Trends of natural organic matter concentrations in river waters of Latvia.

This study revealed significant (P < 0.1 and P < 0.05) increasing trends of total organic carbon (TOC) and water colour in most of the studied Latvian rivers during the last decade. However, over longer time periods, there were pronounced oscillations of TOC concentrations, similar to patterns of long-term changes of river discharge regime. On a yearly basis, there was a positive correlation between parameters of organic matter concentration and discharge in all selected rivers (r (S) = 0.540-0.803; P < 0.01). The impact of discharge on concentrations of organic matter could be masked by other factors, such as changes in precipitation, biological processes, soil types or land use.