Natural levels and photo-production rates of hydrogen peroxide (H2O2) in Andean Patagonian aquatic systems: Influence of the dissolved organic matter pool

Abstract

In aquatic environments the reactive oxygen species hydrogen peroxide (H2O2) is produced through photochemical reactions involving chromophoric dissolved organic matter (CDOM). Andean Patagonian freshwaters experience challenging underwater UV levels, which promote high levels of photochemical weathering. In this investigation, we study natural H2O2 levels and experimentally address the photochemical formation of H2O2 in stream and lake water with a range of dissolved organic matter (DOM) concentrations and quality. The screening of different pristine aquatic systems of Patagonia revealed that H2O2 concentration fluctuates between 8 and 60 nM. Laboratory incubation of different water types in PAR + UV showed photo-production of H2O2. The H2O2 formation rate increased linearly with dissolved organic carbon (DOC) in streams (13.5–20.5 nM h-1) and shallow lakes (25.7–37.8 nM h-1). In contrast, the H2O2 formation rate in deep lakes was much lower (2.1–7.1 nM h-1), and decreased with DOC. The natural potential for H2O2 formation was related to the concentration and quality of the DOM pool. At higher DOC levels, such as those present in shallow lakes, H2O2 production was directly related to DOC, whereas at low DOC levels in deep lakes and streams, two patterns were distinguished in relation to their DOM pool quality. Stream DOM, composed of high molecular weight/size humic compounds, proved to be a reactive substrate, as reflected by their high H2O2 formation rates. On the other hand, deep lake DOM, with its higher relative contribution of small and more processed compounds, was found to be a less reactive substrate, affording lower H2O2 formation rates.