Alfred Wegener Institute for Polar and Marine Research
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Macroscopic and microscopic perspectives on habitat-forming Antarctic hexactinellid sponges
No full textSponges of the class Hexactinellida, commonly known as glass sponges, are conspicuous members of the Antarctic benthos. They grow to considerable sizes and can form dense aggregations on the continental shelf of the Southern Ocean. These sponge grounds are associated with a high biodiversity as the large barrel-shaped hexactinellid sponges provide a complex three-dimensional structure and various micro-habitats for other animals. Their role as habitat-forming species has been well established, but other aspects of their biology remain largely unexplored. This thesis investigates Antarctic hexactinellid sponges from different perspectives that have previously been understudied or neglected. It presents new insights on their in-situ identification, their microbiomes, and their relevance for pelagic top predators.
Ecological studies on Antarctic benthos regularly use seafloor imaging for in-situ observations of habitats and communities. However, in-situ species identification of Antarctic hexactinellid sponges has often been problematic due to unprecise species descriptions focusing on microscopic skeletal spicules while disregarding external macroscopic features, as well as several taxonomic changes. Therefore, the first objective of this thesis was to develop a practical and reliable macroscopic approach to image-based species identification of the common hexactinellid sponges of the Antarctic shelf, comprising Anoxycalyx (Scolymastra) joubini and eight currently accepted species of the genus Rossella.
In order to identify diagnostic macroscopic characteristics of each species, I combined the examination of trawl-collected sponges, previous species descriptions, and a large collection of in-situ photographs and videos. Manuscript I describes all aspects of the external morphology of each species, accounting for their morphological variability and identifying species-specific characteristics. Special emphasis is put on the differentiation of species that have been mixed up in the past. The manuscript shows that each species in fact has unique macroscopic features that can be used to distinguish them in situ and without expert taxonomic knowledge. This provides a basis for species-specific observations on Antarctic hexactinellids in image-based studies.
The second objective of this thesis was to elucidate the role of Antarctic hexactinellid sponges as habitat on different scales beyond the direct macroscopic associations. Therefore, I investigated them from a microscopical perspective and from a top predator’s perspective. It is well known that sponges can host a variety of symbiotic microorganisms in their tissue. The sponge-associated microbial communities are species-specific and tightly coupled with their host regarding metabolic processes, chemical defense, and health. However, the microbial associations of Antarctic hexactinellids have barely been investigated before. Even less explored is the relevance of hexactinellid sponges for pelagic megafauna like seals or other top predators, despite descriptions of various fish species using them as habitat and nursery ground.
In order to describe the microbiomes of Antarctic hexactinellids and to compare them to those of Antarctic demosponges and deep-sea sponges from other regions, I have sampled deep-water hexactinellid and demosponge specimens for inclusion in two studies. Their microbial communities were analyzed by 16S rRNA gene sequencing and generation of amplicon sequence variants (ASVs). In addition, histological and ultrastructural analyses were conducted for several sponges. Manuscript II provides a detailed description of the microbiomes of Antarctic hexactinellids and demosponges in relation to sponge histology. All sponges were characterized as low microbial abundance sponges, but the denser the sponge tissue, the higher was the number of hosted microbes. Furthermore, the microbial community composition in hexactinellids and demosponges was significantly different. Although the hexactinellids showed a lower microbial richness than the demosponges, their microbiome comprised a higher percentage of class-specific ASVs. Manuscript III integrates these findings into a global analysis of deep-water sponge microbiomes. It shows that hexactinellid sponges in general have a characteristic microbiome distinct from that of demosponges. Compared to sponges from other regions, the Antarctic hexactinellids were among the species with the lowest microbial richness. Taken together, both studies show that Antarctic hexactinellid sponges host unique microbial communities.
Scaling up to a wider ecosystem context, Manuscript IV describes a previously unknown interaction of Weddell seals with Antarctic hexactinellid sponges. In a study on the foraging behavior of female Weddell seals during lactation period using animal-borne video recorders, several seals were found to investigate the cavities of large hexactinellid sponges, presumably searching for prey. This behavior was observed for the first time and it is currently unknown how widespread it is. All sponges in the videos were identified as Rossella cf. racovitzae, but other hexactinellid species may likewise serve as foraging ground for seals.
Overall, this thesis shows that the role of Antarctic hexactinellid sponges as habitat extends beyond the well-described associations of benthic fauna and that their relations with other organisms are more complex and diverse than previously recognized. They also host specific microbial communities and are relevant as foraging habitat for Antarctic top predators. Both aspects require further dedicated research to elucidate their ecological implications. As demonstrated in this thesis, changing perspectives may help to further broaden our understanding of the various roles of hexactinellid sponges in Antarctic marine ecosystems
Drivers of winter ice formation on Arctic water bodies in the Lena Delta, Siberia
Get PDFArctic landscapes are characterized by diverse water bodies, which are covered with ice for most of the year. Ice controls surface albedo, hydrological properties, gas exchange, and ecosystem services, but freezing processes differ between water bodies. We studied the influence of geomor-phology and meteorology on winter ice of water bodies in the Lena Delta, Siberia. Electrical conductivity (EC) and stable water isotopes of ice cores from four winters and six water bodies were measured at unprecedented resolution down to 2-cm increments, revealing differences in freezing systems. Open-system freezing shows near-constant isotopic and EC gradients in ice, whereas closed-system freezing shows decreasing isotopic composition with depth. Lena River ice displays three zones of isotopic composition within the ice, reflecting open-system freezing that records changing water sources over the winter. The isotope composition of ice covers in landscape units of different ages also reflects the individual water reservoir settings (i.e., Pleistocene vs. Holocene ground ice thaw). Ice growth models indicate that snow properties are a dominant determinant of ice growth over winter. Our findings provide novel insights into the winter hydro-chemistry of Arctic ice covers, including the influences of meteorology and water body geomor-phology on freezing rates and processes
Parameterized Internal Wave Mixing in Three Ocean General Circulation Models
Get PDFThe non-local model of mixing based on internal wave breaking, IDEMIX, is implemented as an enhancement of a turbulent kinetic energy closure model in three non-eddy resolving general circulation ocean models that differ in the discretization and choice of computational grids. In IDEMIX internal wave energy is generated by an energy flux resulting from near-inertial waves induced by wind forcing at the surface, and at the bottom, by an energy flux that parameterizes the transfer of energy between baroclinic and barotropic tides. In all model simulations with IDEMIX, the mixing work is increased compared to the reference solutions without IDEMIX, reaching values in better agreement with finestructure observations. Furthermore, the horizontal structure of the mixing work is more realistic as a consequence of the heterogeneous forcing functions. All models with IDEMIX simulate deeper thermocline depths related to stronger shallow overturning cells in the Indo-Pacific. In the North Atlantic, deeper mixed layers in simulations with IDEMIX are associated with an increased Atlantic overturning circulation and an increase of northward heat transports toward more realistic values. The response of the deep Indo-Pacific overturning circulation and the weak bottom cell of the Atlantic to the inclusion of IDEMIX is incoherent between the models, suggesting that additional unidentified processes and numerical mixing may confound the analysis. Applying different tidal forcing functions leads to simulation differences that are small compared to differences between the different models or between simulations with IDEMIX and without IDEMIX
Firn on ice sheets
No full textMost of the Greenland and Antarctic ice sheets are covered with firn — the transitional material between snow and glacial ice. Firn is vital for understanding ice-sheet mass balance and hydrology, and palaeoclimate. In this Review, we synthesize knowledge of firn, including its formation, observation, modelling and relevance to ice sheets. The refreezing of meltwater in the pore space of firn currently prevents 50% of meltwater in Greenland from running off into the ocean and protects Antarctic ice shelves from catastrophic collapse. Continued atmospheric warming could inhibit future protection against mass loss. For example, warming in Greenland has already contributed to a 5% reduction in firn pore space since 1980. All projections of future firn change suggest that surface meltwater will have an increasing impact on firn, with melt occurring tens to hundreds of kilometres further inland in Greenland, and more extensively on Antarctic ice shelves. Although progress in observation and modelling techniques has led to a well-established understanding of firn, the large uncertainties associated with meltwater percolation processes (refreezing, ice-layer formation and storage) must be reduced further. A tighter integration of modelling components (firn, atmosphere and ice-sheet models) will also be needed to better simulate ice-sheet responses to anthropogenic warming and to quantify future sea-level rise
Antarctic Macroalgae: Beyond Species Distribution Models to estimate Blue Carbon standing stocks
Get PDFThe West Antarctic Peninsula has been identified as highly vulnerable due to rising temperatures and increased anthropogenic carbon emissions impacting its biodiversity. Glacier retreat is one of the identified responses of the ecosystem, opening up new ice-free areas available for colonization but also changing the environment due to meltwater input, sediment runoff, and ice impact. A habitat is characterized by the combination of environmental parameters that support carbon cycling within a specific blue carbon ecosystem, such as macroalgae forest/assemblages. Species Distribution Models (SDMs) are an efficient tool to link environmental parameters with species presence/absence to identify their habitat suitability and distribution. What if the tool allows us more? We aim to estimate potential carbon standing stock expansion in the recently ice-free areas of a fjord ecosystem under glacial retreat from the determination of habitat suitability of macroalgae species. This study focuses on the SDMs of Chlorophytes, Rhodophytes, and five Phaeophytes species, projecting macroalgae habitat suitability by ensemble modeling applying biomod2 in the Potter Cove fjord. Further estimations of macroalgal expansion and colonization, as well as potential carbon stocks, were conducted through SDMs results, binary transformation, data from published in-situ abundance quadrat surveys, and conversion factors. From 1956 to 2020 Fourcade Glacier opened up ~1,6 km2 free of ice. Our estimation of macroalgal colonization during this time period at Potter Cove shows an expansion of ~0.42 km2 with a total carbon standing stock of 9.73 ± 6.32 tons of C. Through the integration of SDMs and field data, this research provides valuable insights into the dynamic relationship between Antarctic macroalgae, environmental changes, and carbon stocks, contributing to the broader understanding of ecosystem dynamics in the face of global climate change
Flipbook-ENA: Towards a dynamic Ecological Network Analysis under changing environmental conditions
Get PDFChanges in abiotic parameters can affect ecosystem structure and function. Network models with subsequent Ecological Network Analysis (ENA) are often used for the quantification of ecosystem-wide properties with descriptive system indices. However, dynamic abiotic alterations of an ecosystem cannot be resolved with the “state of the art” ENA as the methodology of analyses is static in both space and time. In this study, we present a new, almost dynamic ENA “Flipbook-ENA” which allows for the trend analysis of system indices over a defined range of abiotic factors. Flipbook-ENA enables an approximation of the dynamic system response by discretizing the continuous influence of abiotic factors and by calculating the corresponding changes in the model for each discretization step. ENA indices are therefore obtained as a discrete function of the abiotic conditions. We applied this new concept to two aquatic food web models as case studies, using temperature as the influencing abiotic factor. Flipbook-ENA can be considered an enhancement of ENA flexibility, also facilitating the provision of a quantitative assessment basis for socially, economically and ecologically-balanced management of ecosystems in unstable environmental conditions under the pressure of climate change
The experimental implications of the rate of temperature change and timing of nutrient availability on growth and stoichiometry of a natural marine phytoplankton community
Get PDFClimate change increases the need to understand the effect of predicted future temperature and nutrient scenarios on marine phytoplankton. However, experimental studies addressing the effects of both drivers use a variety of design approaches regarding their temperature change rate and nutrient supply regimes. This study combines a systematic literature map to identify the existing bias in the experimental design of studies evaluating the phytoplankton response to temperature change, with a laboratory experiment. The experiment was designed to quantify how different temperature levels (6°C, 12°C, and 18°C), temperature regimes (abrupt vs. gradual increase), timings of nutrient addition (before or after the temperature change) and nutrient regimes (limiting vs. balanced) alter the growth and stoichiometry of a natural marine phytoplankton community. The systematic map revealed three key biases in marine global change experiments: (1) 66% of the studies do not explicitly describe the experimental temperature change or nutrient regime, (2) 84% applied an abrupt temperature exposure, and (3) only 15% experimentally manipulated the nutrient regime. Our experiment demonstrated that the identified biases in experimental design toward abrupt temperature exposure induced a short-term growth overshoot compared to gradually increasing temperatures. Additionally, the timing of nutrient availability strongly modulated the direction of the temperature effect and strength of growth enhancement along balanced N : P supply ratios. Our study stresses that the rate of temperature change, the timing of nutrient addition and the N : P supply ratio should be considered in experimental planning to produce ecologically relevant results as different setups lead to contrasting directions of outcome
