Seasonal and Inter-Annual Dynamics in Water Quality and Stream Metabolism in a Beaver-Impacted Drought-Sensitive Lowland Catchment

Increasing drought frequency and severity from climate change are causing streamflow to become increasingly intermittent in many areas. This has implications for the spatio-temporal characteristics of water quality regimes which need to be understood in terms of risks to the provision of clean water for public supplies and instream habitats. Recent advances in sensor technology allow reliable and accurate high-resolution monitoring of a growing number of water quality parameters. Here, we continuously monitored a suite of water quality parameters over 3 years in an intermittent stream network in the eutrophic, lowland Demnitzer Millcreek catchment, Germany. We focused on the effects of wetland systems impacted by beaver dams on the diurnal, seasonal and inter-annual variation in water quality dynamics at two sites, upstream and downstream of these wetlands. We then used the data to model stream metabolism. Dissolved oxygen and pH were higher upstream of the wetlands, while conductivity, turbidity, chlorophyll a and phosphorous concentrations were higher downstream. We found clear diurnal cycling of dissolved oxygen and pH at both sites. These dynamics were correlated with seasonal hydroclimatic changes and stream metabolism, becoming increasingly pronounced as temperatures increased and flows decreased in spring and summer. Upstream of the wetlands this corresponded to the stream rapidly becoming increasingly heterotrophic as modelled Gross Primary Production (GPP) was exceeded by Ecosystem Respiration (ER). Downstream, where GPP was lower, the stream was usually strongly heterotrophic and prone to increasingly hypoxic conditions (i.e., insufficient oxygen) before streamflow ceased in summer. This coincided with lower velocities and deeper channels in beaver impacted areas. Seasonal and inter-annual variations in water quality were found to mainly correlate with hydroclimatic factors (particularly temperature) and their influence on streamflow. This study highlights that heterotrophy and hypoxia in lowland rivers in central Europe is an important seasonal feature of intermittent streams where agricultural landscapes continue leaching nutrients. These insights contribute to an evidence base for understanding how climate change will affect the quantity and quality of rural water resources in intermittent lowland streams with wetlands where the presence of beavers requires management responses.