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Showing papers in "Journal of Plankton Research in 2009"


Journal ArticleDOI
TL;DR: Simulations under dynamic conditions include considerations of predation on bacteria, and on microalgae, which show the potential for mixotrophs, but also indicate the importance of using an appropriate description of their physiology, with different mixotrophy configurations having significant effects on system dynamics.
Abstract: A mechanistic model is described for carbon–nitrogen–phosphorous-based interactions within a protistan mixotroph. The model describes interactions between photosynthesis (with photoacclimation), inorganic nutrient acquisition and the consumption of prey, making use of a flexible structure to allow an exploration of alternative modes of interaction. Operation can be varied with respect to differential growth rates under pure phototrophy, phago-heterotrophy or mixotrophy, substitutional or additive interactions between modes of C acquisition, the suppression of digestion by C flow from photosynthesis (including, if applicable, that from kleptochloroplasts), competition for volume within the cell between chloroplasts and food vacuole, the need for some level of obligatory photosynthetic activity, activation of mixotrophy in response to general growth limitation, or to specific nutrient limitations. Simulations under dynamic conditions include considerations of predation on bacteria, and on microalgae. These show the potential for mixotrophs, but also indicate the importance of using an appropriate description of their physiology, with different mixotrophy configurations having significant effects on system dynamics. The potential value of kleptochloroplasts for support of mixotroph growth is highest when the food vacuole is large, when the ingested phototroph prey is of good nutritional status and when digestion of prey is repressed by photosynthesis. Analyses of the behaviour of the new model demonstrate that simulations which do not consider the stoichiometric implications of mixotrophy cannot reflect the reality of the trophic interaction both for the mixotroph and for the associated ecosystem. We dedicate this work to the memory of Mike Fasham FRS, without whose enthusiasm and guidance none of this would have come to pass.

120 citations


Journal ArticleDOI
TL;DR: It is recommended that molecular approaches to identification be developed and extended where possible, that serious effort be committed to ensuring correct identification of species when DNA sequences are published and that new species of plankton should not be named based on morphology alone without supporting molecular information, especially for protists.
Abstract: Precise identification of species is critical for the study of biogeography of plankton and for applying laboratory culture results to the same organism in situ. Traditionally, identification has been based on knowledge of morphological traits transmitted from generation to generation of planktologists in monographs or at the bench. Despite recent rapid growth of molecular methods, taxonomists have been slow to incorporate molecular information in a formal way into species descriptions. Likewise, molecular biologists have often been less than thorough about making precise identifications of the species they sequence, as the large number of sequences in the public databases that are linked to mis- or unidentified species will attest. Although some have advocated for a new taxonomy built solely on a scaffold of DNA, for the present it seems wise to use a “total evidence” approach in identifying plankton, relying on both molecular and morphological information whenever possible. There is a large body of information on morphology, phenotypic variation, distribution and ecology of many species that is recorded in their formal descriptions, and this would be lost in a DNA-only approach. Without a successful marriage of molecular and morphological methods, it will be more difficult to solve the mystery of cryptic species. For now, we recommend that molecular approaches to identification be developed and extended where possible, that serious effort be committed to ensuring correct identification of species when DNA sequences are published and that new species of plankton should not be named based on morphology alone without supporting molecular information, especially for protists.

119 citations


Journal ArticleDOI
TL;DR: In this paper, a trend analysis confirmed a statistically significant east-west decrease of mean temperatures and salinities and an increase of nutrients (P-PO 4 and N-NO 3 ), chlorophyll a, primary productivity and phytoplankton cell concentrations.
Abstract: Environmental and phytoplankton parameters were explored at nine stations located along a large scale (3188 km) east-west longitudinal transect (4°95'E-32°67'E) of the Mediterranean Sea. The trend analysis confirmed a statistically significant east-west decrease of mean temperatures and salinities and an increase of nutrients (P-PO 4 and N-NO 3 ), chlorophyll a, primary productivity and phytoplankton cell concentrations. A total of 216 species were identified: 48 diatoms, 112 dinoflagellates and 56 coccolithophores. On the basis of this analysis, the structure of the microphytoplankton community (Shannon's diversity, species richness, taxa dominance and community dissimilarities) along the transect was investigated. Many species (102) of this community were rare, having a frequency of occurrence < 2.0 in all samples, but they were very important because they controlled the levels of species diversity. A highly significant power relationship between "species number-sample number" was also recorded along the studied transect. The analysis showed that the prevailing east-west gradients of the physical and chemical conditions determined phytoplankton abundance and community diversification.

105 citations


Journal ArticleDOI
TL;DR: The history of why the authors use Michaelis-Menten kinetics in models is traced, and it is shown that this functional form may not be the best representation of nutrient uptake by a diverse and changing phytoplankton community.
Abstract: Planktonic ecosystem models have been used for many decades; modern models are only subtle variations on model structures established in the 1970s or earlier. Here I explore two problems that I see with these models: (i) their formulation and parameterization and (ii) their use. Using nutrient uptake by the phytoplankton as an example, I trace the history of why we use Michaelis-Menten kinetics in our models, and show that this functional form may not be the best representation of nutrient uptake by a diverse and changing phytoplankton community. I then discuss how models are used-not as the hypotheses they are, but more like toasters. I make the point that treating models as hypotheses could lead to much stronger and more robust insights into planktonic dynamics. However, this requires better statistical comparisons of models and data, including the "right" kinds of data. Finally I suggest some ways to move firward, to make planktonic ecosystem models much more powerful tools in our investigations of ocean dynamics.

92 citations


Journal ArticleDOI
TL;DR: An evaluation of data and published results on abundances of doliolids and salps from ocean margins reveals that a considerable degree of prediction is possible, based upon meteorological and boundary current intrusion dynamics.
Abstract: The occurrence of large patches of gelatinous zooplankton has for decades been considered to be unpredictable. An evaluation of our own data and published results on abundances of doliolids and salps from ocean margins reveals that a considerable degree of prediction is possible, based upon meteorological and boundary current intrusion dynamics.

88 citations


Journal ArticleDOI
TL;DR: Abundance and stable isotope composition of large and small mesozooplankton were analyzed in samples taken with 333 and 100 μm nets, respectively, at four sites in the eastern Mediterranean down to 4200 m depth in October 2001 as mentioned in this paper.
Abstract: Abundance and stable isotope composition of large and small mesozooplankton were analyzed in samples taken with 333 and 100 μm nets, respectively, at four sites in the eastern Mediterranean down to 4200 m depth in October 2001. Large mesozooplankton (333 μm nets) was sieved into five size fractions, and the δ 13 C and δ 15 N values of the fractions were measured as well as the δ 15 N values of total small mesozooplankton (100 μm nets) and specific mesozooplankton taxa. These measurements allow insights into the source of the diet and the trophic level relative to sinking and suspended particulate organic matter. Overall, biomass and abundance of zooplankton was low, reflecting the oligotrophic character of the eastern Mediterranean. Stable nitrogen isotope values of mesozooplankton were low (1 ― 4‰) and close to zero in suspended particles at the surface. This indicates that the fixation of atmospheric nitrogen probably contributes to the N-pool in the eastern Mediterranean. Such low values were also found in sinking particles in deep waters and in most zooplankton size classes. However, suspended particles and mesozooplankton in the size class 0.5―1 mm, which was primarily composed of the deep-sea species Lucicutia longiserrata, showed higher values at depths below 1000 m. There is some indication that L longiserrata was able to utilize the suspended particle pool in the deep eastern Mediterranean.

85 citations


Journal ArticleDOI
TL;DR: The pan-global marine appendicularian, Oikopleura dioica, shows considerable promise as a candidate model organism for cross-disciplinary research ranging from chordate genetics and evolution to molecular ecology research, and a detailed culture protocol is tested that permits sustainable long-term maintenance of the animal.
Abstract: The pan-global marine appendicularian, Oikopleura dioica, shows considerable promise as a candidate model organism for cross-disciplinary research ranging from chordate genetics and evolution to molecular ecology research. This urochordate, has a simplified anatomical organization, remains transparent throughout an exceptionally short life cycle of less than 1 week and exhibits high fecundity. At 70 Mb, the compact, sequenced genome ranks among the smallest known metazoan genomes, with both gene regulatory and intronic regions highly reduced in size. The organism occupies an important trophic role in marine ecosystems and is a significant contributor to global vertical carbon flux. Among the short list of bona fide biological model organisms, all share the property that they are amenable to long-term maintenance in laboratory cultures. Here, we tested diet regimes, spawn densities and dilutions and seawater treatment, leading to optimization of a detailed culture protocol that permits sustainable long-term maintenance of O. dioica, allowing continuous, uninterrupted production of source material for experimentation. The culture protocol can be quickly adapted in both coastal and inland laboratories and should promote rapid development of the many original research perspectives the animal offers.

85 citations


Journal ArticleDOI
TL;DR: Video observations of individual prey capture and feeding events showed prey rejection frequencies (caught and then released cells) that did not differ between mixed and mono-specific diets, suggesting that the selection between prey cells occurs prior to capture and that it is based on remote characterization of the cells.
Abstract: Grazing on two red tide dinoflagellates, the potentially toxic Karenia mikimotoi and the non-toxic Gyrodinium instriatum, was examined in two species of marine copepods, Pseudocalanus elongatus and Temora longicornis. Both copepods cleared K. mikimotoi at rates that were a little lower but comparable to those at which they cleared the slightly larger G. instriatum when the two dinoflagellates were offered separately. However, when feeding on mixtures of the two prey species, the clearance rates on K. mikimotoi were substantially reduced in both copepods while their clearances of G. instiatum remained unaltered, suggesting active prey selection. Video observations of individual prey capture and feeding events showed prey rejection frequencies (caught and then released cells) that did not differ between mixed and mono-specific diets. This suggests that the selection between prey cells occurs prior to capture and that it is based on remote characterization of the cells.

76 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of warmer water temperatures on phytoplankton abundance and species composition in three small eutrophic lakes (5.2 -14.9 ha) of the Canadian prairies in Winnipeg, Manitoba, Canada, over two open-water seasons with contrasting spring weather conditions, i.e. 2005-a “normal” spring and 2006-a warm spring.
Abstract: Shallow, polymictic lakes with low heat storage capacity are especially susceptible to warmer spring conditions, predicted for a changing climate. In these lakes, atmosphere to water mass heat transfer is efficient as a result of high wind exposures and large surface areas relative to volumes. We examined effects of warmer water temperatures on phytoplankton and zooplankton abundance and species composition in three small eutrophic lakes (5.2‐14.9 ha) of the Canadian prairies in Winnipeg, Manitoba, Canada, over two open-water seasons with contrasting spring weather conditions, i.e. 2005-a “normal” spring and 2006-a warm spring. Warmer spring and summer water temperatures were associated with decreased water transparency, increased phytoplankton biomass, increased relative filamentous cyanobacteria biomass and shifts in dominant genera from Aphanizomenon to Anabaena and Planktothrix. Zooplankton responded strongly; abundance of Daphnia (D. pulicaria, D. ambigua and D. parvula) decreased while rotifers, Skistodiaptomus oregonensis and Bosmina longirostris increased in abundance. Of several factors influencing phytoplankton dynamics, total dissolved nutrients [nitrogen (N), phosphorus (P) and N:P] and water column stability did not show important changes between years. In contrast, water temperature [described as the metric degree-days (8C day)] was related to changes in phytoplankton and % cyanobacteria biovolume. Daphniid abundance showed a significant negative relationship with an increase in filamentous cyanobacteria biomass and, thus we suggest, was indirectly associated with increased water temperatures.

76 citations


Journal ArticleDOI
TL;DR: It is suggested that sex determination in copepods is under strong environmental control, and it is argued that the food limitation is an important determinant of sex change.
Abstract: Female-biased adult sex ratios are common in copepod species, and have been observed both in wild populations and in cultures. Biased sex ratios are most commonly explained by sex- or stagespecific differences in longevity or mortality. However, neither differential longevity nor differential mortality fully explains skewed sex ratios in planktonic copepod populations. We propose that sex change is an important mechanism determining the adult sex ratio. Though sex change has been proposed for only a few copepod species, intersexuality is widespread. We review the occurrence and causation of intersexuality in planktonic copepods, which is a manifestation of late sex change during development. By way of example, we demonstrate that skewed sex ratios of Acrocalanus gracilis, a common tropical paracalanid copepod, could be explained by sex change alone. Our observations suggest that sex determination in copepods is under strong environmental control, and we argue that the food limitation is an important determinant of sex change.

73 citations


Journal ArticleDOI
TL;DR: This study investigated the dormancy length, germination patterns and cyst progeny fate of Alexandrium catenella and A. tamarense found in Thau lagoon, France, suggesting that cysts may germinate shortly after being recruited from sediment re-suspensions and contributing to the initial seeding of blooms.
Abstract: This study investigated the dormancy length, germination patterns and cyst progeny fate of Alexandrium catenella and A. tamarense found in Thau lagoon, France. In laboratory-produced cysts, the dormancy period was estimated to be less than 2 weeks. A vernalization period was not required prior to germination. However, after 1 month of dark-storage at 6°C, excystment was obtained 2-3 days from re-exposure to favourable conditions thereby suggesting that germination was synchronized. Germination patterns and germling cell viability in laboratory produced cysts were similar to those exhibited in natural cysts. The following conditions optimized germination temperature between 14 and 26°C, salinity between 30 and 38 psu and light exposure for > 1 h at 100 μmoles photons m -2 S -1 Similar conditions prevailed at the commencement of and during Alexandrium blooms in Thau lagoon, suggesting that cysts may germinate shortly after being recruited from sediment re-suspensions. Accordingly, cyst bank germinations contribute to the initial seeding of blooms, and subsequently germinations of newly formed cysts can help to maintain blooms. In cyst banks, the high germination capabilities seem, however, to be counter-balanced by the low viability of cyst progeny, as estimated from the realized seeding ratio. This ratio is proposed as a means of quantifying the potential for cyst banks to regenerate planktonic populations.

Journal ArticleDOI
TL;DR: Qagen DNeasy® Blood & Tissue kit was the most efficient of the tested methods for genomic extraction from both free-living microalgae and microalgai inside copepod guts and the appropriate handling of predator copepods prior to genomic extraction was essential for quantitative gut content estimates.
Abstract: Molecular methods are becoming increasingly common for taxonomic and ecological studies of marine and freshwater plankton. Recently, nucleic acids have been used as target molecules for identification and quantification of prey species in studies of trophic interactions. A critical step in the quantification of mesozooplankton feeding by molecular analysis is the isolation of microalgal DNA from predator guts and in the food environment. It is essential that total genomic DNA extraction provides maximum quantitative yield suitable for downstream analysis. In this study, we compared the efficacy and experimental variability of eight different protocols for total genomic DNA extraction from free-living microalgae and microalgae within the gut of copepods. We also developed and evaluated different sampling procedures for copepods prior to genomic extraction. The optimal protocol was evaluated using real time quantitative polymerase chain reaction (qPCR) and the integrity of the genomic DNA was determined by amplifying PCR targets of increasing size. Considerable variability was observed between purification protocols. Qagen DNeasy® Blood & Tissue kit was the most efficient of the tested methods for genomic extraction from both free-living microalgae and microalgae inside copepod guts. Furthermore, the appropriate handling of predator copepods prior to genomic extraction was essential for quantitative gut content estimates.

Journal ArticleDOI
TL;DR: It is found that at highest densities of the species, in summer, a significant predation on its larvae occurs, this being the major carbon source of adults and overall, these results are discussed in the context of trade-offs M. leidyi faces in the new environment and adverse environmental conditions.
Abstract: The sudden occurrence of the ctenophore Mnemiopsis leidyi has been reported recently from different regions of the Baltic Sea and it has been suggested that the species has invaded the whole basin. Here we provide the first set of quantitative data of seasonal diet composition and life history traits of M. leidyi and its predatory role in the pelagic ecosystem of the Western Baltic Sea. The size structure of the species appeared to be dominated by small size classes and only a few adults were as large as those reported in the native region of the species and in other invaded areas. We show that the species has a high preference for small-sized and slow swimming prey, mainly during the winter low temperature period. Barnacle nauplii appeared to be the main source of carbon for the over-wintering population of M. leidyi. A preference for copepods was only found during August when these prey contributed up to 20% of the gut composition. In summer, planula larvae of the jellyfish Aurelia aurita were the most abundant prey in the gut content (feeding rate of 621 ind. ctenophore ― 1 day ― 1 ). We further found that at highest densities of the species, in summer, a significant predation on its larvae occurs, this being the major carbon source of adults. Overall, these results are discussed in the context of trade-offs M. leidyi faces in the new environment and adverse environmental conditions, which are likely forcing the species toward reduced sizes and also probably reducing its potential predatory impact in the Baltic Sea.

Journal ArticleDOI
TL;DR: Based on the results of N-15 uptake assays, it was concluded that P. delicatissima more readily acquires ammonium than urea under low N conditions, and may use urea as an alternative N source, and comparable photosynthetic rates are attained on either substrate.
Abstract: The influence of organic nutrients on the evolution of Pseudo-nitzschia delicatissima cultures was investigated in an enrichment experiment with high-molecular-weight dissolved organic matter (HMWDOM) and in an uptake assay with N-15-ammonium and N-15-urea. HMWDOM was extracted from seawater collected at a nearby shore station during the decline of a diatom bloom. Four incubations were prepared: L1/5+DOM (P. delicatissima grown in L1 growth medium with 1/5 of the nitrate concentration of standard L1), (L1-N)+DOM (L1 without nitrate, i.e. nitrogen-deficient treatment), L1-DOM (control culture without added DOM) and BV+DOM (bacterial and viral control, free of microalgae). Incubations were carried out for 10 days. Chlorophyll a concentrations differed after day 4 and reached higher levels in the L1-DOM incubation by the end of the experiment; however, similar growth rates were observed in all incubations (1.64 +/- 0.05 divisions day(-1)). The persistently lower cellular chlorophyll content in (L1-N)+DOM during the experiment was consistent with N limitation conditions. The data suggested that the nitrogen needed for the growth of (L1-N)+DOM cells might have originated from the DOM. Based on the results of N-15 uptake assays, it was concluded that P. delicatissima more readily acquires ammonium than urea. Nevertheless, under low N conditions, P. delicatissima may use urea as an alternative N source, and comparable photosynthetic rates are attained on either substrate. Taken together, our results suggest a positive effect of organic nutrients on the growth of P. delicatissima.

Journal ArticleDOI
TL;DR: Production of these toxins showed a circadian rhythm synchronized with light:dark (L:D) cycle and also with a period of 24 h under the three light intensities tested and the circadian rhythm of STX and NSTX production under white light conditions was confirmed.
Abstract: Effects of light intensity and light quality on saxitoxin and neosaxitoxin production by Cylindrospermopsis raciborskii (Woloszynska) Seenaya and Subba Raju (strain T3) were investigated in batch culture conditions. Three light intensities (50, 100 and 150 μmol photons m -2 s -1 ) and three light qualities (white and blue 470 nm and red 630 nm) were used. Growth rates were higher under 100 and 150 when compared to 50 μmol photons m -2 s -1 (control condition). Saxitoxin (STX) and neosaxitoxin (NSTX) production showed an increase during the early stationary phase under 50 μmol photons m -2 s -1 (12 days). Higher concentrations of STX and NSTX were observed in cells under 100 μmol photons m -2 s -1 . Production of these toxins showed a circadian rhythm synchronized with light:dark (L:D) cycle and also with a period of 24 h under the three light intensities tested. The circadian rhythm of STX and NSTX production under white light conditions was confirmed. Red light induced the loss of circadian rhythm in NSTX production. These data demonstrate a circadian rhythm in saxitoxins production modulated by light.

Journal ArticleDOI
TL;DR: The distribution and dynamics of the oceanic water masses seemed to be the most important influence on copepod diversity and production at this subtropical site.
Abstract: We investigated the influence of nutrient-rich oceanic waters in comparison to the estuarine outflow from Santos Bay (SE Brazil) on copepod abundance and production on the adjacent inner shelf. Zooplankton samples were collected with a Multinet in spring 2005 and in summer 2006. Copepod biomass was derived from length―weight regressions, and growth rates were estimated from empirical models. Altogether, 58 copepod taxa were identified. The highest abundances were due to small-sized organisms including nauplii, oncaeids and copepodids of paracalanids and clausocalanids. Biomass and secondary production mirrored copepod abundance, with Temora copepodids accompanying the above-mentioned taxa as major contributors. The contribution of naupliar biomass and production was low (2.2 and 3.8% of the total, respectively). The influence of the Santos Bay outflow was observed only in spring when Coastal Water (CW) dominated at the study site; whereas in summer the inner shelf was occupied by CW in the suface layer and the oceanic South Atlantic Central Water (SACW) in the bottom layer. The SACW intrusion had more of an influence for the increase in copepod production than the Santos Bay plume. The distribution and dynamics of the oceanic water masses seemed to be the most important influence on copepod diversity and production at this subtropical site.

Journal ArticleDOI
TL;DR: It is highlighted that the smallest photosynthetic communities show taxon-specific responses to mixing and resource availability, which affect the structure and dynamics of picophytoplankton.
Abstract: Dynamics of prokaryotic and eukaryotic picophytoplankton were investigated over a 2-year time period using flow cytometry and combined with an in situ experiment in Lake Tahoe, USA to better characterize to which extent environmental factors control these communities. Pronounced seasonal patterns and clear temporal and spatial partitioning were observed between picocyanobacteria and picoeukaryotes. Picocyanobacteria dominated in the nutrient deficient upper water column during the stratified season, while picoeukaryotes reached maximum abundance during isothermal conditions and maintained high numbers in deep-water layers during the stratified season. Picocyanobacteria were more sensitive to high solar and UV radiation compared with picoeukaryotes, which were not affected by high solar radiation and nutrient enrichment stimulated their growth. The opposing response of these two populations is consistent with their vertical distribution: picocyanobacteria dominate below the 30% isolume and above the nitrocline depth, whereas picoeukaryotes increase in the vicinity of the nitrocline and thus increased nutrient concentration. This spatial separation of picophytoplankton groups along environmental gradients in Lake Tahoe is consistent with other deep-oligotrophic lakes and the marine environment, suggesting that these marine and freshwater organisms have similar ecophysiological requirements. These results highlight that the smallest photosynthetic communities show taxon-specific responses to mixing and resource availability, which affect the structure and dynamics of picophytoplankton.

Journal ArticleDOI
TL;DR: New evidence is provided on the critical role played by viruses and small flagellates in the functioning of freshwater microbial food webs and also that these mortality processes vary strongly throughout the seasons.
Abstract: Viruses and small heterotrophic flagellates are known to exert an important control on bacterial populations. In parallel with the study of picoplankton dynamics (abundance and distribution) in surface waters (0-50 m) of Lakes Geneva, Bourget and Annecy, we used a dilution technique during different seasonal periods in order to assess flagellate- versus virus-induced mortality of heterotrophic bacteria, picocyanobacteria and small eukaryotic phytoplankton. Although it was not always possible to detect a significant viral effect (typically in winter), viral lysis and protozoan grazing could be responsible for up to 71 % of the bacterial mortality (in summer). Viral impact, considered alone, never equalled or exceeded predation for heterotrophic bacteria, but could for picocyanobacteria, typically in autumn. In addition, during summer complex interactions between grazing- and virus-induced mortality of bacteria (e.g synergism versus antagonism) could be highlighted for instance with bacterial lysis susceptible to enhance picocyanobacterial growth). The temporal variations observed for experimental viral parasitism and flagellate predation were consistent with the in situ dynamics and statistical relationships found between the targeted communities. This study thus provides new evidence on the critical role played by viruses and small flagellates in the functioning of freshwater microbial food webs and also that these mortality processes vary strongly throughout the seasons.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the nature of water column physical habitat structure (light penetration, thermocline depth and shape and relative thermal resistance to mixing), and in turn, how these structures influenced the distribution of bulk chlorophyll a and the biomass of several major phytoplankton groups across 45 lakes in eastern Canada, within two lake districts which varied in watershed geology and water chemistry.
Abstract: Observations from single lake and experimental studies predict that vertical habitat heterogeneity in lakes can influence phytoplankton community structure. We examined the nature of water column physical habitat structure (light penetration, thermocline depth and shape and relative thermal resistance to mixing), and in turn, how these structures influenced the distribution of bulk chlorophyll a and the biomass of several major phytoplankton groups across 45 lakes in eastern Canada, within two lake districts which varied in watershed geology and water chemistry. Across all lakes, more pronounced temperature gradients favoured the distribution of bulk phytoplankton into more defined layers. The depth at which peak chlorophyll a was observed was affected by temperature heterogeneity and environmental factors related to light penetration. Peak depths and vertical heterogeneity of the major phytoplankton groups were differentially related to epilimnetic water colour and total phosphorus concentration across all lakes. Further insight was gained by comparing the physical structure and phytoplankton responses in the two regions. Lakes from the Laurentians Region had less wind exposure, shallower thermoclines, but greater vertical temperature variability than in the Eastern Townships Region. As a result, total and major phytoplankton group biomass showed more heterogeneous distributions in the Laurentians. The depth of peaks in total biomass and for the major phytoplankton groups was similar in both regions; the exception being a deeper chlorophyte maximum in the ETR, suggesting that there may be important differences between regions in the taxonomic composition of this group.

Journal ArticleDOI
TL;DR: This work sequenced and phylogenetically analysed partial sequences of the 16S rRNA gene, cpcBA-IGS and rpoCI from 31 cyanobacteria isolates of the genera Aphanizomenon and Anabaena and has documented morphotypic characteristics of new and recently isolated strains.
Abstract: Cylindrospermopsin (CYN) is a potent hepatotoxic alkaloid that has been detected in freshwater samples worldwide and is produced by a number of cyanobacterial species, mainly of the genera Cylindrospermopsis, Aphanizomenon and Anabaena. Cylindrospermopsis raciborskii is a morphologically distinctive species which forms a genetically well-defined cluster. In contrast, some species within Aphanizomenon and Anabaena are morphologically not clearly assignable to either genera and both genera are polyphyletic. In the Cylindrospermopsis cluster CYN producing and non-producing strains co-occur, but it is not known if GYM producing and non-producing strains are closely related in Anabaena and Aphanizomenon. Here we attempt to disentangle the phylogenetic relationships of four taxa of the genera Anabaena and Aphanizomenon, some of which are known as CYN producers. We have sequenced and phylogenetically analysed partial sequences of the 16S rRNA gene, cpcBA-IGS and rpoCI from 31 cyanobacteria isolates of the genera Aphanizomenon and Anabaena and have documented morphotypic characteristics of new and recently isolated strains. Our results do not corroborate the separation of Aph. gracile and Aph. flos-aquae into separate species. In contrast, they support the distinction of Ana. bergii and Aph. ovalisporum into distinct taxa. Further, Ana. bergii is most likely not a CYN producing species.

Journal ArticleDOI
TL;DR: The factors listed above require consideration when solar energy input into metabolism of oceanic photoheterotrophic prokaryotes is experimentally quantified and numerically modelled.
Abstract: Despite considerable advances in the understanding of the various microbial photoheterotrophic mechanisms, the role of solar radiation in the metabolism of bacterioplankton in the ocean is difficult to assess. It is already apparent that rates of CO2 fixation by prokaryotic cells may be only a part of the picture. Photophosphorylation is difficult to differentiate from respiratory phosphorylation and other types of ATP synthesis. Solar energy could by-pass ATP synthesis, instead being used to generate a proton-motive force, which in turn could be directly used for cell motility or even for importing molecules into cells. In addition, photoheterotrophic prokaryotes could actively regulate intake and use of solar energy for different metabolic functions depending on the energetic demands of the cell. The factors listed above hence require consideration when solar energy input into metabolism of oceanic photoheterotrophic prokaryotes is experimentally quantified and numerically modelled.

Journal ArticleDOI
TL;DR: The seasonal distribution of the zooplankton community, including copepod and cladocerans, along a eutrophication gradient was examined in this paper to establish if the increase in phytoplank tonnage and productivity has an impact on the biomass and composition of the ZOP community.
Abstract: The seasonal distribution of the major components of the zooplankton community, protozooplankton, copepods and cladocerans, along a eutrophication gradient were examined in order to establish if eutrophication through increases in phytoplankton biomass and productivity has an impact on biomass and composition of the zooplankton community. Data on salinity, temperature, inorganic nutrients, chlorophyll a, phytoplankton biomass and production, protozooplankton and mesozoo-plankton biomass were collected during routine sampling at seven stations representing a eutrophication gradient from open to estuarine waters. Mesozooplankton biomass varied seasonally in all areas with a unimodal pattern characterized by a peak between mid-spring and mid-summer. Total mesozooplankton biomass as well as copepod and cladoceran biomass, decreased with DIN concentration. Salinity and the type of ecoystem (well-mixed shallow estuaries versus stratified open waters) were the major factors controlling the biomass proportion of copepod species. Annual production of the total copepod community as well as the total grazing impact of copepods on primary production was higher in open waters than in estuarine waters. In estuarine type ecosystems, phytoplankton production is underexploited by copepod grazing whereas in the open type ecosystems, the phytoplankton production alone could not satisfy the carbon demand of copepods stressing the potential importance of protozoans in the copepod diet.

Journal ArticleDOI
TL;DR: The results suggest that efforts should be made to estimate the variabiliry of prey abundance at the scale of larval fish foraging rather than using large-scale average abundance estimates, as small-scale prey patchiness likely plays a role in larvalFish feeding dynamics.
Abstract: We quantified the small-scale distribution of larval fish prey (copepod nauplii and copepodites) to (i) investigate the variabiliry in prey abundance across a range of spatial scales and (ii) determine the statistical distribution (normal, negative binomial or Poisson) that best describes the distribution of prey at scales relevant to larval fish foraging. We used a hierarchal sampling program to collect zooplankton at scales from metres to kilometres using a rosette sampler that collected replicate 2.5 L samples (∼ volume searched by larval fish) at 10 m. A generalized linear model framework was used to investigate the underlying distribution of the zooplankton at different spatial scales. The majority of variance (51 %) in zooplankton abundance was found at the metre-scale indicating a high degree of small-scale patchiness was present. The distribution of zooplankton was significantly different from a Poisson distribution but not a normal or negative binomial distribution. Thus, larval fish prey were not randomly distributed within the upper mixed layer, but were more aggregated than predicted by the standard Poisson distribution used in most foraging models. Our results suggest that efforts should be made to estimate the variabiliry of prey abundance at the scale of larval fish foraging rather than using large-scale average abundance estimates, as small-scale prey patchiness likely plays a role in larval fish feeding dynamics.

Journal ArticleDOI
TL;DR: In this article, the authors analyzed the pattern of distribution of three populations of picoplanktonic autotrophs measured using flow cytometry at 58 stations to the south of the Iberian Peninsula including the northeastern Gulf of Cadiz, Strait of Gibraltar and northwestern Alboran Sea, during July 1995.
Abstract: In this paper, we analyze the pattern of distribution of three populations of picoplanktonic autotrophs measured using flow cytometry. Prochlorococcus sp., Synechococcus sp. and eukaryotic picoplankton were investigated at 58 stations to the south of the Iberian Peninsula including the northeastern Gulf of Cadiz, Strait of Gibraltar and northwestern Alboran Sea, during July 1995. There were important differences in the oceanographic conditions in these regions, with warmer, low-nutrient and low-chlorophyll waters in the Atlantic sector and colder, high-nutrient and high- chlorophyll waters in the Alboran sector in which upwelling occurred. The biomass concentration of picoplanktonic cells was two times higher in the Atlantic area than in the richer Alboran Sea waters. The average integrated abundance of picophytoplankton was negatively correlated with chlorophyll a concentration as well as with inorganic nitrogen availability. Prochlorococcus and Synechococcus were the most abundant groups, and they showed a complementarity in their spatial distribution, with a shallower and more coastal distribution of Synechococcus and the presence of Prochlorococcus mainly at offshore stations and at deeper depths. The ratio of autotrophic pico- plankton/total autotrophic biomass showed significant differences on both sides of the Strait, with much higher values in the Gulf of Cadiz sector.

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TL;DR: In this article, a coupled physical-biogeochemical model was used to assess the biological responses to basin-scale climate forcing in the subtropical North Pacific Ocean (North Pacific) based on temporal variations in plankton community structure.
Abstract: Biological responses to basin-scale climate forcing in the subtropical North Pacific Ocean are assessed based on temporal variations in plankton community structure observed at Station ALOHA and results of a coupled physical‐biogeochemical model. Observational data and model simulations for the period 1990‐2004 reveal distinct temporal patterns, with significant increases in net primary productivity, modeled nitrate flux into the euphotic zone and the measured downward flux of particulate nitrogen during 1999‐2004. Concurrent increases in microalgae, cyanobacteria and modeled and measured zooplankton biomass were also observed during this period. We provide evidence that these responses were a consequence of climate forcing that destratified the upper ocean, making it more susceptible to mixing events and nutrient entrainment. These findings underscore the importance of nitrate flux and plankton community structure, as modulated by climate forcing, in regulating particle export over interannual and decadal time scales.

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TL;DR: It is confirmed that a gamma density function is an appropriate fitting function for copepod development time, based on a large data set on development time and can therefore be integrated into individual-based models ofCopepod population dynamics.
Abstract: While the populations of the copepod Eurytemora affinis are often morphologically (i.e. taxonomy) indistinguishable, the species complex is composed of genetically distinct clades, representing divergent evolutionary histories. The most distant clades, genetically and morphologically (i.e. phylogeny), are transatlantic clades: North American and European (Lee, 2000). The study of the life cycle strategies of two populations from St. Lawrence salt-marshes (Canada) and from the Seine estuary (France) at three salinities (5, 15 and 25) revealed differences in their salinity tolerance. Individual from the Seine exhibited high mortality under the highest salinity suggesting that the St. Lawrence population tolerated a wider salinity range. At the lowest salinity, the development time of St. Lawrence individuals was longer than that of individuals from the Seine suggesting that the Seine population was more adapted to low salinity. The clutch size and the longevity of St. Lawrence adults were on average two times higher compared to Seine adults. Thus, the St. Lawrence population exhibited a higher fitness relative to the Seine population. Such differences could be due to genetic differences resulting from divergent evolutionary history, to phenotypic plasticity and/or to the acclimation to culture conditions. We confirmed that a gamma density function is an appropriate fitting function for copepod development time, based on a large data set on development time. It can therefore be integrated into individual-based models of copepod population dynamics.

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TL;DR: The results of this study show that the period of higher river flow, coincident with winter, resulted in changes in the zooplankton community, and short-term temporal variations in the species composition and abundance were also attributed to tidal and diel cycles.
Abstract: In order to study the influence of physical forcing at different temporal scales, zooplankton was sampled at a fixed station located at the mouth of Mondego estuary (southern Europe). Samples were collected during diel cycles, over neap and spring tides. Zooplankton abundance and diversity were estimated for each sampling period Holoplankton were dominated by three taxa: Copepoda, with 48% of total zooplankton abundance, Cladocera (16%) and Medusae (12%). Meroplankton occurred mainly as barnacle and decapod larvae. Copepoda were the most diverse group, represented by 26 species follomed by Decapoda larvae (21) and Medusae (16). In order to assess significant dfferences between seasons, a univariate analysis was carried out. Higher abundance and diversity were found in warm months, particularly at neap tides, when water temperature and salinity were higher. Multivariate analysis revealed significant seasonal differences in species composition. The estuarine community was strongly dependent on allochthonous events, such as tidal exchange and river inflow. The results of our study show that the period of higher river flow, coincident with winter, resulted in changes in the zooplankton community. Short-term temporal variations in the species composition and abundance were also attributed to tidal and diel cycles. Zooplankton reached significantly higher densities at night (P < 0.05), suggesting the occurrence of vertical migrations. By emphasizing the importance of dijferent timescale changes in the zooplankton community of the Mondego estuary, this study will be useful fir the design of more efficient sampling programs, aiming at documenting changes in the zooplankton at a broad but also at a fine temporal scale.

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TL;DR: A method is proposed to evaluate the impact of the number of selected classes, in terms of classification performance, on the basis of a data set of classified zooplankton images, and suggests groupings that improve the performance of the automated classification.
Abstract: Zooplankton biomass and abundance estimation, based on surveys or time-series, is carried out routinely. Automated or semi-automated image analysis processes, combined with machine-learning techniques for the identification of plankton, have been proposed to assist in sample analysis. A difficulty in automated plankton recognition and classification systems is the selection of the number of classes. This selection can be formulated as a balance between the number of classes identified (zooplankton taxa) and performance (accuracy; correctly classified individuals). Here, a method is proposed to evaluate the impact of the number of selected classes, in terms of classification performance. On the basis of a data set of classified zooplankton images, a machine-learning method suggests groupings that improve the performance of the automated classification. The end-user can accept or reject these mergers, depending on their ecological value and the objectives of the research. This method permits both objectives to be equally balanced: (i) maximization of the number of classes and (ii) performance, guided by the end-user.

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TL;DR: Evidence is provided that over short distances, the wind is an important agent for the dispersal of phytoplankton with different colonization potentials, including the bloom-forming toxic cyanobacterium Microcystis aeruginosa.
Abstract: The air-dispersed phytoplankton diversity and colonization potential, in a Mediterranean River-Reservoir System (Aliakmon-Polyphytos, Northern Greece), were studied Phytoplankton samples were collected weekly, from five sampling sites in the river-reservoir system (for 11 weeks) and from jars replaced weekly for 10 successive weeks placed at one colonization site (CS) (for 10 weeks) The algal diversity in the river-reservoir system was high The air-dispersed algae in the CS jars included taxa of the local species pool (reservoir phytoplankton), including taxa of the airborne algae common in most biogeographic regions Both small- and large-sized phytoplankton in the vegetative form of unicells, small coenobia, large colonies and filaments were observed in the CS jars Nanoplanktic algae and among them the known allergenic chlorophyte Chlorella were the most frequent and abundant air-dispersed algae The large-sized chlorophytes Mougeotia sp and Ulothrix sp were also frequently air-dispersed in the CS jars Chlorophytes exhibited the highest colonization potential, followed by diatoms and cyanobacteria Evidence is provided that over short distances, the wind is an important agent for the dispersal of phytoplankton with different colonization potentials, including the bloom-forming toxic cyanobacterium Microcystis aeruginosa

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TL;DR: A Dual frequency IDentification SONar (DIDSON) provided a poweful tool to determine the numerical abundance and spatial distribution of medusae of the common jellyfish Aurelia aurita s l.
Abstract: A Dual frequency IDentification SONar (DIDSON) provided a poweful tool to determine the numerical abundance and spatial distribution of medusae of the common jellyfish Aurelia aurita s. l. in shallow coastal water. The sonar image obtained in high frequency (1.8 MHz) mode enabled us to identify and count individual medusae of 4.1―19.6 cm (mean: 13.1 cm) bell diameter. Deployment of the DIDSON along three ∼4-km-long transects in a shallow brackish-water lake (average depth: 5.1 m) revealed that A. aurita aggregated (e.g > 8. 0 medusae m ―3 ) near the lake center. The medusae occurred throughout the water column, but tended to avoid low salinity surface and deoxygenated bottom layers. The overall average density of medusae estimated by the DIDSON was 3.3 times higher than that estimated by net sampling Use of a DIDSON can facilitate quantitative determination of jellyfish populations that cause problem blooms worldwide in order to better understand their ecological importance.