Nitrogen fluxes and budget seasonality in the Ria Vigo (NW Iberian Peninsula)

Abstract

Inorganic and organic nitrogen fluxes in the Ria Vigo have been quantified in order to recognise the contrasting nitrogen budget scenarios and understand the biogeochemical response to eutrophication events. According to the nitrogen biogeochemical pathways of the ria reservoir (photosynthesis, remineralization, denitrification, PON rain rate and sedimentation), three main seasonal behavioural trends are emphasised: (1) low inorganic nitrogen inputs and low organic nitrogen fluxes, (2) high inorganic nitrogen input and output, (3) high inorganic nitrogen input and high organic nitrogen output. The first scenario occurs in late spring and in summer during non-upwelling situations. The consumption of inorganic nitrogen by net photosynthesis is approximately 2 mol N s−1 and the ria is oligotrophic (12 mgC m−2 h−1). The outgoing estuarine residual current transports phytoplanktonic material towards the mouth of the ria whereupon it sediments and is remineralized as it falls to the lower water layers and the incoming residual current. The regenerated nitrogen is reintroduced to the photic ria layer which leads to the greatest reduction in dissolved oxygen concentration (50% of saturation). Recycled nutrients play an important role in primary production during this oligotrophic state of the ria. Thus, approximately half of the inorganic nitrogen utilised by photosynthesis is ammonium. The majority of PON is deposited inside the ria (0.8 mmol N m−2 d−1) and the denitrification rate is 0.3 mmol N2 m−2 d−1. The other two cases occur in winter and spring—summer with upwelling. In winter, estuarine circulation and freshwater contributions control the nitrogen cycle. The ria mainly exports nitrate (up to 14 mol N s−1) and so there is fertilisation but no eutrophication. In spring and summer, the nitrogen cycle is controlled by upwelling circulation. The inorganic nitrogen consumption by net photosynthesis is high, 7–14 mmol N m−2 d−1, and the ria is a natural eutrophic system (70 mgC m−2 h−1). Accordingly, 90% of organic nitrogen is synthesised from nitrate and the upwelling-increased circulation exports 6.5 mol N s−1 of organic nitrogen.