Ökophysiologie antarktischer Meereis Meiofauna

Sea ice is permeated by small brine channels, which are characterised by sub-zero temperatures and varying salinities. Despite sometimes extreme conditions a relatively diverse fauna and flora thrives within these brine channels. Stephos longipes, Paralabidocera antarctica and Drescheriella glacialis are the dominant copepod species found within Antarctic sea ice. Their life-cycle strategies are well-established, but life cycles of other meiofauna (metazoans > 50 µm) found within sea ice are little explored. Adaptation mechanisms allowing meiofauna species to survive within sea ice are largely unknown. In order to increase our knowledge of the Antarctic sea-ice meiofauna, different microhabitats of sea ice and their metazoan fauna were studied during two cruises with R/V “Polarstern” to the western Weddell Sea. The dominant sympagic copepod species found in the sub-ice layer was Ectinosoma sp., other sympagic copepod species occurring regularly were D. glacialis/racovitzai, Diarthrodes cf. lilacinus, Idomene antarctica and S. longipes. Drescheriella glacialis/racovitzai and Stephos longipes were the dominant members of the surface-layer meiofauna during late spring. Their populations consisted mainly of adults and early naupliar stages in this layer, which points to an active reproduction of these species within the surface layer. Other taxa found in the surface layer were undetermined turbellarians, the gastropod Tergipes antarcticus, a ctenophore and two amphipod species. Sampling records from the Bellingshausen Sea, the Weddell Sea, as well as from the Prydz and the Lützow-Holm Bay indicate that T. antarcticus is widely distributed in Antarctic sea ice. During this study, adults, juveniles, veliger larvae and egg clutches of T. antarcticus were found in sea ice. A thorough morphological and anatomical description of all life stages was performed and the developmental time from egg to veliger larvae was determined as being 31 days (range: 13 to 65 days) at 0 °C. The observed reproduction of D. racovitzai, Idomene antarctica and T. antarcticus within the habitat allows to assign also these species as true members of the sea-ice meiofauna. Adaptation mechanisms to changing salinities and ice formation were studied for P. antarctica, S. longipes and T. antarcticus. The haemolymph of the two copepod species is isosmotic to the medium at salinities from 25 to 45 g/kg (S. longipes) and 25 to 55 g/kg (P. antarctica). Thermal hysteresis (a non-colligative inhibition of ice growth) was found for T. antarcticus and S. longipes, but not for P. antarctica. These are the first reports of thermal hysteresis from gastropods and crustaceans, respectively. The acquisition of thermal hysteresis seems to enable S. longipes to exploit all available microhabitats within sea ice. In particular, S. longipes is found in high abundances in the surface layer, in which stronger temperature fluctuations can occur than in the lowermost centimetres of the ice. P. antarctica seems to be restricted physiologically to the lower layer. T. antarcticus was also found in the surface layer, but the importance of thermal hysteresis for habitat choice in T. antarcticus remains to be shown. Thermal hysteresis is probably also a prerequisite for the ability of S. longipes and T. antarcticus to spawn within the ice. Adaptations to low temperatures and elevated salinties on the transcriptional level were investigated for S. longipes. Two isoforms of a protein were found, which, if recombinantly expressed, confers thermal hysteresis. A high homology to a group of (putative) antifreeze proteins from a bacterium, a snow mold and several diatoms and, in contrast, no homologs in any metazoan lineage suggest that this protein was obtained through horizontal gene transfer. This seems to be a key event in the evolution of S. longipes.Meereis ist durchzogen von kleinen Solekanälen, die durch Temperaturen unter 0 °C und variable Salzgehalte gekennzeichnet sind. Trotz teilweise extremer Bedingungen ist das Solekanalsystem von einer relativ artenreichen Flora und Fauna besiedelt. Stephos longipes, Paralabidocera antarctica und Drescheriella glacialis sind die dominanten Copepodenarten des Antarktischen Meereises. Ihre Lebenszyklen sind gut untersucht, die Lebenszyklen anderer Meereis-Meiofauna Organismen (Metazoen > 50 µm) hingegen nicht. Anpassungsmechanismen, die es Meiofauna-Organismen erlauben, im Meereis zu überleben, sind weitestgehend unbekannt. Um die Kenntnis der Antarktischen Meereis-Meiofauna zu verbessern, wurden während zweier Expeditionen mit dem Forschungsschiff „Polarstern“ in das westliche Weddellmeer verschiedene Mikrohabitate des Meereises und ihre Meiofauna untersucht. Ectinosoma sp. war die dominierende Copepodenart des Untereishabitats im späten Frühjahr. Weitere, regelmäßig vorkommende Arten waren D. glacialis/racovitzai, Diarthrodes cf. lilacinus, Idomene antarctica und S. longipes. Drescheriella glacialis/racovitzai und S. longipes dominierten in der Oberflächenschicht (engl.: surface layer). Ihre Populationen bestanden hauptsächlich aus Adulten und frühen Larvenstadien, was auf eine aktive Reproduktion dieser Arten in der Oberflächenschicht hindeutet. Andere in der Oberflächenschicht nachgewiesene Taxa sind Plathelminthen, die Nacktschnecke Tergipes antarcticus, eine Ctenophore und zwei Amphipoden. Nachweise aus dem Bellingshausen- und dem Weddellmeer, aus der Prydz- und der Lützow-Holm-Bucht weisen darauf hin, dass T. antarcticus im Antarktischen Meereis weit verbreitet ist. Im späten Winter wurden Adulte, Juvenile, Larven und Eiballen von T. antarcticus im Meereis gefunden. Eine eingehende morphologische und anatomische Beschreibung der Art wurde vorgenommen und die Entwicklungszeit vom Einzell-Stadium bis zum Schlüpfen der Veliger-Larve auf 31 Tage (Spannweite: 12 bis 65 Tage) bei 0 °C bestimmt. Die nachweisliche Reproduktion von D. racovitzai, I. antarctica und T. antarcticus im Meereis erlaubt es, auch diese Arten als echte Bestandteile der Meereis-Meiofauna einzustufen. Mechanismen der Anpassung an variierende Salzgehalte und Eisbildung wurden an T. antarcticus, P. antarctica und S. longipes studiert. Die Hämolymphe der beiden Copepodenarten ist isosmotisch zum Außenmedium bei Salzgehalten von 25 bis 45 g/kg (S. longipes) bzw. von 25 bis 55 g/kg (P. antarctica). Thermale Hysterese (die Behinderung von Eiswachstum) wurde bei T. antarcticus und S. longipes gefunden. Dies sind jeweils die ersten Nachweise von thermaler Hysterese für Gastropoden und Crustaceen. Diese physiologische Anpassung scheint es S. longipes zu ermöglichen, alle vorhandenen Habitate des Meereises zu besiedeln. Vor allem in der Oberflächenschicht des Eises, in welcher stärkere Temperaturschwankungen vorkommen können als in der Bodenschicht (engl.: bottom layer), ist S. longipes in hohen Abundanzen zu finden. P. antarctica scheint physiologisch auf die Bodenschicht beschränkt zu sein. T. antarcticus wurde ebenfalls in der Oberflächenschicht gefunden, aber die Bedeutung der nachgewiesenen thermalen Hysterese für die Habitatwahl dieser Art bleibt fraglich. Thermale Hysterese ist wahrscheinlich auch eine Voraussetzung dafür, dass S. longipes und T. antarcticus ihre Eier im Eis ablegen können. Anpassungen an niedrige Temperaturen und erhöhte Salzgehalte wurden auf Transkriptionsebene an S. longipes untersucht. Zwei Isoformen eines Proteins konnten identifiziert werden, welches rekombinant hergestellt thermale Hysterese bewirkt. In den verfügbaren Sequenzdatenbanken lassen sich keine tierischen Homologe finden. Dies und eine hohe Homologie der Proteine zu Gefrierschutzproteinen von einem Bakterium, einem Pilz und einer Meereis-Diatomee deuten darauf hin, dass das Protein durch horizontalen Gentransfer übertragen wurde. Dies scheint ein Schlüsselereignis in der Evolution von S. longipes gewesen zu sein

MorphoCluster: Efficient Annotation of Plankton Images by Clustering

In this work, we present MorphoCluster, a software tool for data-driven, fast, and accurate annotation of large image data sets. While already having surpassed the annotation rate of human experts, volume and complexity of marine data will continue to increase in the coming years. Still, this data requires interpretation. MorphoCluster augments the human ability to discover patterns and perform object classification in large amounts of data by embedding unsupervised clustering in an interactive process. By aggregating similar images into clusters, our novel approach to image annotation increases consistency, multiplies the throughput of an annotator, and allows experts to adapt the granularity of their sorting scheme to the structure in the data. By sorting a set of 1.2 M objects into 280 data-driven classes in 71 h (16 k objects per hour), with 90% of these classes having a precision of 0.889 or higher. This shows that MorphoCluster is at the same time fast, accurate, and consistent; provides a fine-grained and data-driven classification; and enables novelty detection

Ammonium excretion and oxygen respiration of tropical copepods and euphausiids exposed to oxygen minimum zone conditions

Calanoid copepods and euphausiids are key components of marine zooplankton communities worldwide. Most euphausiids and several copepod species perform diel vertical migrations (DVMs) that contribute to the export of particulate and dissolved matter to midwater depths. In vast areas of the global ocean, and in particular in the eastern tropical Atlantic and Pacific, the daytime distribution depth of many migrating organisms corresponds to the core of the oxygen minimum zone (OMZ). At depth, the animals experience reduced temperature and oxygen partial pressure (pO2) and an increased carbon dioxide partial pressure (pCO2) compared to their near-surface nighttime habitat. Although it is well known that low oxygen levels can inhibit respiratory activity, the respiration response of tropical copepods and euphausiids to relevant pCO2, pO2 and temperature conditions remains poorly parameterized. Further, the regulation of ammonium excretion at OMZ conditions is generally not well understood. It was recently estimated that DVM-mediated ammonium supply considerably fuels bacterial anaerobic ammonium oxidation – a major loss process for fixed nitrogen in the ocean. These estimates were based on the implicit assumption that hypoxia or anoxia in combination with hypercapnia (elevated pCO2) does not result in a downregulation of ammonium excretion. Here we show that exposure to OMZ conditions can result in strong depression of respiration and ammonium excretion in calanoid copepods and euphausiids from the Eastern Tropical North Atlantic and the Eastern Tropical South Pacific. These physiological responses need to be taken into account when estimating DVM-mediated fluxes of carbon and nitrogen into OMZs

On the Estimation of Zooplankton-Mediated Active Fluxes in Oxygen Minimum Zone Regions

In the Peruvian upwelling system, the mesopelagic oxygen minimum zone (OMZ) is the main vertically structuring feature of the pelagic habitat. Several zooplankton and nekton species undertake diel vertical migrations (DVMs) into anoxic depths. It has been argued that these migrations contribute substantially to the oxygen consumption and release of dissolved compounds (in particular ammonium) in subsurface waters. However, metabolic suppression as a response to low ambient oxygen partial pressure (pO2) has not been accounted for in these estimates. Here, we present estimates of zooplankton- and nekton-mediated oxygen consumption and ammonium release based on vertically stratified net hauls (day/night, upper 1,000 m). Samples were scanned, followed by image analysis and size-/taxon-specific estimation of metabolic rates of all identified organisms as a function of their biomass as well as ambient temperature and pO2. The main crustacean migrants were euphausiids (mainly E. mucronata) on offshore stations and the commercially exploited squat lobster Pleuroncodes monodon on the upper shelf, where it often undertakes migration to the seafloor during the day. Correction for metabolic suppression results in a substantial reduction of both respiration and ammonium excretion within the OMZ core. Ignoring this mechanism leads to a 10-fold higher estimate of DVM-mediated active export of carbon by respiration to below 100 m depth at deep-water stations. The DVM-mediated release of ammonium by euphausiids into the 200–400 m depth layer ranges between 0 and 36.81 μmol NH4 m−2 d−1, which is insufficient to balance published estimates of ammonium uptake rates due to anammox. It seems critical to account for the modulation of zooplankton metabolic activity at low oxygen in order to correctly represent the contribution of migrating species to the biological pump

Effects of nitrate and phosphate supply on chromophoric and fluorescent dissolved organic matter in the Eastern Tropical North Atlantic: a mesocosm study

In open-ocean regions, as is the Eastern Tropical North Atlantic (ETNA), pelagic production is the main source of dissolved organic matter (DOM) and is affected by dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations. Changes in pelagic production under nutrient amendments were shown to also modify DOM quantity and quality. However, little information is available about the effects of nutrient variability on chromophoric (CDOM) and fluorescent (FDOM) DOM dynamics. Here we present results from two mesocosm experiments ("Varied P" and "Varied N") conducted with a natural plankton community from the ETNA, where the effects of DIP and DIN supply on DOM optical properties were studied. CDOM accumulated proportionally to phytoplankton biomass during the experiments. Spectral slope (S) decreased over time indicating accumulation of high molecular weight DOM. In Varied N, an additional CDOM portion, as a result of bacterial DOM reworking, was determined. It increased the CDOM fraction in DOC proportionally to the supplied DIN. The humic-like FDOM component (Comp.1) was produced by bacteria proportionally to DIN supply. The protein-like FDOM component (Comp.2) was released irrespectively to phytoplankton or bacterial biomass, but depended on DIP and DIN concentrations. Under high DIN supply, Comp.2 was removed by bacterial reworking, leading to an accumulation of humic-like Comp.1. No influence of nutrient availability on amino acid-like FDOM component in peptide form (Comp.3) was observed. Comp.3 potentially acted as an intermediate product during formation or degradation of Comp.2. Our findings suggest that changes in nutrient concentrations may lead to substantial responses in the quantity and quality of optically active DOM and, therefore, might bias results of the applied in situ optical techniques for an estimation of DOC concentrations in open-ocean regions

Effects of varied nitrate and phosphate supply on polysaccharidic and proteinaceous gel particles production during tropical phytoplankton bloom experiments

Gel particles such as the polysaccharidic transparent exopolymer particles (TEP) and the proteinaceous Coomassie stainable particles (CSP) play an important role in marine biogeochemical and ecological processes like particle aggregation and export, or microbial nutrition and growth. So far, effects of nutrient availability or of changes in nutrient ratios on gel particle production and fate are not well understood. The tropical ocean includes large oxygen minimum zones, where nitrogen losses due to anaerobic microbial activity result in a lower supply of nitrate relative to phosphate to the euphotic zone. Here, we report of two series of mesocosm experiments that were conducted with natural plankton communities collected from the eastern tropical North Atlantic (ETNA) close to Cape Verde in October 2012. The experiments were performed to investigate how different phosphate (experiment 1, Varied P: 0.15–1.58 μmol L−1) or nitrate (experiment 2, Varied N: 1.9–21.9 μmol L−1) concentrations affect the abundance and size distribution of TEP and CSP. In the days until the bloom peak was reached, a positive correlation between gel particle abundance and Chl a concentration was determined, linking the release of dissolved gel precursors and the subsequent formation of gel particles to autotrophic production. After the bloom peak, gel particle abundance remained stable or even increased, implying a continued partitioning of dissolved into particulate organic matter after biomass production itself ceased. During both experiments, differences between TEP and CSP dynamics were observed; TEP were generally more abundant than CSP. Changes in size distribution indicated aggregation of TEP after the bloom, while newly formed CSP decomposed. Abundance of gel particles clearly increased with nitrate concentration during the second experiment, suggesting that changes in [DIN] : [DIP] ratios can affect gel particle formation with potential consequences for carbon and nitrogen cycling as well as food web dynamics in tropical ecosystems

Antarctic sympagic meiofauna in winter: Comparing diversity, abundance and biomass between perennially and seasonally ice-covered regions

This study of Antarctic sympagic meiofauna in pack ice during late winter compares communities between the perennially ice-covered western Weddell Sea and the seasonally ice-covered southern Indian Ocean. Sympagic meiofauna (proto- and metazoans >20 μm) and eggs >20 μm were studied in terms of diversity, abundance and carbon biomass, and with respect to vertical distribution. Metazoan meiofauna had significantly higher abundance and biomass in the western Weddell Sea (medians: 31.1×103 m−2 and 6.53mg m−2, respectively) than in the southern Indian Ocean (medians: 1.0×10 103 m−2and 0.06 mg m−2, respectively). Metazoan diversity was also significantly higher in the western Weddell Sea. Furthermore, the two regions differed significantly in terms of meiofauna community composition, as revealed through multivariate analyses. The overall diversity of sympagic meiofauna was high, and integrated abundance and biomass of total meiofauna were also high in both regions (0.6–178.6×103 m−2 and 0.02–89.70mg m−2, respectively), mostly exceeding values reported earlier from the western Weddell Sea in winter. We attribute the differences in meiofauna communities between the two regions to the older first-year ice and multi-year ice that is present in the western Weddell Sea, but not in the southern Indian Ocean. Our study indicates the significance of perennially ice-covered regions for the establishment of diverse and abundant meiofauna communities. Furthermore, it highlights the potential importance of sympagic meiofauna for the organic matter pool and trophic interactions in sea ice

Editorial: Zooplankton and Nekton: Gatekeepers of the Biological Pump

International audienceEditorial on the Research Topic Zooplankton and Nekton: Gatekeepers of the Biological Pump Zooplankton and nekton organisms create and destroy particles in manifold ways. They feed on the diverse components of the plankton community and on detrital matter. They disaggregate these components, but also repackage them into fecal pellets. Zooplankton and nekton thereby contributes to the attenuation, but also to the export of vertically settling particles. Many zooplankton and nekton organisms also ascend to the surface layer of the ocean at dusk to feed during the dark hours, and return to midwater at the break of dawn. This diurnal vertical migration (DVM) shuttles organic matter from the surface ocean to deeper layers, where it is metabolized and excreted. This active flux (as opposed to the passive flux of sinking particles) can contribute substantially to the biological pump, the downward export of carbon and nutrients into the oceans interior. Due to their multiple roles in oceanic particle dynamics, zooplankton and nekton organisms can actually be considered the gatekeepers of the biological pump. Several articles in this Research Topic deal with the contribution of zooplankton and nekton-mediated active flux to the total export of organic matter. Using biomass and enzyme transport system (ETS) assessments of respiratory flux for both mesozooplankton and micronekton communities, Hernández-León et al. estimated the total active transport of carbon (respiration, excretion, mortality, and egestion) along a transect in the Atlantic from the Canary Islands to Brazil. They found that active flux by these communities ranged from 25 to 80% of the total particulate organic carbon export at 150 m depth and that the importance of active flux increased with increasing surface productivity. Kwong et al. compared biomass, diel vertical migration, and active flux of mesozooplankton and micronekton across a range of mesoscale eddy structures along the east-coast of Australia during winter and spring. They found that although all eddy regimes had similar integrated biomass of mesozooplakton and micronekton, the organisms in the individual eddies had different migratory behavior, which resulted in contrasting importance of active flux. Kiko et al. assessed the impact of mesozooplankton DVM on elemental cycling at three stations in the Eastern Tropical North Atlantic. They found that approximately 31 to 41% of the total nitrogen loss from the upper 200 m of the water column was attributable to DVM mediated fluxes. They also suggest that gut flux-the flux created by migrators when they evacuate their gut at DVM-depth-and migrator mortality at DVM-depth contribute to an Intermediate Particle Maximum. In their study conducted in the Peruvian upwelling system (which features a severe midwater oxygen minimum zone), Kiko and Hauss concluded that the metabolic suppressio

S2C2 -- An orthogonal method for Semi-Supervised Learning on ambiguous labels

Semi-Supervised Learning (SSL) can decrease the required amount of labeled image data and thus the cost for deep learning. Most SSL methods assume a clear distinction between classes, but class boundaries are often ambiguous in real-world datasets due to intra- or interobserver variability. This ambiguity of annotations must be addressed as it will otherwise limit the performance of SSL and deep learning in general due to inconsistent label information. We propose Semi-Supervised Classification & Clustering (S2C2) which can extend many deep SSL algorithms. S2C2 automatically estimates the ambiguity of an image and applies the respective SSL algorithm as a classification to certainly labeled data while partitioning the ambiguous data into clusters of visual similar images. We show that S2C2 results in a 7.6% better F1-score for classifications and 7.9% lower inner distance of clusters on average across multiple SSL algorithms and datasets. Moreover, the output of S2C2 can be used to decrease the ambiguity of labels with the help of human experts. Overall, a combination of Semi-Supervised Learning with our method S2C2 leads to better handling of ambiguous labels and thus real-world datasets