Dissolved organic matter composition in a fragmented Mediterranean fluvial system under severe drought conditions

In Mediterranean regions, drought is one of the main factors shaping fluvial ecosystems. Droughts cause a shift from lotic to lentic conditions, triggering a gradual fragmentation of the longitudinal hydrological continuum, and a severe alteration of water chemical properties. However, within a biogeochemical perspective, little is known about how and to which extend droughts modify the chemical properties of dissolved organic matter (DOM). In this study, the variability of DOM properties along a fragmented fluvial system is explored, during a summer severe drought, by means of (a) the ratio between dissolved organic carbon and nitrogen concentrations (DOC:DON); (b) DOC bioavailability (BDOC) and (c) DOM optical properties (SUVA index, fluorescence index, and excitation-emission fluorescence matrices). DOM and water measurements were collected from isolated water parcels that became disconnected from the fluvial continuum at different times, and were compared with data obtained in the following autumn, when the fluvial continuum was re-established. Analysis of DOM chemical properties evidenced that these properties during drought clearly differed from those observed in autumn, but changes did not follow an arbitrary pattern. Thus, the sampling sites with lotic water bodies showed DOM properties similar to those observed in autumn reflecting the dominance of terrestrial inputs. But, once hydrological fragmentation occurred, there was a gradual increase in the contribution of autochthonous DOM as the time elapsed since the pools were established, and the geochemical conditions shifted from oxidized to reduced conditions. In consequence, the fragmentation of fluvial continuum generates a set of distinct biochemical hot spots (i.e., each water parcel), revealing that extreme drought greatly amplifies the qualitative heterogeneity of organic matter in a fluvial system.