Hydrological connectivity controls nitrogen sink-source transitions in Antarctic lakes under warming

ABSTRACT

Glacier retreat and the expansion of ice-free terrain are driving new freshwater ecosystems across Antarctica, yet the role of groundwater in regulating their biogeochemical function has been largely overlooked. Here we show that hydrological connectivity governs whether Antarctic lakes act as sinks or sources of reactive nitrogen. By integrating water- and nitrate-isotope tracers with functional gene evidence, we establish a framework that links groundwater-surface water coupling to nitrogen sources, microbial pathways, and greenhouse-gas potential. Across contrasting lake settings, we find that stable, groundwater-dominated regimes promote denitrification and nitrogen retention, whereas increased surface runoff under warming shortens residence times, depletes organic substrates, and drives incomplete denitrification with nitrite and N₂O accumulation. This demonstrates that climate-driven shifts in hydrological pathways can transform Antarctic groundwater from a long-term nitrogen sink into a source, indicating its role as a regulator of nutrient fluxes and a potential amplifier of climate feedbacks. Our findings provide the first integrative framework for evaluating groundwater’s biogeochemical role in polar lakes and its sensitivity to ongoing cryospheric change.