Nitrogen interchanges generated by biogeochemical processes in a Galician ria

Abstract

The biogeochemistry of nitrogen in the ria of Vigo is approached on the basis of a box model which allows for calculation of the physical transport of nitrate, nitrite, ammonium and organic nitrogen, and the fluxes caused by photosynthesis, remineralisation and sedimentation of nitrogen.

The ria of Vigo receives between 4 and 22 mol s−1 from exterior coastal water, a flux far greater than that of freshwater. The balance of organic nitrogen indicates that there is always an exportation to the outer ria, in the range of 0.5 to 5 mol s−1. The enrichment of the incoming water is produced by photosynthetic activity in spring and summer, and by river and sediment contributions in winter. Inorganic nitrogen is only exported during the rainy season, when phytoplankton activity is low. During the rest of the year, the ria retains 3.5 mol s−1 of the total nitrogen received. As a result of these fluxes, the residence time of nitrogen in the ria varies between 15 and 97 days.

In the spring and summer of 1986, photosynthesis of the ria of Vigo consumed 7.4 mol s−1, 24% of which was derived from the remineralisation in the aphotic zone in the ria. Part of the synthesised PON is sedimented and remineralised on the continental shelf, or is reintroduced into the ria. This plays an important role in fertilising the area, not considered until now in Galician rias. Both remineralisations supply 40% of the inorganic nitrogen to the euphotic layer in the ria.