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The entry of microplastics (MPs) into marine food webs is a major environmental concern. We investigated how the behavior of planktonic copepods influences the risk of MPs to enter marine food webs by applying a trait-based approach and by combining experiments (bottle incubations and video observations) with biogeographical analyses. We aimed to evaluate which type of feeding behavior is most risky in terms of MP ingestion and which marine geographical areas are more susceptible to MP ingestion by planktonic copepods. We used different species as models of the main foraging behaviors in planktonic copepods: feeding-current, cruising, ambush, and mixed behavior feeding. All behaviors showed a similarly low risk of MP ingestion, up to 1 order of magnitude lower than for similar-sized microalgae. We did not observe any influence of the prey type or MP size (8 and 20 μm) on MP ingestion for any of the behaviors. By mapping the global distribution of feeding behaviors, we showed that feeding-current feeding is the most common behavior, but the risk of MP ingestion remains equally low across the global ocean, independently of the predominant behavior. Overall, our results suggest a low risk of MP ingestion by planktonic copepods and therefore a minimal risk of trophic transfer of MPs via marine pelagic copepods in marine ecosystems.

Planktonic copepods are the most abundant animals in the ocean and key players in global biochemical processes. Recent modeling suggests that zooplankton ingestion of microplastics (MPs) can disrupt the biological carbon pump and accelerate a global loss of oceanic oxygen. Here we investigate the behavioral responses and ingestion rates of a model feeding-current generating copepod when exposed to microplastics of different characteristics by small-scale video observations and bottle incubations. We found that copepods rejected 80% of the microplastics after touching them with their mouth parts, in essence exhibiting a kind of taste discrimination. High rejection rates of microplastics were independent of polymer type, shape, presence of biofilms, or sorbed pollutant (pyrene), indicating that microplastics are unpalatable for feeding-current feeding copepods and that post-capture taste discrimination is a main sensorial mechanism in the rejection of microplastics. In an ecological context, taking into account the behaviors of planktonic copepods and the concentrations of microplastics found in marine waters, our results suggest a low risk of microplastic ingestion by zooplankton and a low impact of microplastics on the vertical exportation of fecal pellets.

The copepod Calanus finmarchicus was investigated in four Subpolar Basins, Labrador, Irminger, Iceland, and Norwegian Seas, during spring, covering the time of ascent, grazing, and initiation of reproduction in the area. Lipid content, spawning activity, and stage composition and vital rates, such as egg and faecal pellet production were measured and linked to environmental parameters. Specific egg- and faecal pellet production rates varied with diatom biomass and were negatively correlated with temperature. Comparison of the various biological indicators revealed different life history traits C. finmarchicus has adopted in the different basins. In Labrador Sea, the females have invested in large eggs compared to the remaining basins. Labrador and Irminger Sea C. finmarchicus invest in size that we propose to be adaptation to cope with warmer overwintering habitats resulting in larger potential lipid storage capacity, while the Iceland and Norwegian Sea females can invest their remaining lipid storage in spring to fuel lipid-driven egg production. Grazing pressure on the phytoplankton community was estimated and compared between copepod and two dominating groups of protozooplankton; ciliates and heterotrophic dinoflagellates. Despite approximately the same biomass in the upper 100 m, the grazing impact of the protozoan grazers was an order of magnitude higher than the C. finmarchicus dominated mesozooplankton. This illustrates the importance to also include the smallest grazers when studying the spring bloom in high latitude marine ecosystems if the fate of the primary production should be fully understood.

Microplastics (MPs) were quantified in Danish marine waters of the Kattegat and the southernmost part of Skagerrak bordering to it. Kattegat is a waterbody between Denmark and Sweden that receives inflow from the Baltic Sea and direct urban runoff from the metropolitan area of Copenhagen and Malmö. MPs were measured in 14 continuous transects while steaming between monitoring stations. MP levels tended to be highest close to the Copenhagen-Malmö area, albeit this was more obvious from the abundance of particles rather than mass. The outcome of the measurements allowed a rough MP budget in the Danish Straits region, suggesting that urban waste- and stormwater discharges could not be neglected as potential MP source in these waters. The marine samples were collected by pumping and filtering water over 10 μm steel filters, hereby sampling a total of 19.3 m3. They were prepared and analyzed by FPA-μFTIR imaging, and the scans interpreted to yield MP size, shape, polymer type, and estimated mass. The average concentration was 103 ± 86 items m−3, corresponding to 23.3 ± 28.3 μg m−3 (17–286 items m−3; 0.6–84.1 μg m−3). Most MPs were smaller than 100 μm and fragments dominated the samples. The carbonyl index was assessed for polyolefins, showing that oxidation increased with decreasing MP size, but did not correlate with distance to urban areas. A rough budget of MP in the Danish Straits region suggested that MPs discharged from urban waste- and stormwaters were an import source of MPs.

Plastics contain various types and amounts of additives that can leach into the water column when entering aquatic ecosystems. Some leached plastic additives are hazardous to marine biota at environmentally relevant concentrations. Disparate methodological approaches have been adopted for toxicity testing of plastic leachates, making comparison difficult. Here we propose a protocol to standardize the methodology to obtain leachates from microplastics (MPs) for aquatic toxicity testing. Literature reviewing and toxicity tests using marine model organisms and different types of MPs were conducted to define the main methodological aspects of the protocol. Acute exposure to leachates from the studied plastics caused negative effects on the early life stages of sea urchins and marine bacteria. We provide recommendations of key factors influencing MPs lixiviation, such as MP size (

The discharge of produced water from offshore oil platforms is an emerging concern due to its potential adverse effects on marine ecosystems. In this study, we investigated the feasibility and capability of using marine sediments for the bioremediation of produced water. We utilized a combination of porewater and solid phase analysis in a series of sediment batch incubations amended with produced water and synthetic produced water to determine the biodegradation of hydrocarbons under different redox conditions. Significant removal of benzene, toluene, ethylbenzene and xylene (BTEX) compounds was observed under different redox conditions, with biodegradation efficiencies of 93−97% in oxic incubations and 45−93% in anoxic incubations with nitrate, iron oxide or sulfate as the electron acceptor. Higher biodegradation rates of BTEX were obtained by incubations dominated by nitrate reduction (104−149 nmolC/cm3/d) and oxygen respiration (52−57 nmolC/cm3/d), followed by sulfate reduction (14−76 nmolC/cm3/d) and iron reduction (29−39 nmolC/cm3/d). Chemical fingerprint analysis showed that hydrocarbons were biodegraded to smaller alcohols/acids under oxic conditions compared to anoxic conditions with nitrate, indicating that the presence of oxygen facilitated a more complete biodegradation process. Toxicity of treated produced water to the marine copepod Acartia tonsa was reduced by half after sediment incubations with oxygen and nitrate. Our study emphasizes the possibility to use marine sediment as a biofilter for treating produced water at sea without extending the oil and gas platform or implementing a large-scale construction.

Microplastics (MPs) are ubiquitous pollutants in the ocean, and there is a general concern about their persistence and potential effects on marine ecosystems. We still know little about the smaller size-fraction of marine MPs (MPs −3, with 88% of the MPs being smaller than 300 μm. The most abundant shape of MPs were fragments (56%), and polyester, polypropylene, and polyethylene were the dominant synthetic polymer types. The concentration of MPs shows a significant positive correlation to the seawater density. Furthermore, there was a tendency towards higher MPs concentrations in the Northern and the Southern parts of the study area. The concentration of MPs collected with the UFO sampler was several orders of magnitude higher than those commonly found in samples collected with the Manta net due to the dominance of MP smaller size fractions. Despite the multiple potential sources of MPs in the study area, the level of MPs pollution in the surface waters was low compared (−3) to other regions. The concentrations of MPs found in the studied surface waters were six orders of magnitude lower than those causing negative effects on pelagic organisms based on laboratory exposure studies, thus is not expected to cause any impact on the pelagic food web.

Tyre wear particles (TWP) are some of the dominant sources of microplastics in the aquatic environment. Once TWP enter aquatic systems, they can leach certain plastic additives that can be potentially toxic to biota. However, little is known about the impact of TWP lixiviates on marine phytoplankton, the base of marine food webs. This study aims to determine the acute toxic effect of leachates derived from TWP on three phytoplankton species: the cryptophyte Rhodomonas salina, the diatom Thalassiosira weissflogii and the dinoflagellate Heterocapsa steinii, using the median effect concentration (EC50) for specific growth rate as endpoint. Leachates were obtained by incubating 1 g L−1 of < 250 µm TWP in artificial seawater for 3 days. Each phytoplankton species was exposed to leachates at five different concentrations, and cell concentrations were measured every 24 hours over 3 days. Leachates from TWP were toxic to marine phytoplankton. The dinoflagellate H. steinii was the most sensitive species, with 72-h EC50 of 23% leachate concentration, whereas R. salina and T. weissflogii exhibited EC50 values of 64% and 73%, respectively. Our results suggest that TWP leachates have a negative effect on phytoplankton growth, although more field data on the concentration of TWPs and their leachates is needed to fully evaluate the environmental impact of TWP.

Marine zooplankton are among the most diverse and abundant organisms on earth. Despite a great wealth of knowledge and research on their ecology, the processes promoting coexistence and maintaining their high diversity worldwide are poorly known. In order to understand the processes underpinning coexistence among marine zooplankton, we investigated the existence and degree of niche separation within two pairs of taxonomically and ecologically related species of copepods belonging to the widespread, abundant genera Calanus and Oithona. We compared the spatial variation in species abundances to the abiotic and biotic environment using a multimodel approach and vertically resolved survey data on zooplankton composition, abundances, and environmental conditions in the North Sea. Our results demonstrate pronounced spatial differences between species in each pair, primarily in their vertical abundance distributions. These differences can largely be explained by different preferences for temperature and salinity. This supports the occurrence of environmental niche separation in marine zooplankton and highlights its role as a mechanism reducing interspecific competition and promoting coexistence.

The calanoid copepod Pseudodiaptomus annandalei is used as live feed in aquaculture because of its nutritional value and the ability to cope with environmental fluctuations in outdoor ponds. However, little knowledge exists on its ecology. Here we investigated the ecology of P. annandalei in an aquaculture pond in Vietnam. Temperature, salinity, chlorophyll a and biomass of protozoans and copepods were monitored every other day for 1 month. Experiments on protozoan growth and grazing by P. annandalei were also conducted. Copepod fecal pellet production and temperature-dependent egg hatching rates were likewise quantified. Despite very high phytoplankton biomass, biomass of P. annandalei was surprisingly low. Copepod production was estimated from three independent methods: clearance, weight-specific egg production rate (SEP) and specific fecal pellet production rate. SEP proved to be accurate to predict the in situ population growth in the pond. A simple model for production of P. annandalei based on SEP was developed. Our study extends our knowledge of how environmental conditions in the pond may affect the population dynamics and production of copepods. The results have important implications for pond managements ensuring stable copepod production and harvest.

We know very little about the effects of two global stressors, elevated temperature and contaminants, on the grazing of marine copepods. To address this issue, we tested the hypotheses that the individual and combined effects of these two stressors may reduce grazing rates and may depend on food availability and gender. We exposed male and female Calanus finmarchicus copepods to pyrene at two temperatures (10 and 14 °C) and six food concentrations (25–800 μg C Rhodomonas baltica L−1) and measured fecal pellet size, and grazing rate (GR) from pellet production. Males had smaller fecal pellets and lower GR than did females. Temperature and pyrene exposure had no effect on pellet size. Temperature alone had no effect on GR of males, but females had lower GR at elevated temperature. Pyrene-exposed males and females had lower GR only at the food concentrations of 200–800 μg C R. baltica L−1 and those patterns were independent of temperature. Pyrene-induced reduction in GR was stronger in females than in males. The negative effects of both elevated temperature and pyrene may reduce the abundance and trophic success of C. finmarchicus in a warmer, more polluted future.

Bowhead whales (Balaena mysticetus) visit Disko Bay, West Greenland in winter and early spring to feed on Calanus spp., at a time of year when the copepods are still mostly in diapause and concentrated in near-bottom patches. Combining past observations of copepod abundance and distribution with detailed observations of bowhead whale foraging behaviour from telemetry suggests that if the whales target the highest-density patches, they likely consume 26–75% of the Calanus standing stock annually. A parallel bioenergetic calculation further suggests that the whales' patch selection must be close to optimally efficient at finding hotspots of high density copepods near the sea floor in order for foraging in Disko Bay to be a net energetic gain. Annual Calanus consumption by bowhead whales is similar to median estimates of consumption by each of three zooplankton taxa (jellies, chaetognaths, and predatory copepods), and much greater than the median estimate of consumption by fish larvae, as derived from seasonal abundance and specific ingestion rates from the literature. The copepods' self-concentration during diapause, far from providing a refuge from predation, is the behaviour that makes this strong trophic link possible. Because the grazing impact of the whales comes 6–10 months later than the annual peak in primary production, and because Disko Bay sits at the end of rapid advective pathways (here delineated by a simple numerical particle-tracking experiment), it is likely that these Calanus populations act in part as a long-distance energetic bridge between the whales and primary production hundreds or thousands of km away.

Microplastics (MPs) are polluting the Arctic, but our understanding of their abundance, distribution, and sources is limited. This study quantified MPs down to 10μm in marine waters of the most populated region in Greenland. A new plastic-free pump-filter system was used to collect MPs from surface waters in the fjord Nuup Kangerlua close to Nuuk. Additionally, we took samples by horizontal tows with a bongo net (300μm mesh-size). The median concentrations were 142MPsm−3and 0.12MPsm−3in the pump and bongo samples, respectively. The most abundant polymer was polyester across stations and sampling types. Fibers were the dominant shape in the bongo samples, while non-fibrous particles dominated in the pump samples. MP abundance was lower in the fjord and increased close to Nuuk and towards the open ocean, indicating that Nuuk is an important point source for MPs. In both samples, concentrations of MPs increased with decreasing size, illustrating the importance of quantifying the smallest fraction of MPs. Thus, the use of methods allowing for a quantification of the smallest MPs is vital to reduce the underestimation of MP concentrations in the environment. The smallest size fraction is also most available to plankton-feeding marine invertebrates and an important entry point for MPs into marine food webs. At the found concentrations, immediate adverse effects on the pelagic food webs are unlikely. However, growing anthropogenic activities could increase the risk of MPs to affect the sensitive Arctic ecosystem.

Plastic pollution is considered one of today’s major environmental problems. Current land-based monitoring programs typically rely on beach litter data and seldom include plastic pollution further inland. We initiated a citizen science project known as the Mass Experiment inviting schools throughout The Danish Realm (Denmark, Greenland and the Faeroe Islands) to collect litter samples of and document plastic pollution in 8 different nature types. In total approximately 57,000 students (6–19 years) collected 374,082 plastic items in 94 out of 98 Danish municipalities over three weeks during fall 2019. The Mass Experiment was the first scientific survey of plastic litter to cover an entire country. Here we show how citizen science, conducted by students, can be used to fill important knowledge gaps in plastic pollution research, increase public awareness, establish large scale clean-up activities and subsequently provide information to political decision-makers aiming for a more sustainable future.

Tropical coastal areas are increasingly exposed to temperature extremes from marine heatwaves and contaminants from anthropogenic activities. The interactive effects of these environmental changes on marine life are understudied. We investigated the direct and cross-generational effects of copper (Cu) on F0 and F1 generations of the common tropical copepod Pseudodiaptomus annandalei under extreme temperatures (30 and 34 °C). In F0, Cu exposure reduced survival and nauplii production; these patterns were more pronounced at 34 °C and in females. F0 Copepods produced more faecal pellets at 34 °C than 30 °C, indicating a higher energetic demand. In F1, the number of F1 adults was lower in CuF0 and at 34 °C. Cu-exposed F0 produced larger adult F1, while exposure to 34 °C resulted in smaller adult F1. Our results show that tropical copepods are highly vulnerable to the interactive effects of contaminants and extreme temperatures.

Microplastics (MPs) are contaminants of emerging concern in the Arctic, but knowledge of their potential effects on Arctic plankton food webs remains scarce. We experimentally investigated ingestion and effects of MPs (20 μm polyethylene spheres) on the arctic copepodsCalanus finmarchicus, C. glacialisandC. hyperboreus. These species dominate arctic zooplankton biomass and are relevant target species to investigate the potential impacts of MPs on the Arctic marine ecosystem. Females of each species were exposed to two concentrations of MPs (200 and 20,000 MPs L−1) in combination with different food (diatom) concentrations, reflecting high (3000–5000 cells mL−1, spring phytoplankton bloom) and low (50–500 cells mL−1, pre/post bloom) food conditions. MPs did not affect negatively fecal pellet production rates in any of the species at the studied exposure concentrations. However, egg production rates of copepods exposed to MPs were 8 times higher compared with the controls, which suggests that MP exposure can cause stress-induced spawning in arctic copepods. Microscopic examination of the fecal pellets confirmed ingested MPs in the three species (up to aprox. 1000 MPs cop−1d−1). The number of MPs per pellet decreased exponentially with increasing food concentration. The daily ingestion of MPs per copepod was higher at low- food concentrations (250–500 cells mL−1). At our exposure conditions, the presence of MPs insideC. hyperboreusfecal pellets did not affect their sinking rates. Overall, our experimental research show that 1) acute exposure to virgin polyethylene MPs has a low impact on arcticCalanusspecies at environmentally relevant MP concentrations, independent of food availability, and 2) arctic copepods influence the environmental fate of plankton-sized MPs by exporting buoyant MPs from the surface layer to the sea floor via fecal pellets.

Shallow, tropical marine ecosystems provide essential ecosystem goods and services, but it is unknown how these ecosystems will respond to the increased exposure to the temperature extremes that are likely to become more common as climate change progresses. To address this issue, we tracked the fitness and productivity of a key zooplankton species, the copepod Pseudodiaptomus annandalei, acclimated at two temperatures (30 and 34 °C) over three generations. 30 °C is the mean temperature in the shallow water of the coastal regions in Southeast Asia, while 34 °C simulated a temperature extreme that occurs frequently during the summer period. For each generation, we measured the size at maturity and reproductive success of individuals. In all three generations, we found strong negative effects of warming on all measured fitness-related parameters, including prolonged development time, reduced size at maturity, smaller clutch sizes, lower hatching success, and reduced naupliar production. Our results suggest that P. annandalei are already exposed to temperatures that exceed their upper thermal optimum. Increased exposure to extreme temperatures may reduce the abundance of these tropical marine copepods, and thus reduce the availability of resources to higher trophic levels.

In costal ecosystems, copepods coexist with toxin‐producing phytoplankton. The presence of copepods can amplify the phytoplankton toxin production and thereby increase the overall toxicity of a bloom. Copepods are not always affected by the toxins and can vector the toxins to higher trophic levels. To investigate the interactions between toxin producers and their grazers, we determined the kinetics of grazer‐induced increases in toxin production and the subsequent toxin reduction in a domoic acid (DA)‐producing diatom, Pseudo‐nitzschia seriata. The cellular DA level of the diatom was within the range of in situ measurements. Ten days after removal of the copepods, 28% ± 8% of the cellular DA still remained in the cells. Simultaneously, we monitored the toxicokinetics of DA in two grazers; Calanus finmarchicus and Calanus glacialis. After 144 h of grazing on the toxic diet, the copepods accumulated and retained high concentrations of DA. Nine hours after exposure to the toxic diet was terminated, the copepods had depurated 70% ± 10% of the DA. The depuration lasted 4 ± 2 d and was independent of Calanus species and treatment. We explored the possible physiological responses in copepods after feeding on a purely toxic diet from gene expression profiles of C. finmarchicus. Expression of genes regulating several major metabolic and cellular processes was reduced in copepods feeding on DA‐containing diatoms, and we hypothesize that this is because of exposure to DA.

Grazers can induce toxin (domoic acid, DA) production in diatoms. The toxic response has been observed in two species of Pseudo-nitzschia and was induced by Calanus copepods. In this study, interactions between diatoms and copepods were further explored using different species of diatoms and copepods. All herbivorous copepods induced toxin production, whereas exposure to carnivorous copepods did not. In line with this, increasing the number of herbivorous copepods resulted in even higher toxin production. The induced response is thus only elicited by copepods that pose a real threat to the responding cells, which supports that the induced toxin production in diatoms evolved as an inducible defense. The cellular toxin content in Pseudo-nitzschia was positively correlated to the concentration of a group of specific polar lipids called copepodamides that are excreted by the copepods. This suggests that copepodamides are the chemical cues responsible for triggering the toxin production. Carnivorous copepods were found to produce less or no copepodamides. Among the diatoms exposed to grazing herbivorous copepods, only two of six species of Pseudo-nitzschia and none of the Nitzschia or Fragilariopsis strains responded by producing DA, indicating that not all Pseudo-nitzschia species/strains are able to produce DA, and that different diatom species might have different strategies for coping with grazing pressure. Growth rate was negatively correlated to cellular domoic acid content indicating an allocation cost associated with toxin production. Long-term grazing experiments showed higher mortality rates of grazers fed toxic diatoms, supporting the hypothesis that DA production is an induced defense mechanism.

The copepod Calanus hyperboreus, a key species of Arctic marine ecosystems, has a partially anoxic gut that is suspected to host anaerobic microbial activities. So far, however, only dead specimens have been studied in which gut processes are quickly abolished by microbial carcass degradation. Here, live specimens were exposed to different feeding regimes and ambient oxygen levels to study the controls on copepod-associated microbial nitrogen cycling in 15N-enrichment experiments. Algae-fed copepods exhibited consistently high rates of denitrification and low or variable rates of dissimilatory nitrate reduction to ammonium (or nitrite), nitrification, and nitrous oxide release. Thus, live C. hyperboreus contribute to pelagic fixed-nitrogen loss through denitrificationthat is otherwise absent from oxygenated Arctic surface waters. The feeding-related controls on copepod-associated anaerobic nitrogen cycling aresupported by significantly reduced rates in starved copepods. In addition, freshly released fecal pellets showed similar patterns of nitrogen cycling as fedcopepods. Unlike in previous studies on copepod carcasses, low ambient oxygen levels did not stimulate anaerobic nitrogen cycling in live specimens, supportingthat the C. hyperboreus gut is anoxic, irrespective of ambient oxygen levels. Fecal pellets and carcasses retained denitrification activity for 4 d of simulated sinking. We estimate that during the time of the Arctic spring bloom, pelagic denitrification associated with live specimens, fecal pellets, and carcassesof C. hyperboreus is equivalent to ~12% of the benthic fixed-nitrogen loss. This estimate increases to ~28%, provided that the abundant sibling species C.glacialis and C. finmarchicus exhibit denitrification activities in proportion to their smaller body size

While ongoing warming and sea ice decline threaten unique Arctic ecosystems, they improve the prospect of exploiting fossil fuels in the seafloor. Arctic Calanus copepods can accumulate oil compounds in the large lipid reserves that enable them to cope with highly seasonal food availability characteristic of the Arctic. While spending a significant part of their lives overwintering at depth, their vulnerability to oil contamination during winter remains unknown. We investigated effects of the hazardous crude oil component pyrene on overwintering Calanus glacialis, a key species in Arctic shelf areas. Females were exposed from December to March and then transferred to clean water and fed until April. We showed that long-term exposure during overwintering reduced survival and lipid mobilization in a dose-dependent manner at concentrations previously considered sublethal. After exposure, strong delayed effects were observed in lipid recovery, fecal pellet, and egg production. We showed that 50% lethal threshold concentrations were at least 300 times lower than expected, and that 50% effect thresholds for pellet and egg production were at least 10 times lower than previously documented. Our study provides novel insights to the effects of oil contamination during winter, which is essential to evaluate ecological impacts of Arctic oil pollution.

The subarctic Atlantic phytoplankton spring bloom is one of the largest biological features of the ocean; however, processes initiating the bloom are still not well understood. We hypothesize that the microbial grazing food chain plays an important role in creating a pre-bloom condition with top-down control of small-sized phytoplankton, thus paving the way for a diatomdominated spring bloom. To assess the trophic role of protist grazers during the winter to spring transition, 3 experiments were performed using size-fractionated surface water from the Iceland Basin (March−April 2012). These experiments demonstrated heterotrophic nanoflagellates (HNF) grazing of picophytoplankton to be a key pathway, even though these are rarely considered as important phytoplankton grazers in high-latitude systems. The growth rate of HNF was significantly correlated to the biomass of picophytoplankton and was substantially higher than the growth of the larger microzooplankton (MZP), i.e. ciliates and dinoflagellates. During the first experiment, small phytoplankton dominated and overall protist grazing (HNF + MZP) was low. In the later experiments, MZP grazing on HNF became evident; however, MZP were not able to control the community of larger phytoplankton (>10 μm), which became more abundant. Our experiments thus support the hypothesis that pre-bloom conditions promote a build-up of large phytoplankton, i.e. diatoms. We found that the high growth rates of HNF together with the relaxed MZP grazing pressure allow HNF to respond rapidly to the early primary production by picophytoplankton and maintain a strong top-down control on these. We suggest that this succession may be an important mechanism that allows large diatoms, rather than picophytoplankton, to become the dominant primary producers during the subarctic Atlantic spring bloom. publishedVersion

Shallow, tropical marine ecosystems provide essential ecosystem goods and services, but it is unknown how these ecosystems will respond to the increased exposure to the temperature extremes that are likely to become more common as climate change progresses. To address this issue, we tracked the fitness and productivity of a key zooplankton species, the copepod Pseudodiaptomus annandalei, acclimated at two temperatures (30 and 34 °C) over three generations. 30 °C is the mean temperature in the shallow water of the coastal regions in Southeast Asia, while 34 °C simulated a temperature extreme that occurs frequently during the summer period. For each generation, we measured the size at maturity and reproductive success of individuals. In all three generations, we found strong negative effects of warming on all measured fitness-related parameters, including prolonged development time, reduced size at maturity, smaller clutch sizes, lower hatching success, and reduced naupliar production. Our results suggest that P. annandalei are already exposed to temperatures that exceed their upper thermal optimum. Increased exposure to extreme temperatures may reduce the abundance of these tropical marine copepods, and thus reduce the availability of resources to higher trophic levels.

Background A major cause of phytoplankton mortality is predation by zooplankton. Strategies to avoid grazers have probably played a major role in the evolution of phytoplankton and impacted bloom dynamics and trophic energy transport. Certain species of the genus Pseudo-nitzschia produce the neurotoxin, domoic acid (DA), as a response to the presence of copepod grazers, suggesting that DA is a defense compound. The biosynthesis of DA comprises fusion of two precursors, a C10 isoprenoid geranyl pyrophosphate and l-glutamate. Geranyl pyrophosphate (GPP) may derive from the mevalonate isoprenoid (MEV) pathway in the cytosol or from the methyl-erythritol phosphate (MEP) pathway in the plastid. l-glutamate is suggested to derive from the citric acid cycle. Fragilariopsis, a phylogenetically related but nontoxic genus of diatoms, does not appear to possess a similar defense mechanism. We acquired information on genes involved in biosynthesis, precursor pathways and regulatory functions for DA production in the toxigenic Pseudo-nitzschia seriata, as well as genes involved in responses to grazers to resolve common responses for defense strategies in diatoms. Results Several genes are expressed in cells of Pseudo-nitzschia when these are exposed to predator cues. No genes are expressed in Fragilariopsis when treated similarly, indicating that the two taxa have evolved different strategies to avoid predation. Genes involved in signal transduction indicate that Pseudo-nitzschia cells receive signals from copepods that transduce cascading molecular precursors leading to the formation of DA. Five out of seven genes in the MEP pathway for synthesis of GPP are upregulated, but none in the conventional MEV pathway. Five genes with known or suggested functions in later steps of DA formation are upregulated. We conclude that no gene regulation supports that l-glutamate derives from the citric acid cycle, and we suggest the proline metabolism to be a downstream precursor. Conclusions Pseudo-nitzschia cells, but not Fragilariopsis, receive and respond to copepod cues. The cellular route for the C10 isoprenoid product for biosynthesis of DA arises from the MEP metabolic pathway and we suggest proline metabolism to be a downstream precursor for l-glutamate. We suggest 13 genes with unknown function to be involved in diatom responses to grazers. Electronic supplementary material The online version of this article (10.1186/s12867-019-0124-0) contains supplementary material, which is available to authorized users.

We studied how exposure to oil spill response technologies affect marine microorganisms during Arctic winter and spring. Microorganisms were exposed to chemically dispersed oil (DISP), in situ burnt oil (ISB), and natural attenuated oil (NATT) in mesocosms from February to May. We subsampled the mesocosms and studied the effects of oil in laboratory incubations as changes in biomass of the major functional groups: bacteria, heterotrophic-nanoflagellates, dinoflagellates, ciliates, pico- and nanophytoplankton, and diatoms over two 14-day periods. In winter, the majority of polycyclic aromatic hydrocarbons (PAHs) remained encapsulated in the ice, and the low concentrations of PAHs in water led to minute changes in biomass of the investigated groups. In spring, however, when the PAHs were partially released from the melting ice, the biomass of many functional groups in DISP and NATT decreased significantly, while the changes in ISB were less pronounced. The overall biomass reduction, as observed in this study, could lead to a disrupted transfer of energy from the primary producers to the higher trophic levels in oil affected areas.

In a warmer Arctic with less sea ice and stronger stratification, the environmental changes are expected to impact the pelagic food web, but few biological studies supporting this exist compared with the well-documented physical changes. Here, we analyze a subset of 13 yr of data from Disko Bay, Western Greenland, from the period 1992 to 2018 for trends in the key zooplankton genus Calanus during May and June in relation to physical conditions. In the 1990s, the small North Atlantic species Calanus finmarchicus and the two larger Arctic species Calanus hyperboreus and Calanus glacialis contributed equally to the copepod biomass. With the reduction in sea ice cover, however, the Arctic species have declined, and currently C. finmarchicus dominates the biomass. Because of the species shift, the Calanus community is now dominated by smaller individuals and the lipid content of Calanus females during spring and summer has decreased by 34%. Moreover, during the last decade the annual variation in population size has been prominent, Calanus virtually being absent in some years. Because of the central role of Calanus in the Arctic food web, the changes will likely impact higher trophic levels, including fish, sea birds, and marine mammals.

The oligotrophic subtropical gyre covers a vast area of the Atlantic Ocean. Decades of time-series monitoring have generated detailed temporal information about zooplankton species and abundances at fixed locations within the gyre, but their live/dead status is often omitted, especially in the dynamic subtropical convergence zone (STCZ) where the water column stratification pattern can change considerably across the front as warm and cold water masses converge. We conducted a detailed survey in the North Atlantic STCZ and showed that over 85% of the copepods were typically concentrated in the upper 200 m. Copepod carcasses were present in all samples and their proportional numerical abundances increased with depth, reaching up to 91% at 300–400 m. Overall, 14–19% of the copepods within the upper 200 m were carcasses. Shipboard experiments showed that during carcass decomposition, microbial respiration increased, and the bacterial community associated with the carcasses diverged from that in the ambient water. Combining field and experimental data, we estimated that decomposing copepod carcasses constitute a negligible oxygen sink in the STCZ, but sinking carcasses may represent an overlooked portion of the passive carbon sinking flux and should be incorporated in future studies of carbon flux in this area.

Marine snow aggregates represent heterogeneous agglomerates of dead and living organic matter. Composition is decisive for their sinking rates, and thereby for carbon flux to the deep sea. For oligotrophic oceans, information on aggregate composition is particularly sparse. To address this, the taxonomic composition of aggregates collected from the subtropical and oligotrophic Sargasso Sea (Atlantic Ocean) was characterized by 16S and 18S rRNA gene sequencing. Taxonomy assignment was aided by a collection of the contemporary plankton community consisting of 75 morphologically and genetically identified plankton specimens. The diverse rRNA gene reads of marine snow aggregates, not considering Trichodesmium puffs, were dominated by copepods (52%), cnidarians (21%), radiolarians (11%), and alveolates (8%), with sporadic contributions by cyanobacteria, suggesting a different aggregate composition than in eutrophic regions. Composition linked significantly with sampling location but not to any measured environmental parameters or plankton biomass composition. Nevertheless, indicator and network analyses identified key roles of a few rare taxa. This points to complex regulation of aggregate composition, conceivably affected by the environment and plankton characteristics. The extent to which this has implications for particle densities, and consequently for sinking rates and carbon sequestration in oligotrophic waters, needs further interrogation.

The present study is the first to describe egg cannibalism in the key Arctic copepod species Calanus finmarchicus, Calanus glacialis, and Calanus hyperboreus. Initially, a series of staining experiments evaluated the application of Neutral Red for staining Calanus eggs. The method was effective and applied in subsequent feeding experiments, where adult females were incubated in bottles with their own eggs. The results showed that all Calanus spp. ingested C. finmarchicus and C. glacialis eggs. However, consumers showed a slight preference for C. finmarchicus eggs when incubated with those of both species. The addition of phytoplankton even at high concentrations did not decrease clearance rates for eggs, suggesting that the presence of alternative food does not afford eggs any protection from cannibalism. To evaluate the potential impact of egg cannibalism on the succession of the three species, we calculated and compared field egg mortality rates with potential egg clearance rates for the Calanus complex based on rates from the experiments. Our results show that in Disko Bay cannibalism by Calanus spp., even at its highest level just before the spring bloom, could only account for about 10% of observed in situ egg mortality, and much less for most of the season.

Understanding the processes that affect fish larval survival and recruitment is a fundamental tenant of fisheries science. Small, isolated fjords are ideal study systems for elucidating early life history processes, as population dynamics are well traced in these partially closed systems. We examined the distribution, growth and mortality of eggs and larvae of a fjord population of cod during a 5 mo field campaign in the fjord Kapisigdlit, West Greenland. Cod mainly spawnedearly in the season in the innermost shallow region of the fjord. Egg survival was generally high in the fjord. The high survival may have been driven by relatively high temperature and/or low predation in the inner region. Early in the season, the distribution of eggs and young larvae was mostly restricted to the spawning area. Later in the season, larger larvae had become more evenly distributedin the fjord. This shift in distribution was observed after the seasonal pulse in freshwater outflow following the ice break-up in Kapisigdlit River. There was a positive correlation between the amount of food in a larval stomach and growth, and larval growth was greater in the outer fjord where prey availability was higher. The timing between spawning and freshwater input may be essential for survival and recruitment, this ensuring low dispersal of eggs and younger stages and high dispersal of older, actively feeding stages. Therefore, cod in this area could be vulnerable to future climate change affecting the timing and magnitude of freshwater outflow, by changes in precipitation, temperature or prey availability

The polar night in the Arctic is characterized by up to six months of darkness, low temperatures and limited food availability. Biological data on species composition and abundance during this period are scarce due to the logistical challenges posed when sampling these regions. Here, we characterize the plankton community composition during the polar night using water samplers and zooplankton net samples (50, 64, 200, 1500 μm), supplemented by acoustics (ADCPs, 300 kHz), to address a previously unresolved question–which species of zooplankton perform diel vertical migration during the polar night? The protist community (smallest plankton fraction) was mainly represented by ciliates (Strombidiida). In the larger zooplankton fractions (50, 64, 200 μm) the species composition was represented primarily by copepod nauplii and small copepods (e.g., Microcalanus spp., Pseudocalanus spp. and Oithona similis). In the largest zooplankton fraction (>1500 μm), the euphausiid, Thysanoessa inermis, was the most abundant species followed by the chaetognath Parasagitta elegans. Classical DVM was not observed throughout the darkest parts of the polar night (November–mid-January), although, subtle vertical migration patterns were detected in the acoustic data. With the occurrence of a more distinct day–night cycle (i.e., end of January), acoustical DVM signals were observed, paralleled by a classical DVM pattern in February in the largest fractions of zooplankton net samples. We suggest that Thysanoessa spp. are main responsible for the acoustical migration patterns throughout the polar night, although, chaetognaths and copepods may be co-responsible.

Picoplankton are an ecologically important component of pelagic Arctic marine ecosystems that may be heavily impacted by climate change. In order to assess potential impacts of a changing environment on this group, it is necessary to develop a better understanding of their population dynamics and seasonal distribution. This study, carried out in Disko Bay, West Greenland, during spring 2012, demonstrates that fuco-algae (e.g. chrysophytes, cryptophytes, diatoms and pelagophytes) dominated the picophytoplankton during the spring bloom with minor contributions from haptophytes. In the post-bloom phase, fuco-algae were replaced by haptophytes. In contrast to total chlorophyll a, which varied dramatically over the study period, the picoplanktonic chlorophyll a remained relatively stable despite the variability in picophytoplankton community composition. Based on mostly molecular studies, a general picture has emerged from the literature that mamiellophytes (a group within the green algae) dominate Arctic picophytoplankton. Here, however, green algae were found to contribute with only about 10 % of the picoplanktonic chlorophyll a. We suggest here that differences in cell size may offer a plausible explanation for the contrast between results obtained from molecular studies and those obtained from pigment- and microscopy-based studies.

Along the west coast of Greenland several fish species spawn across a wide latitudinal range encompassing significantly different environments. In an evaluation of the constraints to fish early life in high latitude ecosystems, we compared larval feeding and growth rates of an important species from the areas, the capelin (Mallotus villosus), at two latitudinal distant localities: the subarctic Kapisigdlit Fjord (64°25′N) and the arctic Disko Bay (69°15′N). In these areas, we describe oceanography by series of CTD casts and zooplankton and capelin larvae by fine-meshed nets. Zooplankton abundance and diversity were subsequently described and related to feeding and growth rates of the capelin larvae, assessed from analyses of larval gut contents and otolith ring-patterns. Early life characteristics of larvae differed markedly between the two areas. The onset of zooplankton production and the emergence of larvae took place earlier in Kapisigdlit than in the more northern Disko Bay. In Kapisiglit Fjord the dominant prey items for larvae were small calanoid and cyclopoid copepodites while large-sized calanoid nauplii dominated in the diet of Disko Bay larvae. Larval growth rates were lower in Kapisigdlit Fjord than in Disko Bay. We suggest that the later emergence of capelin larvae in Disko Bay could be compensated by enhanced feeding and thus growth rates, so that larvae from both populations would reach comparable sizes when entering the overwintering phase. However, while larval emergence and onset of plankton production are linked to different environmental cues (temperature increase vs light increase), a former match between emergence of larvae and preferred prey might be disrupted by the present temperature changes in the Arctic.

Copepods are important grazers on toxic phytoplankton and serve as vectors for algal toxins up the marine food web. Success of phytoplankton depends among other factors on protection against grazers like copepods, and same way copepod survival and population resilience relies on their ability to escape predators. Little is, however, known about the effect of toxins on the escape response of copepods. In this study we experimentally tested the hypothesis that the neurotoxin domoic acid (DA) produced by the diatom Pseudo-nitzschia affects escape responses of planktonic copepods. We found that the arctic copepods Calanus hyperboreus and C. glacialis reduced their escape response after feeding on a DA-producing diatom. The two species were not affected the same way; C. hyperboreus was affected after shorter exposure and less intake of DA. The negative effect on escape response was not related to the amount of DA accumulated in the copepods. Our results suggest that further research on the effects of DA on copepod behavior and DA toxicity mechanisms is required to evaluate the anti-grazing function of DA.

Organic aggregates sinking through marine and freshwater water bodies are increasingly perceived as abundant hotspots of anaerobic microbial metabolism that cannot take place in the surrounding oxygenated water. While the research focus is often on phytoplankton-derived aggregates, zooplankton may also provide anoxic microenvironments, such as guts, fecal pellets, and carcasses of copepods. We tested therefore, if anaerobic nitrogen cycling is associated with copepods collected from polar, temperate, and tropical marine or freshwater ecosystems and belonging to different taxonomic groups and size classes. Dissimilatory nitrogen transformations were quantified using 15N tracing with living and dead copepods and with fecal pellets at different ambient oxygen levels to mimic conditions prevailing in the respective ecosystem. Significant anaerobic nitrogen cycling was without exception associated with copepod carcasses and was also evident in living specimens and fecal pellets of Calanus hyperboreus, a key component of Arctic marine zooplankton. In this large copepod species, denitrification was the dominant pathway of anaerobic nitrogen cycling and correlated with the presence or absence of food particles in the gut, but not with ambient oxygen levels. In contrast, the carcasses of the much smaller tropical marine and temperate freshwater copepods displayed high rates of dissimilatory nitrate reduction to ammonium or nitrite that were boosted by low ambient oxygen levels. Extrapolated to their in situ carcass abundance, copepods contribute up to 28% to the total fixed-nitrogen loss in oxic/hypoxic settings and up to 14% in anoxic settings. Overall, copepods greatly expand the water volume in which anaerobic nitrogen cycling is possible.

We present a blue mussel exposure system where the fate of microplastics (polystyrene beads) is tracked during exposure and depuration phases. This enabled the establishment of a complete mass balance. Quantification of beads in mussels was done with a novel enzymatic digestion protocol. We found a similar relative distribution of beads for 2 environmentally realistic concentrations (5 and 100 beads L-1) and no substantial egestion of particles within 2 h of depuration. Environ Toxicol Chem 2019;38:99-105. (c) 2018 SETAC

10 pages, 7 figures, 1 table, supplementary material https://doi.org/10.1016/j.pocean.2018.10.008, In Antarctica, summer is a time of extreme environmental shifts resulting in large coastal phytoplankton blooms fueling the food web. Despite the importance of the microbial loop in remineralizing biomass from primary production, studies of how microzooplankton communities respond to such blooms in the Southern Ocean are rather scarce. Microzooplankton (ciliate and dinoflagellate) communities were investigated combining microscopy and 18S rRNA sequencing analyses in the Amundsen Sea Polynya during an extensive summer bloom of Phaeocystis antarctica. The succession of microzooplankton was further assessed during a 15-day induced bloom microcosm experiment. Dinoflagellates accounted for up to 59 % of the microzooplankton biomass in situ with Gymnodinium spp., Protoperidium spp. and Gyrodinium spp. constituting 89 % of the dinoflagellate biomass. Strobilidium spp., Strombidium spp. and tintinids represented 90 % of the ciliate biomass. Gymnodinium, Gyrodinium and tintinnids are known grazers of Phaeocystis, suggesting that this prymnesiophyte selected for the key microzooplankton taxa. Availability of other potential prey, such as diatoms, heterotrophic nanoflagellates and bacteria, also correlated to changes in microzooplankton community structure. Overall, both heterotrophy and mixotrophy appeared to be key trophic strategies of the dominant microzooplankton observed, suggesting that they influence carbon flow in the microbial food web through top-down control on the phytoplankton community, This work was supported by the Swedish Research Council [grant 2008-6430] to S. Bertilsson and L. Riemann and [grant 824-2008-6429] to P.-O. Moksnes and J. Havenhand, and by the US National Science Foundation through the ASPIRE project [NSF OPP-0839069] to P. Yager

A mesocosm study with oil in ice was performed in Van Mijenfjorden in Svalbard to compare effects of the oil spill responses (OSR) in situ burning, chemical dispersion and natural attenuation on the physiological performance of the Arctic copepod Calanus glacialis. Seawater collected from the mesocosms in winter and spring was used in laboratory incubation experiments, where effects on fecal pellet production, egg production and hatching success were investigated over a period of 14 days. Polycyclic aromatic hydrocarbon (PAH) seawater concentrations were lowest in winter. Brine channel formation in spring resulted in an 18 times increase in PAH concentration in the chemical dispersion treatment (1.67 μg L−1), and a 3 fold increase in the natural attenuation (0.36 μg L−1) and in situ burning (0.04 μg L−1) treatments. The physiological performance of female C. glacialis was unaffected by the PAH seawater concentrations. However, a higher mortality and deformity of nauplii was observed in the chemical dispersion treatment, highlighting the importance of considering secondary effects on next generation in future environmental risk assessment of OSR. This study shows that during the ice-covered period, chemical dispersion of oil spills leads to higher PAH exposure than natural attenuation and in situ burning, with potential consequences for recruitment of Arctic copepods.

Marine productivity hotspots are usually conceived of, or defined as, sources of energy and nutrients to surrounding waters (for example, in the context of marine protected areas that serve as buffers against fisheries catch elsewhere). This study considers whether Arctic hotspots for top predators are indeed sites of net production - and thus standing in contrast to the Arctic system as a whole - or whether they are net sinks at lower trophic levels, sites that collect plankton productivity from other oceans and deliver it in concentrated form to top predators.A new model, Coltrane (Copepod Life history Traits and Adaptation to New Environments) provides a relatively simple mathematical framework for layering multiple levels of mesozooplankton biology on top of oceanographic models. It resolves 1) individual life history (strategy traits controlling growth, development, and size, diagnosed from a broad review of lab studies); 2) population dynamics (the time-dependent energy balance between growth, egg production, and predation mortality); and 3) community assembly (the envelope of viable trait combinations under particular annual cycles of prey and temperature). Advective pathways in the Atlantic and Pacific Arctic were mapped using the BIOMAS oceanographic model, and Coltrane was used to map the energy budget of Calanus populations along northward transport corridors. Results in the Pacific Arctic show a spatial offset between the regions where biomass is transferred from phytoplankton to mesozooplankton and where it is transferred from mesozooplankton to higher predators. These results suggest that the yield of Calanus to predators in the Chukchi Sea is dependent primarily on Bering Sea phytoplankton blooms, although this complex interdependence could change as ice retreats and phytoplankton phenology shifts. Similar model results in Disko Bay, West Greenland are reinforced by a statistical analysis revealing intense predation by bowhead whales on overwintering Calanus: an unexpected trophic link from mesozooplankton to a top predator. Both lines of evidence suggest that Disko Bay is, like the Arctic Ocean as a whole, a net sink for plankton. However, completing this picture and fully testing this hypothesis will require solving difficult problems of scale in both the biology and the hydrodynamics.

The European and American eels spawn in the subtropical convergence zone (STCZ) in the Sargasso Sea, a dynamic and relatively productive area that is strongly influenced by front and eddy formations and subducted high-saline water masses. To understand how the physical and biological environments may affect the early life history of eels, we conducted a detailed bio-physical investigation of the water column at a site of high eel larvae abundance. Diel measurements and sampling in the upper 300 m revealed strong variations in hydrographic conditions and mean depths of different taxonomic groups; however, characteristics patterns of distribution were apparent. Most species showed diel vertical migrations, ascending about 20-30 m at night, whereas examples of night-time downward migration were also seen. European eel larvae were among the species showing more extensive diel vertical migration: their population mean depth changed from 160 m at day to 100 m at night where abundance peaked at 45 m depth. Distribution and migration of eel larvae corresponded to patterns observed for small hydrozoans, supporting a proposed predator-prey linkage. The study demonstrates the diverse and vertically strongly structured plankton community of STCZ where larvae of eel and other fish find a wide range of potential niches.

The larval krill community (Thysanoessa spp.) was investigated along the sub-Arctic Godthåbsfjord, SW Greenland, in June 2010. In addition, the progress of krill development from egg to furcilia was studied from March to August 2010 in a fjord branching off the Godthåbsfjord. Krill spawned from late April until early May, with a second spawning event in early July. Spawningtook place in the warmer, innermost part of the fjord, correlated with phytoplankton blooms. Naupliar abundance peaked immediately after spawning, and naupliar stage duration was 3 d. Sequences of the calyptopis and furcilia stages lasted 22 and 63 d, respectively. The growth rate from metanauplius to calyptopis was 0.12 d−1, while the growth rate across all developmental stages was 0.05 d−1. Mortality rates were calculated as 25% from eggs to nauplii, 48% from eggs to calyptopes and 83% from eggs to furcilia. During development, the larvae were dispersed from the shallow, warmer hatching area in the inner part of the fjord to the main fjord by tidal currents and runoff from land. The study showed that developmental stages of krill are a key group in Greenland coastal waters, one which should be considered in future studies of the pelagic food web

Identification and characterisation of microplastic (MP) is a necessary step to evaluate their concentrations, chemical composition and interactions with biota. MP ≥10μm diameter filtered from below the sea surface in the European and subtropical North Atlantic were simultaneously identified by visual microscopy and Raman micro-spectroscopy. Visually identified particles below 100μm had a significantly lower percentage confirmed by Raman than larger ones indicating that visual identification alone is inappropriate for studies on small microplastics. Sixty-eight percent of visually counted MP (n=1279) were spectroscopically confirmed being plastic. The percentage varied with type, colour and size of the MP. Fibres had a higher success rate (75%) than particles (64%). We tested Raman micro-spectroscopy applicability for MP identification with respect to varying chemical composition (additives), degradation state and organic matter coating. Partially UV-degraded post-consumer plastics provided identifiable Raman spectra for polymers most common among marine MP, i.e. polyethylene and polypropylene.