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This study analyses the effects of the interplay between climate seasonality and hydrogeomorphic (HGM) lake features on dissolved organic matter (DOM) properties in two neighbouring shallow lakes of Andean Patagonia with different connectivities. The survey was conducted over three years at the end of the wet and dry seasons, assessing the seasonal and interannual variation of dissolved organic carbon (DOC) concentration, whole-lake DOC mass, and DOM quality, through chromophoric and fluorescent DOM (CDOM and FDOM, respectively) properties. During the wet season (fall-winter), precipitation and runoff increased water discharge, water level, and inputs of terrestrial DOM with high aromaticity, humic content, and high molecular weight in both lakes. Contrastingly, during the dry season (spring-summer), in which photodegradation promoted by high irradiance and stagnant conditions drove DOM transformation, nonhumic, low-molecular-weight DOM prevailed. Both lakes displayed synchronicity in their DOC mass and CDOM and FDOM properties, indicative of similar responses to climate forcing, although the overall impact was modulated by their HGM features. Conversely, DOC concentration showed asynchronous responses between lakes, due to the higher intensity of the dilution or evapoconcentration processes in the connected lake, highlighting that DOC concentration is not always sensitive to climate-driven forces. Overall, this study emphasizes the importance of variables other than DOC concentration, like whole-lake DOC mass, DOM quality, and HGM features, to better understand the effect of climate variability on DOM dynamics. Our results allow inferring the potential impact of an environmental scenario characterized by lower precipitation and sustained warming on DOM dynamics in Northern Andean Patagonia.

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Terrestrial inputs of dissolved organic matter (DOM) make a significant contribution to the carbon pool of headwaters and the reactivity of this pool depends on its source and diagenetic state, being influenced by photochemical and biological processes. The main goal of this study was to characterise the composition and reactivity of soil and leaf litter DOM from a native forest of Nothofagus pumilio (Nothofagaceae), and from natural stream water, evaluating the effect of degradation processes. Photo‐ and biodegradation laboratory experiments were conducted using DOM leached from soil and leaf litter, while the impact of photodegradation alone was also analysed through laboratory assays using stream water. The effects of photo‐ and biodegradation were evaluated through changes in the concentration of dissolved organic and inorganic carbon (DOC and DIC, respectively) and optical DOM proxies (absorbance and fluorescence). In the initial characterisation, DOM from soil and water leachates showed naturally high humification, aromaticity and lignin content compared with the DOM of leaf litter leachate rich in non‐humic compounds. Photo‐ and biodegradation increased humification of the DOM. DOM from leaf litter leachate was more bioavailable than DOM from soil leachate, as reflected by the higher growth of bacteria, DOC consumption and DIC production. In general, biodegradation increased DOM molecular weight, aromaticity and lignin content. Changes in fluorescent DOM (FDOM) showed a trend characterised by the loss of labile protein‐like compounds and an increase in refractory humic‐like components. Long‐rod shaped bacteria were more abundant in leaf litter leachate, suggesting their preference for labile DOM, whereas cocci dominated in the humic and more biorecalcitrant DOM from soil leachate. This study showed a continuum of DOM humification, with decreasing DOM reactivity from leaf litter leachate towards soil leachate and stream water. Soil leachate DOM was probably the main source of stream water DOM, as reflected by their similar signatures and close positioning in the reactivity continuum, although carbon mineralisation was much lower in soil leachate than stream water.