Estuaries Vol. 21, No. 4A, p. 635-645 December 1998 The Influence of Tidal Action on Porewater Nitrate Concentration and Dynamics in Intertidal Sediments of the Sado Estuary CARLOS ROCHA l A. P. CABRAL University of Algarve Unidade de Ciencias e Tecnologias dos Recursos Aquaticos Campus Gambelas PT-8000 Faro Portugal ABSTRACT: The change in porewater nitrate (N0 2 - + N0 3 -) concentrations during exposure of intertidal sediment was studied at a fixed location in the Sado estuary, southwest Portugal, in November 1994. In order to follow nitrate concentration and dynamics from pre-ebb to post flood, during the day, high vertical resolution profiles (0.2 cm) were studied. As a complement, in February 1995, potential nitrification rates in the sediment were measured by laboratory incubations, with high vertical resolution (0.2 em) up to 3 cm depth. Oxygen penetration was measured with polaro- graphic mini-electrodes. The sediment's texture as well as the organic matter composition in carbon and nitrogen were studied in deeper (30 em) cores. In February 1993, 210Pb activity depth profiles were measured in a core sampled at the beginning of exposure, in order to evaluate the possibility of nonlocal particle exchange. C:N ratios and 210Pb 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 brought from the surface to 7-9 cm depth, cansing enhancement of nutrient concentrations. Results of this study suggest nitrate dynamics in intertidal sediments of the Sado estuary are strongly influenced by tidal action. Periodic submersion and exposure allow for the diversification of pathways of oxygen supply to the sediment. Tidal stress at the sediment-water interface during the arrival (flooding) and departure (exposure) of the tidal front at the site has an important bearing on the effective depth of the nitrification zone. A denitrification rate of 2.16 JLmol N dm- 3 h- 1 was measured directly from the nitrate inventory in the 1.5-6 em depth layer. The schematic 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 availability for nitrification. Introduction Estuaries link freshwater and marine ecosystems. They are highly dynamiC environments, with large spatial and temporal gradients (Nixon 1981; An- derson 1983; Henriksen and Kemp 1988; Klump and Martens 1989; de Jonge and van Beusekom 1995; Middelburg et al. 1996). Intertidal areas of- ten cover large fractions of meso tidal and macro- tidal estuaries, and their importance as major sites of sediment-and associated organic matter and contaminants-accumulation, has been widely rec- ognized (Bradley and Morris 1990; Middelburg et al. 1996 and references therein). Enhanced min- eralization and turnover of nitrogen compounds takes place (Jorgensen 1983; Nielsen 1993) where denitrification is usually coupled to nitrification (Jorgensen 1983), a biochemical loop that repre- sents an important nitrogen sink in coastal systems. Bioturbation and irrigation in the sediments can influence porewater chemistry by providing addi- tional O 2 to depths below the surface, enhancing 1 Corresponding author; tele: +351-89-800900 (ext 7373); fax: +351-89-818353; e-mail: crocha@ualg.pt. © 1998 Estuarine Research Federation 635 ammonification and further oxidation of intersti- tial solutes. Grundmanis and Murray (1977) pro- posed that the activity of benthic fauna resulted in the availability of O 2 to an intermediate depth in Puget Sound sediments, where nitrification took place, and above and below this depth denitrifi- cation predominated. In intertidal sediments, drainage and air intrusion during the exposure pe- riod can allow for the same process to occur, es- pecially in permeable sediment. The oscillating cy- cle of exposure and flood in tidal systems may re- sult in alternating aerobic and anaerobic condi- tions in permeable sediment (Kerner et al. 1990). On a daily basis, intertidal sediments are dewa- tered by evaporation and drainage (Anderson 1983; Anderson and Howell 1984; Allen 1994). This loss is compensated by infiltration of tidal wa- ters during flood and by precipitation during ex- posure (Harvey and Odum 1990). The oscillatory pattern of dewatering and reloading of the sedi- ment water content becomes a major factor con- trolling solute transport and mixing through hy- drodynamic dispersion (Gardner 1975; van der Loeff 1981; Harvey and Odum 1990). This leads