NH4+ turnover in intertidal sediments of Marennes-Oleron Bay (France): effect of sediment temperature

NH4+ turnover plays an important role in benthic compartment as source of n utrient for primary production. The intertidal mudflats of the Marennes-Ole ron Bay are characterized by high fluctuations of sediment temperature that generally ranges from 10 to 30 degreesC during emersion periods. Influence of sediment temperature on NH4+ processes was investigated. Grass ammonifi cation and NH4+ incorporation kinetics were studied using the N-15 isotope dilution method. The method was first adapted to these very fine sediments and applied to 2 upper sediment layers (0-2 and 2-4 cm) at different incuba tion times of up to 120 h. The effect of sediment temperature on gross ammo nification and NH4+ incorporation rates was investigated at 10, 20 and 30 d egreesC. There was a strong increase of both rates as a function of tempera ture, gross ammonification rates ranged from 0 to 17 mug NH4+-N.g(-1) dw.d( -1) and NH4+ incorporation rates from 2 to 13 mug N.g(-1) dw.d(-1) between 10 and 30 degreesC. This study showed that short-term temperature variation s during the emersion period should be taken into account in quantifying NH 4+ turnover in intertidal sediments and that benthic regeneration of NH4+ s ignificantly supports benthic and planktonic primary production in Marennes -Oleron Bay. It suggests also that there is no N-limitation for benthic pri mary production in this bay and that there is up to 44 % of NH4+ produced i n surface sediments, which can diffuse to the water column and become avail able to phytoplankton. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.