Seasonality and landscape factors drive dissolved organic matter properties in Mediterranean ephemeral washes

Dissolved Organic Matter (DOM) is a fundamental component of the aquatic carbon cycle and a key driver of the biogeochemical interactions between terrestrial and aquatic ecosystems. The origin, properties and role of DOM are increasingly characterised in lakes, rivers and streams, but little is known about DOM characteristics in ephemeral washes, which are the most common water flows in Mediterranean landscapes. Here, we examine the patterns in the optical properties of DOM in ephemeral washes draining a small catchment in the island of Menorca, Western Mediterranean. We used concentration data (dissolved organic carbon and nitrogen) and several spectroscopic descriptors (SUVA₂₅₄, absorption coefficient at 440 nm, fluorescence index, and excitation-emission fluorescence matrices) to assess changes in DOM concentrations and quality at both seasonal and spatial scales. Two periods were clearly distinguished in the DOM properties: autumn and winter–spring. In autumn, which includes the first flows of the hydrological year, DOM showed an aromatic character and was spatially homogenous over the catchment. In winter–spring, DOM was smaller and recently produced, and a considerable spatial heterogeneity was observed in all descriptors. The variability in DOM concentrations and quality was driven by hydromorphology and by the landscape features of the catchment, but the influence of these drivers on DOM properties changed along the hydrological year. In autumn, hydromorphology was the main factor determining DOM properties, whereas in winter-spring the land uses in the watershed highly determined the observed differences in DOM quality between subcatchments.