ARJEL - AWI

Gelatinous zooplankton from the Fram Strait region, Arctic Ocean (Photo: Mario Hoppmann/AWI)

Gelatinous zooplankton, comprising cnidarians, ctenophores and tunicates, are known to be major drivers of ecosystem changes. Recently, increases in jelly biomass, referred to as “jellification”, have been observed in several marine ecosystems, causing the collapse of major fisheries. Abundance data on jellies are virtually non-existent for the Arctic, impeding our ability to detect changes of a similar magnitude. To overcome the drawbacks of traditional net sampling, we sample jellies using the most recent technologies in optics and environmental DNA analyses. Such interdisciplinary surveys allow us to link distributional patterns of jellies to sea-ice and oceanographic (sub-mesoscale) features. We apply species and community distribution models to a broader set of archived data as well as on newly obtained distributional data from optical transects. With these models, we aim to understand observed species patterns and to predict changes under future scenarios. The role of jellies in the Arctic food web, their importance for higher trophic levels and their link to the sea-ice trophic pathway will be elucidated with metabarcoding and biomarker studies. We plan to conduct physiological and transcriptomic studies assessing different populations’ thermal responses in order to predict range expansions along a latitudinal, poleward gradient. Using eDNA, we want to monitor “newcomers” and community changes in rapidly changing regions such as Fram Strait, the gateway to the Arctic, and the Atlantic-influenced Kongsfjorden (Svalbard). Eventually, these data on abundances, distribution and prey spectrum will be fed into food web models for evaluating the consequences of range shifts of Atlantic and Arctic jellyfish. A better understanding of jellies´ trophic role is crucial for the rapidly changing Arctic and surrounding shelf seas, responsible for 10% of the global fisheries.

The different objectives of the ARJEL project (Graphic: Charlotte Havermans/AWI)

Current projects

Some of the research topics currently addressed by ARJEL:

How diverse are gelatinous zooplankton communities in the Arctic Ocean?

Since gelatinous zooplankton are fragile and easily fragmented, there are some biases associated with net and trawl sampling. We use integrative field surveys (see also the section “Methods used”) combining nets, optical instruments (imaging or video footage), and environmental DNA (eDNA) studies of water and sediment, to get an accurate estimate of jellyfish diversity, distribution patterns and abundances. During the FramJelly project, a collaboration between AWI and GEOMAR, on PS126 (schematic below), we compared the outcome of jellyfish diversity sampled with depth-stratified nets like the Multinet, eDNA community composition in the water column at different depths, and the results of the annotations of video footage of the Pelagic In Situ Observation System (PELAGIOS, GEOMAR). With these studies in Fram Strait, the Atlantic gateway to the Arctic, we also aim to set a baseline of gelatinous zooplankton diversity for long-term monitoring in this area which acts as a sentinel for future changes further north. In collaboration with GEOMAR, we have samples for eDNA from different years (2019-2021) and aim to continue sampling these same stations on an annual basis.

Integrative field survey of the FramJelly project during the R/V Polarstern cruise PS126 (Graphic: Dmitrii Pantiukhin/AWI)

How do jellyfish survive in the Polar Night? Which species are abundant in winter and on what do they feed?

During our research stay at the AWIPEV research base in winter 2022, we investigated the presence of overwintering jellyfish species, their distribution and feeding ecology. To do so, we used net sampling, environmental DNA of water and sediment, and DNA metabarcoding of jellyfish stomach contents, respectively. Further analyses will bring new insights into the gelatinous zooplankton community composition during this understudied but ecologically relevant period.

Overwintering jellyfish sampled in Kongsfjorden, Svalbard, during the Polar Night (Photo: Charlotte Havermans/AWI)

Which jellyfish species will become the winners and losers in the new Arctic?

Gelatinous zooplankton are reputed to be climate change winners, but this picture is much more complex than usually presented. Gelatinous zooplankton, or jellies, comprise different groups (cnidarian medusae, ctenophores and pelagic tunicates), which are merged together due to their fragile, watery bodies, but actually consist of phylogenetically very different taxa. In the Arctic ecosystem, much of the gelatinous fauna consist of temperate (Atlantic) species extending their distributions into polar waters. Some species have a distribution centered in Arctic waters, and are likely to be cold-adapted, some of which maybe even dependent on sea ice for part of their life cycle or feeding. To investigate this, we harvest observations from public databases and carry out integrative field surveys to get better estimates of species distributions and abundances. Using these data, we apply species and community distribution models for predicting species distributions on the regional and pan-Arctic scale for a better understanding of species affinities to certain environmental parameters. Using these models, we then project their distributions under future climate change scenarios, in order to find out how gelatinous communities will change and which species may become less or more abundant in the future.

Does sea ice play a particular role in Arctic jellyfish ecology?

We evaluate the role of sea ice as a habitat for jellies in order to make predictions on their potential decrease with further sea ice decline. To do so, we use environmental DNA analyses, in combination with ROV transects and footages from other optical devices, to detect jellyfish under the ice, interpret their aggregations, behavior and distributions with regard to sea-ice patterns and other biological parameters measured in situ.

How prevalent is jellyfish predation in Arctic and sub-Arctic fish communities?

Despite several jellyfish groups to be major predators, gelatinous zooplankton has been depicted as a “trophic dead-end” in the food web. Based on their high water content, jellyfish were assumed to be of low nutritional value and of little importance as prey for higher trophic levels. This biased view has also been amplified by the rapid digestion of their soft tissues in predators’ stomachs, making them “invisible” for traditional microscopy. However, this paradigm is shifting these days with the application of modern methods such as in-situ observations and DNA metabarcoding. The latter technique can detect trace DNA of digested tissues in stomach contents, and has revealed that gelatinous zooplankton plays a much bigger role as prey for fish and seabirds than previously assumed. We aim to validate this for Arctic and sub-Arctic regions, by investigating the stomach contents of dominant fish species (e.g., Atlantic cod, polar cod, wolfish, redfish) with DNA metabarcoding to detect jellyfish predation.

How important are “jelly-falls” (sinking jellyfish carcasses) in sustaining the benthic food web?

Jelly can also play a major role in sustaining benthic communities. Because of the combination of an assumingly low predation pressure with the explosive growth rates of many jellyfish species, sinking jelly carcasses may represent an efficient export mechanism of carbon to the seafloor in the shape of so-called “jelly-falls”. On the seafloor, they provide a potential food source for a diverse scavenging community, including bentho-pelagic amphipods. To which extent the benthic community in Arctic fjords and deep-sea systems may be scavenging on jelly-falls, and particularly in winter, remains unexplored. To address this knowledge gap, we collect scavengers with baited traps in different localities, and assess their diet spectrum with DNA metabarcoding.

Scavenging amphipods sampled in Kongsfjorden, Svalbard, during the Polar Night (Photo: Charlotte Havermans/AWI)

Are there other gelatinous organisms besides salps that play a key role in the Southern Ocean ecosystem?

In several marine ecosystems, gelatinous zooplankton are on the rise since recent decades. This so-called “jellification” also holds for the warming region of the Southern Ocean, with its notorious shift from a krill-based to a salp-based pelagic ecosystem in the rapidly warming Antarctic Peninsula region. However, besides salps, other Antarctic gelatinous zooplankton are barely studied, because of the challenges associated to sampling them. By collecting gelatinous zooplankton during Southern Ocean campaigns, and sampling water and sediment for environmental DNA studies, we investigate their regional diversity and distribution patterns. We study the diet of amphipod crustaceans, myctophid and notothenioid fish for detecting jellyfish predation with DNA metabarcoding. Finally, by using the millions of DNA reads, generated by high-throughput sequencing for eDNA and diet studies, we want to conduct a “metaphylogeographic” study, to understand how jellyfish populations are distributed and connected throughout the Southern Ocean. This study is carried out with funding obtained through the German Science Foundation (DFG, SPP1158 on Antarctic Research), with the project HA7627/3-1, in collaboration with University Duisburg-Essen.

Gelatinous zooplankton species sampled in the Southern Ocean (Photo: Charlotte Havermans/AWI)

Target regions:

While our main focus lies on Arctic jellies, we carry out gelatinous zooplankton studies in different regions in the world. Our team has sampled gelatinous zooplankton during several campaigns: RV Heincke HE560 (2020), Walter Herwig WH440 (2020), Polarstern PS126 (FramJelly, 2021) expeditions and the AWIPEV project KOP183 (2022). Sampling for the ARJEL project was also carried out in the central-Arctic Ocean by participants of the MOSAiC campaign. Our next campaigns are the RV Heincke HE605 expedition “RISING”, investigating the species boundaries and range shifts of gelatinous zooplankton along a latitudinal gradient of Norwegian and Svalbard fjords (August-September 2022). We will also carry out jellyfish diversity studies with the secondary user projects “ICE-Jelly” on the Arctic campaign PS131 and “SO-Jelly” in the Southern Ocean during PS134. Finally, we also carry out trophic studies investigating the role of jellies as prey of amphipod crustaceans in the Benguela Upwelling System, in collaboration with the Marine Zoology department of the University of Bremen.

Methods used

Making the invisible visible: using optical surveys and eDNA for assessing distribution patterns of jellyfish

Zooplankton has been traditionally studied with nets, however, these consistently underestimate abundances of the fragile, watery jellies, and also do not cover their entire biodiversity. Hence, ARJEL applies a comprehensive set of advanced technologies to study these elusive critters, estimate their diversity and abundances based on the most recent knowledge in the field.

We use optical instruments to document jellyfish abundances and distribution patterns. As gelatinous zooplankton covers a wide range of sizes, from small appendicularians and hydrozoans of less than a centimetre in size, to huge scyphozoans, with tentacles of several meters, we also need a broad set of instruments. The Underwater Vision Profiler will be used to target the smaller-sized zooplankton whereas we deploy cameras on towed systems or remotely operated vehicles (ROVs) to document the large-sized fraction of the gelatinous communities.

Part of the FramJelly team (collaboration between AWI, GEOMAR & JAMSTEC) onboard R/V Polarstern (PS126), with the towed camera system PELAGIOS from GEOMAR (Photo: Mario Hoppmann/AWI)

PhD students Annkathrin Dischereit and Ayla Murray during a training with the ROV (Photo: Charlotte Havermans/AWI)

We also use state-of-the-art molecular methods to detect the elusive gelatinous component in the pelagic and benthic systems. Environmental DNA (eDNA) represents the DNA shed by organisms in the environment, e.g. in water, or sediment, without the organisms themselves having to be present in the samples. This trace DNA can originate from faeces, dead cells, mucus etc. that organisms leave behind, and can be used to monitor particular species or entire communities, depending on the molecular method used. For assessing communities, we apply DNA metabarcoding, which is a recent application of high-throughput sequencing on eDNA or bulk organismal samples.This methodology targets regions of DNA (barcodes or amplicons) that have enough variability to distinguish different taxonomic groups. Similar to eDNA assessments from water of soil, metabarcoding can also be used to identify DNA of dietary items for estimating prey composition from gut contents or faeces of consumers. This approach has played a crucial role in uncovering prey spectra in high resolution but also provides the only way to detect otherwise undetectable prey. This is particularly useful for detecting gelatinous zooplankton, which is easily overlooked when examining predator stomachs with traditional microscopy. When targeting a single species, we use the quantitative real-time polymerase chain reaction (qPCR) analysis based on species-specific primers.

Water sampling with the CTD rosette on board R/V Polarstern PS126. These water samples originating from different depths will be filtered on Sterivex filters from which the DNA will be isolated and sequenced for determining the jellyfish communities in Fram Strait — Water sampling with the CTD rosette on board R/V Polarstern PS126. (Photo: Charlotte Havermans/AWI)

Environmental DNA sampling in the Polar Night in Kongsfjorden, Svalbard (left: Niskin bottle, middle: water samples from different depths, right: sediment sampling with a Van Veen grab). DNA will be isolated from the filters and sediment, and analyzed for jellyfish community composition — Environmental DNA sampling in the Polar Night in Kongsfjorden, Svalbard (left: Niskin bottle, middle: water samples from different depths, right: sediment sampling with a Van Veen grab). DNA will be isolated from the filters and se (Photo: Charlotte Havermans/AWI)

Alongside the use of these modern techniques, we deploy plankton nets (Multinets, Bongo nets, hand nets), to validate the newly applied methods (eDNA, optics) and collect specimens for morphological and molecular identification. These specimens will also be used for experiments, such as temperature increase/decrease experiments to assess a species or population’s thermal response. We also carry out experiments with different species to calibrate eDNA analyses, looking at eDNA decay over time, as well as the quantitative link between jelly biomass and DNA copies. To do so, we keep the jellyfish in large circulation tanks (“Kreisel tanks”).