Factors controlling field distributions of benthic infauna remain poorly understood. Enoploides longispiculosus is a prominent predacious nematode in sandy sediments of the North Sea and adjacent estuaries. This study assesses the distribution of E. longispiculosus and of prey nematodes in an intertidal flat (the Paulina) relative to variation in sediment characteristics. Predator densities varied strongly, even between sites with only subtle differences in sediment composition. E. longispiculosus abundances were positively correlated with grain size, proportion of fine sand and emersion period during low tides, and negatively with silt, very fine sand content and prey densities. We then tested whether and how grain size, silt content and water content affected predation rates and prey selectivity in E. longispiculosus using microcosm experiments with 2 prey species. Each of these sediment characteristics per se strongly affected predation efficiency but not selectivity. Increases in silt fraction and decreases in grain size and water content strongly reduced predation rate. These effects compared exceptionally well with the range of silt contents and grain sizes in which E. longispiculosus is abundant on the Paulina tidal flat, suggesting that its field distribution is governed at least in part by sedimentary effects on foraging efficiency. Sediment water content also impacted predation rates; however, its effect did not follow a pattern predicted by field data on emersion time. Hence, even small shifts in sediment composition may strongly affect the activity of predacious nematodes, and both temporal and small-scale spatial variability in sediment characteristics likely affects predator-prey dynamics on tidal flats. |
