The influence of long emersion on biota, ammonium fluxes and nitrification in intertidal sediments of Marennes-Oléron Bay, France

Abstract

A comparative study between waterlogged and reflooded intertidal sediments was undertaken in March and June 1999 through statistical analysis of selected sediment parameters (biota, salinity, O 2, E h), pool sizes and benthic fluxes of nutrients (NH 4 +, NO 2 −, NO 3 −) and nitrification rates. In March samples, absence of polychaetes and oligochaetes from upper sediment horizons were due to erosional events sweeping away surface sediments. Presence of richer annelid assemblages in June samples indicated more stable hydrodynamic conditions that favoured the development of benthic microalgae biofilms. Dewatering of sediments during a 3-day emersion period promoted a salinity rise on top layers, migration of pore water ions towards the sediment surface, and created sediment fissures that accelerated water exchange on reflooding. Reflooded and waterlogged sediment systems were comparable with respect to the release of NH 4 + to overlying water but were different with respect to nitrification rates. Sediment-water NH 4 + fluxes were higher ( P=0.011) in March (3.3 mmol m −2 day −1) compared to June (1.4 mmol m −2 day −1) due to higher macrofauna biomasses and lower benthic microalgae concentrations in March samples. Potential nitrification rates (range from 19 to 60 mmol NO 3 − m −2 day −1) were not statistically different between March and June. A thinner oxic layer in reflooded compared with waterlogged systems reflects a decrease of O 2 diffusion into sediment at high salinities which resulted in the fall of the actual nitrification rates ( P<0.05). Our data suggest that long term dessication of intertidal sediments may depress the nitrification process at the ecosystem level.