Changing precipitation patterns affect nitrate input to and subsequent cycling in a subalpine lake

Subalpine lakes are highly sensitive ecosystems that respond rapidly to variations in temperature, precipitation, and hydrological inputs triggered by climate change. These lakes are typically oligotrophic, and the availability of nutrients is highly dependent on nutrient loads with rainfall and runoff and further controlled by in-catchment processes. Altered precipitation patterns, rising (water) temperatures, and ice- and snow-free winters can significantly impact these ecosystems' water balance, stratification, and nutrient dynamics. Understanding these processes is critical, as small environmental changes can affect their biogeochemical cycles and biological communities. Although the effects of warming on subalpine lakes are recognized, the magnitude by which climate change impacts the water balance and nutrient dynamics in these ecosystems remains uncertain. Moreover, subalpine lakes, as part of the headwater catchment, impact water and nutrient availability downstream. In this context, water stable isotopes provide essential insights into the hydrological processes, helping to understand the water balance and mixing processes of lakes. Long-term data from subalpine Lake Lunz, Austria, indicate a decrease in nitrate concentrations (N−NO₃⁻) during the past decade. This study investigates the spatiotemporal patterns of N−NO₃⁻ and stable water isotopes (δ¹⁸O−H₂O and δ²H−H₂O) during two hydrometeorological cycles. Samples were collected monthly from the inflow, outflow, epi-, meta-, and hypolimnion of the lake. Preliminary results showed that precipitation and snowmelt during spring significantly influenced lake water levels and nitrate inputs. Stable water isotope analyses revealed seasonal isotopic stratification, with higher values of δ¹⁸O−H₂O in the epilimnion during summer following an isotopically enriched signal from the catchment. The hypolimnion exhibited stable isotopic values of water with minimal variation, suggesting limited vertical mixing. Nitrate concentrations in the inflow and the epilimnion were higher in winter and spring, coinciding with depleted isotopic values from the water molecule and suggesting snow melt as an essential source of N−NO₃⁻. On the other hand, the hypolimnion showed increased nitrate concentrations as stratification persisted and dissolved oxygen levels declined, possibly due to remineralization of organic matter from the thick layer of fine sediment at the bottom of the lake. These findings indicate the need to study the sensitivity of lake nutrient dynamics to variations in hydrological inputs during climate change.