Response of stable isotopes (δH, δC, δN, δO) of lake water, dissolved organic matter, seston, and zooplankton to an extreme precipitation event.

Lake ecosystems process and cycle organic substrates, thus serving as important bioreactors in the global carbon cycle. Climate change is predicted to increase extreme weather and precipitation events that can flush nutrients and organic matter from soils to streams and lakes. Here we report changes in stable isotopes (δH, δC, δN, or δO) of water, dissolved organic matter (DOM), seston, and zooplankton in a subalpine lake at short time resolution following an extreme precipitation event between early July to mid-August 2021. Water from excess precipitation and runoff remained in the lake epilimnion and coincided with increasing δC values of seston (-30 ‰ to -20 ‰), due to the input of carbonates and terrestrial organic matter. Particles settled into deeper lake layers after two days and contributed to the uncoupling of C and N cycling as the lake responded to this extreme precipitation event. Following the event, there was an increase in bulk δC values of zooplankton (from -35 ‰ to -32 ‰). Throughout this study, δC values of DOM remained stable throughout the water column (-29 ‰ to -28 ‰), while large isotopic fluctuations in DOM δH (-140 ‰ to -115 ‰) and δO (+9 ‰ to +15 ‰) values suggested DOM relocation and turnover. Integrating isotope hydrology, ecosystem ecology, and organic geochemistry offers an element-specific, detailed approach to investigating the impact of extreme precipitation events on freshwater ecosystems and particularly aquatic food webs.