The influence of tidal action on porewater nitrate concentration and dynamics in intertidal sediments of the Sado estuary

The change in porewater nitrate (NO2- + NO3-) concentrations during exposur e of intertidal sediment was studied at a fixed location in the Sado estuar y, southwest Portugal, in November 1994. In order to follow nitrate concent ration and dynamics from pre-ebb to post flood, during the day, high vertic al resolution profiles (0.2 cm) were studied. As a complement, in February 1995, potential nitrification rates in the sediment were measured by labora tory incubations, with high vertical resolution (0.2 cm) up to 3 cm depth. Oxygen penetration was measured with polarographic mini-electrodes. The sed iment's texture as well as the organic matter composition in carbon and nit rogen were studied in deeper (30 cm) cores. In February 1993, Pb-210 activi ty depth profiles were measured in a core sampled at the beginning of expos ure, in order to evaluate the possibility of nonlocal particle exchange. C: N ratios and Pb-210 activity profiles show evidence of nonlocal exchange of solid phase particles between the surface and deeper sediment, most likely due to macrofaunal activity. As a consequence, fresh organic matter is bro ught from the surface to 7-9 cm depth, causing enhancement of nutrient conc entrations. Results of this study suggest nitrate dynamics in intertidal se diments of the Sado estuary are strongly influenced by tidal action. Period ic submersion and exposure allow for the diversification of pathways of oxy gen supply to the sediment. Tidal stress at the sediment-water interface du ring the arrival (flooding) and departure (exposure) of the tidal front at the site has an important bearing on the effective depth of the nitrificati on zone. A denitrification rate of 2.16 mu mol N dm(-3) h(-1) was measured directly from the nitrate inventory in the 1.5-6 cm depth layer The schemat ic model of N cycling in these sediments suggests that 20% of the N pool is denitrified during exposure, and that this process is limited by O-2 avail ability for nitrification.