Seasonal variation in denitrification and dissolved nitrogen fluxes in intertidal sediments of the Tagus estuary, Portugal

Dissolved nitrogen fluxes and denitrification were studied during 1 yr in i ntertidal sediments of the Tagus estuary (Portugal). This study focused on the factors regulating both nitrogen fluxes across the sediment-water inter face and denitrification, and on the effect of microphytobenthos activity i n controlling nitrogen cycling in these areas. Sampling was performed month ly at 2 stations located in inner and outer intertidal areas. Fluxes of O-2 , 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 c onditions, as well as in the dark. At the sediment-water interface, higher DIN fluxes and lower denitrification rates were registered at higher temper atures and lower NO3- concentration in the water column. Oxygen uptake by t he sediment was generally higher than release, particularly in summer. Prim ary productivity displayed a seasonal cycle, positively influenced by tempe rature. Denitrification rates were closely related to NO3- river-input. Tem perature, NO3- concentration in the water column, microphytobenthos, infaun a 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 denitrif ication accounted for 156 mmol m(-2) yr(-1) in the inner station and 482 mm ol 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 s tations, respectively. N2O production was comparatively very low (0.3 to 0. 6 mmol m(-2) yr(-1)). The estimated nitrogen assimilation rates by microphy tobenthos 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 nit rogen was retained within benthic microalgae. The assimilation/denitrificat ion ratio, with a mean value of 2, shows the relative importance of the den itrification role as a N-sink. Apart from denitrification, it is suggested that other processes must be involved in the removal of nitrogen from the e stuary.