Connection between El Niño-Southern Oscillation events and river nitrate concentrations in a Mediterranean river

The causes of interannual nitrate variability in rivers remain uncertain, but extreme climatic events have been suggested as drivers of large nitrate inputs to rivers. Based on a 24-year data set (1983–2006), we suggest that El Niño-Southern Oscillation (ENSO) can affect nitrate behavior in a seasonal extra-tropical stream, the Llobregat (NE Iberian Peninsula), located thousands of kilometers away from the ENSO oscillating system via atmospheric teleconnections. Two commonly used indices, the Southern Oscillation Index (SOI) and the self-calibrating-Palmer Drought Severity Index (scPDSI) showed highly significant correlations with nitrate concentrations, which recurrently increased during La Niña phases, coinciding with severe droughts.

► We analyze relationships between NO3 variations, precipitation, drought and ENSO in a Mediterranean river.
► The most affecting ENSO phase is La Niña, which cause year-long droughts.
► Resulting reduced runoff and edaphic moisture promote higher-than-usual nitrate storage in the catchment.
► Nitrate excess is transferred to the river by autumn precipitation causing nitrate peaking.