Natural levels of hydrogen peroxide (HO) in deep clear South temperate lakes: Field and laboratory evidence of photo- and biotic production.

Hydrogen peroxide (HO) is a ubiquitous reactive oxygen species (ROS) in aquatic systems and is produced mainly in surface water by the interaction of ultraviolet radiation (UVR) and natural dissolved organic carbon (DOC). Andean Patagonian lakes are ultraoligotrophic, clear systems with extended photic zones (~40 m), and are exposed to challenging UVR levels due to their lati-altitudinal situation and extremely low DOC levels. This investigation describes the seasonal levels of HO in relation to DOC quality in the water column of lakes Moreno East (ME) and Moreno West (MW), two deep (ca. 100 m), ultraoligotrophic, low-DOC (<0.7 mg L) systems of Andean Patagonia. HO concentrations recorded in the lakes were below 60 nM, ranging from ~3 to ~60 nM in Lake ME and from ~5 to ~35 nM in Lake MW. In most of the samples of both lakes, the HO levels were higher in the photic zone (surface to 30-40 m) than the aphotic zone (from 30-40 m to 90-100 m), particularly in summer samples. Laboratory experiments evaluated the abiotic (photochemical) and biotic (microbial) production of HO in seasonal (summer, autumn) samples which varied DOM quality due to lake (ME, MW) and depth (photic and aphotic lake layers) provenance. Abiotic and biotic production of HO attained higher levels in summer samples from the photic zones of both lakes. Humic DOM from deep layers (particularly from summer samples) was more susceptible to both photo- and bio-degradation than DOM from upper lake layers, which was characterized by stronger signs of degradation and progress in diagenesis.