Reply on RC1

Abstract

Dynamic, sandy environments are generally less vulnerable to mechanical stress compared to silty, low energy habitats. Biogenic reef communities, however, may provide an exception to this. This study explores the physical, biological and biogeochemical effects of electric pulse and tickler chain rigged beam trawls on a coastal ecosystem dominated by the tube building polychaete, Lanice conchilega. With a before-after-control-impact (BACI) in situ study, we detected a ~1 cm bathymetric deepening after trawling associated with significant losses in benthic chlorophyll a caused from both fishing gears. Trawled sediments exhibited physical mixing (DB) values similar to areas with higher bioturbation. Tickler chain trawls significantly reduced sediment oxygen consumption (57 %), total organic matter mineralization (56 %), denitrification (61 %), nitrification (60 %), and total benthos densities (52 %) while pulse trawls had no statistically significant impact on these parameters. Before trawling, significant relationships could be found between L. conchilega and very fine sand fractions, oxygen and nitrate fluxes, taxon densities and species richness, however, the trawl disturbances from both gears disrupted these connections. Our results suggest a reduced average effect for pulse compared to tickler chain beam trawls for several ecological and biogeochemical characteristics though their impact was still significant for L. conchilega and associated species. This study also suggests that faunal-mediated ecosystem functions in L. conchilega dominated habitats may be sensitive to relatively shallow sediment penetration from trawl gears and should be considered when assessing habitat vulnerability.