Three-dimensional spatial structure: nematodes in a sandy tidal flat

Benthic infauna live in a three-dimensional world, and patterns of horizontal distribution may vary with depth in the sediment, as may the mechanisms that determine those patterns. This study examines how horizontal spatial structure varies with depth in a semi-exposed coarse-sand flat adjacent to St Martin's, one of the Isles of Scilly, UK. Nematode samples were collected from 3 sediment depth horizons (0 to 5, 5 to 10 and 10 to 15 cm) in a nested hierarchical design, with samples 0.1, 1 and 10 m apart. Results show that spatial patterns are strongest amongst samples of intermediate depth (5 to 11) cm), and generally insignificant amongst samples collected from 10 to 15 cm. There is short-order spatial autocorrelation in surface (0 to 5 cm) samples, with samples collected 10 cm apart being significantly more alike than samples collected further apart. We postulate that this is because spatial structure of nematodes in surface sediments is determined by patchiness induced by interactions between the water column and the physical structure of the surface layers of the sediment, which are constantly reworked by wind and tides. The deeper layers, 10 to 15 cm below the surface, are constantly reworked by a dense population of heart urchins Echinocardium cordatum. It is only in the intermediate horizon that relatively stable conditions allow nematode assemblages to establish spatial structure over a range of scales. It is concluded that the interaction between sediment permeability, and therefore interfacial advection, and bioturbation in controlling infaunal diversity and spatial patterns is potentially important.