In addition, mesoscale dynamics have also been shown to strongly modulate the phytoplankton phenology. The first ocean color images over theLigurianSeainthe early 1980s have revealed phytoplankton heterogeneity at scales similar to mesoscale eddies over this area during winter and spring [MOR 91a], suggesting a strong impact by mesoscale dynamics. The region is indeed the location of intense mesoscale activity, which originates primarily from the baroclinic instability of the current surrounding the deep-convection area. One of the first biogeochemical modeling studies conducted at mesoscale resolution aimed at understanding the physical processes involved in this physical control [LÉV 00, LÉV 98b, LÉV 99]. The model, although highly idealized, was able to reproduce the spatial heterogeneity inthe onset of the bloom, which started from the border of the convective area before amplifying in its core. Interestingly, the onset of the bloom around the convective area occurred a few weeks in advance of seasonal stratification. This was due to the restratificaying action of mesoscale eddies that acted to flatten the isopycnals around the convective area, preventing vertical mixing and enabling an early bloom there. A decade later, this process was very elegantly confirmed by in situ observations inthe North Atlantic [MAH 12]. Another impact of the mesoscale and sub-mesoscale dynamics is through the associated frontal vertical velocities, which constitute an efficient pathway to supply nutrients to the euphotic layer. Model results suggest that this process constitutes a year-long source of new nutrients to the euphotic layer that adds up to the
Fig. 1. Transect of the BIOSOPE cruise from the Marquesas Islands to Chile. Long-term process stations are indicated in red. Numbers indicates short-term stations, for which only numbers have been indicated to simplify presentation, and not the complete code as in Table 3. For instance 1 is STB1 and 21 is STA21.
stable water body (Claustre and Maritonera, 2003). To date, however, no investigation on the biogeochemistry of this wa- ter body has taken place. The aim of the BIOSOPE (Bio- geochemistry and Optics South Pacific Experiment) project was to conduct a pluridisciplinary exploration of this gyre as well as their eastern (Chilean coastal upwelling) and western (Marquesas plateau) borders, allowing the examination of a very large range of trophic conditions. Hyperoligotrophic conditions were observed at the centre of the gyre, with the clearest natural waters ever described (Morel et al., 2007), and a deep chlorophyll maximum reaching 180 m (Ras et al., 2007). The aim of the present study was to determine the abundance and activity of heterotrophic bacteria across the South Pacific Ocean, and to relate bacterial heterotrophic ac- tivity to phytoplankton primaryproduction. We further dis- cuss the techniques involved for determining the coupling be- tween primary and bacterial heterotrophic production.
tions of the environmental factors that contribute to determine the phytoplankton assemblage ( Stenuite et al., 2007 ).
A key feature of Lake Kivu, already pointed out by Spigel and Coulter (1996) , is the weak thermal gradient inthe mixolimnion, due to its location at high altitude, where air temperature is lower than inthe re- gions of the other East African great lakes. Then the density gradient in surface waters is always weak ( Schmid and Wüest, 2012 ). Strati ﬁcation conditions are therefore not constant during the rainy season and short mixing events may alternate with episodes of strati ﬁcation ( Fig. 2 C). This leads to short periods of phytoplankton development out of the dry season, as observed by example during the rainy season 2006. On average, half of the annual primaryproduction occurred nevertheless during the 4 months of the dry season. But exceptions to this general pattern are not rare: dry season phytoplankton peaks were not ob- served during 3 years (2002, 2005 and 2007) out of 7 ( Fig. 2 ). Conse- quently, the absence of the dry season peak directly heavily impacted the mean annual primaryproduction. For example, the absence of a complete mixing within the mixolimnion during the dry season 2007 led to a low annual primaryproduction (143 g C m −2 y −1 ) compared to the situation in 2008 where a complete overturn of the mixolimnion was observed during the dry season, producing a high phytoplankton biomass over the whole mixed layer and an annual production of 278 g C m −2 y −1 .
The special issue presents a collection of papers dealing with these different themes. The issue is introduced by a review paper written by John Marra (2015 –in this issue) . His review highlights the progress that has been made inthe knowledge of Ocean Productivity since the ﬁrst Liège colloquium, 45 years ago. He emphasizes that the conclusions on net heterotrophy or net autotrophy of the open ocean are strongly dependent on the estimation of the depth of the euphotic zone, a deeper productive layer (compared to what is traditionally assumed) would fa- vour net autotrophy. On the other hand, the paper points out that the quantity of ﬁxed carbon that gets diverted to the dissolved pool (i.e., dis- solved primaryproduction) could have been underestimated inthe past, complicating the view of the metabolic balance inthe ocean. Merlivat et al. (2015 –in this issue) ; Shadwick et al. (2015 –in this issue) and Stanley et al. (2015 –in this issue) estimated the net commu- nity production (NCP) using mass balance approaches of several tracers in different marine environments. Merlivat et al. (2015 –in this issue)
Our measurements of bacterial production, though among the lowest reported for the
open ocean – excluding high latitude, cold waters-, clearly do not represent minimum values. If bacterial activity is similar among open ocean oligotrophic environments, is this also the case for primaryproduction? A comparison among studies is not simple due to differences inthe incubation conditions. Our IPPdeck values were generally lower than those obtained by “standard” in situ incubations (IPP in situ , Table 2). It is well known
[ 15 ] New sterile 260-mL plastic bottles were filled gently using a new silicone tube washed with 0.5 N HCl and rinsed with double-distilled water. The tracer solution was prepared according to the procedure recommended by Fitzwater et al. . Na 2 CO 3 was added to double- distilled water up to pH 10. 14 C-NaHCO 3 was diluted in this water and immediately frozen in Teflon bottles. Before each experiment, one frozen aliquot of tracer solution was thawed to ambient temperature, and 0.5 or 1 mL (5 to 10 mCi of 14 C) was added to each experimental bottle. For each experiment, one to several samples were inoculated with 14 C solution and immediately filtered to determine abiotic particulate 14 C incorporation. Thein situ array was launched before sunrise, and picked up at about 1800. One series of replicate samples was immediately filtered, giving daytime primaryproduction values. The other series was placed for the night in a shipboard incubator to obtain net production per day. After incubation, samples were col- lected on 25-mm GF/F filters using a vacuum pressure <50 hPa. Duplicate samples were fractionated onto 3-mm Nuclepore filters following the same procedure as that for pigments. At the end of filtration, filters were immediately rinsed with filtered seawater, taking the recommendations of Goldman and Dennett  into account, and placed in 7-mL scintillation glass picovials. After addition of 100 mL of 0.5 N HCl, the vials were dried for 24 hours at 50C before 5 mL of Ultima Gold scintillation liquid were added to each vial. 14 C-CO 2 uptake was determined on board by using 6-min counts on a liquid scintillation counter Packard model TRI-CARB 1600-TR with a quenching correction by a 14 C internal standard. Several series of samples were counted a second time after a 24-hour delay, showing no significant difference from the first counts. For each experiment, total activity introduced into incubation samples was measured by counting several 50-mL replicates of the tracer solu- tion in vials containing 5 mL of Instagel and 50 mL of Carbo-Sorb. Since insignificant variability of 14 C activity was detected among experiments (cv = 1.3% on 102 analyses), we used a mean value for total activity intheproduction calculations.
[ 39 ] As discussed before, the HE 1 period has been chosen to test the effects of increased insolation and decreased ice sheet influence on productivity, just after the Last Glacial Maximum. At high latitudes, light availability inthe ocean surface is very sensitive to seasonal effects, constituting one of the limitations for phytoplankton blooms [Sverdrup, 1953]. During MIS2, light was reduced by lower summer insolation and higher sea ice cover conditions. At the oppo- site HE 1 lies on the onset of deglaciation that coincides with a maximum in obliquity and a period of increasing summer insolation at high northern latitudes, peaking between 12 and 15 ka B.P. [Berger, 1978]. Productivity is minimum in all the northern cores at the end of the Last Glacial Maximum, and tends to increase during deglaciation in parallel with the increase in insolation, the HE 1 drop being more or less marked depending on the cores. At the opposite, south of about 50°N, within the Ruddiman belt, HE 1 presents a well marked minimum in productivity, and its increase occurs only at the end of the event. It is tempting to attribute this latitudinal difference to the changes inthe relative impor- tance of insolation versus surface hydrology to control primary productivity, insolation change playing its major role only at high northern latitudes, where light is limited or absent for a large part of the year.
of a food chain. This becomes a reliable indication to the animal trophic position ( De Niro and Epstein, 1981 ).
Isotopic signatures have also recently been used to trace the contaminants transfer inthe food chain ( Hobson et al., 2002; Borrell et al., 2006 ). Yet, few studies have focused on heavy metals ( Cabana and Rasmussen, 1994; Das et al., 2003a; Das et al., 2004 ). In this work, metal concentrations (total and organic mercury, cadmium, lead, copper, iron, manganese, selenium and zinc) and stable nitrogen and carbon signature were measured inthe tissues and organs of six different ceta- cean species from theLigurianSea: 2 fin whales (B. physalus, Linnaeus, 1758), 1 sperm whale (P. macrocephalus, Linnaeus, 1821), 3 Risso's dolphins (G. griseus,Cuvier, 1812), 3 striped dolphins (S. coeruleoalba, Meyen, 1833), 2 bottlenose dolphins (T. truncatus, Montagu, 1821) and 1 Cuvier's beaked whale (Z. cavirostris, Cuvier, 1823). These animals stranded inthe “Cetaceans' Sanctuary of the Mediterranean Sea” along theLiguriansea, or were found dead offshore during 1990 –2004. The small number of specimens available in this kind of studies, which are sometimes carried out on single animals, is very common, and it is for this reason that studies do not offer statistical analysis data. Nevertheless, they provide useful information, since it is to be borne in mind that it is difficult to collect these samples, always belonging to individuals found dead, and not caught.
Chapter 3 Nesting
A system of nested models is applied to the Mediterranean Sea with two successive zooms of the Liguro-Proven¸cal basin and of theLigurianSea. Two strong and variable currents, the Western Corsican Current and the Eastern Corsican Current (hereafter WCC and ECC) enter the domain of theLigurianSea 3.1. Both advect Modified Atlantic Water (MAW) at the surface and the ECC also transports the denser Levantine Intermediate Water (LIW). The variability of these currents has been studied by e.g. Astraldi and Gasparini (1992) and Sammari et al. (1995) and shows a seasonal cycle and a dependence on local atmospheric forcings. These currents join and give birth to the Northern Current (hereafter NC) following the French and Spanish coast. NC and WCC describe a cyclonic circulation along the Liguro-Proven¸cal front. Especially during the winter, a high mesoscale activity associated with meanders inthe NC, eddy formation or displacements intheLigurian-Proven¸cal front can be observed (Sammari et al., 1995).
et al. 2004), and numerical models predict that the Arc- tic Ocean could be free of ice in summer by the end of the 21st century (Serreze et al. 2007) or as early as 2040 (Holland et al. 2006). Since 2002, the Arctic Ocean has experienced record low sea-ice extents, with a new maximum in summer open water in September 2007 (Comiso et al. 2008), suggesting an accelerating loss of sea-ice cover. The Arctic Ocean is also strongly influ- enced by large river inflow (Macdonald et al. 2004), and the freshwater inputs will likely increase through the intensification of the hydrological cycle (Peterson et al. 2002); therefore, the Arctic Ocean’s marginal seas would be even more sensitive to climate change impact (ACIA 2005). Arctic ecosystems are expected to be affected by climate changes, e.g. shifts inthe bio- diversity and the food web structure (Gradinger & Bluhm 2005), though it is unclear how these changes will impact the components and pathways of carbon cycling (Wassmann 2004). Analysis of climate-related changes in arctic ecosystems and model validation need to be based on historical measurements in order to identify where the impacts of climate change are most likely to be observed. However, such data are still missing in some biologically active regions and sea- sons inthe Arctic Ocean (Carmack & Wassmann 2006). Arctic marine environments are characterized by large seasonal variations in solar radiation and sea-ice cover (Sakshaug & Slagstad 1991). Indeed, pelagic phyto- plankton production is usually constrained to the sum- mer months between sea-ice melt in spring and the freeze-up in autumn, and high phytoplankton produc- tion and standing stocks are restricted to relatively short periods within the ice-free season (Sakshaug 2004). Moreover, phytoplankton is the most important mediator of carbon flow in pelagic ecosystems, and phytoplankton cell size is a critical factor inthe fate of carbon through the food web (Legendre & Le Fèvre 1995). For example, carbon export and transfer to higher trophic levels are favoured by large cell production (Legendre & Le Fèvre 1995). However, blooms of large microphytoplankton cells are often constrained to a couple of weeks and smaller cells, i.e. nano- and picophytoplankton, often dominate outside these periods (Not et al. 2005).
[ 4 ] Although low primaryproduction is a defining char-
acteristic of oligotrophic systems, the large area covered by such oceanic regions (with a dominant contribution from the subtropical gyres) means that they contribute roughly half of the total marine export production [Jenkins and Doney, 2003]. As a consequence they play a crucial role inthe biogeochemistry of the ocean. The paucity of nutrients inthe surface waters of oligotrophic regions, especially during summer months, means that the local phytoplankton are very dependent on the rapid recycling of organic matter. Sufficient ammonium can be produced by this process to make ammonium-fueled regenerated production a very significant fraction of total primaryproductionin such settings. Ammonium is also nitrified into nitrate, however. Therefore, should nitrification be a significant flux in surface waters, its contribution relative to the total produc- tion at a site may be greatest in oligotrophic regions where recycling is so active. For this reason we focus our study on the subtropical gyre of the North Atlantic as representative of oligotrophic systems. More specifically we focus our study on the Bermuda Atlantic Time-series Study (BATS) site, located at 32N 64W inthe western subtropical North Atlantic. It is a classic seasonally oligotrophic site with extremely low concentrations of nutrients in surface waters throughout the summer months. Monthly sampling at this site since 1988 provides one of the most valuable biogeo- chemical time series for the open ocean. Overviews of the work that it has supported are given, for example, by Michaels and Knap , Siegel et al. , Lipschultz  and Steinberg et al. . Data can be accessed from the website http://www.bbsr.edu.
• The profile ends inthe central part of the basin, where voluminous salt domes disturb the geometry of the upper series and make interpretation of deep structures more challenging. Further effort is needed to improve the seismic image at depth here.
Three crustal domains (the continental, transitional, and central oceanic domains) were previously defined intheLigurian Basin, based on available geophysical and geological data [ 9 , 11 ] and references therein (Figures 3 and 5 a). Some correspondence can be found inthe SEFASILS morphostructural observables, although not implying first order causality: (1) the continental slope is part of the continental domain made of thinned continental crust; (2) the Var canyon/channel and ridge/levee are part of the transitional domain, whose nature is discussed, possibly including exhumed lithospheric mantle [ 10 ]; and (3) the distal basin is part of the central domain where sparse wide-angle data indicate a basement made of anomalously thin (~4 km) oceanic crust, classically labelled “atypical” [ 9 ] or, alternatively, hyperextended (less than 3 km-thick) continental crust, possibly to the point of having outright exhumed serpentinised continental mantle beneath basinal sediments [ 14 ]. In this latter view, Dannovski et al. [ 14 ] conclude that no oceanic spreading would have taken place intheLigurianSea. The processing of the SEFASILS wide-angle seismic data will bring some much-needed information on these basement domains and their seismic velocity signatures, from the northern margin to far into the basin.
Based on the consistency of the size distributions and modal diameters throughout the course of the campaign, we ob- served few effects on aerosol physical parameters from the mesocosm enrichments. The modal diameters observed in this work for the number distributions (25, 49, 105 nm) are similar to those observed previously using similar bubble production devices. In acidified seawater mesocosm experi- ments performed inthe Mediterranean, Schwier et al. (2015) observed the same four log-normal modal diameters (18.5, 37.5, 91.5, 260 nm) during both pre-bloom and non-bloom (oligotrophic) conditions. Fuentes et al. (2010a) observed four modes (14, 48, 124, 334 nm) from similar laboratory experiments with artificial seawater. Modal diameters of 15, 45, 125 and 340 nm were observed with artificial seawater and western African coastal seawater samples (Fuentes et al., 2010b). Wave-channel experiments with modified Pacific coastal seawater showed three modes (∼ 90, 220, 1000 nm) (Collins et al., 2013). Sellegri et al. (2006) observed three modes (45, 110, 300 nm) by using a weir to bubble syn- thetic seawater at 23 ◦ C. Hultin et al. (2011) observed either two log-normal modes (site: Askö, 86, 180 nm) or three log- normal modes (site: Garpen, 93, 193, 577 nm) with Baltic seawater. Bubbling systems used in these various studies vary significantly in terms of the number of jets and dis- tance between them, height of the jets to the surface of the seawater, and the presence of a blowing air jet above the sur- face, etc. The variability of the bubbling systems can largely explain the discrepancies between the bubble size distribu- tions within the seawater, and hence thesea spray size dis- tribution. Overall, however, most sea spray size distributions show an Aitken mode around 40–50 nm and a small accu- mulation mode around 100 nm. Similar systems such as the ones used in Sellegri et al. (2006), Fuentes et al. (2010a, b) and Schwier et al. (2015) show very similar size distributions to the ones reported inthe present study, with an additional ultra-fine mode around 15–25 nm. Inthe present study, we did not detect the larger accumulation mode at 300 nm, due to the smaller upper-size cut used in our DMPS system.
Carapus acus and Carapus mourlani are able to live inside sea cucumbers and sea stars respectively. Unlike other carapids whose sounds have been recorded ( C. boraborensis , C. homei and Encheliophis gracilis ), these two species have a central constriction in their swimbladder and are unlikely to encounter heterospecific carapids within their hosts. We evoked sound productionin Carapus acus and Carapus mourlani by adding several individuals to a tank with a single host and found that their sounds differ substantially from the sounds emitted by other carapids in pulse length, peak frequency and sharpness of tuning ( Q 3 dB ). Unlike the other carapids, C. mourlani and C. acus produce shorter and less repetitive sounds and do not produce sounds when they enter their host. Since sounds produced within a sea cucumber have the potential to be heard by distant carapids and are typically recorded outside thesea cucumber, we examined the effect of thesea cucumber tegument on acoustic transmission. Attenuation by the tegument was negligible at the frequencies within carapid sounds. Therefore, carapids have the potential to call from the relative safety of a sea cucumber without sacrificing the distance over which their transmissions are heard.
of distribution of different benthic organisms. The limita- tions related to the use of SeaWiFS data to estimate the ir- radiance reaching thesea floor are described in Sect. 4.1. There are also biological and sedimentological sources of uncertainty. The method of estimating benthic irradiance as- sumes that there is no shading from other erect organisms nor epibionts. The effects of backscaterring within the sediment, which can result in a 50% increase of the light exposure of some microphytobenthic communities (K¨uhl and Jørgensen, 1992), are also neglected. Finally, tidal effects were ignored, which in areas subject to large tidal amplitude, can induce hourly, daily and seasonal variations in light penetration by altering the height of the water column and turbidity (e.g., Dring and L¨uning, 1994). Data on both the maximum depth of occurrence of species and the irradiance at this depth were compiled from the literature to determine the surface area where benthic primary producers are not light limited. Often the benthic irradiance was not reported but either the attenu- ation coefficient or the percent light penetration was (some- times in another paper); in this case, the benthic irradiance was estimated by combining this value with the surface PAR value from SeaWiFS.
The Ross Sea is known for showing the greatest sea-ice increase, as observed globally, particularly from 1979 to 2015. However, corresponding changes insea-ice thickness and productioninthe Ross Sea are not known, nor how these changes have impacted water masses, carbon fluxes, bio- geochemical processes and availability of micronutrients. The PIPERS project sought to address these questions during an autumn ship campaign in 2017 and two spring airborne campaigns in 2016 and 2017. PIPERS used a multidisciplinary approach of manned and autonomous platforms to study the coupled air/ice/ocean/biogeochemical interactions during autumn and related those to spring conditions. Unexpectedly, the Ross Sea experienced record low sea ice in spring 2016 and autumn 2017. The delayed ice advance in 2017 contributed to (1) increased ice production and export in coastal polynyas, (2) thinner snow and ice cover inthe central pack, (3) lower sea- ice Chl-a burdens and differences in sympagic communities, (4) sustained ocean heat flux delay- ing ice thickening and (5) a melting, anomalously southward ice edge persisting into winter. Despite these impacts, airborne observations in spring 2017 suggest that winter ice production over the continental shelf was likely not anomalous.
We aim to verify how surface currents obtained by 2 WERA high-frequency radars can improve a regional model intheLigurianSea.
100 ROMS instances form an ensemble simulation. An observation operator extracts the radial currents (centered on the radars) from the hourly-averaged model forecasts,
maximum that manifests as a peak in beam attenuation or backscattering (Mignot et al., 2014). Both ends of this trophic gradient can be found inthe Mediterranean Sea along its well-known longitudinal trend in nutrient availability, phy- toplankton biomass and productivity (Antoine et al., 1995; D’Ortenzio and Ribera d’Alcalà, 2009; Lavigne et al., 2015). Using data from Biogeochemical-Argo (BGC-Argo) profil- ing floats deployed throughout the Mediterranean, Barbieux et al. (2019) established general patterns inthe distribution and seasonal dynamics of biomass (estimated from the par- ticulate backscattering coefficient) and chlorophyll subsur- face maxima. They found that inthe western Mediterranean Sea, during late spring and summer, a subsurface biomass maximum develops that coincides with a chlorophyll max- imum and is located roughly at the same depth as the nu- tricline and above the 0.3 mol quanta m −2 d −1 isolume. In contrast, inthe Ionian and Levantine seas the DCM, which has a smaller magnitude, arises solely from photoacclima- tion and is located well above the nutricline at a depth that corresponds closely with the 0.3 mol quanta m −2 d −1 isol- ume (Barbieux et al., 2019). The presence of a subsurface or deep biomass maximum may suggest that a particularly favourable combination of light and nutrients occurs at that depth, leading to enhanced phytoplankton growth and new production. It remains unknown, however, whether phyto- plankton growth and biomass turnover rates are actually higher at the depth of the biomass maximum. An additional source of uncertainty is that both the particulate attenuation and backscattering coefficients relate not only to phytoplank- ton abundance but to the entire pool of particles, including non-algal and detrital particles, which are known to con- tribute significantly to total suspended matter in oligotrophic regions (Claustre et al., 1999). Combining direct and specific measurements of phytoplankton production (with the 14 C up- take technique) and biovolume (with flow cytometry) offers a way to determine photoautotrophic biomass turnover rates (Kirchman, 2002; Marañón et al., 2014) and thus gain further insight into the dynamics and underlying mechanisms of the DCM. By investigating concurrently the vertical variability in heterotrophic prokaryotic productionin relation to phyto- plankton standing stocks and productivity, it is also possible to ascertain potential implications of the DCM for trophic coupling within the microbial-plankton community.
7.1. Magmatism: Underplating or ‘‘Undercrusting’’? [ 43 ] High-velocity levels (HVL) are described on several
Atlantic-type passive margins, which are interpreted either as underplating resulting from the partial melting of the upper mantle during rifting on volcanic margins [e.g., White and McKenzie, 1989; Eldholm and Grue, 1994; Holbrook et al., 1994], or as partly serpentinized peridotites of the upper mantle exhumed inthe continental breakup zone on nonvolcanic passive margins (West Iberia margin, ‘‘under- crusting’’ of Boillot et al. , Pinheiro et al. , Whitmarsh et al. , Brun and Beslier , Pickup et al. ; Labrador Sea, Chian et al. ; Bay of Biscay, Thinon ). As underlined by Mauffret et al. , the Liguro-Provenc¸al margins do not display the main characteristics of volcanic margins, since neither seaward dipping reflectors nor thick HVL are found. More- over, the Tyrrhenian Sea, which opened in a similar geo- dynamic context, depicts an atypical oceanic crust corresponding to a very thin basaltic crust overlying ser- pentinized mantle [Mascle and Re´hault, 1990; ODP Leg 107 Shipboard Scientific Party, 1987; Bonatti et al., 1990; Kastens and Mascle, 1990]. From our data set only, we cannot unambiguously discount previously proposed hypotheses. However, the above mentioned observations, together with the range of acoustic velocities and the geometry and the location of the thin HVL found almost systematically along theLigurian margin, suggest that the HVL could represent partly serpentinized peridotites. Accordingly, volcanism would be limited to ovoid bodies inthe TD of theLigurian basin, which indeed is further documented by magnetic data (Figure 11). The synrift exhumation of the mantle may have occurred either by simple shear along reactivated inherited compressive struc- tures [Mauffret et al., 1995], or as a consequence of litho- spheric boudinage [Brun and Beslier, 1996].
suggesting an incomplete utilization of nitrate by phytoplankton at the bottom of the euphotic zone due to the lack of phosphate (Raimbault and Coste, 1990). From November to February the N/P ratio is below 20 for relatively shallow waters (0-40 m) and over 20 in deeper layers (90-200 m). In this period, which is characteristic of low stratification, the limitation factor is essentially due to nitrate in surface waters (where most of theproduction occurs). Different study (Marty et al., 2002; Barlow et al., 1993; Claustre and Marty, 1995) indicate that the dominant group of phytoplankton, inthe western Mediterranean Sea, is the prymnesiophytes and that this predominance is independent of seasons. The 10 years study of Marty et al., indicates a large abundance of diatoms and, this abundance, is variable with time particularly associated to blooms. The diatoms are opportunistic species (Fogg, 1991), which bloom as soon as of stratification breaks down and nutrients are already present, due to winter mixing. Diatoms are well known to be responsible of spring blooms and likely main contributors to the new productioninthe open ocean (Goldman, 1993; Claustre, 1994), and to high particle fluxes associated to the bloom period (Miquel et al., 1994).