Sediments in a Portuguese coastal area - pool sizes of mobile and immobile forms of nitrogen and phosphorus

Sediment samples collected in April 1993 and July 1994 from the Tejo and Sado coastal area have been characterized in terms of porosity, C-org, N and P contents. Features of this region have revealed enrichment of Tejo sediments relative to Sado sediments and deposition of fine organic material with high concentrations of these constituents in both Lisboa and Setubal canyons. Lack of stratification in the top 9 cm of sediment reflects bottom mixing, favoured by the dynamic conditions in the zone. Loosely bound forms of P and N were measured and the respective mobile pools estimated as far as 6 cm depth. The derived values of 159 mmol m(-2) for PO43- and 65 mmol m(-2) for NH4+ in the Tejo coastal area and 50 mmol m(-2) for PO43- and 32 mmol m(-2) for NH4+ in the Sado coastal area suggest that a relatively large proportion of P in the sediment, particularly in the Tejo area (20%), is available to be mobilized into the water column and so could contribute to the enrichment in this nutrient which is known to limit the biological productivity of most of the area during spring.