Testing the resilience of dead maerl infaunal assemblages to the experimental removal and re-lay of habitat

Abstract

An experimental trial to mitigate dredging impact was undertaken within Falmouth Harbour, UK, removing a surface layer of dead maerl for storage on a barge and allowing the channel to be deepened before re-laying the maerl. The resilience (resistance and recovery) of the habitat and faunal assemblage to this disturbance was assessed. Six sites each had 2 conditions - a manipulated treatment area where maerl (25 m2 plots, top 0.3 m) was removed, stored on a barge and re-laid by backhoe dredger and a control area - which were cored at 0 (before), 5 and 4 wk after re-lay. PERMANOVA was used to test for differences between condition and time using a 2-factor design. Results should not be extrapolated to live maerl habitats or to large, longlived fauna that may live within them. Following the mechanical disturbance, the maerl matrix structure was altered through loss of fine sediment from the lower half of cores (>10 cm). There was also a significant reduction in the number of taxa and abundance of infauna and a change in the assemblage composition. By Week 44, however, no such significant differences were evident, indicating that the infauna was in a state of recovery. The only response variable showing recovery was annelid biomass. The trial demonstrated that removing and re-laying the top 0.3 m of maerl habitat is technically feasible, and whilst some differences in the habitat structure following re-laying were evident, this did not affect the habitat quality enough to prevent recolonisation of infauna.