The herbivorous, detrital and microbial pathways are major components of marine food webs. Although it is commonly recognized that these pathways can be linked in several ways, elucidating carbon transfers between or within these pathways remains a challenge. Intertidal flat communities are driven by a wide spectrum of organic matter sources that support these different pathways within the food web. Here we reconstruct carbon pathways using inverse analysis based on mass balancing, stable isotope signatures and tracer data. Data were available on biomass, total carbon production and processing, integrated diet information from stable isotope signatures and the transfer of recently produced carbon through the food web from an isotope tracer study. The integration of these data improved the quality of the inverse food web reconstruction considerably, as demonstrated explicitly by an uncertainty analysis. Deposition of detritus (detrital pathway) from the water column and subsequent assimilation and respiration by bacteria and to a lesser extent by microbenthos (microbial pathway) dominated the food web. Secondary production was dominated by bacteria (600 mg C m −2 d −1 ), but transfer to higher trophic levels was limited to 9% and most bacterial carbon was recycled back to dissolved organic carbon (DOC) and detritus. Microbenthos secondary production (77 mg C m −2 d −1 ) was supported by DOC (73%) and detritus (26%) and was entirely transferred up the food web. The higher trophic levels consisting of nematodes, meiobenthos (copepods, ostracods and foraminifera) and macrobenthos fed highly selectively and relied primarily on microphytobenthos and pelagic primary production (herbivorous pathway). Deposit feeding is a common feeding mode among these sediment dwelling fauna, but detritivory was negligible due to this selective feeding. This strong resource selectivity implies that the herbivorous and detrital-microbial pathways function more or less autonomously, with limited interaction.

Ilse H a m e l s ', Koen S a b b e ', Koenraad M u y l a e r t ', Christiane B a r r a n g u e t 2-4, C atherine L u c a s \ Peter H e r m a n 4 & W im V y v e r m a n 1

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Observational and experimental studies have shown that phagotrophic ciliates are highly selective predators. However, little is as yet known about the actual mechanisms involved in prey selection. We used direct behavioral observations to study the mechanism of prey selection in benthic algivorous ciliates feeding on mixed assemblages of diatom species. Four ciliate species, viz. three Strombidium species and a Pseudochilodonopsis species, and three diatom species from intertidal sediments in the Schelde estuary were used for the experiments. In each experiment, a single ciliate species was offered a mixture of two diatom species. The feeding preferences of the ciliates were estimated as well as relative encounter rates, attack probabilities, and capture successes for the prey species. The feeding preferences of the ciliates were distinctly predator-specific and highly discerning with respect to the nature of the prey species. They appeared to result mainly from active selection at the time of the encounters and between the encounter and attack of the diatoms. Our observations suggest that selective encounters with the diatoms were caused by noncontact detection of individual prey items, at least for the Strombidium species. Additional experiments confirmed that these ciliates were able to distinguish between diatom species on the basis of soluble chemical cues. Grazing was also constant, i.e., prey switching was not observed. Moreover, neither total prey density nor feeding history influenced the preferences. We hypothesize that these highly specialized trophic interactions may be an important driving force in shaping structure and diversity of benthic diatom communities in intertidal sediments.