Temporal and spatial variation of NO production from estuarine and marine shallow systems of Cadiz Bay (SW, Spain).

There is still much uncertainty regarding the global oceanic emissions of NO, and particularly emissions from coastal regions, because spatio-temporal datasets have limited coverage. The concentration of dissolved NO in surface waters and the associated fluxes to the atmosphere have been studied in three coastal systems located near Cadiz Bay (southwestern coast of Spain) over different time scales. The three systems present different hydrodynamic characteristics (an estuary and two marine systems) that influence the distribution of NO in the water column. Nutrients, oxygen, and particulate organic nitrogen were also measured to investigate the processes responsible for NO production in the water column. Data on dissolved NO has been obtained in each system from i) two-year monitoring at fixed station; ii) four seasonal samplings along the longitudinal length of the system; and iii) daily sampling in summer. The concentration of NO ranges between 1.1 and 292.0nM indicating very high spatio-temporal variability. In general, the concentration of NO increased during the rainy season associated with the precipitation regime that, in turn, increases the lateral inputs of organic matter and nutrients from both natural sources (discharges into rivers and adjacent marshes) and anthropogenic activities (agriculture, urban effluents and fish farming). Dissolved NO also varied with the tides: the highest concentrations were measured during the ebb, which suggests that the systems export NO to the Bay and adjacent Atlantic Ocean. In addition nitrification seems to be an important process for NO formation in the water column, which also explains some of the variability in the dataset. The mean atmospheric flux of NO reveals that entire study area was a net source of NO to the atmosphere. The fluxes ranged between 0.5 and 313.2μmolmday in the estuarine system, and between -7.2 and 97.8μmolmday in the two marine systems.