Trawling effects on biogeochemical processes are mediated by fauna in high-energy biogenic-reef-inhabited coastal sediments

Abstract

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 bottom trawl fishing on a coastal ecosystem dominated by the tube-building polychaete Lanice conchilega. Two specific gear types, both used to exploit North Sea sole (Solea solea), were compared: electric pulse trawls and tickler chain-rigged beam trawls. We detected a ∼1 cm bathymetric deepening after trawling associated with significant losses in benthic chlorophyll a caused by both fishing gears. 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, macrobenthos 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 habitats dominated by L. conchilega may be sensitive to relatively shallow sediment penetration from trawl gears and should be considered when assessing habitat vulnerability.