Seasonal cycle of N 2O vertical distribution and air–sea fluxes over the continental shelf waters off central Chile (∼36°S)

Abstract

The continental shelf off central Chile is subject to strong seasonal coastal upwelling and has been recognized as an important outgassing area for, amongst others, N 2O, an important greenhouse gas. Several physical and biogeochemical variables, including N 2O, were measured in the water column from August 2002 to January 2007 at a time series station in order to characterize its temporal variability and elucidate the physical and biogeochemical mechanisms affecting N 2O levels. This 4-year time series of N 2O levels reveals seasonal variability associated basically with hydrographic and oceanographic regimes ( i. e., upwelling and non-upwelling). However, a noteworthy temporal evolution of both the vertical distribution and N 2O levels was observed repeatedly throughout the entire study period, allowing us to distinguish three stages: winter/early spring (Stage I), mid-spring/mid-summer (Stage II), and late summer/early autumn (Stage III). Stage I presents low N 2O, the lowest surface saturation ever registered (from 64% saturation) in a period of high O 2, and a homogeneous column driven by strong wind; this distribution is explained by physical and thermodynamic mechanisms. Stage II, with increasing N 2O concentrations, agrees with the appearance of upwelling-favourable wind stress and a strong influence of oxygen-poor, nutrient-rich equatorial subsurface waters (ESSW). The N 2O build-up creates a “hotspot” (up to 2426% N 2O saturation) and enhanced concentrations of NO 2 - (up to 3.97 μM) and NH 4 + (up to 4.6 μM) at the oxycline (4–28 μM) (∼20–40 m depth). Although the dominant N 2O sources could not be determined, denitrification (mainly below the oxycline) appears to be the dominant process in N 2O accumulation. Stage III, with diminishing N 2O concentrations from mid-summer to early autumn, was accompanied by low N/P ratios. During this stage, strong bottom N 2O consumption (from 40% saturation) was suggested to be mainly driven by benthic denitrification. Consistent with the evolution of N 2O in the water column over time, the estimated air–sea N 2O fluxes were low or negative in winter (−9.8 to 20 μmol m −2 d −1, Stage I) and higher in spring and summer (up to 195 μmol m −2 d −1, Stage II), after which they declined (Stage III). In spite of the occurrence of ESSW and upwelling events throughout stages II and III, N 2O behaviour should be a response of the biogeochemical evolution associated with biological productivity and concomitant O 2 levels in the water and even in the sediments. The results presented herein confirm that the study area is an important source of N 2O to the atmosphere, with a mean annual N 2O flux of 30.2 μmol m −2 d −1; however, interannual variability could not yet be properly characterized.