Bacterivory by benthic organisms in sediment : 1 quantification using 15 N enriched bacteria

11 The fate of benthic bacterial biomass in benthic food webs is a topic of major importance 12 but poorly described. This paper describes an alternative method for evaluation of bacterial 13 grazing rate by meiofauna and macrofauna using bacteria pre-enriched with stable isotopes. 14 Natural bacteria from the sediment of an intertidal mudflat were cultured in a liquid medium 15 enriched with 15 NH4Cl. Cultured bacteria contained 2.9 % of 15 N and were enriched 16 sufficiently to be used as tracers during grazing experiments. Cultured-bacteria presented a 17 biovolume (0.21 !m 3 ) and a percentage of actively respiring bacteria (10 %) similar to those 18 found in natural communities. The number of Operational Taxon Units found in cultures 19 fluctuated between 56 and 75 % of that found in natural sediment. Despite this change in 20 community composition, the bacterial consortium used for grazing experiments exhibited 21 characteristics of size, activity and diversity more representative of the natural community 22 than usually noticed in many other grazing studies. The bacterial ingestion rates of three 23 different grazers were in the range of literature values resulting from other methods: 1149 24 ngC ind -1 h -1 for the mud snail Hydrobia ulvae, 0.027 ngC ind -1 h -1 for the nematode 25 community, and 0.067 ngC ind -1 h -1 for the foraminifera Ammonia tepida. The alternative 26 method described in this paper overcomes some past limitations and it presents interesting 27 advantages such as short time incubation and in situ potential utilisation. 28