Agriculture-induced increase in nitrate concentrations in stream waters of a large Mediterranean catchment over 25 years (1981–2005)

Abstract

Anthropogenic activities influence past and present nitrate levels recorded in European stream waters,

posing a threat to aquatic biota and human beings. Scarce information on temporal trends of nitrate

concentration and its causes is available for Mediterranean catchments. This study presents the evolution of

nitrate concentrations over 25 years in stream waters of the Ebro River Basin (Spain), a large Mediterranean

catchment involving 85,566 km2. Nitrate concentration increased with time in 46% of the 65 sites involved in

the study. Agricultural cover of 30 hydrologically independent sub-catchments was the main land use related

to nitrate concentration (R2=0.69). Throughout the 25 year-period, the sites showing increased nitrate

concentrations with time (trend sites) also presented an enhanced influence of agricultural cover on nitrate

concentrations along the time frame of the study. As a result of these temporal changes, at the end of the

studied period nitrate concentrations in stream waters responded similarly to agricultural cover in both

trend and non-trend sites, showing non significant differences in the slope of the resultant regression

models. At this time, agricultural cover explained 82% of the variability found in nitrate levels. If these trends

remain unchanged, in 2015 many of the water bodies considered in this study would not comply with the

requirements of the European Union Water Framework Directive (WFD). Therefore management decisions,

mainly associated to agricultural practices, should be implemented as soon as possible at the catchment level

to meet WFD objectives.