such as Lake Geneva, flood-driven turbidity currents are thought to contribute to the replenishment of deep oxygen by significant transport of river waters saturated with oxygen into the hypolimnion. The overarching aim of this study was to test this long-standing hypothesis directly. It combines direct observational data collected during an extreme flooding event that occurred in May 2015 with dark bioassays designed to evaluate the consequences of river-borne inputs for the hypolimnetic respiration. The exceptional precipitation events of May 2015 caused floods with an annual return time for the Rhône River, the dominant tributary of Lake Geneva, and with 50-year return time for the Dranse River, the second-most important tributary. Sediment-loaded river flows generated turbidity currents plunging into the lake hypolimnion. The observed river intrusions contributed to the redistribution of dissolved oxygen, with no net gain, when occurring in the lowermost hypolimnetic layer. In the uppermost hypolimnion above the last deep-mixing event, the intrusions coincided with a net oxygen deficit. Consistent with field observations, dark bioassays showed that 1 to 50 % substitution of riverine organic matter to deep (< 200 m) hypolimnetic water did not affect microbial respiration, while the addition of 1 to 10 % of riverine water to the uppermost hypolimnetic waters resulted in a respiration over-yielding, i.e. excess respiration of both river-borne and lacustrine organic matter. The results of our study conflict with the hypothesis that flood-driven turbidity currents necessarily increase hypolimnetic oxygen stocks in Lake Geneva. In contrast, results show that flood-driven turbidity currents can be potential hot spots for priming effect in lakes.
Highlights
In thermally stratified lakes, river water inflow occurs under two different modes
Assuming that the sediment load was predominantly made of quartz, the density of the Rhône River was estimated as ρR, tot = ρw (S, T ) +(1-ρsed / ρw (S, T )) aQb = 1000.7 kg m−3, where ρw (S, T ) is the density of the water depending on the temperature and salinity (Chen and Millero, 1986)
This value was slightly lower than the density of the lake water at the deepest location (1001.4 kg m−3) and did not account for the later entrainment of lake water into the intrusion
Summary
River water inflow occurs under two different modes. Under normal (i.e dry-weather) flow conditions, the river water is injected at the interface between the warm, upper layer (epilimnion) and the cold, lower layer (hypolimnion), forming an interflow in the upper thermocline (Fischer, 1979) or at the surface. During flood events, high concentrations of suspended sediments in rivers increase the density of inflowing waters and generate turbid density currents following the slope and flowing into the hypolimnion (i.e. hyperpycnal plume). Their dynamics are divided into three distinct stages (Alavian et al, 1992; Cortés et al, 2014; Hogg et al, 2013). The river dense water pushes the ambient lake water until the resulting baroclinic pressure created by the local density difference between the river and the lake water balances the force of the momentum inflow. The flow eventually reaches the depth of neutral buoyancy, separates from the lake bottom and intrudes into the lake (intrusion stage)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
