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

Numerous submarine canyons around the world are preferential conduits for episodic dense shelf water cas- cading (DSWC), which quickly modifies physical and chem- ical 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 in- vestigate 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 >2100m 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 (af- ter a major event). We report here that the late winter–early spring 2005 cascading led to a reduction of the organic mat- ter 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 re- moved from the shallower continental shelf, canyon floor and flanks, and also the adjacent open slope was rapidly trans- ported 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 ecosys- tems, which may eventually challenge their resilience

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

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

Persistent, depth-intensified mixing during the Western Mediterranean Transition's initial stages

Piñeiro, S., González-Pola, C., Fernández-Díaz, J. M., Naveira-Garabato, A. C., Sánchez-Leal, R., Puig, P., et al. (2021). Persistent, depth-intensified mixing during the Western Mediterranean Transition's initial stages. Journal of Geophysical Research: Oceans, 126, e2020JC016535. https://doi.org/10.1029/2020JC016535. © 2020. American Geophysical Union. All Rights Reserved.© 2020. American Geophysical Union. All Rights Reserved. Major deep-convection activity in the northwestern Mediterranean during winter 2005 triggered the formation of a complex anomalous deep-water structure that substantially modified the properties of the Western Mediterranean deep layers. Since then, evolution of this thermohaline structure, the so-called Western Mediterranean Transition (WMT), has been traced through a regularly sampled hydrographic deep station located on the outer continental slope of Minorca Island. A rapid erosion of the WMT's near-bottom thermohaline signal was observed during 2005–2007. The plausible interpretation of this as local bottom-intensified mixing motivates this study. Here, the evolution of the WMT structure through 2005–2007 is reproduced by means of a one-dimensional diffusion model including double-diffusive mixing that allows vertical variation of the background mixing coefficient and includes a source term to represent the lateral advection of deep-water injections from the convection area. Using an optimization algorithm, a best guess for the depth-dependent background mixing coefficient is obtained for the study period. WMT evolution during its initial stages is satisfactorily reproduced using this simple conceptual model, indicating that strong depth-intensified mixing (K ∞ (z) ≈ 22 × 10−4 m2 s−1; z ⪆ 1,400 dbar) is a valid explanation for the observations. Extensive hydrographic and current observations gathered over the continental slope of Minorca during winter 2018, the first deep-convective winter intensively sampled in the region, provide evidence of topographically localized enhanced mixing concurrent with newly formed dense waters flowing along-slope toward the Algerian sub-basin. This transport-related boundary mixing mechanism is suggested to be a plausible source of the water-mass transformations observed during the initial stages of the WMT off Minorca.CTM2014-54374-R. BES-2015-074316.Versión del editor3,17

Observations of open-ocean deep convection in the northwestern Mediterranean Sea: Seasonal and interannual variability of mixing and deep water masses for the 2007-2013 Period

We present here a unique oceanographic and meteorological data set focus on the deep convection processes. Our results are essentially based on in situ data (mooring, research vessel, glider, and profiling float) collected from a multiplatform and integrated monitoring system (MOOSE: Mediterranean Ocean Observing System on Environment), which monitored continuously the northwestern Mediterranean Sea since 2007, and in particular high‐frequency potential temperature, salinity, and current measurements from the mooring LION located within the convection region. From 2009 to 2013, the mixed layer depth reaches the seabed, at a depth of 2330m, in February. Then, the violent vertical mixing of the whole water column lasts between 9 and 12 days setting up the characteristics of the newly formed deep water. Each deep convection winter formed a new warmer and saltier “vintage” of deep water. These sudden inputs of salt and heat in the deep ocean are responsible for trends in salinity (3.3 ± 0.2 × 10−3/yr) and potential temperature (3.2 ± 0.5 × 10−3 C/yr) observed from 2009 to 2013 for the 600–2300 m layer. For the first time, the overlapping of the three “phases” of deep convection can be observed, with secondary vertical mixing events (2–4 days) after the beginning of the restratification phase, and the restratification/spreading phase still active at the beginning of the following deep convection event

Sediment transport along the Cap de Creus Canyon flank during a mild, wet winter

Cap de Creus Canyon (CCC) is known as a preferential conduit for particulate matter leaving the Gulf of Lion continental shelf towards the slope and the basin, particularly in winter when storms and dense shelf water cascading coalesce to enhance the seaward export of shelf waters. During the CASCADE (CAscading, Storm, Convection, Advection and Downwelling Events) cruise in March 2011, deployments of recording instruments within the canyon and vertical profiling of the water column properties were conducted to study with high spatial-temporal resolution the impact of such processes on particulate matter fluxes. In the context of the mild and wet 2010–2011 winter, no remarkable dense shelf water formation was observed. On the other hand, the experimental setup allowed for the study of the impact of E-SE storms on the hydrographical structure and the particulate matter fluxes in the CCC. The most remarkable feature in terms of sediment transport was a period of dominant E-SE winds from 12 to 16 March, including two moderate storms (maximum significant wave heights = 4.1–4.6 m). During this period, a plume of freshened, relatively cold and turbid water flowed at high speeds along the southern flank of the CCC in an approximate depth range of 150–350 m. The density of this water mass was lighter than the ambient water in the canyon, indicating that it did not cascade off-shelf and that it merely downwelled into the canyon forced by the strong cyclonic circulation induced over the shelf during the storms and by the subsequent accumulation of seawater along the coast. Suspended sediment load in this turbid intrusion recorded along the southern canyon flank oscillated between 10 and 50 mg L−1, and maximum currents speeds reached values up to 90 cm s−1. A rough estimation of 105 tons of sediment was transported through the canyon along its southern wall during a 3-day-long period of storm-induced downwelling. Following the veering of the wind direction (from SE to NW) on 16 March, downwelling ceased, currents inside the canyon reversed from down- to up-canyon, and the turbid shelf plume was evacuated from the canyon, most probably flowing along the southern canyon flank and being entrained by the general SW circulation after leaving the canyon confinement. This study highlights that remarkable sediment transport occurs in the CCC, and particularly along its southern flank, even during mild and wet winters, in absence of cascading and under limited external forcing. The sediment transport associated with eastern storms like the ones described in this paper tends to enter the canyon by its downstream flank, partially affecting the canyon head region. Sediment transport during these events is not constrained near the seafloor but distributed in a depth range of 200–300 m above the bottom. Our paper broadens the understanding of the complex set of atmosphere-driven sediment transport processes acting in this highly dynamic area of the northwestern Mediterranean Sea

Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea

Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies

Effects of temperature in juvenile seabass (Dicentrarchus labrax L.) biomarker responses and behaviour: implications for environmental monitoring

The effects of temperature on European seabass (Dicentrarchus labrax L.) juveniles were investigated using a 30-day bioassay carried out at 18 and 25 °C in laboratory conditions. A multiparameter approach was applied including fish swimming velocity and several biochemical parameters involved in important physiological functions. Fish exposed for four weeks to 25 °C showed a decreased swimming capacity, concomitant with increased oxidative stress (increased catalase and glutathione peroxidase activities) and damage (increased lipid peroxidation levels), increased activity of an enzyme involved in energy production through the aerobic pathway (isocitrate dehydrogenase) and increased activities of brain and muscle cholinesterases (neurotransmission) compared to fish kept at 18 °C. Globally, these findings indicate that basic functions, essential for juvenile seabass surviving and well performing in the wild, such as predation, predator avoidance, neurofunction and ability to face chemical stress may be compromised with increasing water temperature. This may be of particular concern if D. labrax recruitment phase in northwest European estuaries and coastal areas happens gradually inmore warm environments as a consequence of global warming. Considering that the selected endpoints are generally applied in monitoring studies with different species, these findings also highlight the need of more research, including interdisciplinary and multiparameter approaches, on the impacts of temperature on marine species, and stress the importance of considering scenarios of temperature increase in environmental monitoring and in marine ecological risk assessment

Diameter growth of few African rainforest species

This paper summarises the results of a study on the growth of 17 timber species that are commonly logged in African rainforest areas: aningre (Aningeria altissima), obeche (Triplochiton scleroxylon), bosse clair (Guarea cedrata), dabema (Piptadeniastrum africanum), dibetou (Lovoa trichilioides), iroko (Milicia excelsa), kosipo (Entandrophragma candollei), kotibe (Nesogordonia papaverifera), limba (Terminalia superba), longhi (Gambeya boukokoensis), moabi (Baillonella toxisperma), niangon (Heritiera utilis), okoume (Aucoumea klaineana), sapelli (Entandrophragma cylindricum), sipo (Entandrophragma utile), tali (Erythrophleum ivorense) and tiama (Entandrophragma angolense). The data are compared with the results of growth ring analyses carried out in Cameroon, the Central African Republic and Gabon. The data were collected in a series of sampling trips to measure trunk girths in tree plots (Cote d'Ivoire, Cameroon, Central African Republic, Ghana and Gabon). They will provide a basis for calculating rotations between potential successive logging operations within the framework of forest development projects