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

Abstract

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 d¹Arcachon, French Atlantic coast. Rates of gross community primary productivity 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 incubations. 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 abolished 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 productivity, 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 supply 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 15N-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 consistently 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 denitrification coupled to nitrification were consistently low, probably due to the competition between nitrifying 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 inputs 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 conservative 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.