Variation in detrital enrichment causes spatio- temporal variation in soft-sediment assemblages

We investigated the importance of algal detritus in determining complex patterns of spatio-temporal variation in annelid assemblages in sites separated by 10s of meters on mudflats on Long Island, New York. We used field sampling to test the hypothesis that spatio-temporal variation in annelid assemblages contributes substantially more to total variation than consistent spatial differences among sites (pure spatial variation). While this pattern was generally shown by dominant surface-feeding annelids (e.g. Paranais litoralis and Streblospio benedicti), this was not the case for the deeper-feeding orbinid polychaete Leitoscolopolos robustus. We enriched the sediment with Ulva rotundata detrius to test the hypothesis that variation in detrital enrichment creates complex spatio-temporal patterns in annelid assemblages. Initially, the diversity and abundance of annelids was lower in enriched sites compared to untouched sites because of anoxic conditions at the sediment–water interface in the manipulated treatment. After about 1 mo, however, populations of opportunistic surface-feeding annelids (e.g. P. litoralis and Capitella capitata) increased in enriched sites to peak significantly higher than those in unmanipulated sediment, indicating resourcelimitation. The annelid assemblages in different treatments then converged in June/July when the excess detrital resources were sufficiently depleted. Overall, we demonstrate the important role that deposition of detritus plays in determining variation in soft-sediment communities. More than just simple spatial variation, however, we show that detritus enrichment can generate the complicated spatio-temporal patterns observed in natural annelid assemblages.