Hydrological connectivity dynamics in a mixed land use lowland catchment drive intra- and inter-annual variation in water quality in an intermittent stream network under drought conditions

This study investigates the spatio-temporal dynamics of water quality in a 70 km2 mixed land use, lowland catchment in NE Germany over a four-year period (2018-2022). During this period with a consistent negative rainfall anomaly compared to the long-term average, the intermittent stream network exhibited three distinct hydrological phases each year, with important implications for water quality. Autumn and early winter featured a connecting phase, where spatially variable stream flows responded to rising water tables following increased rainfall and reduced evapotranspiration. The winter and early spring saw a fully connected phase, marked by increased stream flows throughput the catchment. Late spring and early summer experienced a disconnecting phase as flow gradually reduced and stopped in various parts of the catchment before ceasing altogether. A peat wetland in the centre of the catchment exhibited both the earliest and latest stream flows. Water quality was characteristic of a eutrophic lowland catchment and displayed spatial variations linked to catchment soils and land use. During the connecting phase, stream water quality mirrored that of groundwater and saw mobilization of dissolved organic carbon from wetland areas. In the fully connected phase, stream water became enriched with contributions from soil water and a higher nitrate load from agricultural areas. The disconnecting phase was characterized by lower flows and higher temperatures, contributing to increasingly anoxic conditions which saw nitrate reduction, mobilization of redox elements (Fe and Mn) and release of P. Intermittency caused a transition in stream water quality from hydrological process control in the connecting phase to joint control of hydrological and biogeochemical processes in the fully connected phase and then to biogeochemical process control in the disconnecting phase. Inter-annual water quality variation was associated with hydroclimate and catchment wetness dynamics, involving flushing and accumulation. Considering intermittency as an influencing variable changed the inter-annual characteristics of flow-concentration relationships compared with the previous perennial river stage, especially for nitrate. These findings have significant implications for the ecology and management strategies in similar catchments, highlighting the need to consider the seasonal hydrological phases for effective water quality management and ecological preservation.