This study examines the resource use and trophic position of nematodes and harpacticoid copepods at the genus/species level in an estuarine food web in Zostera noltii beds and in adjacent bare sediments using the natural abundance of stable carbon and nitrogen isotopes. Microphytobenthos and/or epiphytes are among the main resources of most taxa, but seagrass detritus and sediment particulate organic matter contribute as well to meiobenthos nutrition, which are also available in deeper sediment layers and in unvegetated patches close to seagrass beds. A predominant dependence on chemoautotrophic bacteria was demonstrated for the nematode genus Terschellingia and the copepod family Cletodidae. A predatory feeding mode is illustrated for Paracomesoma and other Comesomatidae, which were previously considered first-level consumers (deposit feeders) according to their buccal morphology. The considerable variation found in both resource use and trophic level among nematode genera from the same feeding type, and even among congeneric nematode species, shows that the interpretation of nematode feeding ecology based purely on mouth morphology should be avoided.

Acidification treatment has been used in most stable isotope studies to eliminate inorganic non-dietary carbon, allowing d13C measurement of organic carbon needed for trophic studies. Because only limited information about the acidification effects on isotopic ratios exists in the literature, we provide an assessment of such effects for a wide range of benthic macroinvertebrates from estuarine intertidal sediments with sparse Zostera noltii Hornem. vegetation. Our results revealed only few significant d13C shifts for macrobenthos; however, d13C decrease after acidification was substantial for several species. We conclude that acidification is not necessary for removing calcium carbonate in a majority of macrobenthos, but we also suggest preliminary testing before deciding it can be omitted. The magnitude of the d15N shifts was substantial for some species and variability in d15N values of replicate acidified samples exceeded that of non-acidified samples. Therefore, sample acidification should be avoided for d15N determination. Moreover, acidification effects vary considerably among species belonging to a single class, rendering generalisations from single-species information potentially spurious.