Infaunal and biogeochemical response to sand capping: insights from a laboratory study

Managed realignment (MR) is a nature restoration approach, converting reclaimed areas threatened by sea-level rise into marine habitats. However, the ecological functioning of newly formed marine ecosystems remains poorly understood, particularly concerning the suitability of newly inundated soils to support marine life. This study bridges that knowledge gap by examining how two infaunal species are affected by sand capping of fine-grained and compact sediment after MR of Gyldensteen Coastal Lagoon, Denmark. Understanding these effects is crucial for successful MR outcomes. Our study focuses on the burrow-dwelling polychaetes, Hediste diversicolor and Marenzelleria viridis, to elucidate the interplay between sand capping, sediment properties, and polychaete behavior. Results showed that sand capping influenced the two polychaetes differently. Hediste diversicolor had higher recovery without sand capping, whereas M. viridis showed higher recovery with sand capping. Bioturbation analysis also revealed differences in burrow ventilation and bioirrigation between the two species. Hediste diversicolor caused higher net ventilation in uncapped than sand capped sediment, while the opposite was apparent for M. viridis, corresponding with their highest (83 and 50%) recovery of individuals. The bioirrigation coefficient (α) from non-local transport models similarly revealed that sand capping increased bioirrigation for M. viridis while inhibiting solute transport for H. diversicolor. Sand capping impacts benthic metabolism and biogeochemistry, with implications for species survival and ecosystem dynamics. Capping fine-grained sediment with coarse sand favors certain infaunal species while hampering others. While this study was of relatively short duration, further research is needed to understand the long-term consequences of sand capping.