Temporal trends in nitrogen concentrations in Estonian rivers

The nutrient content in streams and rivers depend on many interacting processes such as hydro-geographical conditions and land use practices. The aim of this study was to investigate the current status of Estonian rivers and determine any trends in the concentrations of total nitrogen (TN) and nitrate-nitrogen (NO 3 -N) between 1992 and 2013. This study involved 43 monitoring sites and 32 rivers in Estonia. The temporal trends were assessed using the partial Mann-Kendall (PMK) test, which was adapted to account for the influence of water discharge. Most of the studied streams and sites did not show any trend in nitrogen concentrations. The statistically significant downward trend in TN was identified at 13 monitoring stations and upward trend at four monitoring sites. The results for NO 3 -N showed a statistically significant downward trend at three sampling sites while the upward trend was found at nine monitoring stations, particularly at four sites located within the nitrate vulnerable zone (NVZ). Overall, the increasing nitrate content in surface waters can most probably be attributed to the intensification of agricultural activities in rivers catchments during the last ten years. However, there are still many uncertainties in nutrient loss processes. Thus, the national monitoring programmes should be further developed.