Temporal and spatial variations in the isotope hydrobiogeochemistry (H, C, O, S, Ra) of a managed river draining a lowland towards the Baltic Sea

<p>The flow path of a river draining a lowland in the southern Baltic Sea, the Warnow River, was investigated to evaluate its freshwater composition as a source of dissolved substances to regional coastal waters. A spatial study was carried out once to follow the variations from the source to the estuary. A temporal study in the composition as a function of the season, during 6 years (2017-2022), was carried out at a site just before the river reaches the estuary. Surface water was sampled to analyze major and tracer elements, stable (H, C, O, S), and unstable (Ra) isotopes. The results show that the composition of the Warnow River along the flow path is controlled by a complex interplay between in-situ processes, exchange with the atmosphere, diffuse groundwater, and surface water inlets. On a temporal scale, pH, nutrient, and redox sensitive trace element concentrations are strongly impacted by pelagic primary production in spring. During summer and autumn, influences occurred by benthic microbial activity, associated diffusive release from soils/sediments, and surface water inlets. Throughout the investigation period, the Warnow River was a source of isotopically light CO2 to the atmosphere and DIC to the estuarine waters. The delivered DIC concentrations seem to vary with the season due to changes in biological pelagic and benthic activity. DOC was derived from a mixture of C3 organic sources and fertilizers. From concentration-discharge relationships, examples of dilution, mobilization, and chemostasis trends were found. Discharge-controlled seasonal trends are superimposed by system-internal processes and the hydrological consequences of river and drainage management. Our analysis thus provides new insights into the controls on the variations of water and solutes in a managed river at the land-sea interface as part of the regional hydrological cycle of a lowland catchment coastal water system.</p> <p>&#160;</p> <p>The study was supported by the DFG research training group BALTIC TRANSCOAST, DAAD ,&#160; and the BMBF project CARBOSTORE/COOLSTYLE</p>