NITROGEN INTERCHANGES GENERATED BY BIOGEOCHEMICAL PROCESSES IN A GALICIAN RIA

The biogeochemistry of nitrogen in the ria of Vigo is approached on th e basis of a box model which allows for calculation of the physical tr ansport of nitrate, nitrite, ammonium and organic nitrogen, and the fl uxes caused by photosynthesis, remineralisation and sedimentation of n itrogen. The ria of Vigo receives between 4 and 22 mol s-1 from exteri or coastal water, a flux far greater than that of freshwater. The bala nce 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 an d 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. I n the spring and summer of 1986, photosynthesis of the ria of Vigo con sumed 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 sedimen ted and remineralised on the continental shelf, or is reintroduced int o the ria. This plays an important role in fertilising the area, not c onsidered until now in Galician rias. Both reminemlisations supply 40% of the inorganic nitrogen to the euphotic layer in the ria.