Surveys of he biogeochemistry of phosphate were performed in the ria of Vigo. A two-layer box model was applied to the data obtained from seven transects made during 1986. Based on the flux of phosphate in the water of the ria, the fluxes of phosphate consumed by photosynthesis in the upper layer, regenerated by the remineralization of organic phosphorus in the lower layer and interchanged with the sediment by precipitation-adsorption and desorption-redissolution of phosphate, were calculated. The ria behaves like a biogeochemical phosphate reactor which annually receives 350 × 106 gP from oceanic waters, mainly by upwelling, and 80 × 106 gP from land inputs. During winter, the ria flushes into the ocean more phosphate than it receives, approximately 0·3 molP s-1, whereas during the rest of the year the opposite occurs. Between 0·12 and 0·78 molP s-1 is consumed by photosynthesis, of which from 0·10 to 0·30 molP s-1 are normally remineralized within the ria. The net result of this annual biogeochemical activity in the ria makes the bed a phosphorus trap, that retains 38% of phosphorus inputs to the ria. The organic removal of phosphorus was estimated at 150 × 106 gP year-1, and 30 × 106 gP year-1 was the inorganic removal of phosphate.

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