Effects of trawling disturbances on temporal and spatial structure of benthic soft-sediment assemblages in Gullmarsfjorden, Sweden.

Hypotheses on the effects of shrimp trawling on large benthic macrofauna in a previously protected Swedish fjord were tested in a manipulative experiment. Three trawled sites and three untrawled (control) sites were sampled 1–4 months before, and 8–12 months after, experimental trawling on a weekly basis. Multivariate analyses indicate large temporal changes in assemblages of benthic fauna at both types of sites. The Bray-Curtis dissimilarity measure was used to test the hypothesis that changes in assemblages through time at trawled sites were different from those at untrawled sites. Although changes in average assemblages (centroids) from the start to the end of the experiment were larger at trawled sites, there were marked differences among sites, and differences between trawled and untrawled sites were not significant. There were, however, differences in temporal and spatial variability in structure of benthic assemblages. Variability at untrawled sites tended to be smaller. Thus, spatial and temporal variability in the structure of assemblages after one year of trawling was larger at the trawled sites than at the untrawled sites. Trawling with this particular type of gear at the experimental frequency for approximately one year appears to cause relatively subtle changes in the overall structure of assemblages of large macrofauna compared with changes caused by other factors. Furthermore, the results suggest that tests of hypotheses of changed patterns of variability may be sensitive to detecting effects of impacts of disturbance from trawling. [ABSTRACT FROM PUBLISHER]