Dissolved organic matter dynamics in the pristine Krka River estuary (Croatia)

International audienceIn many estuarine systems, riverine discharge is the main source of dissolved organic matter (DOM). In contrast, karstic Krka River, with self-purifying ability, is characterized by very low DOM concentrations (~30 µM) and a diluting effect on DOM in the estuary. Interestingly, within the estuary a marked in-situ production is observed, particularly in summer. Pristine nature of the estuary makes it a natural laboratory to study the DOM dynamics, originated from 3 sources: the river the seawater and in-situ production. With this work we aim at reporting the first information on DOM dynamics in the Krka estuary in two contrasting seasons, winter and summer 2019. The quality of DOM was assessed using absorbance and fluorescence measurements coupled with PARAFAC analysis. During winter, the estuary is characterized by extremely low DOC concentrations, for which optical properties indicate that is mainly of terrestrial origin. In summer, due to the low riverine discharge and high temperature, DOC concentrations are higher than in winter and a marked increase in absorption and protein-like fluorescence is observed in the estuary. The permanent stratification determines DOC accumulation in the surface layer with values up to 147 µM, suggesting a decoupling between production and removal processes. Our work opens intriguing questions about the main processes responsible for DOC accumulation in this system and highlights the need of new studies combing chemical and biological information