Major consequences of an intense dense shelf water cascading event on deep-sea benthic trophic conditions and meiofaunal biodiversity

Abstract

Numerous submarine canyons around the world are preferential conduits for episodic dense shelf water cascading (DSWC), which quickly modifies physical and chemical ambient conditions while transporting large amounts of material towards the base of slope and basin. Observations conducted during the last 20 yr in the Lacaze-Duthiers and Cap de Creus canyons (Gulf of Lion, NW Mediterranean Sea) report several intense DSWC events. The effects of DSWC on deep-sea ecosystems 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 (after a major event). 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, canyon floor and flanks, and also the adjacent open slope was rapidly transported to the deep margin. During the cascading event the meiofaunal abundance and biodiversity in the studied deep-sea sediments were significantly lower than after the event. Benthic assemblages during the cascading were significantly different from those in all other sampling periods in both the canyon and deep margin. After only six months from the cessation of the cascading, benthic assemblages in the impacted sediments were again similar to those observed in other sampling periods, thus illustrating a quick recovery. Since the present climate change is expected to increase the intensity and frequency of these episodic events, we anticipate that they will increasingly affect benthic bathyal ecosystems, which may eventually challenge their resilience

Deep-Sea Bioluminescence Blooms after Dense Water Formation at the Ocean Surface

“…Recent models [37], [38] based on the A2 IPCC scenario indicate a strong reduction in the convection intensity in the Mediterranean Sea for the end of the 21 st century, which will induce a massive reduction in organic matter supply and ventilation of the deep basin. Hence changes in the deep Mediterranean ecosystem more intense than those already observed in both the Eastern [39], [40] and the Western Mediterranean [27] basins are forecasted for the near future, a situation that could also occur but remain unnoticed in other sensitive areas of the world ocean. Our results illustrate the potentially far-reaching multidisciplinary scientific and societal benefits of the installation of cabled deep-sea observatories in critical ocean areas.…”

“…The newly-formed deep water (nWMDW) resulting from both dense shelf water cascading and open-sea convection has been observed to spread over the deep basin floor within months [22], [24], [25], [26]. Studies about the response of deep ecosystems to such processes are scarce and focus on the impact of dense shelf water cascading on benthic and epi-benthic organisms [27], [28]. Other recent works highlight how deep water formation triggers the resuspension of deep-sea sediments, including organic matter [21], and the development and spreading of a thick bottom layer loaded with resuspended particulate matter across the NW Mediterranean Basin as a result of dense shelf water cascading [29].…”

The Whittard Canyon – A case study of submarine canyon processes

“…9b). This lack of bathymetric pattern has been observed in several canyons, independent of geographical region or canyon-scale environmental conditions (Bianchelli et al, 2010;Ingels et al, 2013a;Romano et al, 2013;Leduc et al, 2014;Pusceddu et al, 2013) and is likely reminiscent of canyon heterogeneity and associated environmental variability exerting influence on benthic assemblages. More important are small-scale environmental conditions that act on the scale of meiofauna and nematodes, such as those associated with sediment grain size and sediment depth, or the amount and availability of food (Ingels et al, 2013b, Leduc et al, 2012, 2014b.…”

Is the meiofauna a good indicator for climate change and anthropogenic impacts?

“…An example of such events is the dense shelf water cascading (DSWC), particular buoyancy-driven currents causing dense water to rapidly sink into deep basins due to the excess density gained by cooling, evaporation and freezing in the surface layer (Shapiro et al 2003). A recent study in the Gulf of Lions (Pusceddu et al 2013) reported that the 2005 DSWC event resulted in lower abundance, biomass and richness of all meiofaunal groups compared with other periods (Pusceddu et al 2013). …”