Daily, seasonal, and flood-dependent dynamics of water quality in a lowland urban stream in a Mediterranean climate

Water quality in streams provides fundamental information on the stream's ecosystem functioning and status. High-frequency measurements, taken with sensors, can reveal details about hydrological and biogeochemical processes in these streams, which are difficult to obtain through other sampling methods. Such measurements are generally lacking in streams in Mediterranean climates, and thus, their temporal water quality patterns are not entirely clear. This study aimed to quantify and characterize the daily, seasonal, and flood-dependent water quality dynamics in a lowland urban Mediterranean stream, and to compare them to water quality dynamics in streams located in temperate climates. Data of dissolved oxygen, carbon dioxide, nitrate, cDOM, chlorophyll a, turbidity, electrical conductivity, pH, water level, and light intensity in the Yarkon Stream, a lowland urban stream in Israel, were measured in 15-minute intervals beginning in July 2019. It was found that water quality dynamics, under baseflow conditions, were controlled by the input from wastewater treatment plants and biogeochemical reactions and not by fluctuations in discharge. In addition, water quality variability, under baseflow conditions, was more distinctly observed through diel patterns than through comparisons of average seasonal values. This was especially true for dissolved oxygen, pH, CO₂, chlorophyll a, and turbidity. Floods were dominated by dilution processes, while flushing incidents occurred only during specific events, mostly during floods that followed closely after a preceding one. The insights gained from this study are expected to provide a foundational basis for developing a conceptual model of water quality in a lowland urban Mediterranean stream. They are also expected to enhance our understanding of the similarities and differences between lowland urban Mediterranean streams and those in temperate climates, for which more data are available.