Effects of regional climate, hydrology and river impoundment on long-term patterns and characteristics of dissolved organic matter in semi-arid northern plains rivers

Diverse environmental and anthropogenic factors, such as the ongoing reservoir constructions may influence riverine dissolved organic matter (DOM) properties. This has important implications for river water quality, particularly when reservoirs are a source of drinking water. Simultaneous studies of multidecadal trends in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are scarce. We studied the patterns in DOC and DON concentration in two major rivers of the South Saskatchewan River (SSR) basin over a 42-year period (1978-2019). We also examined the impact of a large reservoir on riverine DOC properties. Contrary to many studies, we did not find a long-term in-crease in DOC and DON concentration, and DOC and DON patterns were not always synchronous. In an agriculture dominated watershed like the SSR basin, agricultural land use (e.g., nitrogen-fertilizer application) could influence DOC and DON concentration differently, potentially resulting in asynchronous patterns over time. River discharge was an important driver of DOM patterns. Regional precipitation in the lower SSR basin may also influence DOM pat-terns in locations where runoff contribution is greater. These regional factors explained greater variability in DOM compared to global scale indices (e.g., Pacific decadal oscillation) due to their direct control on DOM. A travel time corrected approach to account for the lengthy reservoir turnover time showed that a large reservoir caused a reduction in allochthonous DOC characteristics through photodegradation and perhaps, an increase in autochthonous character-istics. Our results illustrate: 1) the increase in DOM concentrations seen in the northern hemisphere is not present in semi-arid prairie rivers, 2) Controls on different DOM components could be different, and 3) large reservoirs may mod-ify riverine DOC composition due to longer water residence time.

Automated summary

Diverse environmental and anthropogenic factors can influence the properties of dissolved organic matter (DOM) in rivers, which is important for water quality. Studies have shown that the trends in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in rivers can vary, and agricultural land use and river discharge are important drivers of DOM patterns. Large reservoirs can also modify riverine DOC composition through longer water residence time.