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The NW Mediterranean Sea with an inset showing the Bay of Villefranchesur-Mer and the sampling station Point B
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ABSTRACT: Nitrogen transformations were investigated In the Bay of Villefranche-sur-Mer, on the south coast of France, during autumn 1990 and spring 1991, using '"N-isotope techniques on sizefractionated sea water samples. Besides measuring uptake of ammonium and nitrate, and ammonium regeneration, the biological composition in 4 size fractions (<...
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... only previous I5N-based studies of new and regenerated production in the NW Mediterranean Sea of which we know were performed in the Gulf of Lions ( Owens et al. 1989, Woodward et al. 1990), an area adjacent to the Ligurian Sea (where the present work was undertaken, see map, Fig. 1). In the Ligurian Sea, 15N-based uptake studies had never been performed before, but new production has indirectly been esti- mated to ca 25 % as a yearly mean, and ca 40 % during spring bloom ( Minas et al. 1988, Fowler et al. 1991 Fowler et al. (1991) concluded that the Ligunan Sea was ...
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... microbial loop has been investigated for a long time in the Ligunan Sea, and especially in the Bay of V~llefranche-sur-Mer (see map, Fig. 1). These waters are dominated by pico-and nanoplankton, mainly con- sisting of cyanobactena (Synechococcus sp.), hetero- trophic bactena and nanoflagellates [Rassoulzadegan 1979, Hagstrom et al. 1988, Lins da Sllva 1991. Not even dunng the spring bloom, however, does the abundance of microphytoplankton become very large. It is the ...
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... Nitrogen uptake experiments were performed on the following dates: 9 and 30 November 1990;12, 24 and 30 April;15 and 31 May 1991. Ammonium regeneration studies were performed on 4 December 1990, 15 April, 17 and30 May 1991. Water was collected in the morning at 5 m depth at Point B in the Bay of Villefranche-sur-Mer, France [see map, Fig. 1) using a 30 1 Niskin sampler, and the expenmental work was performed the same day as sampling. The water was size-fractionated using in- verse filtration according to Sheldon & Rassoulzadegan (1987), with 100 Fm filter mesh or 1 and 10 pm Nuclepore filters (0 = 142 mm). For the uptake ex- penments the water was post-fractionated, for ...
Citations
... study, the changes in NH 4 + /NO 3 − ratio showed a very strong positive relationship with PROC and SYN in CL 1 and CL 3. Wafar et al. (2004) found that N uptake by APP was mainly supported by regenerated nitrogen originating from NH 4 + (66%) and urea (17%). A strong preference of APP for a reduced form of nitrogen (NH 4 + ) (Probyn et al., 1990;Selmer et al., 1993) is particularly evident in autotrophic prokaryotes (PROC and SYN). For example, Moore et al. (2002) reported that almost all isolates of PROC and SYN are restricted to NH 4 + as a nitrogen source. ...
The mechanisms responsible for the development of various structural and functional features of the microbial food web (MFW) and their dynamics at spatial and temporal scales, which are important for predicting their responses to future environmental changes, are largely unknown. More than 3000 datasets of environmental and microbial variables collected over a decade on a seasonal and large spatial scale in the Adriatic Sea were analyzed. The sets of environmental variables were classified into four clusters (representing different environmental states) using Neural Gas analysis and the differences in MFW structure between the clusters were analyzed. Different variants of MFW evolve in the different clusters in terms of the abundance of MFW components, their ratios, growth and grazing rates, predator preference in prey selection, the strength of predator-prey interaction, and the relative importance of top-down and bottom-up control. However, these clusters are neither spatially nor temporally fixed; rather, the area studied represents a mosaic of different environmental conditions that alternate from one state to another on a time scale. In each of the environmental states, a distinct structure of MFW develops that shows consistent and repeatable changes that strictly follow the switching in environmental conditions from one state to another.
... curvisetus, resulting in lower Nrt2 expression (Fig. 7B). In marine ecosystems, nanoflagellates are major contributors of regenerated ammonium (Selmer et al., 1993). This mechanism suggests a negative correlation between Nrt2 expression and nanoflagellate abundance, as revealed by our observation at the offshore station (Fig. 7C). ...
In this study, the mRNA levels of a diatom nitrate transporter gene, Nrt2, were investigated in a coastal ecosystem on the western boundary of the northwestern Pacific Ocean. During the period between June 2011 and June 2013, 14 cruises were conducted, and samples were collected for nutrient-manipulation incubations, quantitative polymerase chain reactions (qPCRs), and metatranscriptome construction. Based on metatranscriptomes generated through next-generation sequencing, the dominant diatoms and sequences associated with Nrt2 as well as EFL, a qPCR reference gene, were identified. Next, primer pairs for a dominant bloom-forming diatom, Chaetoceros cf. curvisetus, were designed, and the expression levels of Nrt2 were measured by qPCR in both original (untreated) and nitrogen-deprived samples. By calculating the ratio of Nrt2 expression between these 2 samples, a Log (fNrt2) index was developed to be used as a molecular indicator of nitrogen demand. At our study site, the nitrate and nitrite concentrations displayed a clear seasonal cycle, with high (8.75 μM) and low (0.05 μM) concentrations occurring in December and June, respectively. The Log (fNrt2) index also varied with time in 2012, with a high value of approximately −1.44 occurring in October or November; this variation was not in phase with the nitrate trend. The regression analysis indicated that Log (fNrt2) was inversely correlated with nitrate and nitrite concentrations. On the other hand, Log (fNrt2) was positively correlated with bacterial abundance. This regression equation properly included values of Log (fNrt2) that were not well explained by the nitrate and nitrite concentrations. When the Chaetoceros abundance increased, the associated low value of Log (fNrt2) suggested the repression of Nrt2 by ammonia; apparently, the diatom utilizes ammonia as the nitrogen source to maintain its dominance in the later phase of the bloom. Since bacteria and nanoflagellates are actively involved in the uptake and release of nitrogenous nutrients, these two functional groups also control the expression of Nrt2 through important ecological processes such as competition and regeneration.
... Wafar et al. [103] found that N uptake by APP was mainly supported by regenerated N originating from NH4 + (66%) and urea (17%). A strong preference of APP for a reduced form of nitrogen (NH4 + ) [104,105] is particularly expressed in autotrophic prokaryotes (PROC and SYN). Thus, Moore et al. [106] reported that nearly all PROC and SYN isolates are limited to NH4 + as their source of nitrogen. ...
A recent analysis of the Mediterranean Sea surface temperature showed significant annual warming. Since small picoplankton microorganisms play an important role in all major biogeochemical cycles, fluxes and processes occurring in marine systems (the changes at the base of the food web) as a response to human-induced temperature increase, could be amplified through the trophic chains and could also significantly affect different aspects of the structure and functioning of marine ecosystems. In this study, manipulative laboratory growth/grazing experiments were performed under in situ simulated conditions to study the structural and functional changes within the microbial food web after a 3 °C increase in temperature. The results show that a rise in temperature affects the changes in: (1) the growth and grazing rates of picoplankton, (2) their growth efficiency, (3) carrying capacities, (4) sensitivity of their production and grazing mortality to temperature, (5) satisfying protistan grazer carbon demands, (6) their preference in the selection of prey, (7) predator niche breadth and their overlap, (8) apparent uptake rates of nutrients, and (9) carbon biomass flow through the microbial food web. Furthermore, temperature affects the autotrophic and heterotrophic components of picoplankton in different ways.
... A strong preference of S-Phy for reduced nitrogen (ammonium) (Chang et al., 1989;Probyn et al., 1990;Selmer et al., 1993) is particularly expressed in prokaryotes. Moore et al. (1998Moore et al. ( , 2002 reported that nearly all Prochlorococcus isolates are limited to AMM as their source of nitrogen. ...
... Although S-Phy (especially cyanobacteria) is an important microbial component in AMM uptake (Wan et al., 2018), it has often been suggested that a substantial fraction of AMM assimilation in marine microbial populations is associated with non-photosynthetic organisms (Kirchman, 2000), largely heterotrophic bacteria (Selmer et al., 1993;Rodrigues and Williams, 2002). When ambient AMM concentration was relatively high, AMM contributed to about 80% to the bacterial DIN utilization (Rodrigues and Williams, 2002). ...
Vertical mixing and stratification are among the most important physical processes controlling nutrient dynamics, the dominant category of primary producers and consequently the dominant types of food web, and are therefore important for the assessment of the marine ecosystem's response to global climate change. This study showed consistent short-term cyclic successions of the plankton food web types, governed by the dynamics of water column stability changes, occurring in generally oligotrophic, phosphate deficient surface waters of the open middle Adriatic Sea. The biogeochemical nitrogen cycle appeared as a key driving force responsible for the food web structure changes. The ‘herbivorous food web’ dominated during the nitrate-rich mixed water column period (winter) and gradually changed to ‘multivorous food web’ where large phytoplankton still constitute a significant fraction of phytoplankton. This intermediate type of food web lasted for a short time and quickly changed to the typical ‘microbial food web’, which then dominated during the stratified water column period (summer) and was characterised by a large proportion of picoplankton size-fraction organisms in total plankton biomass and production. Furthermore, at the very end of summer, the high bacterial carbon flux through the ‘microbial loop’ was established. The succession of food web types affects the mechanisms of bacterial control in a way that ‘bottom up’ control dominated during the mixed water column period and ‘top-down’ control prevailed during the stratified period. Since the ongoing global warming is expected to change water column stability dynamics and thereby significantly affect the supply of nutrients in surface waters, this study helps to understand the possible direction of changes in the plankton food webs of the Adriatic Sea, and consequent changes in marine nitrogen and carbon biogeochemical cycles.
... Oxygen concentration was measured using a SBE43 sensor mounted on the CTD and calibrated using the Winkler method (Strickland and Parsons, 1968). Apparent oxygen utilization (AOU) was calculated as the difference between the oxygen solubility values at saturation with respect to the atmosphere and the measured dissolved oxygen concentration (computed in Ocean Data View software; Schlitzer, 2006). Photosynthetically active radiance (PAR) and fluorescence were measured using a Biospherical and SeaPoint Fluorometer sensors, respectively. ...
The phytoplankton community composition, structure, and biomass were investigated under stratified and oligotrophic conditions during summer for three consecutive years in the Mediterranean Sea. Our results reveal that the phytoplankton community structure was strongly influenced by vertical stratification. The thermocline separated two different phytoplankton communities in the two layers of the euphotic zone, characterized by different nutrient and light availability. Picoplankton dominated in terms of abundance and biomass at all the stations sampled and throughout the photic zone. However, the structure of the picoplanktonic community changed with depth, with Synechococcus and heterotrophic prokaryotes dominating in surface waters down to the base of the thermocline, and Prochlorococcus and picoeukaryotes contributing relatively more to the community in the deep chlorophyll maximum (DCM). Light and nutrient availability also influenced the communities at the DCM layer. Prochlorococcus prevailed in deeper DCM waters characterized by lower light intensities and higher picophytoplankton abundance was related to lower nutrient concentrations at the DCM. Picoeukaryotes were the major phytoplankton contributors to carbon biomass at surface (up to 80%) and at DCM (more than 40%). Besides, contrarily to the other phytoplankton groups, picoeukaryotes cell size progressively decreased with depth. Our research shows that stratification is a major factor determining the phytoplankton community structure; and underlines the role that picoeukaryotes might play in the carbon flux through the marine food web, with implications for the community metabolism and carbon fate in the ecosystem.
... In the Bay of Villefranche-sur-mer (South of France), nutritive conditions can be seen as oligotrophic to mesotrophic (Selmer et al., 1993). Recent blooms of O. cf. ...
Environmental factors that shape dynamics of benthic toxic blooms are largely unknown. In particular, for the toxic dinoflagellate Ostreopsis cf. ovata, the importance of the availability of nutrients and the contribution of the inorganic and organic pools to growth need to be quantified in marine coastal environments. The present study aimed at characterizing N-uptake of dissolved inorganic and organic sources by O. cf. ovata cells, using the ¹⁵N-labelling technique. Experiments were conducted taking into account potential interactions between nutrient uptake systems as well as variations with the diel cycle. Uptake abilities of O. cf. ovata were parameterized for ammonium (NH4⁺), nitrate (NO3⁻) and N-urea, from the estimation of kinetic and inhibition parameters. In the range of 0 to 10 μmol N L⁻¹, kinetic curves showed a clear preference pattern following the ranking NH4⁺ > NO3⁻ > N-urea, where the preferential uptake of NH4⁺ relative to NO3⁻ was accentuated by an inhibitory effect of NH4⁺ concentration on NO3⁻ uptake capabilities. Conversely, under high nutrient concentrations, the preference for NH4⁺ relative to NO3⁻ was largely reduced, probably because of the existence of a low-affinity high capacity inducible NO3⁻ uptake system. Ability to take up nutrients in darkness could not be defined as a competitive advantage for O. cf. ovata. Species competitiveness can also be defined from nutrient uptake kinetic parameters. A strong affinity for NH4⁺ was observed for O. cf. ovata cells that may partly explain the success of this toxic species during the summer season in the Bay of Villefranche-sur-mer (France).
... Comparatively, systems outside the bay receive more new nutrients coming from the sea (eventually through upwelling) and contribution of other nutrients may be lower. Regenerated nutrients and nutrients coming from terrigenous sediment are both 15 N-enriched compared to new nutrients coming from the sea (Selmer et al. 1993;Pantoja et al. 2002). Hence, from spring the embayment of site 2 may increase the relative contribution to primary production of 15 Nenriched nutrients, which may increase the δ 15 N baseline in site 2 and explain the global 15 Nenrichment of all trophic entities. ...
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Background. In Mediterranean subtidal rocky reefs, macrophytes belonging to the genus Cystoseira (Phaeophyceae) form structurally complex habitats, called Cystoseira forests. Due to the occurrence of multiple anthropogenic stressors, Cystoseira forests disappeared from numerous localities in the Mediterranean Sea and are deteriorating in other localities. Cystoseira forests are usually replaced by structurally less complex habitats such as shrublands, turfs and barren grounds. The consequences of such habitat shifts on fish assemblage structure are still poorly known.
Aim. The present work aimed at comparing juvenile and adult fish assemblage structures between Cystoseira forests and structurally less complex habitats, and at investigating the ecological processes underlying fish distribution patterns. Special attention was paid to the processes related to two putative ecosystem functions of Cystoseira spp.: 'habitat-former' and 'basal source of organic material'.
Methods. To achieve these goals, multiple complementary approaches were used: macrophytes and fish field surveys, prey and piscivorous fish habitat-choice and predation experiments in tanks, food web analysis by using stable isotopes and piscivorous fish stomach contents analyses.
Results. Field surveys highlighted that Cystoseira forests, compared to less structurally complex habitats, present higher densities of both juveniles and adults fish belonging to (1) the crypto-benthic Blenniidae, Gobiidae, Trypterigidae and Cliniidae, (2) the necto-benthic Labridae, and (3) the necto-benthic Serranidae and Scorpaenidae. Fish of the latter group prey upon macrofauna and fish of the two former groups, as evidenced by stable isotopes and stomach contents analyses. It thus appeared that prey and piscivorous fish co-habit in higher densities in Cystoseira forests.
Tank experiments highlighted that this co-habitation was explained, at least partially, by the function 'habitat former' of Cystoseira spp., which induces (1) reduced mortality of prey fish in forests due to higher shelter availability, and (2) net immigration into forests of both prey and piscivorous fish due to their preference for the high structural complexity of forests.
Food web analysis based on stable isotopes suggested that Cystoseira forest food web may rely heavily on Cystoseira spp. as basal source of organic material. However, this was not clearly demonstrated since carbon and nitrogen isotopic compositions did not enable to distinguish Cystoseira spp. from some other possible sources such as particulate organic matter.
Conclusions. The present work evidence that threatened Cystoseira spp., as habitat formers and maybe also as primary producers, support high diversity and densities of fish, including (1) juvenile fish, and consequently the potential for fish population replenishment, as well (2) socio-economically important fish species such as Scorpaena spp. and Serranus spp.. This stresses the need to better manage human activities impacting Cystoseira spp..
... The Rade de Villefranche displays a trophic status between oligotrophic (Bustillos-Guzmán et al. 1995;Lacroix and Nival 1998) and mesotrophic (Selmer et al. 1993). Its food web is based largely on the microbial loop (e.g., Dolan et al. 1995;Thingstad et al. 1998;Bonilla-Findji et al. 2010). ...
The Service d'Observation de la Rade de Villefranche-sur-Mer is designed to study the temporal variability of hydrological conditions as well as the abundance and composition of holo- and meroplankton at a fixed station in this bay of the northwest Mediterranean. The weekly data collected at this site, designated as "Point B" since 1957, represent a long-term time series of hydrological conditions in a coastal environment. Since 2007, the historical measurements of hydrological and biological conditions have been complemented by measurements of the CO2-carbonic acid system parameters. In this contribution, CO2-carbonic acid system parameters and ancillary data are presented for the period 2007-2011. The data are evaluated in the context of the physical and biogeochemical processes that contribute to variations in CO2 in the water column and exchange of this gas between the ocean and atmosphere. Seasonal cycles of the partial pressure of CO2 in seawater (pCO2) are controlled principally by variations in temperature, showing maxima in the summer and minima during the winter. Normalization of pCO2 to the mean seawater temperature (18.5 °C), however, reveals an apparent reversal of the seasonal cycle with maxima observed in the winter and minima in the summer, consistent with a biogeochemical control of pCO2 by primary production. Calculations of fluxes of CO2 show this area to be a weak source of CO2 to the atmosphere during the summer and a weak sink during the winter but near neutral overall (range -0.3 to +0.3 mmol CO2 m-2 h-1, average 0.02 mmol CO2 m-2 h-1). We also provide an assessment of errors incurred from the estimation of annual fluxes of CO2 as a function of sampling frequency (3-hourly, daily, weekly), using data obtained at the Hawaii Kilo Nalu coastal time-series station, which shows similar behavior to the Point B location despite significant differences in climate and hydrological conditions and the proximity of a coral reef ecosystem.
... Using the hierarchical classification, the discrimination of the sampling depths suggested that the anticyclonic eddy has lead to a modification of nutrient conditions in the water column. Low concentration of nutrients may suggest that the ecosystem can be supported by a microbial loop (regenerated production) in the upper layer (Selmer et al., 1993). Consequently, high abundance of heterotrophic prokaryotes is expected in the epipelagic zone as previously reported in some anticyclonic eddies (Lasternas et al., 2013). ...
... Using the hierarchical classification, the discrimination of the sampling depths suggested that the anticyclonic eddy has lead to a modification of nutrient conditions in the water column. Low concentration of nutrients may suggest that the ecosystem can be supported by a microbial loop (regenerated production) in the upper layer (Selmer et al., 1993). Consequently, high abundance of heterotrophic prokaryotes is expected in the epipelagic zone as previously reported in some anticyclonic eddies (Lasternas et al., 2013). ...
The distribution of ultraphytoplankton was investigated in the western North Pacific Subtropical Gyre (NPSG) during La Niña, a cold phase of El Niño Southern Oscillation (ENSO). Observations were conducted in a north-south transect (33.6–13.25° N) along the 141.5° E meridian in order to study the ultraplankton assemblages in various oligotrophic conditions. Analyses were performed at the single cell level by analytical flow cytometry. Five ultraphytoplankton groups (Prochlorococcus, Synechococcus, picoeukaryotes, nanoeukaryotes and nanocyanobacteria-like) defined by their optical properties were enumerated in three different areas visited during the cruise: the Kuroshio region, the subtropical Pacific gyre and a transition zone between the subtropical Pacific gyre and the Warm pool. Prochlorococcus outnumbered the other photoautotrophs in all the investigated areas. However, in terms of carbon biomass, an increase in the relative contribution of Synechococcus, picoeukaryotes and nanoeukaryotes was observed from the centre of the subtropical gyre to the Kuroshio area. In the Kuroshio region, a peak of abundance of nanoeukaryotes observed at the surface suggested an increase in nutrients likely due to the vicinity of a cold cyclonic eddy. In contrast, in the salinity front along the isohaline 35 and anticyclonic eddy located around 22.83° N, the mainly constant distribution of Prochlorococcus from the surface down to 150 m characterised the dominance by these microorganisms in high salinity and temperature zone. Results suggested that the distribution of nanocyanobacteria-like is also closely linked to the salinity front rather than low phosphate concentration. The maximum abundance of ultraphytoplankton was located above the SubTropical Counter Current (STCC) at depths > 100 m where higher nutrient concentrations were measured. Finally, comparison of the ultraphytoplankton concentrations during El Niño (from the literature) and La Niña (this study) conditions seems to demonstrate that La Niña conditions lead to higher concentrations of Synechococcus in the Subtropical gyre and a lower abundance of Synechococcus in the Kuroshio region. Our results suggest that the west part of NPSG is a complex area, where different water masses, salinity fronts and eddies lead to a heterogeneous distribution of ultraphytoplankton assemblages in the upper layer of the water column.
