Small-scale spatial distribution of marine meiobenthos: the effects of decaying macrofauna.

To evaluate the effects of decaying animals on small-scale horizontal distribution of meiobenthos in muddy habitats, a laboratory experiment was performed at the Askö Laboratory in the northwestern Baltic Sea. A microcosm (35×55×28 cm) containing a ca. 7-cm thick layer of sieved (0.5 mm) sublittoral mud was established in June 1990. Three months later specimens of the bivalve Macoma balthica were collected and killed in boiling water. The sediment inside the microcosm was implanted with empty shell, empty shell and dead animal or left alone. At the end of the experiment (17 days) visual examination of the microcosm revealed black spots at the sediment surface where dead animals had been implanted. The densities of nematodes, the most abundant group (98%), were not significantly different between areas. However total non-nematode fauna was found in much lower numbers (P<0.01) in the black spot areas. A multivariate analysis (detrended correspondence ordination) of nematode species abundance data separated samples from the black spot areas from the others. Of the 25 nematode species recorded in the microcosm, there was a significant difference between areas for four species. The decaying animals clearly attracted Monhystera disjuncta which was almost 6 times as abundant in the black spot areas compared to control and shell areas. Both the overall dominant species, Leptolaimus elegans and Calomicrolaimus honestus were found in lower numbers in the areas of dead Macoma than in control and shell areas. Sabatieria pulchra was found in lower numbers in the control areas compared to shell and dead animal areas. The overall structure of the nematode assemblage indicated a shift to lower dominance in the dead animal areas and it is speculated that decomposing animal tissue may be of primary importance regarding spatial distribution of meiobenthos.