1,720,973 research outputs found
Trophic ecology of Arctic gelatinous zooplankton
No full textThe overarching goal of this dissertation is to reject the hypothesis of gelatinous zooplankton (GZP) as a trophic dead end for the Arctic and sub-Arctic seas. To do so, I investigated the role of GZP in the diet of different invertebrate and fish species, during different seasons and at different locations. To do so, I used DNA metabarcoding of the stomach contents of bentho-pelagic amphipods and demersal and bathypelagic fish species. I also, aimed to prove the usefulness of DNA metabarcoding in diet studies and the detection of GZP, which are rapidly digested and due to that often overlooked with traditional methods. Furthermore, I aimed to investigate the role of GZP species in the marine food web, which was investigated using stable isotope analysis, to determine the trophic level of GZP and fish species in Porsangerfjorden. Overall, I was able to show that GZP was consumed regularly by many predators in the Arctic and sub-Arctic seas during different seasons. Furthermore, I showed that GZP should be considered as different groups in food web models rather than being pooled into one term
Investigating the prey spectrum of two co-occurring Themisto amphipods in the Fram Strait (Atlantic-Arctic gateway) using DNA metabarcoding
Full text linkPelagic amphipods are a key zooplankton group in polar regions. In the Arctic Ocean, the two hyperiid amphipod species Themisto libellula and Themisto abyssorum are dominating the pelagic community. They are not only an important food source for higher trophic levels, but also important predators able to control the zooplankton standing stock in some Arctic regions. In recent years, several studies using stereomicroscopy and biomarkers were conducted to study the diet of T. libellula and T. abyssorum. These studies suggested a diet mainly consisting of the most abundant zooplankton species including copepods, euphausiids and chaetognaths. It was also found that the two amphipods are covering different niches in the Arctic ecosystem, with T. libellula being more dependent on the ice-algal pathway. This leads to the assumption that the two amphipods are differently impacted by the ongoing Atlantification and sea ice retreat. In this study, DNA metabarcoding was used to assess the prey spectrum at high taxonomic resolution and potentially detect so far overlooked gelatinous zooplankton in the diet of these predators. The results indicate that the two predators are feeding on different zooplankton and ichthyoplankton species and hence, do not compete for food. Additionally, calanoid copepods do not seem to be as important as assumed in the diet in the summer months. The diet within one predator species different between the different sampling localities. T.libellula’s diet consisted of ice-associated species like Calanus glacialis and Boreogadus saida in regions with cold, Arctic waters, while these prey species were not found at stations with Atlantic waters. This leads to the assumption that a sympagic fueled diet for this species is rather linked to the location than to a preferred prey type. The diet of T. abyssorum was dominated by the chaetognath Eukrohnia hamata, but high variability was observed between the stations. At locations with Atlantic influence, RRA (relative read abundances) for Calanus finmarchicus were higher than at stations with Arctic impact. In some samples of both predators, sequences belonging to several hydrozoan species, e.g., Nanomia cara and Aglantha digitale, were detected. Those findings were not linked to certain locations, but they show that the amphipods are able to feed on gelatinous zooplankton, although the do not make up a major part of their diet. The broad prey spectrum found for T. libellula shows that this flexible species may be able to adapt its diet to changes in the zooplankton community caused by climate change and sea ice retreat. To fully understand Themisto’s feeding behavior, more sampling is needed, ideally combining DNA metabarcoding and biomarkers, to assess then both short-term and long-term diet
Trophic ecology of Arctic gelatinous zooplankton
No full textGelatinous zooplankton (GZP), comprising ctenophores, cnidarians, and tunicates, gained more interest in recent years. During favourable conditions, GZP is known to rapidly increase in biomass and several dominant species are able to exploit the zooplankton standing stock. Since GZP biomass may increase with ongoing warming in the Arctic Ocean, similar to elsewhere in the World Ocean, it is likely that GZP range shifts will take place concomitant with the Atlantification of the Arctic. Thus, it is crucial to gain knowledge about their role in the Arctic marine food web. Because of GZP having low nutritional values, they were considered to be a “trophic dead end”, until modern methods like video logging, biomarkers, and molecular diet studies introduced a paradigm shift regarding the role of jellyfish as prey for different fish and seabird species worldwide. In this project, we will use DNA metabarcoding to reveal the role of GZP in the diet of Arctic and sub-Arctic fish species. Additionally, the role of so-called “jellyfalls” as a carbon source for the Arctic deep-sea benthic communities will be investigated by applying DNA metabarcoding on the stomach contents of scavenging benthic amphipods. The role of GZP as predators in the Arctic Ocean during different seasons will also be investigated. To do so, DNA metabarcoding of the gastric pouch of dominant GZP species in the Arctic Ocean will be performed, to detect, amongst others, predation on ichthyoplankton. Additionally, biomarkers will be used to determine the trophic position of GZP in the Arctic marine food web and to reveal the role of the ice-algal pathway in the diet of GZP. The obtained data will reveal which fish species may be competitors of predators on jellyfish and, hence, will contribute to improve food web models of the Arctic marine ecosystem, currently neglecting the role of GZP
Investigating the prey spectrum of two co-occurring Themisto amphipod species in the Fram Strait using DNA metabarcoding
No full textThe pelagic amphipods Themisto libellula and T. abyssorum represent a key zooplankton group in the polar regions. These two hyperiid amphipods are an important food source for higher trophic levels, and being carnivores, they are able to control the zooplankton standing stock in some Arctic regions. T. abyssorum is a boreal species reaching the Arctic with incoming Atlantic water and is thus likely to benefit from the ongoing Atlantification of the Arctic Ocean, whereas it may heavily impact the genuine Arctic T. libellula. Since the Arctic is undergoing drastic changes in terms of warming and sea ice loss, it is crucial to gain knowledge about the prey spectrum of these two important amphipods to predict the future of pelagic food web in a changing Arctic.
In recent years, several studies were conducted concerning the diet of T. libellula and T. abyssroum. In those studies, mainly using biomarkers or stereomicroscopy, a diet consisting of the most abundant zooplankton species including calanoid copepods, euphausiids and chaetognaths, was suggested. T. abyssorum and T. libellula were also found to occupy distinct niches in the Arctic ecosystem.
In this study, DNA metabarcoding was used to compare the species-level prey spectrum of the two Themisto amphipods and compare its regional variation. We found that T. libellula feeds on larvae of fish like Boreogadus saida, while this was not the case for T. abyssorum. The chaetognath Eukrohnia hamata was detected in most samples of T. abyssorum, as well as DNA of hydrozoans like Aglantha digitale or Nanomia cara, suggesting that gelatinous zooplankton represent a food source for those amphipods
Investigating the prey spectrum of two co-occurring Themisto amphipods in the Fram Strait (Atlantic-Arctic gateway) using DNA metabarcoding
No full textPelagic amphipods are a key zooplankton group in polar regions. In the Arctic Ocean, the two hyperiid amphipod species Themisto libellula and Themisto abyssorum are dominating the pelagic community. They are not only an important food source for higher trophic levels like birds, fish and marine mammals, but they are also important predators able to control the zooplankton standing stock in some Arctic regions. Due to its association with incoming warm Atlantic waters, T. abyssorum will be benefitting from the ongoing Atlantification of the Arctic Ocean whereas the Arctic water associated T. libellula may suffer from it. Since the Arctic is undergoing drastic changes in terms of warming and sea ice loss, it is crucial to gain knowledge about the prey spectrum of these two important amphipods, to predict the future of pelagic communities and the food web in a changing Arctic.
In recent years, several studies were conducted concerning the diet of T. libellula and T. abyssorum. These studies suggested a diet mainly containing the most abundant zooplankton species including copepods, euphausiids and chaetognaths. As these studies were often conducted using stereomicroscopy they are limited, as it remains impossible to identify soft-bodied organisms and strongly digested prey. Other studies using a biomarker approach like stable isotopes or fatty acids, suggested that the two Themisto species are covering different niches in the Arctic ecosystem, with T. abyssorum covering a slightly higher trophic position than T. libellula. Additionally, they suggested that T. libellula has a diet associated to the sea-ice algal pathway, leading to the assumption that it will be more heavily affected by the ongoing sea-ice retreat. But these approaches are also limited to a certain point since there are only specific markers for primary production and primary consumers and so the whole prey spectrum of these organisms cannot be estimated with these methods.
We used DNA barcoding to compare the species-level prey spectrum of the two Themisto amphipods and investigate its regional variation. Therefore, samples from different locations in the Fram Strait were taken, and stomach contents from 30-75 individuals per station were extracted. When comparing prey spectra between locations, we aim to identify drivers impacting the diet composition of both species and to estimate how environmental changes will affect the food web structure. Finally, we would like to detect whether and to which extent Themisto feeds on gelatinous zooplankton, known to be very abundant in the region
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Presence data of gelatinous zooplankton in the Polar Night in Kongsfjorden, Svalbard, in 2022
No full textDuring the KOP183 project Arctic jellyfish in the Polar Night at the AWIPEV station in January-February 2022, we assessed the Polar Night gelatinous zooplankton community and their overwintering strategies. By means of net catches from WP2 and WP3 plankton nets deployed from MS Teisten, and hand nets deployed from the pontoon in Ny-Alesund, we characterized the species diversity of gelatinous zooplankton, including hydrozoan and scyphozoan medusae, as well as ctenophores. At least 15 different species of gelatinous zooplankton (6 Hydrozoa of which 2 Siphonophora, 2 Scyphozoa, 8 Ctenophora), were observed, based on morphological examination. Several of these species were so far not known to overwinter. DNA barcoding of these specimens is currently ongoing and will likely reveal an even higher species diversity, knowing that a number of unidentified species were sampled
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