Denitrification, nitrogen fixation, community primary productivity and inorganic-N and oxygen fluxes in an intertidal Zostera noltii meadow

Rates of denitrification, N-fixation, gross community primary productivity, inorganic-N and oxygen fluxes were determined in February, May and October 1997 in an intertidal Zostera noltii meadow of the Bassin d'Arcachon, French Atlantic coast. Rates of gross community primary produc- tivity were high, 0.09 to 0.40 g C m -2 h -1 ; high P:R ratios of 1.64 to 2.82 define the system as highly autotrophic and indicate significant losses of carbon via export and/or burial of biomass. Fluxes of DIN, nitrate and ammonium were large (-0.8 to -2.4, -0.1 to -2.2 and -0.1 to -0.7 mmol N m -2 h -1 , respectively) and always directed towards the plants/sediment during both light and dark incuba- tions. The contributions of nitrate, nitrite and ammonium to total DIN fluxes reflected their relative abundance in the water column, indicating that there was no assimilatory selection of inorganic-N sources by the plants. The DIN fluxes were dominated by the N-assimilation activity of the plants even during dark incubations, as removal of the plant shoots prior to incubations essentially abol- ished nitrate fluxes and reversed ammonium fluxes, resulting in substantial effluxes. Thus, inorganic- N fluxes were controlled principally by the Z. noltii and epiphyte biomasses and their primary pro- ductivity, rather than the water column concentrations of DIN. Surprisingly, the plant community showed a high dark assimilation activity for inorganic-N, and differences in light and dark fluxes of DIN, nitrate and ammonium were never significant. Data indicate that, whilst DIN fluxes could sup- ply the N-demand of primary production in spring, the plants became increasingly dependent upon sediment N-pools, N-fixation and internal N-reserves through summer into autumn. Denitrification rates determined by the 15 N-isotope pairing technique were extremely low, ranging between 2 and 6 µmol N m -2 h -1 . Rates of denitrification of nitrate diffusing from the overlying water were consis- tently below 2 µmol N m -2 h -1 during both light and dark incubations and represented only 0.1 to 0.7% and 0.2 to 1.3% of the total light and dark nitrate fluxes, respectively. Similarly, rates of denitri- fication coupled to nitrification were consistently low, probably due to the competition between nitri- fying bacteria and the Z. noltii roots for ammonium. N-fixation rates varied between 4 and 17 µmol N m -2 h -1 and were substantially greater than N-losses via denitrification in all seasons, with net N2 in- puts ranging between 2.5 and 14.6 µmol N m -2 h -1 and 0.5 and 3.8 µmol N m -2 h -1 , during light and dark incubations. Overall, our data demonstrate that the Z. noltii meadows represent a highly conser- vative environment for nitrogen, where the N-cycle is dominated by the primary productivity of the plant community and the associated assimilatory demand for fixed-N to support this productivity. Conversely, N-losses via denitrification are extremely low and are more than balanced by N-inputs from N-fixation. Thus, in this macro-tidal lagoon, export of nitrogen as plant biomass and/or N-burial in the sediments are probably the major loss mechanisms for anthropogenic N-inputs.