Glacial outburst floods sustain microbial ecosystem in Lake Untersee, Antarctica

Abstract:

Antarctica hosts a number of nutrient-poor ice-covered lakes with benthic microbial communities accounting for the majority of biomass. Climate-driven changes to their ice cover and peripheral glacial systems is predicted to impact ecosystem structure and function within these lakes. Here, we show that glacial outburst floods (GOF) can replenish carbon-starved lakes with nutrients and maintain the photosynthetic primary production of microbial mats and conical stromatolitic structures. Over the past few hundred years, Lake Untersee has been in hydrological steady-state with poorly-buffered high pH water and TIC-TOC amongst the lowest reported for Antarctic lakes. In January 2019, Untersee received water from a GOF causing a rapid rise of 2 m of the lake surface, adding 1.75×10⁷ m^-3 of water (or +3% increase in volume) and modifying its water chemistry and increasing its TIC load. The microbial mats preserved a record of past GOFs. Through high-resolution grain size and carbon isotope analyses, we show that at least five GOFs occurred over the Holocene. Carbon mass balance calculations and numerical modeling of ¹⁴C evolution in the water column and mats demonstrate that GOF are crucial to sustain the long-term productivity levels within this unique terrestrial lake.