Spatial and temporal variations in the isotope hydrobiogeochemistry of a managed river draining towards the southern Baltic Sea

The flow path of a river draining a lowland into the southern Baltic Sea, the Warnow River, was investigated to evaluate its freshwater composition and potential to be a source of dissolved substances for regional coastal waters. A spatial study was conducted in April 2019 to follow the variations from the source to the estuary. A temporal study of the composition, for five years (2017–2021), was carried out at a site just before the river reaches the estuary. Surface water was sampled to analyse major and trace elements, stable (H, C, O, S), and short-lived (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 due to 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 seemed to vary with season due to changes in biological pelagic and benthic activities. DOC was derived from a mixture of dominated C₃ organic sources and potentially from fertilizers. From concentration-discharge relationships, examples of dilution, mobilization, and chemostasis trends were found. Discharge-controlled seasonal trends were superimposed by system internal processes and the hydrological consequences of the river 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.