Deep-sea ecosystem response to bottom trawling impacts: changes in sediment bioturbation patterns

Abstract

Studies conducted in La Fonera (Palamós) submarine canyon (NW Mediterranean) evidence that trawling activities along the canyon flanks cause resuspension and transport of sediments towards the canyon axis. The analysis of a sediment core collected at 1750 m in 2002 showed a doubling of the sediment accumulation rate since the 1970s, coincident with the rapid industrialization of the local trawling fleet. This occurred along with a change from bioturbated texture to well-preserved sedimentary layering, interpreted as depositional pulses of trawling-induced sediment gravity flows. The good preservation of these structures was explained as a temporary hampering of biological mixing, resulting from suffocation of bioturbating fauna. The same canyon area was revisited a decade later (2011) and new data are consistent with the sedimentary regime shift during the 1970s, but also suggest that the accumulation rate during the last decade is even greater than expected, approaching ~2.4 cm/y (compared to ~0.25 cm/y pre-1970s). The X-radiograph image from the 2011 sediment core also indicates that bioturbation, if hampered in the 2002 core, has been reactivated later. Various ichnofabrics observable in the 2011 core testify to a diverse bioturbating fauna, including a scolitia resulting from the burrowing activity of an endobenthic echinoid. Remotely operated vehicle images collected along the canyon axis seafloor evidenced a large presence and densities of the burrowing sea urchin Brissopsis lyrifera. This species has also been found in high densities in the neighbor Blanes canyon axis, while it was absent from the adjacent open slope. However, historical data indicate that B. lyrifera was present in high numbers in the slope region in previous decades, suggesting that trawling activities have modified the distribution of this burrowing species and that the canyon environment is currently functioning as refuge area that offers a suitable environment for their proliferation