Effects of sediment microtopography on small-scale spatial distributions of meiobenthic nematodes

Abstract

The influence of sediment microtopography on the small-scale spatial distribution of meiobenthic nematodes was investigated using time series statistical techniques to analyze data obtained from numerous small cores. Two transects, each 1 m long and comprised of contiguous 6 mm cores, were collected from an intertidal sand flat characterized by regularly spaced, asymmetrical sediment ripples of 8 cm wavelength. Autocorrelation coefficients and power spectral estimates showed a statistically significant periodicity at the wavelength of the sediment ripples in the fluctuation of total nematode abundance. Within each transect, several separate series of density oscillations were present, all with successive density peaks spaced 8 cm apart, but phase-shifted with respect to each other. Cross-correlation coefficients between nematode abundance and ripple height revealed that the highest densities of nematodes tend to be associated with ripple crests. These results indicate that nematodes can respond to small-scale environmental heterogeneity produced by sediment microtopography, although the exact mechanism by which this occurs remains unknown. A sequence of events extending over several tidal cycles probably generates this distribution pattern. Observations of stained sediments and fecal pellets showed that organic material accumulates in troughs at low tide and is subsequently buried by migrating ripple crests during the succeeding high tide. As this process is repeated, multiple series of organically enriched zones are laid down, all with the same spacing. Each series is displaced from the one laid down previously by a distance equal to the total movement of the ripples between their times of deposition. It is hypothesized that nematodes are attracted to this buried organic material.