Multi-vehicle adaptive 3D mapping for targeted ocean sampling.

Expanding spatial presentation from two-dimensional profile transects to three-dimensional ocean mapping is key for a better understanding of ocean processes. Phytoplankton distributions can be highly patchy and the accurate identification of these patches with the context, variability, and uncertainty of measurements on relevant scales is difficult to achieve. Traditional sampling methods, such as plankton nets, water samplers and in-situ vertical sensors, provide a snapshot and often miss the fine-scale horizontal and temporal variability. Here, we show how two autonomous underwater vehicles measured, adapted to, and reported real-time chlorophyll a measurements, giving insights into the spatiotemporal distribution of phytoplankton biomass and patchiness. To gain the maximum available information within their sensing scope, the vehicles moved in an adaptive fashion, looking for the regions of the highest predicted chlorophyll a concentration, the greatest uncertainty, and the least possibility of collision with other underwater vehicles and ships. The vehicles collaborated by exchanging data with each other and operators via satellite, using a common segmentation of the area to maximize information exchange over the limited bandwidth of the satellite. Importantly, the use of multiple autonomous underwater vehicles reporting real-time data combined with targeted sampling can provide better match with sampling towards understanding of plankton patchiness and ocean processes.

Long-term dynamics, population structure and connectivity of the helmet jellyfish Periphylla periphylla in a Norwegian fjord and adjacent waters.

Mass occurrences of Periphylla periphylla in Norwegian fjords cause major concerns related to potential regime shifts that could affect ecosystem stability. 15 years of trawl data (2006-2015), complemented with comprehensive sampling in different areas and seasons (2018-2021) allowed new insights on the dynamics, structure and connectivity of P. periphylla populations within and beyond Trondheimsfjorden. Despite assumed population bursts, no clear trend on P. periphylla population size in Trondheimsfjorden were identified. Sampling frequency and population size suggest a local reproduction of P. periphylla, especially in the inner part of the fjord where young-of-the-year (YOY) individuals occur. Size variations occurred in relation to sampling month, thus pointing at seasonal patterns in growth and reproduction. No distinct population structure of P. periphylla populations within Trondheimsfjorden and over larger spatial scales (> 100 km) along the Norwegian coast was observed. Such poor geographic population structure provides evidence for a strong dispersal of P. periphylla, potentially triggered by frequent deep-water renewals of the fjords' basins that enable a high gene flow. Data on P. periphylla long-term dynamics, population structure and connectivity provide valuable information for ecosystem state assessments and enable the advancement of ecosystem management approaches, thus accounting for both stakeholder and ecosystem demands.

Limited effects of macro-nutrient ratios on thiamin content and transfer in phytoplankton and copepods.

Vitamin B1 (thiamin) is primarily produced by bacteria, phytoplankton and fungi in aquatic food webs and transferred to higher trophic levels by ingestion. However, much remains unknown regarding the dynamics this water-soluble, essential micronutrient; e.g. how it relates to macronutrients (carbon, nitrogen and phosphorous). Nutrient limitation has been found to be related to periods of thiamin deficiency as well as in models. Hence, thiamin transfer to copepods from three phytoplankton species from different taxa was investigated, along with the effect of various nutrient regimes on thiamin content. Nutrient levels did not affect thiamin content of phytoplankton nor the transfer to copepods. Instead, phytoplankton displayed species-specific thiamin and macronutrient contents and whilst a higher thiamin content in the prey lead to higher levels in copepods, the transfer was lower for Skeletonema compared to Dunaliella and Rhodomonas. In all, thiamin transfer to copepods is not only dependent on thiamin content of the prey, but also the edibility and/or digestibility is of importance. Thiamin is essential for all organisms, and this study offers insights into the limited effect of macronutrients on the dynamics and transfer of thiamin in the aquatic food webs.

Environmental niche overlap in sibling planktonic species Calanus finmarchicus and C. glacialis in Arctic fjords.

Knowledge of environmental preferences of the key planktonic species, such as Calanus copepods in the Arctic, is crucial to understand ecosystem function and its future under climate change. Here, we assessed the environmental conditions influencing the development stages of Atlantic Calanus finmarchicus and Arctic Calanus glacialis, and we quantified the extent to which their niches overlap by incorporating multiple environmental data. We based our analysis on a 3-year seasonal collection of zooplankton by sediment traps, located on moorings in two contrasting Svalbard fjords: the Arctic Rijpfjorden and the Atlantic-influenced Kongsfjorden. Despite large differences in water temperature between the fjords, local realized ecological niches of the sibling Calanus species overlapped almost perfectly. The exception was the earliest copepodites of C. glacialis in Rijpfjorden, which probably utilized the local ice algal bloom in spring. However, during periods with no sea ice, like in Kongsfjorden, the siblings of both Calanus species showed high synchronization in the population structure. Interestingly, differences in temperature preferences of C. finmarchicus and C. glacialis were much higher between the studied fjords than between the species. Our analysis confirmed the high plasticity of Calanus copepods and their abilities to adapt to highly variable environmental settings, not only on an interannual basis but also in a climate warming context, indicating some resilience in the Calanus community.

Contrasting phytoplankton-zooplankton distributions observed through autonomous platforms, in-situ optical sensors and discrete sampling.

Plankton distributions are remarkably 'patchy' in the ocean. In this study, we investigated the contrasting phytoplankton-zooplankton distributions in relation to wind mixing events in waters around a biodiversity-rich island (Runde) located off the western coast of Norway. We used adaptive sampling from AUV and shipboard profiles of in-situ phytoplankton photo-physiology and particle identification (copepods, fecal pellets and the dinoflagellate Tripos spp.) and quantification using optical and imaging sensors. Additionally, traditional seawater and net sampling were collected for nutrient and in-vitro chlorophyll a concentrations and phytoplankton and meso-zooplankton abundances. Persistent strong wind conditions (~5 days) disrupted the stratification in offshore regions, while stratification and a subsurface chlorophyll maximum (SCM) were observed above the base of the mixed layer depth (MLD ~30 m) in inshore waters. Contrasting phytoplankton and zooplankton abundances were observed between inshore (with the presence of a SCM) and offshore waters (without the presence of a SCM). At the SCM, phytoplankton abundances (Tripos spp., the diatom Proboscia alata and other flagellates) were half (average of 200 cell L-1) of those observed offshore. On the contrary, meso-zooplankton counts were ~6× higher (732 ind m-3 for Calanus spp.) inshore (where a SCM was observed) compared to offshore areas. In parallel, fecal pellets and ammonium concentrations were high (>1000 ind m-3 for the upper 20 m) at the SCM, suggesting that the shallow mixed layer might have increased encounter rates and promoted strong grazing pressure. Low nutrient concentrations (< 1µM for nitrate) were found below the MLD (60 m) in offshore waters, suggesting that mixing and nutrient availability likely boosted phytoplankton abundances. The size of the absorption cross-section (σPII') and yield of photosystem II photochemistry under ambient light (φPII') changed according to depth, while the depth-related electron flow (JPII) was similar between regions, suggesting a high degree of community plasticity to changes in the light regime. Our results emphasize the importance of using multiple instrumentation, in addition to traditional seawater and net sampling for a holistic understanding of plankton distributions.

Hiding in plain sight-Euplokamis dunlapae (Ctenophora) in Norwegian waters.

Cydippid ctenophores of genus Euplokamis have been rarely reported from the north-east Atlantic in the scientific literature. The conspicuous lack of previous records is likely attributable to methodological constraints detrimental to sampling ctenophores, including the use of plankton nets and preservation of samples as well as poor identification literature and a lack of taxonomic expertise on gelatinous zooplankton. Here, we have compiled published and novel records as well as documented diver observations, of Euplokamis spp. in Norwegian waters. Despite scant earlier reports, our data suggest that the genus Euplokamis is widely distributed and relatively common along the entire Norwegian coast, including Svalbard. Euplokamis was recorded from samples taken from several hundred meters depth to surface, from fjords as well as offshore. Most of the observations reported in this study are from the period between April and July, whereas specimens have been found nearly throughout the year. Specimens from Norwegian waters were morphologically most similar to Euplokamis dunlapae, and conservative 18S rDNA sequences of some specimens had a 100% match with an E. dunlapae specimen from Friday Harbor, USA, the type locality for the species. However, the morphological and molecular variation of Euplokamis demonstrates the need for systematic global sampling of multiple individuals of many ctenophore species.

Zooplankton and sediment fluxes in two contrasting fjords reveal Atlantification of the Arctic.

Svalbard fjords are facing a significant increase in Atlantic water inflow, which influences all ecosystem components, thus the objective of this study was to assess how recent Atlantification impacts the functioning of zooplankton community. For this purpose, two year-round operating sediment traps and associated hydrographic instruments, providing continuous time series of zooplankton and sediment fluxes, were deployed in the Atlantic-influenced Kongsfjorden and the typical high Arctic fjord Rijpfjorden. We used multivariate statistical methods to analyze how environmental variables, including the sediment fluxes, influence the zooplankton communities in the fjords. We found out that sedimentation rates were an order of magnitude higher in Kongsfjorden (reaching 39.7 g m-2 d-1 in December) and increased in autumn, while in Rijpfjorden, they peaked in late winter - early spring (2.9 g m-2 d-1 in February). Such sediment flux patterns might result from the redeposition of sediments from shallower, subtidal areas and were probably connected to autumn/winter storms. According to multivariate analyses, zooplankton in Kongsfjorden were significantly influenced by water temperature, which explained 22% of their variation, and the flux of organic and mineral sediments explaining 15% and 7.8%, respectively; while in Rijpfjorden, it was sea ice (25.3%), water temperature (16.2%), salinity (8.1%), and mineral sedimentation (6.3%). The structure of zooplankton communities in both fjords was similar in winter; in Kongsfjorden, zooplankton kept developing through spring and summer, while in the Arctic Rijpfjorden, the community paused until the onset of phytoplankton bloom and sea ice break-up in summer, to finally achieve, in autumn, a similar species and development stage structure as summer in the Atlantic-influenced fjord. Our study demonstrates how integrating multiple pieces of information can provide key insights into the relations between Atlantification, sediment flux, and zooplankton community, thus helping to assess the functioning of high Arctic ecosystems under climate change conditions.

Deficiency syndromes in top predators associated with large-scale changes in the Baltic Sea ecosystem.

Vitamin B1 (thiamin) deficiency is an issue periodically affecting a wide range of taxa worldwide. In aquatic pelagic systems, thiamin is mainly produced by bacteria and phytoplankton and is transferred to fish and birds via zooplankton, but there is no general consensus on when or why this transfer is disrupted. We focus on the occurrence in salmon (Salmo salar) of a thiamin deficiency syndrome (M74), the incidence of which is highly correlated among populations derived from different spawning rivers. Here, we show that M74 in salmon is associated with certain large-scale abiotic changes in the main common feeding area of salmon in the southern Baltic Sea. Years with high M74 incidence were characterized by stagnant periods with relatively low salinity and phosphate and silicate concentrations but high total nitrogen. Consequently, there were major changes in phytoplankton and zooplankton, with, e.g., increased abundances of Cryptophyceae, Dinophyceae, Diatomophyceae and Euglenophyceae and Acartia spp. during high M74 incidence years. The prey fish communities also had increased stocks of both herring and sprat in these years. Overall, this suggests important changes in the entire food web structure and nutritional pathways in the common feeding period during high M74 incidence years. Previous research has emphasized the importance of the abundance of planktivorous fish for the occurrence of M74. By using this 27-year time series, we expand this analysis to the entire ecosystem and discuss potential mechanisms inducing thiamin deficiency in salmon.

Sea-ice eukaryotes of the Gulf of Finland, Baltic Sea, and evidence for herbivory on weakly shade-adapted ice algae.

To determine community composition and physiological status of early spring sea-ice organisms, we collected sea-ice, slush and under-ice water samples from the Baltic Sea. We combined light microscopy, HPLC pigment analysis and pyrosequencing, and related the biomass and physiological status of sea-ice algae with the protistan community composition in a new way in the area. In terms of biomass, centric diatoms including a distinct Melosira arctica bloom in the upper intermediate section of the fast ice, dinoflagellates, euglenoids and the cyanobacterium Aphanizomenon sp. predominated in the sea-ice sections and unidentified flagellates in the slush. Based on pigment analyses, the ice-algal communities showed no adjusted photosynthetic pigment pools throughout the sea ice, and the bottom-ice communities were not shade-adapted. The sea ice included more characteristic phototrophic taxa (49%) than did slush (18%) and under-ice water (37%). Cercozoans and ciliates were the richest taxon groups, and the differences among the communities arose mainly from the various phagotrophic protistan taxa inhabiting the communities. The presence of pheophytin a coincided with an elevated ciliate biomass and read abundance in the drift ice and with a high Eurytemora affinis read abundance in the pack ice, indicating that ciliates and Eurytemora affinis were grazing on algae.

The use of DNA barcodes in food web construction-terrestrial and aquatic ecologists unite!

By depicting who eats whom, food webs offer descriptions of how groupings in nature (typically species or populations) are linked to each other. For asking questions on how food webs are built and work, we need descriptions of food webs at different levels of resolution. DNA techniques provide opportunities for highly resolved webs. In this paper, we offer an exposé of how DNA-based techniques, and DNA barcodes in particular, have recently been used to construct food web structure in both terrestrial and aquatic systems. We highlight how such techniques can be applied to simultaneously improve the taxonomic resolution of the nodes of the web (i.e., the species), and the links between them (i.e., who eats whom). We end by proposing how DNA barcodes and DNA information may allow new approaches to the construction of larger interaction webs, and overcome some hurdles to achieving adequate sample size. Most importantly, we propose that the joint adoption and development of these techniques may serve to unite approaches to food web studies in aquatic and terrestrial systems-revealing the extent to which food webs in these environments are structured similarly to or differently from each other, and how they are linked by dispersal.

Arctic complexity: a case study on diel vertical migration of zooplankton.

Diel vertical migration (DVM) of zooplankton is a global phenomenon, characteristic of both marine and limnic environments. At high latitudes, patterns of DVM have been documented, but rather little knowledge exists regarding which species perform this ecologically important behaviour. Also, in the Arctic, the vertically migrating components of the zooplankton community are usually regarded as a single sound scattering layer (SSL) performing synchronized patterns of migration directly controlled by ambient light. Here, we present evidence for hitherto unknown complexity of Arctic marine systems, where zooplankton form multiple aggregations through the water column seen via acoustics as distinct SSLs. We show that while the initiation of DVM during the autumnal equinox is light mediated, the vertical positioning of the migrants during day is linked more to the thermal characteristics of water masses than to irradiance. During night, phytoplankton biomass is shown to be the most important factor determining the vertical positioning of all migrating taxa. Further, we develop a novel way of representing acoustic data in the form of a Sound Image (SI) that enables a direct comparison of the relative importance of each potential scatterer based upon the theoretical contribution of their backscatter. Based on our comparison of locations with contrasting hydrography, we conclude that a continued warming of the Arctic is likely to result in more complex ecotones across the Arctic marine system.