DYFAMED Station Research Articles

Temporal evolution of 137Cs, 237Np, and 239+240Pu and estimated vertical 239+240Pu export in the northwestern Mediterranean Sea

The evolution of 137Cs, 237Np and 239+240Pu at the DYFAMED station (NW Mediterranean) is discussed in relation to physical processes, downward fluxes of particles, and changes in the main input sources. The data set presented in this study represents the first complete 237Np vertical profiles (0.12–0.27μBqL−1), and constitutes a baseline measurement to assess future changes. A similar behavior of Cs and Np has been evidenced, confirming that Np behaves conservatively. While the 137Cs decrease has been driven by its radioactive decay, the vertical distribution of 237Np has not substantially changed over the last decade. In the absence of recent major inputs, a homogenization of their vertical distribution occurred, partly due to deep convection events that became more intense during the last decade. In contrast, 239+240Pu surface levels in the NW Mediterranean waters have fallen in the past four decades by a factor of 5. This decrease in surface has been balanced by higher concentrations in the deep-water layers. A first estimate of the downward 239+240Pu fluxes in the NW Mediterranean Sea is proposed over more than two decades. This estimation, based on the DYFAMED sediment trap time-series data and published 239+240Pu flux measurements, suggests that sinking particles have accounted for 60–90% of the upper layer (0–200m) Pu inventory loss over the period 1989–2013. The upper layer residence time of Pu is estimated to be ~28years, twice as long as the residence time estimated for the whole western Mediterranean (~15years). This difference highlights the slow removal of Pu in the open waters of the NW Mediterranean and confirms that most of the Pu removal occurs along the coastal margin where sedimentation rates are high.

Excess of 236U in the northwest Mediterranean Sea

In this work, we present first 236U results in the northwestern Mediterranean. 236U is studied in a seawater column sampled at DYFAMED (Dynamics of Atmospheric Fluxes in the Mediterranean Sea) station (Ligurian Sea, 43°25′N, 07°52′E). The obtained 236U/238U atom ratios in the dissolved phase, ranging from about 2×10−9 at 100m depth to about 1.5×10−9 at 2350m depth, indicate that anthropogenic 236U dominates the whole seawater column. The corresponding deep-water column inventory (12.6ng/m2 or 32.1×1012 atoms/m2) exceeds by a factor of 2.5 the expected one for global fallout at similar latitudes (5ng/m2 or 13×1012 atoms/m2), evidencing the influence of local or regional 236U sources in the western Mediterranean basin. On the other hand, the input of 236U associated to Saharan dust outbreaks is evaluated. An additional 236U annual deposition of about 0.2pg/m2 based on the study of atmospheric particles collected in Monaco during different Saharan dust intrusions is estimated. The obtained results in the corresponding suspended solids collected at DYFAMED station indicate that about 64% of that 236U stays in solution in seawater. Overall, this source accounts for about 0.1% of the 236U inventory excess observed at DYFAMED station. The influence of the so-called Chernobyl fallout and the radioactive effluents produced by the different nuclear installations allocated to the Mediterranean basin, might explain the inventory gap, however, further studies are necessary to come to a conclusion about its origin.

Effects of the Western Mediterranean Transition on the resident water masses: Pure warming, pure freshening and pure heaving

Winter 2004–05 marks the beginning of the Western Mediterranean Transition, involving a change in the physical properties of the water masses in the Western Mediterranean. Temperature and salinity decreased in the intermediate water and increased suddenly in the bottom layer. All these changes are related to strong deep water convection events in winters 2004–05 and 2005–06 in the Gulf of Lions and the Ligurian subbasin respectively. A CTD time series collected at the DYFAMED station (Ligurian subbasin) has been analysed to study the effect of the ventilation of the water column due to the large volume of new deep water formed in both winters. Also the impact of the new saltier and warmer deep water mass formed in both winters on the resident deep and intermediate waters has been evaluated. Temperature and salinity changes have been decomposed into changes along isopycnals and vertical displacements of isopycnals (pure warming, pure freshening and pure heaving according to the nomenclature in Bindoff and McDougall, 1994). The results of this analysis show that the formation of a large volume of new deep water caused the upward displacement of the resident deep water. Therefore, the decrease of temperature and salinity in the intermediate water was not only due to actual water mass changes, but also to the uplifting of Western Mediterranean Deep Water. Isopycnals situated at the bottom of the water column before the Western Mediterranean Transition became cooler and fresher after the event. A combination of pure warming and salting is the most plausible mechanism explaining this temperature and salinity decrease observed on isopycnals.

Natural and anthropogenic trace metals in sediments of the Ligurian Sea (Northwestern Mediterranean)

The magnitude and the chronology of anthropogenic impregnation by Hg and other trace metals of environmental concern (V, Cr, Ni, Cu, Zn, Ag, Cd and Pb, including its stable isotopes) in the sediments are determined at the DYFAMED station, a site in the Ligurian Sea (Northwestern Mediterranean) chosen for its supposed open-sea characteristics. The DYFAMED site (VD) is located on the right levee of the Var Canyon turbidite system, at the end of the Middle Valley. In order to trace the influence of the gravity current coming from the canyon on trace metal distribution in the sediment, we studied an additional sediment core (VA) from a terrace of the Var Canyon, and material collected in sediment traps at the both sites at 20 m above sea bottom. The patterns of Hg and other trace element distribution profiles are interpreted using stable Pb isotope ratios as proxies for its sources, taking into account the sedimentary context (turbidites, redox conditions, and sedimentation rates). Major element distributions, coupled with the stratigraphic examination of the sediment cores point out the high heterogeneity of the deposits at VA, and major turbiditic events at both sites. At the DYFAMED site, we observed direct anthropogenic influence in the upper sediment layer (< 2 cm), while on the Var Canyon site (VA), the anthropization concerns the whole sedimentary column sampled (19 cm). Turbiditic events superimpose their specific signature on trace metal distributions. According to the 210Pb xs-derived sedimentation rate at the DYFAMED site (0.4 mm yr − 1 ), the Hg-enriched layer of the top core corresponds to the sediment accumulation of the last 50 years, which is the period of the highest increase in Hg deposition on a global scale. With the hypothesis of the absence of significant post-depositional redistribution of Hg, the Hg/C org ratio changes between the surface and below are used to estimate the anthropogenic contribution to the Hg flux accumulated in the sediment. The Hg enrichment, from pre-industrial to the present time is calculated to be around 60%, consistent with estimations of global Hg models. However, based on the chemical composition of the trapped material collected in sediment traps, we calculated that epibenthic mobilization of Hg would reach 73%. Conversely, the Cd/C org ratio decreases in the upper 5 cm, which may reflect the recent decrease of atmospheric Cd inputs or losses due to diagenetic processes.

High downward flux of mucilaginous aggregates in the Ligurian Sea during summer 2002: similarities with the mucilage phenomenon in the Adriatic Sea

AbstractThe DYFAMED sediment trap station in the Ligurian Sea (NW Mediterranean) has been active since 1986 and today comprises the longest time‐series of downward particle flux in the Mediterranean Sea. As such, it provides valuable information on the interannual variability of the particle flux, and also documents possible recent changes in the NW Mediterranean pelagic ecosystems. We report an unprecedented episode of downward flux of mucilaginous material at the DYFAMED station during summer 2002 in association with singular hydrometeorological conditions. The rain of mucilaginous aggregates clogged a PPS5 sediment trap at 260 m depth and was also clearly detected at 1080 m depth. The possible factors governing the development and sinking of the mucilaginous material are discussed. A very sharp increase of sea surface temperature during June and the presence of freshened waters in the surface the following month resulted in a stronger than usual stratification of the upper water column throughout the summer season. We suggest that the steepness of the vertical density gradient was responsible for the unusual accumulation of mucous aggregates. Additionally, a diatom bloom took place during the nutrient‐depleted conditions typical of summer, a factor which may have contributed to feed the pycnocline with transparent exopolymer substances. A storm occurring in the beginning of August relaxed the stratification and promoted the deposition of the mucilaginous aggregates accumulated in the upper water column during the preceding months. Important similarities of ambient conditions preceding the apparition of mucilaginous material in our open‐sea site and those reported in the Adriatic Sea during major mucilage events, suggest that general climatic conditions, rather than local factors, drive the occurrence of major accumulations of mucilaginous material in the water column at both sub‐basins of the Mediterranean Sea. In this regard, the strength of the air temperature increase during the onset of the stratified season is proposed as a major controlling factor.

Structure and function of the mesopelagic microbial loop in the NW Mediterranean Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 57:351-362 (2009) - DOI: https://doi.org/10.3354/ame01370 AME Special: 'Rassoulzadegan at Villefranche-sur-Mer: 3 decades of aquatic microbial ecology' Structure and function of the mesopelagic microbial loop in the NW Mediterranean Sea Tsuneo Tanaka* Aix-Marseille University, CNRS, LOPB-UMR 6535, OSU/Centre d’Océanologie de Marseille, Campus de Luminy-Case 901, 13288 Marseille Cedex 09, France *Email: tsuneo.tanaka@univmed.fr ABSTRACT: The mesopelagic layer is where the majority of the particulate organic carbon exported from the epipelagic layer is remineralized to CO2. Recent studies at the DYFAMED time-series station in the NW Mediterranean Sea have shown that dominance of prokaryotes (Bacteria and Archaea) within the microbial community represented by biomass data increases with depth. The studies have also shown that the depth-integrated biomass of protists (heterotrophic flagellates and ciliates) in the mesopelagic layer is as great as that in the epipelagic layer, and that biomass of mesopelagic prokaryotes is controlled by both substrate availability (bottom-up control) and predation and viral infection (top-down control). Data on prokaryotic growth efficiency (derived from the prokaryotic heterotrophic production and the loss of organic carbon flux in the mesopelagic layer on an annual scale) suggest that mesopelagic prokaryotes are not simply remineralizers of organic carbon, but also play an important role in supporting the production of organisms belonging to higher trophic levels. Analysis using a simple food chain model suggests that heterotrophic nanoflagellates may be important remineralizers of organic carbon produced by prokaryotes, and that the viral loop may enhance remineralization of organic carbon in the mesopelagic layer. There are, however, uncertain elements in measuring organic carbon flux and biomass and activity of the microbial components in the mesopelagic layer. By comparing the results obtained at the DYFAMED station with those obtained from other environments, this study summarizes our current knowledge on the structure and function of the mesopelagic microbial loop in the NW Mediterranean Sea as well as in other oceanic regions. KEY WORDS: Mesopelagic microbial loop · Protists · Carbon flux · NW Mediterranean Sea Full text in pdf format PreviousCite this article as: Tanaka T (2009) Structure and function of the mesopelagic microbial loop in the NW Mediterranean Sea. Aquat Microb Ecol 57:351-362. https://doi.org/10.3354/ame01370Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 57, No. 3. Online publication date: November 24, 2009 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2009 Inter-Research.

228Ra/ 226Ra and 226Ra/Ba ratios in the Western Mediterranean Sea: Barite formation and transport in the water column

226Ra, 228Ra and Ba distributions as well as 228Ra/ 226Ra and 226Ra/Ba ratios were measured in seawater, suspended and sinking particles at the DYFAMED station in the Western Mediterranean Sea at different seasons of year 2003 in order to track the build-up and fate of barite through time. The study of the 228Ra ex/ 226Ra ex ratios (Ra ex = Ra activities corrected for the lithogenic Ra) of suspended particles suggests that Ba ex (Ba ex = Ba concentrations corrected for the lithogenic Ba, mostly barite) formation takes place not only in the upper 500 m of the water column but also deeper (i.e. throughout the mesopelagic layer). Temporal changes in the 228Ra ex/ 226Ra ex ratios of sinking particles collected at 1000 m depth likely reflect changes in the relative proportion of barite originating from the upper water column (with a high 228Ra/ 226Ra ratio) and formed in the mesopelagic layer (with a low 228Ra/ 226Ra ratio). 228Ra ex/ 226Ra ex ratios measured in sinking particles collected in the 1000 m-trap in April and May suggest that barite predominantly formed in the upper water column during that period, while barite found outside the phytoplankton bloom period (February and June) appears to form deeper in the water column. Combining ratios of both the suspended and sinking particles provides information on aggregation/disaggregation processes. High 226Ra ex/Ba ex ratios were also found in suspended particles collected in the upper 500 m of the water column. Because celestite is expected to be enriched in Ra [Bernstein R. E., Byrne R. H. and Schijf J. (1998) Acantharians: a missing link in the oceanic biogeochemistry of barium. Deep-Sea Res. II 45, 491–505], acantharian skeletons may contribute to these high ratios in shallow waters. The formation of both acantharian skeletons and barite enriched in 226Ra may thus contribute to the decrease in the dissolved 226Ra activity and 226Ra/Ba ratios of surface waters observed between February and June 2003 at the DYFAMED station.

&lt;sup234&lt;/sup&gt;Th measured particle export from surface waters in north-western Mediterranean: comparison of spring and autumn periods

234Th was used to quantify the short-term variability of particle dynamics and of POC export during transition periods in surface waters over the north-western Mediterranean. As a part of DYNAPROC I and II cruises, two intensive time-series of 234Th were carried out near the DYFAMED station (43° 25´N–7°51´E) during late spring (May, 1995), when the system changes towards oligotrophy, and during autumn (October, 2004), when the stratification is disturbed by wind. Particulate fluxes derived from 234Th measured in the upper water column and in drifting sediment trap showed large differences between the two situations: the flux decreased from high to low values during late spring, at the difference of the autumnal situation where the fluxes were always low. 234Th-derived POC fluxes, calculated from the 234Th238U disequilibrium in the water column and POC/234Th ratio on trapped material, and export ratios (ThE: ratio of 234Th-derived POC export to primary production) showed a large range, from 8 to 110 mgC m−2 d−1 and 3–55%; the lowest values were observed at the end of the productive period (end May) and in automn. The 234Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity. From these experiments during transition periods, it is not obvious that renewal of nutrients by wind events is strong enough to sustain significant export after the end of the productive period or to initiate significant export in autumn.

Use of lipids and their degradation products as biomarkers for carbon cycling in the northwestern Mediterranean Sea

Changes in phytoplankton composition and degradation of particulate organic matter (POM) in the northwestern Mediterranean Sea were studied using time-series sediment trap samples collected during the spring of 2003 at the DYFAMED station. Lipid biomarkers (pigments, fatty acids, sterols, acyclic isoprenoids, alkenones and n-alkanols) were used to identify the main contributors to the POM produced during two phytoplankton blooms, while the effects of photooxidation, autoxidation and biodegradation were differentiated using characteristic lipid degradation products. Traps collected material corresponding to pre-bloom, bloom and post-bloom periods. Pigment analyses in the integrated (0-200 m) water column samples indicated that diatoms dominated the initial stages of the bloom event, with smaller amounts of haptophytes and pelagophytes. During the second part of bloom event there was a switch to haptophyte dominance with significant contributions from diatoms and pelagophytes, and an increased contribution from cryptophytes. Fatty acid distributions in the trap samples reflected contributions from marine bacteria, phytoplankton and zooplankton. Photooxidation and autoxidation products of monounsaturated oleic, cis-vaccenic and palmitoleic acids were detected along with photooxidation products from the chlorophyll side-chain. The relatively good correlation between the variation of U 37 K ′ index and specific phytol autoxidation product percentage allowed us to attribute the alterations of U 37 K ′ observed during the pre-bloom period and in the deeper traps to the involvement of selective autoxidative degradation processes. A variety of sterol oxidation products formed by biohydrogenation, autoxidation and photooxidation were detected. Sterol degradation products appeared to be less suited than oxidation products of monounsaturated fatty acids for the precise monitoring of the degradation state of POM, but their stable functionalized cyclic structure constitutes a useful tool to estimate the part played by biotic and abiotic processes. In these waters, biotic degradation generally predominates, but abiotic degradation is not negligible and, as expected, the extent of biotic degradation increases with depth. To obtain a more complete picture of POM degradation, the use of a pool of lipid degradation products (i.e. from unsaturated fatty acids, the phytyl side-chain and sterols) should be employed.

Biomarkers derived from heterolytic and homolytic cleavage of allylic hydroperoxides resulting from alkenone autoxidation

Laboratory incubation of alkenone mixtures with tert-butyl hydroperoxide and di- tert-butyl nitroxide (radical initiator) in hexane, as a means to simulate alkenone autoxidation processes, rapidly led to the formation of allylic hydroperoxides, whose presence was recently demonstrated in Emiliania huxleyi cells. After incubation in seawater and subsequent reduction with NaBH 4 (to reduce residual hydroperoxides before analysis), these reaction products quickly disappeared and were replaced by complex mixtures of n-alcohols, fatty acids, alkyldiols and hydroxyacids. Methyl alkenones produced saturated n-alkan-1-ols and fatty acids ranging from C 13 to C 16 and two series of C 13−C 16 (ω-1)-hydroxyacids and (1,ω-1)-diols. Ethyl alkenones also afforded C 13−C 16 saturated n-alkan-1-ols and fatty acids, accompanied by the production of C 14−C 17 (ω-2)-hydroxyacids and (1,ω-2)-diols. Deuterium labelling allowed us to show that most of the n-alkan-1-ols, hydroxyacids and alkyldiols resulted from the reduction during the NaBH 4 treatment of the corresponding aldehydes, ketoxyacids and ketoxyaldehydes formed from heterolytic or homolytic cleavages of allylic hydroperoxyl groups resulting from the oxidation of the double bonds of di- and triunsaturated alkenones. Amongst these products, the (ω-1)- and (ω-2)-hydroxyacids formed after NaBH 4 reduction of the (ω-1)- and (ω-2)-ketoxyacids were selected as potential biomarkers for alkenone autoxidation. Re-examination of lipid extracts of post-bloom seawater particulate matter samples from the DYFAMED station in the Ligurian Sea (where strong autoxidative alteration of the lipid distributions had previously been detected) showed the presence of significant amounts of 12-hydroxytetradecanoic, 13-hydroxytetradecanoic, 14-hydroxyhexadecanoic and 15-hydroxyhexadecanoic acids thus providing good evidence that these autoxidative processes occur in natural samples.

Model complexity and performance: How far can we simplify?

Handling model complexity and reliability is a key area of research today. While complex models containing sufficient detail have become possible due to increased computing power, they often lead to too much uncertainty. On the other hand, very simple models often crudely oversimplify the real ecosystem and can not be used for management purposes. Starting from a complex and validated 1D pelagic ecosystem model of the Ligurian Sea (NW Mediterranean Sea), we derived simplified aggregated models in which either the unbalanced algal growth, the functional group diversity or the explicit description of the microbial loop was sacrificed. To overcome the problem of data availability with adequate spatial and temporal resolution, the outputs of the complex model are used as the baseline of perfect knowledge to calibrate the simplified models. Objective criteria of model performance were used to compare the simplified models’ results to the complex model output and to the available data at the DYFAMED station in the central Ligurian Sea. We show that even the simplest (NPZD) model is able to represent the global ecosystem features described by the complex model (e.g. primary and secondary productions, particulate organic matter export flux, etc.). However, a certain degree of sophistication in the formulation of some biogeochemical processes is required to produce realistic behaviors (e.g. the phytoplankton competition, the potential carbon or nitrogen limitation of the zooplankton ingestion, the model trophic closure, etc.). In general, a 9 state-variable model that has the functional group diversity removed, but which retains the bacterial loop and the unbalanced algal growth, performs best.

A mechanistic modelling and data assimilation approach to estimate the carbon/chlorophyll and carbon/nitrogen ratios in a coupled hydrodynamical-biological model

Abstract. The principal objective of hydrodynamical-biological models is to provide estimates of the main carbon fluxes such as total and export oceanic production. These models are nitrogen based, that is to say that the variables are expressed in terms of their nitrogen content. Moreover models are calibrated using chlorophyll data sets. Therefore carbon to chlorophyll (C:Chl) and carbon to nitrogen (C:N) ratios have to be assumed. This paper addresses the problem of the representation of these ratios. In a 1D framework at the DYFAMED station (NW Mediterranean Sea) we propose a model which enables the estimation of the basic biogeochemical fluxes and in which the spatio-temporal variability of the C:Chl and C:N ratios is fully represented in a mechanical way. This is achieved through the introduction of new state variables coming from the embedding of a phytoplankton growth model in a more classical Redfieldian NNPZD-DOM model (in which the C:N ratio is assumed to be a constant). Following this modelling step, the parameters of the model are estimated using the adjoint data assimilation method which enables the assimilation of chlorophyll and nitrate data sets collected at DYFAMED in 1997.Comparing the predictions of the new Mechanistic model with those of the classical Redfieldian NNPZD-DOM model which was calibrated with the same data sets, we find that both models reproduce the reference data in a comparable manner. Both fluxes and stocks can be equally well predicted by either model. However if the models are coinciding on an average basis, they are diverging from a variability prediction point of view. In the Mechanistic model biology adapts much faster to its environment giving rise to higher short term variations. Moreover the seasonal variability in total production differs from the Redfieldian NNPZD-DOM model to the Mechanistic model. In summer the Mechanistic model predicts higher production values in carbon unit than the Redfieldian NNPZD-DOM model. In winter the contrary holds.

Can biogeochemical fluxes be recovered from nitrate and chlorophyll data? A case study assimilating data in the Northwestern Mediterranean Sea at the JGOFS-DYFAMED station

One of the principal objectives of studying biogeochemical cycles is to obtain precise estimates of the main fluxes, such as total, new and export oceanic productions. Since models can incorporate the a priori knowledge of the most important processes, they are increasingly used for this purpose. However, biogeochemical models are characterized by a large number of poorly known parameters. Moreover, the available data are rather sparse in both time and space, and represent concentrations, not fluxes. Therefore, the major challenge is to constrain the relevant fluxes using information from a limited number of observations and from models incorporating poorly known internal parameters. The present study attempts to meet this challenge. In a 1D framework at the DYFAMED station (NW Mediterranean Sea), near-monthly nitrate and chlorophyll profiles and daily surface chlorophyll concentrations are assimilated in a coupled dynamical–biological model using the tangent linear and adjoint models. Following sensitivity analyses that show that some parameters cannot be recovered from the data set used, assimilation of observed 1997 data is performed. The first inversion considered clearly shows that, in agreement with previous studies, (1) the data impose a C/Chl ratio that varies with depth (i.e. light) and (2) the “initial” conditions (e.g. winter nitrate profile) strongly constrain the annual biogeochemical fluxes. After assimilation of the 1997 data, the agreement between the data and the model is quantitatively improved in 1995 and 1996, which can be considered a good validation of the methodology. However, the order of magnitude of the biogeochemical fluxes, and especially of the particulate export and regenerated production, are not correctly recovered. An analysis of the simulations shows that this result is associated with a strong decrease in zooplankton concentrations. An additional constraint of maintaining acceptable levels of zooplankton is therefore added. The results are improved, but remain unsatisfactory. A final inversion, which takes into account the a priori estimates of the major annual fluxes, is then performed. This shows that there is no inconsistency between the NO 3 and chlorophyll data, the order of magnitude of the fluxes and the model. The work therefore demonstrates that recovering biogeochemical fluxes from available data of concentrations and stocks is not a straightforward exercise: the coverage and type of observations, and the nonlinearities of the biogeochemical model all contribute to this difficulty.

Strong seasonality in particle dynamics of north-western Mediterranean surface waters as revealed by 234Th/ 238U

234Th was used to quantify sinking fluxes and residence times of particles in surface waters of the north-western Mediterranean Sea. Measurements of dissolved and particulate 234Th were made at the DYFAMED station (43°25′N–7°51′E, JGOFS-France program). Sampling covered 1 year on four cruises in 1994 (February 9, April 29, June 3, October 1) and focused on a transition period in mid-spring with six repeated profiles collected during May 1995. 234Th was nearly in equilibrium with its parent 238U most of the year, except in spring. The intensive sampling in May shows a rapid evolution throughout the month from a moderate 234Th deficit to near-equilibrium values. The time-series of 234Th were treated with steady-state and non-steady-state models. 234Th particulate fluxes clearly indicate large variability in export, with the highest values observed in spring. Particle residence times in the upper 40 m range from <10 to >250 days, and could increase by a factor of 10 within 2 weeks. POC fluxes from the upper 40 m and export ratios (ThE: ratio of 234Th-derived POC export to primary production), derived from the 234Th/ 238U disequilibrium in the water column and POC/ 234Th ratio on trapped material, decrease from about 9.5 mmol C m −2 d −1 and >22% in early May to less than 5 mmol C m −2 d −1 and 15% after mid-May. The 234Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity.

Processes controlling annual variations in the partial pressure of CO2 in surface waters of the central northwestern Mediterranean Sea (Dyfamed site)

Abstract Measurements of the partial pressure of CO2 (pCO2) in surface waters, and other water properties were performed monthly during a 2-year period from February 1998 through February 2000, at a station in the open northwestern Mediterranean Sea (Dyfamed Station). On seasonal timescale, the pCO2 minimum of 300 μatm in winter was followed by an increase of 120 μatm (pCO2 reaching 420 μatm) related to warming of surface waters in summer. Estimates of the underlying processes (mixing, biological activity and air–sea gas exchange) governing the monthly variations of the upper layer pCO2 were obtained from observed variations in total inorganic carbon content (TCO2) in the surface, and from the vertical distribution of physical parameters and TCO2. Monthly variations in TCO2 due to gas exchange were determined from wind speed and from the air–sea pCO2 gradient. The impact of biological activity was estimated from the difference between the observed variations in TCO2 and the evaluations of air–sea exchange and carbon supply by physical processes. Mixing at the base of the mixed layer counteracts the late winter to summer TCO2 drawdown (about 80 mmol m−3) due to a net organic production of about 100 mmol m−3. The carbon consumption continues until early summer despite the absence of nutrients in the upper layer from April or May. The net carbon production in the mixed layer during the warming period exceeds by a factor of 1.6 the carbon production deduced from nitrate fluxes and using the usual Redfield C:N ratio of 6.6:1. The TCO2 increase during the autumn is primarily associated with convective vertical mixing induced by upper layer cooling and deepening. On the other hand, the contribution of air–sea gas exchange to TCO2 variations remains relatively small aside from summer months, when the CO2 oversaturation is high and the mixed layer is only 15–20 m depth.

The relevant time scales in estimating the air–sea CO2 exchange in a mid-latitude region

A 1D biogeochemical model simulating the nitrogen and carbon cycles is validated, using continuous observations obtained with the Carioca buoy (sea surface temperature (SST), pCO2; fluorescence) at the DYFAMED station (NW Mediterranean Sea) in 1995–1997, as well as other in situ data. Although the average surface pCO2 is generally slightly over-saturated (878matm), the NW Mediterranean Sea is a weak sink for atmospheric CO2 (0.1570.07 mol C m � 2 yr � 1 ), because the highest fluxes occur in winter and spring during the period of undersaturation when the winds are strong. The seasonal cycle is modulated by synoptic events (Mistral bursts), which can have a strong impact on the behaviour of the system and on the fluxes in winter and spring. While the atmospheric forcing constrains the annual balances and fluxes of tracers and CO2, the winter pre-conditioning of the bloom plays a major role in driving the interannual variability. Sensitivity analyses indicate that atmospheric forcing in this highly variable region should be averaged over periods of no longer than one week to simulate the biological and air–sea CO2 fluxes correctly. Also, because the model used is rather simple, this time scale is an upper limit. The sampling period must be no greater than a few days in order to estimate the CO2 fluxes with an error smaller than 20%. In the NW Mediterranean Sea, it is difficult to use ‘‘satellite proxies’’ (SST; sea colour) to estimate annual air–sea CO2 fluxes because SST does not vary much in winter and during the beginning of the bloom, while chlorophyll can either remain low (winter) or change rapidly (beginning of the bloom). At least several direct observations of pCO2 are needed during winter to define ‘‘initial conditions’’. r 2002 Published by Elsevier Science Ltd.

Annual to interannual variations of &lt;I&gt;f&lt;/I&gt;CO&lt;SUB&gt;2&lt;/SUB&gt; in the northwestern Mediterranean Sea: Results from hourly measurements made by CARIOCA buoys, 1995-1997

A time series of fCO 2 , SST, and fluorescence data was collected between 1995 and 1997 by a CARIOCA buoy moored at the DyFAMed station (Dynamique des Flux Atmospheriques en Mediterranee) located in the northwestern Mediterranean Sea. On seasonal timescales, the spring phytoplankton bloom decreases the surface water fCO 2 to approximately 290 patm, followed by summer heating and a strong increase in fCO 2 to a maximum of approximately 510 μatm. While the ΔfCO 2 shows strong variations on seasonal timescales, the annual average air-sea disequilibrium is only 2 μatm. Temperature-normalized fCO 2 shows a continued decrease in dissolved CO 2 throughout the summer and fall at a rate of approximately 0.6 μatm d -1 , The calculated annual air-sea CO 2 transfer rate is -0. 10 to -0.15 moles CO 2 m -2 y -1 , with these low values reflecting the relatively weak wind speed regime and small annual air-sea fCO 2 disequilibrium. Extrapolating this rate over the whole Mediterranean Sea would lead to a flux of approximately -3 X 10 12 to -4.5 X 10 12 grams C y -1 , in good agreement with other estimates. An analysis of the effects of sampling frequency on annual air-sea CO 2 flux estimates showed that monthly sampling is adequate to resolve the annual CO 2 flux to within approximately ± 10 - 18% at this site. Annual flux estimates made using temperature-derived fCO 2 based on the measured fCO 2 -SST correlations are in agreement with measurement-based calculations to within ± 7-10% (depending on the gas transfer parameterization used), and suggest that annual CO 2 flux estimates may be reasonably well predicted in this region from satellite or model-derived SST and wind speed information.

Consumption and production on scales of a few days of inorganic carbon, nitrate and oxygen by the planktonic community: results of continuous measurements at the Dyfamed Station in the northwestern Mediterranean Sea (May 1995)

Continuous measurements between 0 and 200 m depth were performed every 2 h over two separate periods of four days at a station in the open northwestern Mediterranean Sea (Dyfamed Station) during the Dynaproc cruise in May 1995. Estimates of the daily variations in profiles of temperature, partial pressure of CO 2, oxygen, chlorophyll a and nutrients were obtained. The distributions of the various physical and chemical properties were clearly different during the two time series, which were separated by a period of 11 days during which a wind event occurred. The mean daily utilization or production due to biological processes of dissolved inorganic carbon (DIC), nitrate+nitrite and oxygen were calculated along isopycnals using a vertical diffusion model. Between the surface and about 20 m depth, DIC was consumed and O 2 released during the two time series while the nitrate+nitrite concentrations as well as supplies were zero. After the wind event, the O 2 : C : N ratios of consumption (or production) were, on average, near the Redfield ratios, but during the first time series, the C : N utilization ratio between 20 and 35 m was two to three times that of Redfield stoichiometry and the oxygen release was low. The integrated net community production (NCP) in terms of carbon was equivalent during the two time series, whereas the chlorophyll a biomass was twice as high, on average, during the first time series but did decrease. These results imply that the production systems were different during the two periods. The first time series corresponds to a period at the end of production, due to the nutrient depletion in the euphotic layer. The formation of degradation products of the living material in dissolved organic form is probably important as indicated by the high C : N utilization ratios. The second time series corresponds to a reactivation of the primary production due to the upward shift of nutrients after the wind event.

Simulation of primary production and export fluxes in the Northwestern Mediterranean Sea

A biogeochemical model, BIOMELL (BIOgeochemical Model of the Euphotic Layer of Lodyc), has been developed to simulate the temporal evolution of the main nitrogen stocks and e uxes at the DyFAMed station (Dynamique des FluxAtmospheriques en Mediterranee), located in the Northwest- ern Mediterranean Sea. This mainly oligotrophic region is characterized by a strong seasonal cycle, and a signie cant export of dissolved organic matter compared to the particulate export measured by sediment traps. Validation of the model is made using temperature, nitrate and chlorophyll proe les acquired at DyFAMed approximately every month in 1991. Extended datasets from specie c years are also used to validate seasonal variations of other variables, for which the coverage in 1991 was rather poor (new and total production, particulate export, dissolved organic matter export, bacteria, zooplankton). Sensitivity studies on selected parameters are carried out in order to give an idea of the margin of error in the model predictions. The model is then used to analyze the behavior of the biogeochemical system during various production regimes (winter, spring bloom, oligotrophy, autumn bloom). It is shown that several processes, which are often neglected in biogeochemical models, must be taken into account: phytoplankton growth limitation by deep mixing, C:Chl ratio dependence on light, nitrie cation, and semi-refractory dissolved organic matter accumulation in the surface layer.