Hydrological connectivity and biogeochemical dynamics in the function and management of the lower Oder floodplain

Abstract

Rivers and their floodplains experience complex, dynamic hydrological regimes resulting in closely-coupled interactions. Like other large floodplains, the Lower Oder Valley National Park, Germany, provides important wetland habitats for fauna and flora but are vulnerable to hydrological disconnection and pollution events, such as the recent water quality disaster in August 2022, where high inputs of mine water caused lethal toxicity levels for fish. This study investigated the role of hydrological connectivity dynamics on biogeochemistry in the Oder river-floodplain system through a multi-proxy approach to quantify water sources, ages and evaporation losses (via water stable isotopes and tritium), water quality (by chemical analysis), as well as surface water (remote sensing) and sub-surface connectivity (geophysical surveys). During elevated levels of the Oder River in winter, open floodgates allow inundation of the floodplain polders delivering a variety of solutes into its associated wetlands. This high connectivity is reflected in low spatial variation in isotopes and hydrochemistry. Solutes are delivered from the Oder River to the floodplain, and water is well-oxygenated (with dissolved O₂ concentrations ∼ 15 mg/L). After flooding recedes, enhanced respiration and photosynthesis in the floodplain intensifies local biogeochemical gradients. Nitrogen is consumed (NO₃ falling from ∼ 4 to ∼ 0.1 mg/L), sulfate (SO₄^2- from ∼ 90 to ∼ 50 mg/L) is reduced, and carbon (DIC increased from ∼ 30 to ∼ 50 mg/L) and phosphorus (SRP increasing from ∼ 1 to ∼ 300 mg/L) are released through the decomposition of organic matter. During this non-inundation period, groundwater discharge to the floodplain’s water bodies is limited and concentrations of chloride and base cations increase in summer due to high evaporation. Low precipitation, dis-connectivity among water ponds in the polders and low groundwater recharge result in high but spatially variable evaporation fractions as reflected by stable water isotopes. We developed a conceptual model of these dynamics of hydrological connectivity and solute transport between the Oder river-floodplain system, and the summer evolution of dominant biogeochemical processes. Understanding these patterns, connections and processes is a prerequisite to sustaining vulnerable wetland habitats under changing climatic, hydrological and water quality conditions.