Deep-sea benthic ecosystem collapse and recovery after an intense Dense Shelf Water Cascading event

Abstract

Abstract. Submarine canyons of several regions of the world are preferential conduits for Dense Shelf Water Cascading (DSWC), which quickly modify physical and chemical conditions while transporting large amounts of material towards the adjacent deep margin. Observations conducted during the last 15 yr in the Cap de Creus Canyon (Gulf of Lion, NW Mediterranean Sea) reported several intense events of DSWC. Their effects on the deep-sea biodiversity and ecosystem functioning are almost unknown. To investigate the effects of these episodic events we analysed changes in the meiofaunal biodiversity inside and outside the canyon. Sediment samples were collected at depths varying from ca. 1000 to > 2100 m in May 2004 (before a major event), April 2005 (during a major cascading event) and in October 2005, August 2006 April 2008 and April 2009. We report here that the late winter-early spring 2005 cascading led to a reduction of the organic matter contents in canyon floor sediments down to 1800 m depth, whereas surface sediments at about 2200 m depth showed an increase. Our findings suggest that the nutritional material removed from the shallower continental shelf open slope and canyon was transported to the deep margin. During the cascading event the meiofaunal abundance and biodiversity in the studied deep-sea sediments dropped down by a factor of 5 to 10. Benthic assemblages in the impacted seafloor recovered to pre-cascading conditions after only six months from the cessation of the cascading. Since the present climate change is expected to increase the intensity and frequency of these episodic events, we anticipate that they will increasingly impact biodiversity and functioning of the benthic bathyal ecosystems, which may eventually challenge their resilience.