Fabiane Gallucci

Dual stable isotope abundances unravel trophic position of estuarine nematodes

The role and quantitative importance of free-living nematodes in marine and estuarine soft sedimentsremain enigmatic for lack of empirical evidence on the feeding habits and trophic position of most nematodespecies. Here we use natural abundances of carbon and nitrogen stable isotopes of some abundantnematode species/genera from estuarine intertidal sediments to assess their trophic level and major foodsources. In all stations, d15N of diierent dominant nematode species/genera spanned a range of 3.6 to6.3 ppt, indicating that at least two trophic levels were represented. The large nematodes Enoplus brevis,Enoploides longispiculosus and Adoncholaimus fuscus consistently had high d15N, in line with mouth-morphologybased predictions and empirical evidence on their predacious feeding modes. Daptonema sp., Metachromadoraremanei, Praeacanthonchus punctatus and Chromadoridae (dominated by Ptycholaimellus ponticus) hadcomparatively lower d15N, and d13C suggesting that microphytobenthos (MPB) is their major carbonsource, although freshly sedimented particulate organic matter may also contribute to their nutrition insilty sediments. The trophic position of Sphaerolaimus sp., a genus with documented predacious feedingmode, was ambiguous. Ascolaimus elongatus had d15N signatures indicating a predacious ecology, which isat variance with expectations from existing feeding type classi¢cations. Our study shows thatdespitelimitations imposed by the biomass requirements for EA-IRMS (elemental analyserisotope ratio massspectrometry)natural isotope abundances of carbon and nitrogen are powerful tools to unravel trophicstructure within nematode communities. At the same time, the prominence of diierent trophic levelsresults in a large span of d15N, largely invalidating the use of nitrogen isotope abundances to assess foodsources and trophic level of whole nematode communities.