MEPS 202:51-65 (2000)  -  doi:10.3354/meps202051

ABSTRACT: Dissolved nitrogen fluxes and denitrification were studied during 1 yr in intertidal sediments of the Tagus estuary (Portugal). This study focused on the factors regulating both nitrogen fluxes across the sediment water interface and denitrification, and on the effect of microphytobenthos activity in controlling nitrogen cycling in these areas. Sampling was performed monthly at 2 stations located in inner and outer intertidal areas. Fluxes of O₂, NO₃^-, NO₂^-, NH₄⁺ and N₂O, and denitrification (determined by the nitrogen-isotope pairing technique) were measured simultaneously in closed chambers incubated in the laboratory under simulated in situ temperature and light conditions, as well as in the dark. At the sediment water interface, higher DIN fluxes and lower denitrification rates were registered at higher temperatures and lower NO₃^- concentration in the water column. Oxygen uptake by the sediment was generally higher than release, particularly in summer. Primary productivity displayed a seasonal cycle, positively influenced by temperature. Denitrification rates were closely related to NO₃^- river-input. Temperature, NO₃^- concentration in the water column, microphytobenthos, infauna and tidal height were the key parameters involved in controlling nitrogen cycling at the sediment water interface in the Tagus estuary. A comparison of annual nitrogen fluxes and denitrification rates between sites was made, taking into account tidal immersion periods. Hence, N-removal by denitrification accounted for 156 mmol m^-2 yr^-1 in the inner station and 482 mmol m^-2 yr^-1 in the outer station. These rates represent respectively ca 3 and 9% of total DIN available in the estuarine water column at those 2 stations, respectively. N₂O production was comparatively very low (0.3 to 0.6 mmol m^-2 yr^-1). The estimated nitrogen assimilation rates by microphytobenthos were 707 mmol m^-2 yr^-1 in the inner station and 333 mmol m^-2 yr^-1 in the outer station, indicating that a considerable amount of nitrogen was retained within benthic microalgae. The assimilation/denitrification ratio, with a mean value of 2, shows the relative importance of the denitrification role as a N sink. Apart from denitrification, it is suggested that other processes must be involved in the removal of nitrogen from the estuary.

KEY WORDS: DIN fluxes · Denitrification · Microphytobenthos · Intertidal sediments · Tagus estuary