Long-term trends in Rickleån : Relationship between concentrations of organic matter and water flow 1970-2019

Dissolved organic matter (DOM) in surface waters affect many important ecological functions. For instance, transporting metals, nutrients, carbon, affecting pH and water colour. In large parts of the Northen hemisphere, surface waters have become browner as a consequense of increasing DOM concentrations and, to some extent, iron. Therfore, altering ecological functions in waters. As such, knowing the causes and extent of the increase is of great importance. This paper used monthly data from the national Swedish monitoring program to investigate trends in DOM-concentrations in the Rickleå river, Västerbotten, Sweden. Results showed a large increase in concentrations of DOM from 1970-2019. However, the increase was most pronounced between 1970-1990, showing no trend after 2003 and indications of a decrease from 2009. DOM-quality changed as well to larger and more colored molecules during 1987-2002 and less coloured, smaller molecules after 2003. Changes in water flow could be an explanation for short-term fluctuations in DOM-concentration, but did not correlate well to increasing trends. However, a large lake at the river inlet as well as hydroelectric dams along the watershead complicates the interpretation between flow and DOM correlation. Climate change did not seem to be an important driver of long term increases in DOM. Further investigations should be carried out to test this hypothesis as well to investigate the cause for the increase