Trophic Regimes Research Articles

Is diapause not needed? Long-term storage potential of zoophytophagous plant bug Dicyphus errans in relation to day length, temperature, and trophic regimes

ABSTRACT Dicyphus errans (Wolff) (Heteroptera: Miridae) is a biocontrol agent of small plant-sucking pests. The study was aimed to estimate its storage potential. Nymphs of the Italian strain were reared on tobacco (Nicotiana tabacum L.) plants and fed on eggs of the grain moth Sitotroga cerealella (Olivier) (Lepidoptera: Gelechiidae) and bee-collected flower pollen under different photoperiodic/thermal/trophic conditions. Then adults were stored at 3, 6, 9 or 12°C for 0–120 days in the dark and their subsequent longevity and fecundity were assessed. The life span of the D. errans adults was significantly affected by storage temperature, trophic regime, and sex of adults, but not the photoperiodic conditions during pre-adult development. Our results indicate that experimental conditions that we tested probably induced a weak diapause that terminated spontaneously and quickly and that did not increase the long-term storage potential of D. errans. When nymphs developed under the long day conditions and adults were stored at 8°C and fed on eggs of the grain moth (the optimal combination of factors), survival of females after 30 days of storage was about 80% and their fecundity was not significantly different from that of the control (not stored) bugs.

Plant cell culture: Triterpenoid production in the context of trophic regimes

Plant cell culture: Triterpenoid production in the context of trophic regimes

Influence of lateral advection on phytoplankton size-structure and composition in a Mediterranean coastal area

Phytoplankton size is a significant functional trait affecting many ecological processes and influencing the carbon fate and the trophic regimes of the ecosystems. In this study, we examined the effect of the lateral advection of nutrient-rich Messina Mixed Waters on the phytoplankton size structure and community composition in the euphotic layer of the Gulf of Augusta (Western Ionian Sea) in early autumn. The chlorophyll a concentration ranged from 0.08 to 0.8 mg m−3 and the phytoplankton abundances ranged from 21 × 103 to 245 × 103 cells L−1. The size structure of phytoplankton biomass, obtained by sequential filtration of water samples, suggested the dominance of micro-planktonic diatoms in an active Deep Chlorophyll Maximum (DCM) whereas smaller forms were mainly represented by phytoflagellates. A short-term tidal-induced variability of the assemblage structure was observed at a “sentinel” station located at the Gulf shelf-break where lateral advection due to meso-scale dynamics sustained most of the phytoplankton biomass.Our research shows that, in stratified conditions, nutrient enrichment by lateral advection at DCM modifies the size-structure of phytoplankton with respect to that expected from the reference baseline oceanic model validated in open ocean waters. In particular, while smaller fraction incidences reveal a surface oligotrophic system, an increase of the micro-sized fraction is triggered at DCM: this indicates the prevalence of “new production” processes which is a quite unusual condition for the Mediterranean DCM in early autumn, when trophic web is typically sustained by “regenerate production”. Our observations suggest that both “microbial” and “herbivore” pathways could coexist and regulate the carbon flow in the Gulf in this season.

Eutrophication as a driver of microbial community structure in lake sediments.

Lake sediments are globally important carbon sinks. Although the fate of organic carbon in lake sediments depends significantly on microorganisms, only few studies have investigated controls on lake sedimentary microbial communities. Here we investigate the impact of anthropogenic eutrophication, which affects redox chemistry and organic matter (OM) sources in sediments, on microbial communities across five lakes in central Switzerland. Lipid biomarkers and distributions of microbial respiration reactions indicate strong increases in aquatic OM contributions and microbial activity with increasing trophic state. Across all lakes, 16S rRNA genes analyses indicate similar depth-dependent zonations at the phylum- and class-level that follow vertical distributions of OM sources and respiration reactions. Yet, there are notable differences, such as higher abundances of nitrifying Bacteria and Archaea in an oligotrophic lake. Furthermore, analyses at the order-level and below suggest that changes in OM sources due to eutrophication cause permanent changes in bacterial community structure. By contrast, archaeal communities are differentiated according to trophic state in recently deposited layers, but converge in older sediments deposited under different trophic regimes. Our study indicates an important role for trophic state in driving lacustrine sediment microbial communities and reveals fundamental differences in the temporal responses of sediment Bacteria and Archaea to eutrophication.

Seasonal distributions of ocean particulate optical properties from spaceborne lidar measurements in Mediterranean and Black sea

Assessing the oceanic surface layer's optical properties through CALIOP has been one of the reasons of the extension of the CALIOP mission for 3 more years (2018–2020). This is the first work evaluating the potential use of CALIOP for ocean applications at regional scale in mid-latitude regions (i.e. Mediterranean, MED, and Black Sea, BS) and investigating the added information on ocean particles given by the column integrated depolarization ratio (δT) parameter. We implemented and refined a retrieval procedure to estimate this parameter at 1/4 degree of spatial resolution, comparing 7 years of CALIOP observations (2011–2017) to the corresponding Copernicus multi-sensor L3 ocean colour products of the surface particle backscattering coefficient (bbp) and chlorophyll-a concentration (Chl-a). This study pointed out that the current CALIOP sampling is inadequate to detect subtle day-night difference due to plankton diel variability for these basins. At a basin scale, δT covaries with bbp for bbp ≥ 0.0015 m−1. This is more evident for BS (R = 0.84) than for MED (R = 0.61). The analysis of seasonal distributions confirm this result for BS, where δT has a semi-annual cycle in very good agreement with bbp. In the MED, characterized by different trophic regimes, δT shows also some similarities with Chl-a annual cycle. The combined characterization in the MED bioregions of the annual patterns of bbp:Chl-a, δT:Chl-a and δT:bbp ratios suggested that δT parameter can provide valuable information about the non-sphericity and the size of ocean particles.

Litter-dwelling invertebrates in natural and plantation forests in the southern Carpathians, Romania

Forest plantations usually have a poorer fauna than native forests of the same region. Exceptions seem to appear in colder areas of Europe with few native forests. The Jiu Gorge National Park (JGNP) is situated in the Southern Romanian Carpathians, where numerous native, especially beech, forests are present. However, in the southern part of JGNP there are plantations with non-native species, like pine or black locust, where previous studies had reported a poor fauna. Based on this information, we supposed that this fauna poverty in plantations will be obvious also in the case of the litter macrofauna. This was verified by analyzing the litter macrofauna from 15 forests in the JGNP (10 natural forests and five forest plantations). We collected 12,950 individuals belonging to 28 invertebrate groups. The highest number of taxa was registered in the sessile oak forest, while the highest individual number and taxa diversity was observed in two beech forests. In the beech and birch reforestation areas the fauna was poorer than in old mature forests of the same tree species. In contrast to native forests, the fauna in plantations was much poorer, especially in the pine plantations. Detritiphagous taxa were the most affected by factors of the plantations. In the plantations, more mobile groups with various trophic regimes prevail. The plantations that are present in the southern areas of JGNP, replaced the original sessile oak forests, compared to which they have a much poorer fauna. Forest plantations from JGNP have low value for biodiversity, compared to the northern European areas where natural forests became very rare and the fauna is recent. JGNP is not in the same situation, having extended native forests, which are present in the area since the glacial periods. Therefore, these forests shelter the native fauna of the region. There are few plantations, present only in the disturbed part of JGNP, but even there they have only a very low importance for biodiversity. The data from JGNP confirms the fact that zones with high biodiversity and native forests should be conserved.
 Key words: Carpathians, beech, forest plantations, past, native, invertebrates, leaf litter, forestry.

Эволюция геоэкологического состояния Отказненского водохранилища Ставропольского края

This study was conducted to monitor and assess the environmental status of the Otkaznensky Reservoir of the Stavropol Territory from 1978 to 2018 and is mainly based on the analysis of satellite remote sensing data using GIS technology, archives materials and information from open sources of information. Anthropogenic activity of the second half of the twentieth century for the North Caucasus, it was accompanied by significant regulation and changes in river flow, the creation of a powerful artificial hydrographic network, changes in trophic state and hydrochemical regime of water bodies. By the beginning of the 1970s, the hydrographic network was replenished with a water management complex consisting of main canals, for example, the Greater Stavropol and Tersko-Kumsky. The total length of all channels was 4000 km (for comparison, the length of the Kuma River is ~ 800 km, the Kalaus River is ~ 400 km). The volume of annual water transfer through canals reached 2500 mn m3, and up to 90 % is the water of the Kuban River. Data from satellite remote sensing made it possible to establish the features of the environmental status, including long-term and seasonal dynamics of the area and other morphometric characteristics of the plain reservoir for the entire period of operation. Analysis of publications on the topic of remote monitoring and assessment of the dynamics of the state of water bodies and hydrographic networks in general has shown the high efficiency and prospects of using remote sensing methods. They provide multiscale and multi-time study. Ecosystems of water bodies are perfectly manifested in satellite images, they are not hidden in any way, they are single-tier, they are well decrypted both in texture and in spectral characteristics.

Morphometric response of late Aptian planktonic foraminiferal communities to environmental changes: A case study of Paraticinella rohri at Poggio le Guaine (central Italy)

The Aptian-Albian boundary interval represents a pivotal time of pronounced climate/ocean changes associated with Oceanic Anoxic Event 1b (OAE 1b) that seriously influenced marine pelagic ecosystems. The sedimentary successions record the first and most significant species turnover in the evolutionary history of planktonic foraminifera, consisting of the selective extinction/disappearance of large Aptian species and resulting in inter-specific size reduction (the post-extinction dwarfing as in the Lilliput effect) in the earliest Albian assemblages. In this study we investigate how the planktonic foraminifera respond to crucial environmental stress and if any intra-specific decrease in test size predate the extinction (pre-extinction dwarfing). Paraticinella rohri was chosen as the most representative of the Aptian planktonic foraminiferal taxa and its test-size evolution was compared with the high-resolution record of redox and productivity geochemical proxies through the uppermost Aptian - lowermost Albian pelagic succession of Poggio le Guaine (central Italy). Our geochemical data document long-term fluctuations characteristic of a system with transitions among different states (oligotrophic-mesotrophic-eutrophic and oxic-dysoxic-anoxic). The strong correlation of the large test-size changes of Pa. rohri during its total range with higher productivity (increased Baexcess concentrations) suggests that the trophic regime was the key factor controlling robust abundance and size variability in this taxon. The progressive increase in surface water fertility during the late Aptian offered optimum ecological conditions for planktonic foraminifera, that increased in abundance, size, and specific diversity. After the transient anoxic 113/Jacob sub-event, smaller to dwarfed specimens of Pa. rohri and depauperate planktonic foraminifera assemblages faced high selective pressures due to more oligotrophic conditions. Transition to a more eutrophic ocean in the latest Aptian irreversibly eroded their resilience, and Aptian dwarfed k-strategist taxa became extinct. The planktonic foraminifera turnover has been therefore the result of an irreversible and drastic regime shift toward a more eutrophic ecosystem that promoted the thriving of small-sized opportunistic/disaster planktonic foraminiferal species and the siliceous radiolaria.

Manipulation of trophic capacities in Haematococcus pluvialis enables low-light mediated growth on glucose and astaxanthin formation in the dark

Haematococcus pluvialis is an industrial green microalga, which presents the best photosynthetic source of the keto-carotenoid astaxanthin due to its high content, reaching over 4% of dry weight. Astaxanthin production by Haematococcus is currently achieved by a two-stage photoautotrophic process, which involves the transition of green biomass to nitrogen starvation under high light to induce astaxanthin accumulation. This production mode is associated with high costs, due to relatively low biomass productivities and other factors. The heterotrophic mode of cultivation, based on organic carbon assimilation under controlled conditions, may offer a solution to the disadvantages of phototrophic cultivation. We created transgenic strains of H. pluvialis by transformation of the hexose uptake protein (HUP1) gene from the green microalga Parachlorella kessleri into the nuclear genome. This manipulation resulted in the ability of modified H. pluvialis strains to proliferate in the dark and under very low illumination on nutrient media supplemented with glucose. Furthermore, modified strains were able to grow and produce astaxanthin under specific conditions. All tested transformants were susceptible to the toxic glucose analogue 2‑deoxyglucose, supporting that the foreign glucose carrier was functional. Here, we report the initial characterization of the transformants cultivated on solid and in liquid media, and address the implications of manipulating trophic regimes in H. pluvialis for biotechnological applications.

Spatial and Temporal Long-Term Patterns of Phyto and Zooplankton in the W-Mediterranean: RADMED Project

It is widely accepted that the Mediterranean is an oligotrophic sea where winter mixing favors the proliferation of diatoms and high values of zooplanktonic biomass, mainly associated with the growth of copepods. Stratified conditions from mid-spring to late autumn are dominated by the picophytoplanktonic groups and the increment of cladoceran abundances. This general picture has important exceptions. A regionalization of the Mediterranean Sea can be established, distinguishing oligotrophic and mesotrophic areas and different blooming periods. The RADMED monitoring program covers a large area from the southwestern limit of the Mediterranean to the Catalan Sea. The analysis of phyto and zooplankton time series extending from 1992 to 2016 in some cases, and from 2007 to 2016 in others, have shown that the Spanish Mediterranean waters have differentiated areas and trophic regimes as a result of the existence of several fertilizing mechanisms which include winter mixing, tidal mixing in the Strait of Gibraltar, cyclonic circulation cells and frontal systems. The present work describes these different mechanisms acting on the Spanish Mediterranean waters, and also the potentiality of monitoring programs for providing statistics suitable for operational activities or the initialization/validation of ecological models.

Interaction between the land and the sea: sources and patterns of nutrients in the scattered coastal zone of a eutrophied sea

A long-term trophic development of three geographical transects—including a river mouth, an estuary, and an archipelago—were studied in the southern Finnish coast in the Baltic Sea. Each transect was studied to clarify how far off the coast the land-based nutrient sources (catchment factor, CF) had a decisive role in shaping the wintertime regimes of dissolved inorganic nitrogen and dissolved inorganic phosphorus and where the marine processes (marine factor, MF) start to play a major role. Generally, CF controlled the nutrient regime from the coast to the outer brink of the inner coastal area, after which MF started to dominate. The estuaries exhibited steep vertical nutrient gradients, above which the riverine input dominated the nutrient regime. The extent of the area where CF dominated the nutrient regime was therefore decisively dependent on estuarine stratification, i.e., whether the conclusions were drawn based on the surface layer data, including the riverine impact, or on the data beneath that layer, including the marine impact. This result deviates from the current consensus that the trophic regime of the sea is most directly assessed by the surface layer nutrient content. The estuarine nutrient regime is unrepresentative to that of a typical coastal water body due to the strong land-based impact on the estuary. Therefore, any generalization of the trophic condition of an estuary to represent areas farther off the coast should be done cautiously. The estuaries should also be defined as belonging to transitional waters according to the typology related to European Marine Legislation.

Impact of decadal reversals of the north Ionian circulation on phytoplankton phenology

Abstract. In the north Ionian, water circulation is characterized by a decadal alternation of cyclonic and anticyclonic regime driven by the mechanism called BiOS (bimodal oscillating system). The circulation regimes affect both vertical dynamics and the nutrient distribution. The north Ionian is then a good study area to investigate how changes in circulation can affect phytoplankton dynamics in oligotrophic regions. From in situ observations, for each circulation regime the averaged distribution of isopycnals is provided, and a depth difference of about 80 m is estimated for the nitracline between the cyclonic and anticyclonic regime. Based on phytoplankton phenology metrics extracted from annual time series of satellite ocean color data for the period 1998–2012, the cyclonic and anticyclonic regimes are compared. Results show that the average chlorophyll in March, the date of bloom onset and the date of maximum chlorophyll were affected by circulation patterns in the north Ionian. In the center of the north Ionian gyre, the bloom started in December and chlorophyll was low in March when circulation was anticyclonic, whereas during the cyclonic circulation regime, a late chlorophyll peak, likely resulting from different phytoplankton dynamics, was commonly observed in March. An additional analysis shows that the winter buoyancy losses, which govern the mixed layer depth (MLD), also contribute to explaining the interannual variability in bloom onset and intensity. Two trophic regimes were then identified in the north Ionian gyre (NIG) and they could be explained with the relative position of the MLD and nitracline. The first one is characterized by an early winter bloom onset and the absence of a chlorophyll peak in March. It was observed when circulation was anticyclonic or when winter MLD was relatively shallow. Dominant regenerated production all year and an absence of significant nutrient supplies to surface waters are proposed to explain this trophic regime. Conversely, the second trophic regime is marked by a bloom onset in late winter (i.e., February) and a chlorophyll peak in March. The chlorophyll increase was interpreted as a direct response to the nutrient enrichment of surface waters. This winter–spring bloom was observed when circulation was cyclonic and when winter mixing was relatively strong.

In-depth characterization of diazotroph activity across the western tropical South Pacific hotspot of N<sub>2</sub> fixation (OUTPACE cruise)

Abstract. Here we report N2 fixation rates from a ∼ 4000 km transect in the western and central tropical South Pacific, a particularly undersampled region in the world ocean. Water samples were collected in the euphotic layer along a west to east transect from 160∘ E to 160∘ W that covered contrasting trophic regimes, from oligotrophy in the Melanesian archipelago (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. N2 fixation was detected at all 17 sampled stations with an average depth-integrated rate of 631 ± 286 µmolNm-2d-1 (range 196–1153 µmolNm-2d-1) in MA waters and of 85 ± 79 µmolNm-2d-1 (range 18–172 µmolNm-2d-1) in GY waters. Two cyanobacteria, the larger colonial filamentous Trichodesmium and the smaller UCYN-B, dominated the enumerated diazotroph community (> 80 %) and gene expression of the nifH gene (cDNA > 105 nifH copies L−1) in MA waters. Single-cell isotopic analyses performed by nanoscale secondary ion mass spectrometry (nanoSIMS) at selected stations revealed that Trichodesmium was always the major contributor to N2 fixation in MA waters, accounting for 47.1–83.8 % of bulk N2 fixation. The most plausible environmental factors explaining such exceptionally high rates of N2 fixation in MA waters are discussed in detail, emphasizing the role of macro- and micro-nutrient (e.g., iron) availability, seawater temperature and currents.

Complementarity of the multidimensional functional and the taxonomic approaches to study phytoplankton communities in three Mediterranean coastal lagoons of different trophic status

We used the individual-based multidimensional functional diversity and the taxonomic approaches in a complementary way to describe phytoplankton communities in three coastal lagoons with different eutrophication status in the South of France. We sampled communities during three seasons, i.e., in autumn, spring, and summer. Using classical taxonomy, 107 taxa/morphotypes were identified in the nine communities. The individual-based functional approach allowed grouping these individuals into 20 functional entities according to their values for 5 traits related to trophic adaptations (cell size, mobility, trophic regime, coloniality, and pelagic/benthic life). Some species (e.g., Prorocentrum micans) emerged in multiple functional entities, showing the importance to consider intraspecific variability. The functional description of phytoplankton communities better reflected the hydrological functioning and the different eutrophication status of the lagoons than the taxonomic approach. Specific functional adaptations were identified in the nine communities. For example, phytoplankton organisms with heterotrophic and potentially mixotrophic abilities occurred when the availability of inorganic nutrient decreased, or when organic matter and small preys were potentially the main nutrient resources. The limitation has also favored small cells highly competitive for nutrients. Using functional indices together with taxonomic description has also helped revealing important aspects of community assembly, such as competitive exclusion in summer.

The soluble guanylate cyclase CYG12 is required for the acclimation to hypoxia and trophic regimes in Chlamydomonas reinhardtii.

Oxygenic phototrophs frequently encounter environmental conditions that result in intracellular energy crises. Growth of the unicellular green alga Chlamydomonas reinhardtii in hypoxia in the light depends on acclimatory responses of which the induction of photosynthetic cyclic electron flow is essential. The microalga cannot grow in the absence of molecular oxygen (O2 ) in the dark, although it possesses an elaborate fermentation metabolism. Not much is known about how the microalga senses and signals the lack of O2 or about its survival strategies during energy crises. Recently, nitric oxide (NO) has emerged to be required for the acclimation of C.reinhardtii to hypoxia. In this study, we show that the soluble guanylate cyclase (sGC) CYG12, a homologue of animal NO sensors, is also involved in this response. CYG12 is an active sGC, and post-transcriptional down-regulation of the CYG12 gene impairs hypoxic growth and gene expression in C.reinhardtii. However, it also results in a disturbed photosynthetic apparatus under standard growth conditions and the inability to grow heterotrophically. Transcriptome profiles indicate that the mis-expression of CYG12 results in a perturbation of responses that, in the wild-type, maintain the cellular energy budget. We suggest that CYG12 is required for the proper operation of the photosynthetic apparatus which, in turn, is essential for survival in hypoxia and darkness.

Dynamics of the Photo-Trophic Termination of Reproductive Diapause in Females of the Multicolored Asian Ladybird Harmonia axyridis (Pallas) (Coleoptera, Coccinellidae)

Females of certain aphidophagous ladybirds in the absence of natural protein food (aphids) enter reproductive diapause. Reactivation of diapausing beetles is possible only after consuming the food which is suitable for oogenesis of females and development of larvae. The influence of diet and photoperiod on the dynamics of weight and on the rate of reproductive maturation of reactivating females of Harmonia axyridis was studied under laboratory conditions. The experiments were conducted at combinations of two day lengths (12 and 18 h) with 6 trophic regimes with the following mean numbers of daily consumed aphids: 0 (aphids were absent over the whole experiment), 0.1 (1 aphid was provided every 10 days), 0.5 (1 aphid every second day), 1 (1 aphid every day), 10 (10 aphids every day), and 50 (about 50 aphids every day). Judging from the state of ovarian development estimated by dissection 20 days after the beginning of the experiment, the threshold of the trophic termination of reproductive diapause under the long day conditions lies between the regimes of 0.1 and 0.5 aphids per day, which is approximately equal to the earlier investigated threshold of the trophic induction of diapause. Short day slowed reactivation and has increased the threshold of the trophic response up to 1-10 aphids per day. Based on these data, we conclude that under natural conditions females which entered reproductive diapause because of the absence of aphids are capable of reactivation in the presence of even a minimal amount of natural protein food. Moreover, in spring and summer (when the probability of the increase in aphid abundance is relatively high) reactivation starts at lower prey population density than in autumn, when the appearance of the stable aphid colony (which is necessary to complete the development of the emerged larvae) is less probable.

Ecological indicators and functional groups of copepod assemblages

Copepods are important ecological indicators of ecosystem functioning. In this study, we analyzed the structure of copepod assemblages and cross-shelf patterns based on functional traits and indicator value analysis. Copepod samples were collected from 54 stations distributed along the inner, middle, and outer continental shelves covering a wide geographical area (20,100km2). Overall, 38 species were identified: 20 Calanoida, 14 Cyclopoida, and 4 Harpacticoida. Copepod density was the highest in the inner shelf profile, with richness and diversity being higher on the outer shelf close to the shelf break. The results suggest that there is a gradient of zooplankton distribution even on narrow (<50km) continental shelves. The cluster analysis of the stations showed a tendency to form three groups largely corresponding to physical location. Two major functional groups were identified, sorted by spawning strategy. These were subdivided according to the trophic regime and feeding strategy. Temora turbinata (alien species), Clausocalanus furcatus, and Ditrichocorycaeus amazonicus (native species) were found to be ecological indicators. These species have different functional traits and distinct cross-shelf distributions, where the alien species dominates the turbid coastal waters, and C. furcatus is an indicator of outer-shelf waters. Thus, these species can be considered ecological indicators of the different continental shelf waters. The results indicate that it is necessary to understand the functional diversity and ecological indicators of copepods, considering their importance for the basis of marine food webs.

Preface: The Oligotrophy to the UlTra-oligotrophy PACific Experiment (OUTPACE cruise, 18 February to 3 April 2015)

Abstract. The overall goal of OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) was to obtain a successful representation of the interactions between planktonic organisms and the cycle of biogenic elements in the western tropical South Pacific Ocean across trophic and N2 fixation gradients. Within the context of climate change, it is necessary to better quantify the ability of the oligotrophic ocean to sequester carbon through biological processes. OUTPACE was organized around three main objectives, which were (1) to perform a zonal characterization of the biogeochemistry and biological diversity of the western tropical South Pacific during austral summer conditions, (2) to study the production and fate of organic matter (including carbon export) in three contrasting trophic regimes (increasing oligotrophy) with a particular emphasis on the role of dinitrogen fixation, and (3) to obtain a representation of the main biogeochemical fluxes and dynamics of the planktonic trophic network. The international OUTPACE cruise took place between 18 February and 3 April 2015 aboard the RV L'Atalante and involved 60 scientists (30 onboard). The west–east transect covered ∼ 4000 km from the western part of the Melanesian archipelago (New Caledonia) to the western boundary of the South Pacific gyre (French Polynesia). Following an adaptive strategy, the transect initially designed along the 19° S parallel was adapted along-route to incorporate information coming from satellite measurements of sea surface temperature, chlorophyll a concentration, currents, and diazotroph quantification. After providing a general context and describing previous work done in this area, this introductory paper elucidates the objectives of OUTPACE, the implementation plan of the cruise and water mass and climatological characteristics and concludes with a general overview of the other papers that will be published in this special issue.

Shelf‐basin gradients shape ecological phytoplankton niches and community composition in the coastal Arctic Ocean (Beaufort Sea)

AbstractThe contiguous Arctic shelf is the green belt of the Arctic Ocean. Phytoplankton dynamics in this environment are driven by extreme physical gradients and by rapid climate change, which influence light and nutrient availability as well as the growth and ecological characteristics of phytoplankton. A large dataset collected across the Canadian Beaufort Shelf during summer 2009 was analyzed to assess how the interplay of physical and biogeochemical conditions dictates phytoplankton niches and trophic regimes. Nonmetric multidimensional scaling and cluster analysis demonstrated marked partitioning of phytoplankton diversity. Elevated phytoplankton biomass (∼ 2.41 μg Chl a L−1) was observed in association with the surface mixed layer near the coast, close to the mouth of the Mackenzie River, and at the shelf‐break as a result of nutrient‐rich Pacific water intrusions. The coastal communities were supported by high levels of nitrogen nutrients and were taxonomically uniform, with diatoms accounting for 95% of total cell numbers. By contrast, adjacent oceanic waters were characterized by low autotrophic biomass near the surface (∼ 0.09 μg Chl a L−1) and below the mixed layer (∼ 0.23 μg Chl a L−1) due to mainly nutrient limitation. However, the oceanic community was more diverse with a mixed assemblage of diatoms and small mixotrophs/heterotrophs near the surface and a predominance of autotrophic nanoflagellates at depth. We conclude that as climate change intensifies freshening and stratification in the Western Arctic Ocean, coastal hotspots of high autotrophic productivity may play an even greater role in supporting Arctic marine ecosystems while offshore environments become increasingly oligotrophic.

Assessing ultraphytoplankton and heterotrophic prokaryote composition by flow cytometry in a Mediterranean lagoon

In the eutrophic Ghar El Melh Lagoon (GML, Tunisia), the distribution of heterotrophic prokaryotes, pico- and nanophytoplankton was studied at five stations in November 2012 at the single cell level, along with environmental factors. Flow cytometry analysis of ultraplankton (<10μm) resolved (i) two heterotrophic prokaryote groups, low and high nucleic acid contents (LNA and HNA, respectively), and (ii) eight to nine ultraphytoplankton groups (cryptophyte-like cells, two nanoeukaryote subgroups, two picoeukaryote subgroups and three Synechococcus-like cells subgroups). Prochlorococcus was not detected. According to redundancy analysis (RDA), a significant difference was found in the distribution of the ultraplankton between stations (F=2.61, p<0.05); maximum proliferations of heterotrophic prokaryotes were observed in the inner parts of the lagoon at stations 3, 4 and 5 affected by urban, agricultural and industrial discharges. Ultraphytoplankton concentrations were the highest near the outlet of the lagoon at stations 1 and 2 influenced by freshwater outflow and oligotrophic Mediterranean water inflow, respectively. At station 1, the large ultraphytoplankton concentration derives from the high abundance of cryptophyte-like cells favoured by the freshwater outflow whereas at station 2, the input of oligotrophic Mediterranean water enhanced the abundance of Synechococcus and picoeukaryotes at the expense of nanoeukaryotes. Two trophic regimes were thus differentiated in GML.