The Ria Formosa is a meso-tidal coastal lagoon experiencing enhanced nutrient concentrations. Assessment of sediment-seawater interaction is essential if nutrient dynamics and the risk of eutrophication are to be fully understood. Pore water concentrations of dissolved inorganic and organic phosphorus, ammonium, nitrate and nitrite were determined in cores from six sites. Changes in nutrients concentrations were measured in intertidal pools on sand and mud between tides. Dissolved inorganic phosphorus (DIP) concentrations (~200 lmol l -1 ) and effluxes (123 ± 14 lmol m -2 h -1 ) were greater from sand than mud (37 ± 10 lmol m -2 h -1 ), possibly due to the binding of P with the <63 lm fraction. NH 4 + effluxes were high outside the Ancã o Basin (821 ± 106 lmol m -2 h -1 ) and were associated with Enteromorpha sp. mats. The greatest NO 3 -efflux was from sediments near a salt marsh (170 ± 67 lmol m -2 h -1 ). These sediment fluxes of P were not sufficient to account for elevated P concentrations seen by other workers on the ebb tide from the Ancã o Basin. Intertidal pools were sinks for Dissolved Inorganic Nitrogen (DIN) and DIP over the 6 h exposure period. Thus, tidepools may be an important route of nutrients into sediments that enhances the effects of sediments on seawater nutrient concentrations. AbbreviationsDIN Dissolved inorganic nitrogen DM Deep mud DS Deep sand DOP Dissolved organic phosphorus MP Mud pool NH 3 Ammonia MR Mud replicate NH 4 + Ammonium OA Outside Ancã o NO 2 -Nitrite SM Salt marsh NO 3 -Nitrate SP Sand pool P Phosphorus SR Sand replicate

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