Seasonal vanations in oxygen, phosphate and arnmonium fluxes between the sediment and overlying water were measured in a small shaiiow estuary (Palmones River Estuary, southern Spain) from Juiy 1996 to Juiy 1997. Fluxes at the sediment-water interface were measured in situ and compared with those obtained from the Pore water gradients. Oxygen consurnption rates ranged from 62.5 to 87.4 mmol m-' d-', and were highly correlated with phosphate (226 to 1597 pmol m-' d-') and ammonium (3189 to 9561 pmol m-' d-') fluxes. The diffusive fluxes of phosphate and ammonium, computed from the Pore water gradient, were lower than those measured in situ (from 7 to 24 tirnes for phosphate and from 21 to 55 tirnes for ammonium). Nutnent fluxes are weii correlated with temperature and with the C:N ratio of organic matter within the upper 2 cm of the sediment.

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The effects of Nereis sp. on the flux of dissolved phosphate across the sediment-water interface has been studied in Palmones River estuary using benthic flux-chambers and intact cores . Diffusive fluxes of phosphate were calculated from pore water gradient concentration and compared with those obtained from benthic chambers experiments . The high abundance of Nereis in the upper sediment layers appears to play an important part in the dissolved oxygen profiles in the overlying water, but had no effect on the redox potential . A negative relationship was found between the Nereis abundance and the phosphate gradient ; this gradient ranged between 40 tmol 1 -1 cm -1 with 340 Nereis individuals m -2 and 20 imol 1 -1 cm -1 with 900 Nereis individuals M-2 . The ratio of the in situ flux to the flux gradient concentration for dissolved phosphate increased with the abundance of Nereis (from 1 . 7 at low abundance to 5 . 8 at high abundance) . 387