Short-scale temporal variability of physical, biological and biogeochemical processes in the NW Mediterranean Sea: an introduction

International audienceIn the framework of the PROOF-PECHE project (http://www.obs-vlfr.fr/proof/vt/op/ec/peche/pec.htm) a multidisciplinary team performed experiments and collected samples during the DYNAPROC2 cruise aboard the RV Thalassa from September to October in 2004. The cruise provided data on the functioning of the pelagic food web by sampling over a month long period in the NW Mediterranean Sea at a fixed station subject to weak horizontal advection currents during a period of hydrological stability. This paper describes the background of the cruise and provides an overview of the results derived from the campaign which constitute the special section. The major objective of the cruise was to assess the relative importance and variability of the pathways of carbon in the open ocean. Intensive sampling through 4 periods of 5 days each was accomplished at a site near the DYFAMED time-series site. The site was near stable in terms of hydrodynamics as there was some evidence of an intrusion of low-salinity coastal water. The cruise yielded a comprehensive data set acquired by sampling over a vertical spatial dimension (0–1000 m) and at high frequencies (ranging from every 3, 6, 12 and/or 24 h), unique for the summer to autumn transition in the North Western Mediterranean. Parameters investigated included the biochemical composition of dissolved organic matter (lipids), and the structure of bacterial communities, phytoplankton and zooplankton community compositions and abundances, as well as zooplankton metabolism, and particulate organic carbon fluxes. Nearly all the parameters described in this section, as well as reports appearing elsewhere, showed time-course variabilities of Correspondence to: J. R. Dolan ([email protected]) similar magnitude to those known from a previous study of the spring-summer seasonal transition, a period of marked hydrological change, at the same study site. Remarkably, the least variable characteristic of the system appeared to be the identities of the dominant taxa across several trophic levels (copepods, phytoplankton, ciliates, and bacteria) throughout the study period despite large shifts in stock sizes and fluxes. Thus, the studies of DYNAPROC 2 documented considerable temporal variability of stocks and rates in a system which was, from a hydrological and taxonomic point of view, relatively stable

Seasonal to hour variation scales in abundance and production of total and particle-attached bacteria in the open NW Mediterranean Sea (0–1000 m)

We present the vertical and temporal dynamics of total <i>vs.</i> particle-attached bacterial abundance and activity over a 5 week period under summer to autumn transition in NW Mediterranean Sea. At a weekly time scale, total bacterial biomass and production in the euphotic layers was significantly correlated with phytoplanktonic biomass. At an hourly time scale, total bacterial biomass responded very rapidly to chlorophyll <i>a</i> fluctuations, suggesting a tight coupling between phytoplankton and bacteria for resource partitioning during the summer-autumn transition. In contrast, no influence of diel changes on bacterial parameters was detected. Episodic events such as coastal water intrusions had a significant positive effect on total bacterial abundance and production, whereas we could not detect any influence of short wind events whatever the magnitude. Finally, we show that particle-attached bacteria can represent a large proportion (up to 49%) of the total bacterial activity in the euphotic layer but display rapid and sporadic changes at hourly time scales. In the mesopelagic layers, bacterial abundance and production linearly decreased with depth, except some production peaks at 400–750 m. This study underlines the value of large datasets covering different temporal scales to clarify the biogeochemical role of bacteria in the cycling of organic matter in open seawater

Short-term changes in particulate fluxes measured by drifting sediment traps during end summer oligotrophic regime in the NW Mediterranean Sea

Short-term changes in the flux of particulate matter were determined in the central north western Mediterranean Sea (near DYFAMED site) using drifting sediment traps at 200 m depth in the course of the DYNAPROC 2 cruise (14 September–17 October 2004). In this period of marked water column stratification, POC fluxes varied by an order of magnitude, in the range of 0.03–0.29 mgC m<sup>−2</sup> h<sup>−1</sup> over the month and showed very rapid and high variations. Particulate carbon export represented less than 5% of integrated primary production, suggesting that phytoplankton production was essentially sustained by internal recycling of organic matter and retained within the photic zone. While PON and POP fluxes paralleled one another, the elemental ratios POC/PON and POC/POP, varied widely over short-term periods. Values of these ratios generally higher than the conventional Redfield ratio, together with the very low chlorophyll a flux recorded in the traps (mean 0.017 μg m<sup>−2</sup> h<sup>−1</sup>), and the high phaeopigment and acyl lipid hydrolysis metabolite concentrations of the settling material, indicated that the organic matter reaching 200 m depth was reworked (by grazing, fecal pellets production, degradation) and that algal sinking, dominated by nano- and picoplankton, made a small contribution to the downward flux. Over time, the relative abundance of individual lipid classes in organic matter (OM) changed from glycolipids-dominated to neutral (wax esters, triacylglycerols) and phospholipids-dominated, suggesting ecosystem maturation as well as rapid and continual exchanges between dissolved, suspended and sinking pools. Our most striking result was documenting the rapid change in fluxes of the various measured parameters. In the situation encountered here, with dominant regenerated production, a decrease of fluxes was noticed during windy periods (possibly through reduction of grazing). But fluxes increased as soon as calm conditions settle

Role of environmental factors for the vertical distribution (0–1000 m) of marine bacterial communities in the NW Mediterranean Sea

Bacterioplankton plays a central role in energy and matter fluxes in the sea, yet the factors that constrain its variation in marine systems are still poorly understood. Here we use the explanatory power of direct multivariate gradient analysis to evaluate the driving forces exerted by environmental parameters on bacterial community distribution in the water column. We gathered and analysed data from a one month sampling period from the surface to 1000 m depth at the JGOFS-DYFAMED station (NW Mediterranean Sea). This station is characterized by very poor horizontal advection currents which makes it an ideal model to test hypotheses on the causes of vertical stratification of bacterial communities. Capillary electrophoresis single strand conformation polymorphism (CE-SSCP) fingerprinting profiles analyzed using multivariate statistical methods demonstrated a vertical zonation of bacterial assemblages in three layers, above, in or just below the chlorophyll maximum and deeper, that remained stable during the entire sampling period. Through the use of direct gradient multivariate ordination analyses we demonstrate that a complex array of biogeochemical parameters is the driving force behind bacterial community structure shifts in the water column. Physico-chemical parameters such as phosphate, nitrate, salinity and to a lesser extent temperature, oxygen, dissolved organic carbon and photosynthetically active radiation acted in synergy to explain bacterial assemblages changes with depth. Analysis of lipid biomarkers of organic matter sources and fates suggested that bacterial community structure in the surface layers was in part explained by lipids of chloroplast origin. Further detailed analysis of pigment-based phytoplankton diversity gave evidence of a compartmentalized influence of several phytoplankton groups on bacterial community structure in the first 150 m depth

Occurrence and distribution of hydrocarbons in the surface microlayer and subsurface water from the urban coastal marine area off Marseilles, Northwestern Mediterranean Sea

Aliphatic (AHs) and polycyclic aromatic hydrocarbons (PAHs) were analyzed in dissolved and particulate material from surface microlayer (SML) and subsurface water (SSW) sampled at nearshore observation stations, sewage effluents and harbour sites from Marseilles coastal area (Northwestern Mediterranean) in 2009 and 2010. Dissolved and particulate AH concentrations ranged 0.05-0.41 and 0.04-4.3 mu g l(-1) in the SSW, peaking up to 38 and 1366 mu g l(-1) in the SML, respectively. Dissolved and particulate PAHs ranged 1.9-98 and 1.9-21 ng l(-1) in the SSW, amounting up 217 and 1597 ng l(-1) in the SML, respectively. In harbours, hydrocarbons were concentrated in the SML, with enrichment factors reaching 1138 for particulate AHs. Besides episodic dominance of biogenic and pyrogenic inputs, a moderate anthropisation from petrogenic sources dominated suggesting the impact of shipping traffic and surface runoffs on this urbanised area. Rainfalls increased hydrocarbon concentrations by a factor 1.9-11.5 in the dissolved phase. (C) 2011 Elsevier Ltd. All rights reserved

Impact of nutrient starvation on the biochemical composition of the marine diatom Thalassiosira weissflogii: from the whole cell to the frustule fraction

Interactions between carbon and silica in the diatom frustule play an important role in carbon export through their impact on diatom remineralization (carbon degradation and biogenic silica dissolution). To ameliorate model prediction of the fate of Si and organic matter during sedimentation, there is a need to first understand the origin and nature of Si-OC interactions, their impact on diatom remineralization and their variability with environmental conditions. In this study we focus on the impact of nutrient starvations on the formation and nature of these interactions in an ubiquitous diatom, Thalassiosira weissflogii. Fluorescence reveals the strong impact of all starvations on diatom metabolism while Fourier transformed infrared (FTIR) spectroscopy clearly showed that starvations altered the composition of the different diatom fractions. The relative compositions of whole cells were almost not impacted by starvations except Si(OH)4 starvation that slightly increased proteins relative contribution while decreasing carbohydrate. Starvation impacts became obvious looking at the composition of the different part of the diatom. The relative biochemical composition of the organic coating, protecting the frustule from the environment, was strongly affected by starvation. Under nitrate starvation, carbohydrate contribution increased while protein contribution decreased. Inversely, phosphate starvation increased the proportion of proteins and decreased carbohydrates contribution. Starvations also modified the different frustule phases. bSiO2 contribution decreased in the less reactive phase under silicate and phosphate starvation whereas nitrate starvation rather increased carbohydrate and protein pools. Phosphate starvation also led to an important shift of dominance among protein groups between amide I and amide II which compounds are suspected to play a key role in the frustule synthesis and architecture. Nutrient starvations affected the relative biochemical composition of diatom frustule fractions and organic coating which could imply a strong impact on frustule structure and architecture but also on frustule mechanical and chemical resistance

Diel and seasonal variations in abundance, activity, and community structure of particle-attached and free-living bacteria in NW Mediterranean Sea

International audienceDiel and seasonal variations in abundance, activity, and structure of particle-attached vs free-living bacterial communities were investigated in offshore NW Mediterranean Sea (0-1000 m). Attached bacteria were always less abundant and less diverse but generally more active than free-living bacteria. The most important finding of this study was that the activity of attached bacteria showed pronounced diel variations in the upper mixed water column with higher activities at night. Under mesotrophic conditions, the contribution of attached bacteria to total bacterial activity increased from less than 10% at day time to 83% at night time. At high chlorophyll a concentration, the highest cell-specific activities and contribution to total bacterial activity were due to free-living bacteria at day and to attached bacteria at night. Under summer oligotrophic conditions, free-living bacteria dominated and contributed to the most important part of the bacterial activity at both day and night, whereas attached bacteria were much less abundant but presented the highest cell-specific activities. These diel and seasonal variations in activities were concomitant to changes in bacterial community structure, mainly in the upper layer. The number of attached ribotypes was fairly constant suggesting that particles are colonized by a relatively limited number of ubiquitous ribotypes. Most of these ribotypes were also free-living ribotypes suggesting that attached bacteria probably originate from colonization of newly formed particles by free-living bacteria in the upper layer. These results reinforce the biogeochemical role of attached bacteria in the cycling of particulate organic carbon in the NW Mediterranean Sea and the importance of diel variability in these processes

Cross-slope variations of dissolved organic carbon in the Gulf of Cadiz, NE Atlantic Ocean (February 1998)

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Bacterial dynamics during the transition from spring bloom to oligotrophy in the Northwestern Mediterranean Sea

International audienceThe variability in microbial communities (abundance and biomass), bacterial production and ectoaminopeptidase activity, particulate and dissolved organic carbon (POC, DOC), and particulate and dissolved lipids was examined in spring 1995 in the northwestern Mediterranean, where a transition from the end of a bloom to pre-oligotrophic conditions was observed. Four time series of 36 h each and 4 h sampling intervals were performed at 5 m and at the chlorophyll maximum (30 m) between 11 and 31 May. Simultaneous measurements of pigments, abundance of hetero- and autotrophic flagellates, bacteria and POC enabled the estimation of living POC (defined as autotrophic-C plus heterotrophic-C biomass), and thus the detrital organic carbon. During the first 2 time series (11 to 15 May), the bacterial-C biomass was higher than the autotrophic-C biomass at 5 m (ratio 1.4 and 1.7), whereas the opposite trend was observed in the chlorophyll peak (ratio 0.7 for the first cycle). However, at the end of May, autotrophic-C biomass was equivalent to bacterial-C biomass at both depths studied. The detrital pool remained a more or less constant fraction of the POC (52, 53 and 47% on 11–12 May, 14–15 May and 30–31 May) at the chlorophyll peak, whereas it decreased significantly with time (62 to 53%) at 5 m. Relationships between bacterial activities and evolution of available resources were not systematically evidenced from our 36 h diel cycle data. Nevertheless, at the monthly scale, comparison of bacterial carbon demand (BCD) to potential carbon resources (detrital POC and DOC) showed that bacteria fed differently on the various pools. From ectoaminopeptidase turnover rates and detrital POC, the potential hydrolysis rate of detritus was calculated. Depending on the choice of conversion factors for bacterial production and estimates of hydrolysis turnover rates, it was shown that bacterial hydrolysis of detritus could be one of the DOC accumulation sources. We observed that the percentage of BCD supplied by detrital POC hydrolysis increased in the surface and decreased in the chlorophyll peak. An index of lipid degradation in POC, the lipolysis index, increased during the month at 5 m, also indicating a higher hydrolysis of POC. The opposite trend was observed in the chlorophyll maximum layer. The selective decrease in dissolved lipids in DOC in the chlorophyll maximum layer, particularly free fatty acids, also suggests that bacteria utilized increased fractions of carbon sources from the DOC. We concluded that partitioning between DOC and detritus as resources for bacteria can change during the rapid transition period from mesotrophy to oligotrophy in the northwestern Mediterranean