Effects of dredging disturbance on seagrass coverage, sediment composition and infaunal assemblages within a SW England Zostera marina bed

Abstract

A previously surveyed Zostera marina (L.) bed in South West England was mapped using SCUBA in September- October 2011 to assess changes in bed extent, environmental and faunal characteristics following disturbance from a scallop dredger in 2006. Surveys indicated a continued absence of seagrass within the dredged area, suggesting no recovery five years after the dredging damage occurred, but new growth was apparent at the shoreward edges of the bed. Within the surveyed area, five conditions were sampled: bare sand (BS), the dredge scar (DS), low density seagrass (LOW: 0-50 shoots 25cm-­­­­2), medium density (MED: 50-100) and high density (HIGH: >100). Seagrass and sediment parameters were measured within each condition, and core samples for infaunal invertebrates were obtained using SCUBA divers. Seagrass and epiphyte biomass increased with higher densities of seagrass, as did the abundance and diversity of associated infauna; univariate and multivariate analyses indicated significant differences in seagrass and epiphytic biomass, sediment parameters and infaunal assemblages between conditions. Whilst assemblages within the dredge scar had lower diversity and abundance than in dense seagrass, these measures were higher than in bare sand. The assemblage composition was more similar to seagrass conditions than bare sand, potentially due to fine sediment material present within the DS condition, indicating the existence of seagrass fauna within the scar. Continued disturbance of this site through anchoring may be reducing the rate of recovery, however, implying the need for protection and future management strategies for this seagrass bed.