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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 (Ϫ4 to Ϫ6‰, on average) compared to the wide range in 13 C/ 12 C ratios of carbon sources found in the studied marshes. At the Spartina site of the Waarde Marsh (The Netherlands), biomarker 13 C/ 12 C 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.

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Oxygen consumption in the profundal gyttja and littoral sands of Lake Vechten was determined with pore-water analyses and core incubations. Methane (58-64%), ammonium (lo-13%), iron (2-6%), and sulfide (< 12%) oxidation accounted for -75% of the oxygen consumed in the profundal sediments. Almost the complete diffusive flux of CH, out of the anoxic layer into the oxic surface layer (7.9-9.4 mmol mm2 d-l) was oxidized, while only 27-36% of the NH,+ flux (5.1-5.2 mmol m-I d-l) was oxidized in the oxic surface layer and 64-73% diffused into the overlying water. In contrast to the profundal sediments, oxidation of reduced end products of anoxic respiration was of minor importance (< 15%) in littoral sandy sediments. Also, sediment oxygen consumption rate was higher in the profundal gyttja (29 mmol m-* d-l at 7°C) during overturn than in the littoral sandy sediments in both winter (8.7 mmol m-2 d-' at 7°C) and summer (18.6 mmol me2 d..' at 22°C). Poisoning the sediment cores with formaldehyde stopped bacterial activity, but oxygen profile measurements showed that this treatment does not reveal the bacterial or chemical nature of the oxidative processes.

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