Limnol. Oceanogr., 44(2), 1999, 309–319

Stable carbon isotope ratios of bacterial biomarkers were determined to infer sources of organic carbon used by bacteria in the sediments of three salt marshes. Biomarkers studied were polar lipid-derived fatty acids (PLFA), mainly bacteria-specific, methyl-branched i15 : 0 and a15 : 0. Experiments showed that isotopic fractionation between substrate and biomarkers was relatively constant (24 to 26‰, on average) compared to the wide range in 13C/12C ratios of carbon sources found in the studied marshes. At the Spartina site of the Waarde Marsh (The Netherlands), biomarker 13C/12C ratios were depleted by approximately 6‰ more than expected for bacteria growing on Spartina litter and were similar to an unvegetated control sediment. This pattern suggested that local macrophyte production was of little importance and that other material (probably of algal origin) was the dominant carbon source for bacterial growth. Spartina contributed about half of the carbon in bacterial PLFA at the Kattendijke Marsh (The Netherlands) and dominated at the Great Marshes (U.S.). The variation in bacterial carbon sources in these marshes was probably related to estimated inputs of nonmacrophyte organic matter to the sediment. At the Waarde Marsh, a clear plant species effect was found as coupling between plant and bacteria was more important in Scirpus maritimus than in Spartina anglica. The contribution of local plant production to bacterial biomass in salt-marsh sediments is highly variable between marshes and depends on the input of nonmacrophyte material by sedimentation in comparison to local plant input, which in turn may differ among plant species. Tidal salt marshes are highly productive ecosystems whose productivity is dominated by macrophytes. This high productivity supports an active microbial community both on standing dead shoots and in salt-marsh sediments (Teal and Kanwisher 1961; Howarth 1993; Newell 1993). Some of the highest mineralization rates have been detected in saltmarsh sediments, and it is generally believed that a close coupling exists between macrophyte productivity and microbial processes in these ecosystems (Howarth 1993). For instance, Howes et al. (1984) found that root and rhizome productivity was similar to sedimentary carbon mineralization rates in Spartina vegetation. Sulfate reduction, the dominant mineralization process in salt-marsh sediment (Howes et al. 1984; Howarth 1993), is influenced by the growth phase of the vegetation (Hines et al. 1989). Besides macrophyte-derived material, other sources of organic matter may be important in salt-marsh sediments. In many of the European and American marshes that have a mineral sediment, the stable carbon isotope ratio of the sediment organic matter does not reflect the macrophyte material produced in these systems. For example, being a C4