Presence of nitrous oxide hotspots in the coastal upwelling area off central Chile: an analysis of temporal variability based on ten years of a biogeochemical time series

Abstract

Seasonal and inter-annual variabilities of biogeochemical variables, including nitrous oxide (N _2 O), an important climate active gas, were analyzed during monthly observations between 2002 and 2012 at an ocean Time-Series station in the coastal upwelling area off central Chile (36° 30.8′; 73° 15′). Oxygen, N _2 O, nutrients and chlorophyll-a (Chl-a) showed clear seasonal variability associated with upwelling favorable winds (spring–summer) and also inter-annual variability, which in the case of N _2 O was clearly observed during the occurrence of N _2 O hotspots with saturation levels of up to 4849%. These hotspots consistently took place during the upwelling-favorable periods in 2004, 2006, 2008, 2010 and 2011, below the mixed layer (15–50 m depth) in waters with hypoxia and some ${\rm N}{{{\rm O}}_{2}}^{-}$ accumulation. The N _2 O hotspots displayed excesses of N _2 O ( Δ N _2 O) three times higher than the average monthly anomalies (2002–2012). Estimated relationships of Δ N _2 O versus apparent oxygen utilization (AOU), and Δ N _2 O versus ${\rm N}{{{\rm O}}_{3}}^{-},$ suggest that aerobic ammonium oxidation (AAO) and partial denitrification are the processes responsible for high N _2 O accumulation in subsurface water. Chl-a levels were reasonably correlated with the presence of the N _2 O hotspots, suggesting that microbial activities fuelled by high availability of organic matters lead to high N _2 O production. As a result, this causes a substantial N _2 O efflux into the atmosphere of up to 260 μ mol m ^−2 d ^−1 . The N _2 O hotspots are transient events or hot moments, which may occur more frequently than they are observed. If so, this upwelling area is producing and emitting greater than expected amounts of N _2 O and is therefore an important N _2 O source that should be considered in the global atmospheric N _2 O balance.

Keywords

• nitrous oxide hotspot
• seasonal and inter-annual N2O variability
• coastal upwelling
• central Chile