Recolonization of defaunated sediments: Fine versus gross sand and dredging versus experimental trays

A field study using experimental trays and small-scale dredging was carried out to: (1) estimate the short-term recovery time of defaunated sediment, (2) evaluate the sediment grain size influence (fine versus gross sand) in the recolonization processes and (3) compare the recolonization in experimental dredged areas and experimental trays. A total of 48 trays (20 × 25 × 7 cm3) with defaunated fine and gross sand were used and six squares of 1 m2 were dredged. Spatial and temporal changes of macrobenthic communities in defaunated sediments and dredged areas were analysed using univariate and multivariate statistics, and compared with those in undisturbed natural sediment at the same site. Recolonization of macrobenthos occurred extremely fast indicating the importance of the bedload transport of juveniles and adults, otherwise necessary for many taxa which presented direct development. This quick recolonization of defaunated sediments could have important environmental implications since natural and anthropogenic perturbations may cause partial or total defaunation of marine sediments. Most of the species were able to recolonize both substrates (fine and gross sand) but some species showed clear preferences for one of the sediment types, and the gross sand supported a higher number of species and individuals than fine sand during the short-term recolonization. Multivariate analysis showed that dredged areas (1 m2) were recolonized in approximately 15 days, recovering similar biotic and abiotic characteristics to the control area (restoration). But in the case of experimental trays, although recovery of abundance and number of species was also quick (30 days), the community structure differed from the control and dredged area, mainly due to the massive recolonization of peracaridean crustaceans in the trays; newly available sediment in the experimental trays acted like patches and allowed more species to colonize than sediment pre-occupied by an established community.