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Ecosystem effects of dense waterformation on deep Mediterranean Seaecosystems: an overviewAntonio Pusceddu a , Marianna Mea a , Cristina Gambi a , SilviaBianchelli a , Miquel Canals b , Anna Sanchez-Vidal b , AntoniCalafat b , Serge Heussner c , Xavier Durrieu De Madron c , JéromeAvril c , Laurenz Thomsen d , Rosa Garcìa d e & Roberto Danovaro aa Dipartimento di Scienze del Mare, Università Politecnica delleMarche, Via Brecce Bianche, 60131 Ancona, Italyb GRC Geociències Marines, Departament d’Estratigrafia,Paleontologia i Geociències Marines, Universitat de Barcelona,E-08028 Barcelona, Spainc CEFREM, UMR 5110 CNRS-University of Perpignan, F-66860Perpignan Cedex, Franced Jacobs University Bremen, Campus Ring 1, 28759 Bremen,Germanye Department of Global Change Research, IMEDEA (CSIC-UIB)Instituto Mediterráneo de Estudios Avanzados, Miquel Marqués 21,07190 Esporles, Spain

Version of record first published: 05 Jul 2010

To cite this article: Antonio Pusceddu, Marianna Mea, Cristina Gambi, Silvia Bianchelli, MiquelCanals, Anna Sanchez-Vidal, Antoni Calafat, Serge Heussner, Xavier Durrieu De Madron, JéromeAvril, Laurenz Thomsen, Rosa Garcìa & Roberto Danovaro (2010): Ecosystem effects of dense waterformation on deep Mediterranean Sea ecosystems: an overview, Advances in Oceanography andLimnology, 1:1, 67-83

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Advances in Oceanography and LimnologyVol. 1, No. 1, June 2010, 67–83

Ecosystem effects of dense water formation on deep Mediterranean Sea

ecosystems: an overview

Antonio Pusceddua*, Marianna Meaa, Cristina Gambia, Silvia Bianchellia,Miquel Canalsb, Anna Sanchez-Vidalb, Antoni Calafatb, Serge Heussnerc,

Xavier Durrieu De Madronc, Jerome Avrilc, Laurenz Thomsend, Rosa Garcıade andRoberto Danovaroa

aDipartimento di Scienze del Mare, Universita Politecnica delle Marche, Via Brecce Bianche,60131 Ancona, Italy; bGRC Geociencies Marines, Departament d’Estratigrafia, Paleontologia iGeociencies Marines, Universitat de Barcelona, E-08028 Barcelona, Spain; cCEFREM, UMR5110 CNRS-University of Perpignan, F-66860 Perpignan Cedex, France; dJacobs UniversityBremen, Campus Ring 1, 28759 Bremen, Germany; eDepartment of Global Change Research,IMEDEA (CSIC-UIB) Instituto Mediterraneo de Estudios Avanzados, Miquel Marques 21,

07190 Esporles, Spain

(Received 1 February 2010; final version received 16 February 2010)

Natural episodic events, such as gravity flows, submarine landslides, and benthicstorms can determine severe modifications in the structure and functioning ofdeep-sea ecosystems. Here, we report and compare the ecosystem effectsproduced by dense water formation events that occurred in the Gulf of Lions(NW Mediterranean) and the Aegean Sea (NE Mediterranean). In both regions,the rapid sinking of cold dense waters, driven by regional meteorological forcings,results in important immediate modifications that can be summarised in: (i)increased organic matter content in the deep basin; (ii) diminished benthicabundance; and (iii) changes of benthic biodiversity. At longer time scale theanalysis reveals, however, different resilience times in the two regions. The Gulf ofLions is characterized by a very fast (months) recovery whereas the Aegean Seashows much longer (45 years) resilience time. New long-term studies are furtherneeded to identify the potential effects that changes in the duration, intensity andfrequency of episodic events could have on the structure, biodiversity andfunctioning of the deep Mediterranean Sea under environmental and climatechange scenarios.

Keywords: dense shelf water cascading; ecosystem functions; deep sea;Mediterranean Sea; climate change

1. Introduction

The Mediterranean Sea is a land-locked sea affected by strong interactions with thesurrounding lands. These interactions determine, on the one hand, important effects on thecoastal climate and, on the other one, impact the basin with matter and energy fluxesconveyed through river and coastal runoff down to its deepest part [1]. As such, theMediterranean Sea provides a case study for addressing the relationships between local

*Corresponding author. Email: a.pusceddu@univpm.it

ISSN 1947–5721 print/ISSN 1947–573X online

� 2010 Taylor & Francis

DOI: 10.1080/19475721003735765

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climate and coastal oceanography, and evolves as a miniature ocean in response to theeffects of climate and environmental change [2].

The Mediterranean Sea experienced, over the last 50 years, an exponential increase ofsevere environmental problems and injuries (including habitat destruction, overexploita-tion of renewable resources, eutrophication, contamination and pollution, introductionof allochthonous species, loss of seascape quality) [3]. These, in turn, entraineddeleterious ecological consequences that became increasingly evident since the end ofthe last century [4] and are expected to worsen in the future along with global and climatechange [5,6].

In the period 1982–2006, the Mediterranean Sea underwent a net increase of seasurface temperature of 0.71�C, which makes this sub-oceanic sea one of the Large MarineEcosystems strongly affected by present global warming [7]. While the causes of such anincrease have been identified (which include, inter alia, the nearby land warming andseveral heavily populated industrialized countries along its shores) [7,8], the ecologicalconsequences of sea warming have been only recently postulated. In some limited cases,warming has been recognised to be effective on marine biodiversity and to be the causativeagent of several signs of marine ecosystem degradation [5,6,9,10]. Moreover, it is expectedthat such a strong and fast warming of the Mediterranean Sea surface temperature willprovoke changes in the spatial and temporal patterns of physical-chemical characteristicsat the basin scale. For example, model outputs recently postulated that an enhancedsummer stratification of the Mediterranean Sea waters due to sea surface warming coulddiminish benthic biomass by 435% [5]. The lengthening of the summer stratificationperiod would lead to a reduction of the organic particles flux to the sea floor, which wouldresult in a decreased amount of food available for the benthos and in severe energeticconstraints to the whole benthic ecosystem functioning. Sea surface warming will likelyalter also other particle transport mechanisms, including lateral advection and dense waterformation and cascading.

Besides sole sedimentation, several physical diffusive and advective processesredistribute both dissolved and particulate organic matter from the surface layer tothe sea floor. Diffusive transfer through diapycnal mixing is unlikely to dominate withinthe upper ocean or explain the exchanges of elements between the euphotic zone and theunderlying water column [11]. These exchanges are primarily related to three-dimensional time-varying advective processes, such as open-ocean convection and denseshelf water cascading (Figure 1). The stratification of the upper water column plays animportant role in the intensity of the vertical advective exchanges. During summertime, theupper water column is strongly stratified, with marked thermocline and pycnocline thattend to isolate the surface water from the intermediate and deep water masses. Converselyduring wintertime, convective overturning and turbulent mixing contribute to theventilation of intermediate and deep waters of the oceans, which favour vertical transferof organic matter. The Mediterranean Sea is one of the rare oceanic regions around theworld where both open-ocean convection (OOC) and dense shelf water cascading (DSWC)take place (Figure 2).

OOC is the result of durable and local vertical mixing reaching the mesopelagic orbathypelagic layers. Following an autumn pre-conditioning phase allowing mixing in theupper layer of the ocean and destratification of the water column, a strong mixing phasetakes place during the winter period entraining surface water down to intermediate(51000m) or deep (41000m) levels and vice versa (Figure 1). Spreading of the newlyformed intermediate and deep water masses occurs during the spring re-stratification.

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Open-ocean deep convection is known to occur only in restricted areas of theMediterranean, Arctic (Labrador and Greenland Seas) and Antarctic regions (Weddelland Ross Seas), where retention of surface water by cyclonic circulation and exposition tobuoyancy loss and mixing from strong atmospheric forcing coexist [12,13].

Figure 1. Schematic diagram showing the cascading of dense shelf water, open-ocean convectionprocesses controlling the exchanges of water between surface (0–200m) and intermediate (0–1000m)or deep (41000m) layers. Buoyancy fluxes through the surface are represented by curly arrows.Isopycnals characterizing stratification/outcropping are delineated by continuous blue lines. Thecascading schema depicts the down-slope pathways of a dense shelf water plume escaping the bottomand following isopycnals at its equilibrium level or proceeding down the deep basin depending on thedensity contrast. A lateral compensating flow takes place at the surface and entrainment of ambientwater within the plume takes place along its track. The open ocean deep convection scheme depictsthe exchanges of dense surface water in the vertical due to the overturning of the upper part or entirewater column, and the lateral transfer by eddies.

Figure 2. Location of Mediterranean areas where dense shelf water cascades are observed.

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DSWC is a near-bottom gravity current, where the dense water, originating fromcooling, evaporation, freezing and salinization on the continental shelf overflow the shelfedge and flows along of the continental slope until its density reaches the ambient density(Figure 1). This current produces an irreversible exchange of shelf water and elements tothe mesopelagic or bathypelagic zones [14–17]. Extreme events can also transfer densewater down to the deep basin [1]. Cascading has been identified in about 70 sites aroundthe world [18,19]. It is found primarily in polar regions and mid-latitudes regions, andmore scarcely in tropical and subtropical regions. Submarine features, such as canyons andridges, clearly enhance cascading of dense water from the shelf to the deep ocean [20].

Submarine canyons, valleys deeply cut in the continental slope, dissect most ofEurope’s continental margins [21]. The topography and peculiar location of submarinecanyons with their head close to the shore makes them sites of intense exchanges betweenthe coastal area, the continental shelf and the adjacent continental margin and deep basin[1]. Submarine canyons can intercept and trap coastal sediment drifts, thus acting as maindrivers of local sediment transport and deposition and funnelling materials towards theadjacent deep basin [21–28]. Submarine canyons are generally more active in thedownward transfer of material than open slopes [29,30]. However, sediment transportalong canyons and across open slopes is neither constant nor unidirectional, and ratherresults in a pulse-like process. For example, cycles of sediment resuspension and transportin the Nazare canyon off Portugal have been seen to alternate with intervals of sedimentaccumulation on the canyon seabed [31]. One of the most dramatic evidences of thepulsation mechanisms that drive large amounts of sediment and organic matter (OM)during short-lived episodes (several weeks) is the cascading of dense shelf waters thatmakes canyons to behave as flushing conduits [1,28,30,32].

Waters overlying submarine canyons are often sites of enhanced productivity[21,29,33–39], so that the flushing of material through these canyons might have a greatinfluence on deep-sea biodiversity, food webs, ecosystem functioning and benthictrophodynamics [29,38–48].

Following forcing by cold northern winds in winter, DSWC occurs in three differentareas in the Mediterranean Sea [1]: the Gulf of Lions (forced by the Tramontana andMistral winds), the Adriatic Sea (forced by the Bora wind), and the Aegean Sea (forced bythe Etesian wind) (Figure 2). In all three regions, deep water formation has beendocumented using different and independent physical and geophysical approachesincluding water column and near-bottom measurements.

In this overview we summarize the available information and add some new data onthe ecosystemic effects of cascading, with the aim of comparing known and expectedmodifications in the Gulf of Lions and Cretan deep-sea areas, two regions experiencingdeep water formation events. We also pinpoint the gaps in knowledge to be addressed toprovide compelling future scenarios.

2. Effects of dense water formation on deep-sea ecosystem

2.1. The Gulf of Lions case study

To date, the effect of dense water formation on deep ecosystems in the Gulf of Lions in theNW Mediterranean Sea, has been only studied/described for DSWC. The Gulf of Lionspresents the highest density of submarine canyons in the entire basin, some of whichextending for more than 100 km, cutting the entire continental slope and reaching depths

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deeper than 2000m [1,49]. Hydrological measurements collected since 1950 in the Lacaze-

Duthiers canyon, which is located at the SW outlet of the crescent-shaped shelf, revealed

the spreading of dense shelf water plumes [1,27,32,50] that reached equilibrium depths

between 170 and 800m depth. Continuous temperature measurements carried out since

1993 in the same canyon revealed that DSWC reached 500m depth almost every winter.

Moreover, in 1979–1980, 1987–1988, 1998–1999 and, later, during winters 2004–2005 and

2005–2006, particularly intense DSWC occurred triggered by abnormally cold, strong and

persistent winds, and lower than average freshwater inputs from rivers. During these years

of intense cascading dense waters flowing down-slope over the bottom reach the lower

continental slope and basin at depths in excess of 2000m [32]. These results highlight that

DSWC in the Gulf of Lions is an annually recurrent phenomenon that peaks at decadal to

sub-decadal time scales. In the last years several studies conducted in this region aimed at

identifying the consequences of cascading on the sediment and OM transported to the

deep sea during these episodic though recurrent episodes [30,50]. It is worthy to note that

strong open-ocean convection took place concomitantly to the extreme DSWC events that

occurred in 1998–1999, 2004–2005 and 2005–2006.Several signals of consequent ecosystem change were documented in the south

westernmost canyons (Lacaze-Duthiers and Cap de Creus) of the Gulf of Lions during the

major flushing events observed in 1998–1999, 2004–2005 and 2005–2006: a rapid drop in

deep water temperature, the raise of down-canyon currents (peaks up to 1m s–1), the

concurrent increase of bottom water turbidity, and a dramatic increase in downward

fluxes of material exported from the shelf and upper slope [32]. These events were able to

resuspend and transport several million tons of fine sedimentary particles down canyon (a

mass comparable to the mean annual solid transport of all rivers opening into the Gulf of

Lions) [1,51–53]. During the cascading, both fine and coarse shelf and upper canyon

sediments contributed to the mass flux, whereas advection of fine material via nepheloid

layers dominated down-slope fluxes during pre- and post-cascading. The resulting change

in grain-size affected the flux of mineral-bound terrigenous organic carbon (OC),

indicating that the down-canyon transport of land-derived OM did not occur as bulk but

rather its composition is driven by sediment sorting associated with different transport

mechanisms [53]. While export of degraded sedimentary OM dominated during the early

stage of DSWC, export of more labile marine OC took place during the last stage of

DSWC due to the phytoplankton bloom that occurred in late winter on the shelf [32,50].

Hence, the significant export of modern marine organic C observed in the canyons after

the pulsed input of terrigenous OC, suggests that the off-shelf marine export during the

cascading season refuels the adjacent deep-sea basin with fresh OM [50] (Figure 3).The analysis of the quantity and biochemical composition of OM in the sediments of

the Cap de Creus canyon before (May 2004) and after (April 2005) the 2004–2005 DSWC

revealed that the cascading event entrained a significant impoverishment of the organic

content of canyon floor sediments between 1000 and 1900m depth, in accordance with the

measurements of downward fluxes (Figure 4) [21,28,32]. Over the same time span, deep-

sea sediments at 2200m depth in the Sete Canyon off the confluence of the Cap de Creus

and the Lacaze-Duthiers canyons showed a significant increase in OC concentrations.

Although these data do not demonstrate a direct flushing of OM, this observation fits with

the observed decrease in concentration in the upper part of the canyon. These results

suggest that DSWC events can represent one of the most important processes fuelling the

deep sea with fresh resources that are able to sustain high levels of ecosystem functioning,

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Figure 3. Mooring time series of the 2005–2006 DSWC event. (a) Temporal evolution of near-bottom in situ temperature and (b) Current speed at the Cap de Creus canyon axis at 300m of waterdepth. (c) Boxplot of near bottom organic carbon (OC) flux recorded by sediment traps deployedalong the axis of the Cap de Creus and Lacaze-Duthiers canyons and the adjacent open slope. Thecaps at the end of each box indicate the extreme values, the box is defined by the lower and upperquartiles, and the line in the center of the box is the median. (d) Boxplot of marine organic carbon(OCMAR) percentage obtained following a mixing model at the same stations, and sea surface (Chl a)concentration 1 km off Banyuls sur Mer (42.48�N, 3.13�E). The negative temperature anomaly is thesign of the occurrence of a major DSWC event, and the concomitant maximal OC fluxes are thedirect evidence of the capacity of the strong cascading currents to carry and transport massiveamounts of material of different origin (i.e. terrestrial and marine) hundreds of kilometres offshorein a few days. Note the concurrence of the second pulse of cascading waters with bloom conditions insurface waters, which refuels the deep sea. Modified from [50] and [85].

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Figure 4. Concentrations of biopolymeric C in the sediment (top first cm) of the Cap de Creuscanyon in May 2004 (DSWC absent) and April 2005 (DSWC present). Biopolymeric C is the sum ofprotein, carbohydrate and lipid contents in the sediment [86].

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which are over imposed to the seasonally modulated background downward flux ofsinking organic particles from the productive upper water column [54].

Studies addressing the benthic faunal response to DSWC are scant and insufficient todraw general conclusions. An open question remains whether these episodic events,especially those of high intensity, have positive or negative effects on the benthos, byfuelling the deep sea floor with large amounts of bioavailable particles or by disrupting thebenthic habitats, respectively. Results from a multidisciplinary investigation carried out inthe Gulf of Lions in the framework of the EU funded HERMES (Hotspot EcosystemResearch on Europe’s Deep-Ocean Margins) and HERMIONE (Hotspot EcosystemResearch and Man’s Impact on European Seas) projects revealed that meiofaunalabundance in canyon sediments during the 2004–2005 DSWC event was up to one order ofmagnitude lower than during periods characterised by the lack of dense shelf waterformation (Figure 5). However, whether meiofauna almost disappeared due to cascading(i.e. flushed away entrained with seafloor erosion under the effect of sediment-ladenturbulent flows) or was diluted by the ‘azoic’ sediments brought in by the cascadingremains still unclear. In a recent study conducted in the Catalan Sea, west of the Gulf ofLions, it has been shown that DSWC can cause the collapse of the catches of the highlypraised deep-sea shrimp Aristeus antennatus. But it also showed that the population of thisspecies was able to recover in a relatively short period (around 3 years) [47]. This processcould be interpreted as a disturbance event with regenerative properties of renewableresources, in a way similar to what has been reported for fires in forests [55]. Themechanisms behind such a ‘crush-and-rise’ dynamics of fisheries after DSWC couldinclude altered prey–predator relationships, selective elimination of adults vs. juveniles orlarvae and increased availability of particulate food, but no direct evidence has beenprovided to date [1,47].

Whatever the direction of DSWC effects on the structure and functioning of the deep-sea region off the Gulf of Lions and nearby areas is, new studies are needed to identify thepotential effects that changes in the intensity and frequency of these episodic events

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Figure 5. Changes in abundance of metazoan meiofauna in sediments from the Cap de CreusCanyon in periods characterised by the presence (April 2005) and absence (average� standard errorof measurements carried out in October 2005 and August 2006) of DSWC.

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(both DSWC and OOC), as expected under the most recent scenarios of environmentalchange [56,57], will have on deep Mediterranean Sea ecosystems at the basin scale. If, asexpected, the frequency and intensity of deep water formation events will change as aresult of local climate change and altered water column structure [53,57,58], unpredictableand wide fluctuations in the population dynamics of species commercially relevant to thelocal fisheries could be expected, with potential consequences on the local coastaleconomy.

2.2. The Cretan Sea case study

Observations of deep-sea population responses to climate change have been spatiallysparse and/or temporally discontinuous till the end of last century. However, recentobservations highlighted that, contrarily to what was previously thought, deep-seaecosystems can respond rapidly, even to minor changes in deep water characteristics[59,60]. One of the most striking and well-documented local climate change observed in theMediterranean Sea is the so-called Eastern Mediterranean Transient (EMT), whichdetermined drastic changes in the thermohaline circulation of the Eastern MediterraneanSea [61,62]. Changes in the deep waters in the Eastern Mediterranean Sea occurred in twodistinct phases [2,61]: during the first phase, between 1987 and 1992, there was a massiveformation of dense and saltier water in the south Aegean Sea (the so-called Cretan DeepWater); during the second phase, from 1992 to 1994, a drop in deep-water temperature ofabout 0.4�C resulted in even denser deep water being formed [2,63,64]. During this secondphase, the old eastern Mediterranean Deep and Bottom Waters were uplifted by severalhundred metres [2,63] and formed a distinct nutrient-rich intermediate-water layer (theTransitional Mediterranean Water) [63,65], which, under the influence of cycloniccirculation, reached shallower depths (100–150m; i.e. close to the euphotic zone)[63,65,66]. These nutrient-enriched waters stimulated primary production which increased3-fold from the values in early 1980s (20–25 gCm–2 year–1) to the 1994–1995 season[67,68], when primary production reached values comparable with those typically observedin mesotrophic environments (i.e. 60–80 gCm�2 year�1) [59]. Rising primary productionwas also associated with remarkable changes in the composition of phytoplanktonassemblages, which were characterised by an increasing average phytoplankton cell size byfrom two to five times [59,68]. Increased biological production in the upper part of thewater column and the increased phytoplankton cell size concurred along with thecascading of dense and cold waters to enhance vertical fluxes of phytodetritus and OC todeep-sea sediments, which, in turn, resulted in the accumulation of OC and labile organiccompounds (e.g. proteins) in deep-sea sediments (Figure 5) [59]. Despite the increase infood availability for benthic consumers, the abundance of benthic prokaryotes andmeiofauna dropped down, as a result of the prolonged exposure to cold waters producedafter the EMT event (Figure 6). Several years after the EMT, the modified conditions inthe deep-sea sediments of the Eastern Mediterranean Sea were still partly persisting [59].Another striking effect observed after the EMT event was the rise in benthic biodiversity,which was however associated with a simplification of benthic food webs and the increaseof similarity of species assemblages with the colder deep-Atlantic faunas (Figure 7) [60]. Aschematic representation of the complex ecosystem response to cold waters cascadingoccurred during the EMT in the deep-sea ecosystem of the Eastern Mediterranean isillustrated in Figure 8.

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2.3. Comparison of dense water cascading events, future scenarios and perspectives

The episodic events of deep water formation that occurred in the Gulf of Lions and the

Cretan Sea, derived from the cooling of surface waters forced by atmospheric conditions.But one originated from the continental shelf (in the Gulf of Lions) and the other from off-

shore waters (the Cretan Sea). In spite of such a different origin of dense waters, thecomparison of ecosystem effects in the two different regions of the Mediterranean Seahighlights that the formation and cascading of dense cold waters produced important

effects on the properties of deep-sea ecosystems flushed by these episodic events. In bothcases the sinking of cold dense water resulted in increased inputs of OM to the deep seaand its accumulation on the seafloor. In the Catalan Sea and Gulf of Lions the increased

OM inputs were produced by an accelerated downward transport of organic-enrichedparticles (resuspended sediments plus sequestered production) flushed from the

continental shelf and the upper continental slope including canyon heads. In theEastern Mediterranean Sea the increased inputs were produced indirectly by the increasedavailability of nutrients through bottom waters moved and lifted by the cascading of dense

and cold superficial waters. These differently originated OM inputs, however, exertedsimilar consequences in terms of accumulation of OM in the deepest part of the two

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Figure 6. Changes in the quantity and biochemical composition of sedimentary organic matter(OM) from 1989 to 1998 measured in deep-sea sediments of the Cretan Sea. Illustrated are organiccarbon (OC) C and protein concentrations from January 1989 to September 1998. Bars indicatestandard deviations. Data are extracted from [59].

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geographical areas and evident and remarkable drops in benthic abundance and biomass.However, after the DSWC in the Gulf of Lions, the abrupt changes in benthic biomass andfisheries recovered in a relatively short period from months (e.g. for meiofaunalabundance; Figure 5) up to a few years (for fisheries) [47]. On the other end, benthicbiomass in the deep Cretan Sea did not recover to pre-event values for more than five years[59,60]. Such a difference, on the one hand, suggests that the ecological responses to thetwo events, similar in shape but lasting for different periods, might have differentialconsequences on the overall ecosystem organization and functioning of the interestedbasins. On the other hand, the different duration of these episodic events could also bedriven by or associated with the different origin of the dense waters, with a shorter lastingin the case of shelf waters (as in the Gulf of Lions) and a longer one for off-shore waters(as in the Aegean Sea). It is also to be noticed that dense waters formed in the WesternMediterranean Sea are easily exported to the Atlantic Ocean as part of the MediterraneanOutflow, and also that surface waters that sank down because of density increase areefficiently replaced by the entering Modified Atlantic Water.

Whatever the origin (shelf vs. offshore waters), the intensity and duration of deep waterformation events, our overview pinpoints that these events have the potential to function:(i) as a major engine in the formation of deep Mediterranean waters [1]; (ii) as conveyorbelts of food from the surface to the sea bottom [50]; and (iii) as stochastic events of

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Figure 7. Abundance of benthic bacteria and meiofauna in the sediments of the Cretan Sea from1989 to 1998. Bars indicate standard deviations. Data are extracted from [60].

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disturbance of deep-sea ecosystems and of spatial and temporal patterns of deep-seabiodiversity [60]. Although hydrological changes of the bottom water associated to thedeep water formation in the Gulf of Lions have been seen to spread over the entire deepWestern Mediterranean Sea [69,70], how and whether the consequent ecosystem changesapply only at the local scale or have important consequences at the whole Mediterraneanbasin scale is still mostly unknown and under investigation in the framework of severalresearch EU-funded projects such as HERMIONE and SESAME (Southern EuropeanSeas: Assessing and modelling ecosystem changes). In spite of these uncertainties, somepotential consequences of DSWC on the deep Mediterranean ecosystems, based onpredictions of climate change that will affect this ‘miniature’ ocean, can be hypothesized.

Atmospheric forcings that are behind the deep water formation events couldsignificantly vary along with climate change [56,57]. In fact, evidences of marineecosystem shifts associated with local and/or climate change are increasingly accumulating[58]. Predictions based on a IPCC-A2 scenario, with sea surface and deep water warming

Sept 1989

1994-95 Season

Sept 1998

11%

21%

Sept 1997

32%

33%

Pre-EMT season

Deep water cooling

25%

Sept 1989

1994-95 Season

Deep

Atlantic

Ocean

Sept 1998

Re-warming

Sept 1997

2%

16%

13%

11%

Figure 8. Changes in the similarity between meiofaunal benthic assemblages before, during (coolingphase) and after (warming phase) the EMT in the Eastern Mediterranean Sea (A) and similarity withAtlantic ocean communities (B). Panels illustrate the outputs of nMDS scaling produced usingspecies/abundance lists of Nematodes (the most abundant meiofaunal taxa in the sediments of theEastern Mediterranean Sea). Percentage values refer to the similarity between assemblages indifferent periods or oceanic regions.

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and saltening (by þ3.1�C and þ0.48 psu and þ1.5�C and þ0.23 psu on a 100-yearaverage), indicate that the Mediterranean thermohaline circulation could be stronglyweakened as the result of a decrease of surface density (by an estimated 40%) and anoticeable decrease (by 80%) in deep-water formation. Such a scenario would dramaticallyreduce the intensity and depth penetration of the DSWC and OOC.

The hypothesis that a loss of biodiversity might threaten ecosystem functioning [71] hasstimulated deep-sea research on the relationship between biodiversity and ecosystemfunctioning [72,73]. There is a wide consensus that marine ecosystems are experiencingimpacts that can directly and indirectly cause alterations of biodiversity, structure andorganization of marine ecosystems [74–79]. The loss of biodiversity might have differentconsequences in different ecosystem types, being able to impair the sustainable functioningof some ecosystems, while having null or idiosyncratic effects in others [80,81]. However, arecent study carried out along the European continental margins demonstrated that indeep-sea ecosystems even a minor loss of biodiversity could result in a major ecosystemcollapse [82], with a predictable impairing of ecosystem functioning and decreasedavailability of goods and services, including fisheries [58,83,84]. Whether and to whichextent the relationships between biodiversity and ecosystem functioning in the deep sea aremodulated or controlled by atmospheric forcing and climate change and/or fluctuations inthe frequency, intensity and duration of dense water cascading is still matter ofinvestigation.

Sed

imen

tD

eep

wat

ers

Pho

tyc

laye

r

Direct effects

Surface watersinking

Deep-watercooling

Decreasedbenthic biomass

and diversity

Reduced organic matterconsumption and mineralization

Decreasedprokaryote

biomass and activity

Increasedorganic matter

content

Indirect effects

Increased primary production

Increased verticalfluxes

Nutrient uplift

Increased biodiversityand decreased functional

diversity

Figure 9. Schematic representation of deep-sea ecosystem responses to cold water cascading duringthe EMT event in the Eastern Mediterranean (modified after [59] and [60]).

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Long-term studies are obviously needed to comprehend the ecosystem responses toshort, episodic or extreme events. The kind of results presented here represents a basis toidentify the future effects of climate-induced disturbances on deep-sea ecosystemfunctioning.

Acknowledgements

This work has been financially supported by the EU-funded integrated projects HERMES,HERMIONE and SESAME, the national projects VECTOR (FIRB 2007, Italy), OBAMA(PRIN2008, Italy). PROMETEO (CTM2007-66316-C02-01/MAR, Spain), GRACCIE (CSD2007-00067, Consolider Ingenio 2010 Program, Spain) and REDECO (CTM2008-04973-E, Spain). UBacknowledges a Generalitat de Catalunya ‘Grups de Recerca Consolidats’ grant (2009 SGR-1305).

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