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

Abstract

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 resource-limitation. 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.