Inorganic nitrogen dynamics in intertidal rocky biofilms and sediments of the Douro River estuary (Portugal)

Abstract

In this study rates of oxygen, ammonium (NH4+), nitrate (NO-), nitrite (NO0-), and nitrous oxide (N20) fluxes, nitrogen (N) fixation, nitrification, and denitrification were compared between two intertidal sites for which there is an abundant global literature, muddy and sandy sediments, and two sites representing the rocky intertidal zone where biogeochemical processes have scarcely been investigated. In almost all sites oxygen production rates greatly exceeded oxygen consumption rates. During daylight, NH4+ and NO3- uptake rates together with ammonification could supply the different N requirements of the primary producer communities at all four sites; N assimilation by benthic or epilithic primary producers was the major process of dissolved inorganic nitrogen (DIN) removal; N fixation, nitrification, and denitrification were minor processes in the overall light DIN cycle. At night, distinct DIN cycling processes took place in the four environments, denitrification rates ranged from 9 ? 2 to 360 ? 30 ilmol N2 m-' h-', accounting for 10-48% of the water column NOs- uptake; nitrification rates varied from 0 to 1712 ? 666 gmol NH4' m-2 h-'. A conceptual model of N cycle dynamics showed major differences between intertidal sediment and rocky sites in terms of the mean rates of DIN net fluxes and the processes involved, with rocky biofinm showing generally higher fluxes. Of particular significance, the intertidal rocky biofilms released 10 times the amount of N20 produced in intertidal sediments (up to 17 ? 6 imol N20 m-" h-'), representing the highest N20 release rates ever recorded for marine systems. The biogeochemical contributions of intertidal rocky substrata to estuarine and coastal processes warrant future detailed investigation.