Short-term variations in particulate matter sedimentation off Kapp Norvegia,Weddell Sea,Antarctica: relation to water mass advection,ice cover,plankton biomass and feeding activity

Summary A multi-cup sediment trap was deployed at 250m in the shelf area off Kapp Norvegia, Weddell Sea (630 m water depth) to determine the relative importance of water mass advection, sea ice movement, phytoplankton biomass and plankton feeding. Short-term fluctuations in sedimentation were determined using a sampling frequency of 2.7 days over 54 days during January and February 1988. Three periods of enhanced sedimentation were associated with water mass exchange, settling of diatoms following break-up of ice cover and release of fecal matter by krill feeding on particulate matter derived from phytoplankton and ice algae. An initial sedimentation pulse (28 Jan) was mainly due to sinking pelagic diatoms and krill fecal strings containing algae released from sea ice passing over the trap position. The ¹³C-composition of the sedimented organic carbon was about-24. The isotope ratio decreased sharply by about 5.5 at the end of the first pulse indicating the source of sinking matter becoming pelagic diatoms of the retreating ice-edge. At this time the diatom Corethron criophilum contributed a very high proportion of the organic flux causing an increase of the opal/C_org ratios. The second pulse (6 Feb) was due to empty diatom frustules, minipellets and small planktonic aggregates. Much of the organic carbon was transported by round fecal pellets. During the third pulse (14 Feb), round fecal pellets transported even more; the percentage of C. criophilum to the diatom organic carbon flux was more than 80% (>2mg C m^–2 day^–1).Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Large,Omega-3 Rich,Pelagic Diatoms under Arctic Sea Ice: Sources and Implications for Food Webs

Pelagic primary production in Arctic seas has traditionally been viewed as biologically insignificant until after the ice breakup. There is growing evidence however, that under-ice blooms of pelagic phytoplankton may be a recurrent occurrence. During the springs of 2011 and 2012, we found substantial numbers (201–5713 cells m⁻³) of the large centric diatom (diameter >250 µm) Coscinodiscus centralis under the sea ice in the Canadian Arctic Archipelago near Resolute Bay, Nunavut. The highest numbers of these pelagic diatoms were observed in Barrow Strait. Spatial patterns of fatty acid profiles and stable isotopes indicated two source populations for C. centralis: a western origin with low light conditions and high nutrients, and a northern origin with lower nutrient levels and higher irradiances. Fatty acid analysis revealed that pelagic diatoms had significantly higher levels of polyunsaturated fatty acids (mean ± SD: 50.3±8.9%) compared to ice-associated producers (30.6±10.3%) in our study area. In particular, C. centralis had significantly greater proportions of the long chain omega-3 fatty acid, eicosapentaenoic acid (EPA), than ice algae (24.4±5.1% versus 13.7±5.1%, respectively). Thus, C. centralis represented a significantly higher quality food source for local herbivores than ice algae, although feeding experiments did not show clear evidence of copepod grazing on C. centralis. Our results suggest that C. centralis are able to initiate growth under pack ice in this area and provide further evidence that biological productivity in ice-covered seas may be substantially higher than previously recognized.     

A Fatty Acid Based Bayesian Approach for Inferring Diet in Aquatic Consumers

We modified the stable isotope mixing model MixSIR to infer primary producer contributions to consumer diets based on their fatty acid composition. To parameterize the algorithm, we generated a ‘consumer-resource library’ of FA signatures of Daphnia fed different algal diets, using 34 feeding trials representing diverse phytoplankton lineages. This library corresponds to the resource or producer file in classic Bayesian mixing models such as MixSIR or SIAR. Because this library is based on the FA profiles of zooplankton consuming known diets, and not the FA profiles of algae directly, trophic modification of consumer lipids is directly accounted for. To test the model, we simulated hypothetical Daphnia comprised of 80% diatoms, 10% green algae, and 10% cryptophytes and compared the FA signatures of these known pseudo-mixtures to outputs generated by the mixing model. The algorithm inferred these simulated consumers were comprised of 82% (63-92%) [median (2.5th to 97.5th percentile credible interval)] diatoms, 11% (4-22%) green algae, and 6% (0-25%) cryptophytes. We used the same model with published phytoplankton stable isotope (SI) data for δ¹³C and δ¹⁵N to examine how a SI based approach resolved a similar scenario. With SI, the algorithm inferred that the simulated consumer assimilated 52% (4-91%) diatoms, 23% (1-78%) green algae, and 18% (1-73%) cyanobacteria. The accuracy and precision of SI based estimates was extremely sensitive to both resource and consumer uncertainty, as well as the trophic fractionation assumption. These results indicate that when using only two tracers with substantial uncertainty for the putative resources, as is often the case in this class of analyses, the underdetermined constraint in consumer-resource SI analyses may be intractable. The FA based approach alleviated the underdetermined constraint because many more FA biomarkers were utilized (n < 20), different primary producers (e.g., diatoms, green algae, and cryptophytes) have very characteristic FA compositions, and the FA profiles of many aquatic primary consumers are strongly influenced by their diets.     

Diatom-Specific Highly Branched Isoprenoids as Biomarkers in Antarctic Consumers

The structure, functioning and dynamics of polar marine ecosystems are strongly influenced by the extent of sea ice. Ice algae and pelagic phytoplankton represent the primary sources of nutrition for higher trophic-level organisms in seasonally ice-covered areas, but their relative contributions to polar marine consumers remain largely unexplored. Here, we investigated the potential of diatom-specific lipid markers and highly branched isoprenoids (HBIs) for estimating the importance of these two carbon pools in an Antarctic pelagic ecosystem. Using GC-MS analysis, we studied HBI biomarkers in key marine species over three years in Adélie Land, Antarctica: euphausiids (ice krill Euphausia crystallorophias and Antarctic krill E. superba), fish (bald notothens Pagothenia borchgrevinki and Antarctic silverfish Pleuragramma antarcticum) and seabirds (Adélie penguins Pygoscelis adeliae, snow petrels Pagodroma nivea and cape petrels Daption capense). This study provides the first evidence of the incorporation of HBI lipids in Antarctic pelagic consumers. Specifically, a di-unsaturated HBI (diene) of sea ice origin was more abundant in ice-associated species than in pelagic species, whereas a tri-unsaturated HBI (triene) of phytoplanktonic origin was more abundant in pelagic species than in ice-associated species. Moreover, the relative abundances of diene and triene in seabird tissues and eggs were higher during a year of good sea ice conditions than in a year of poor ice conditions. In turn, the higher contribution of ice algal derived organic matter to the diet of seabirds was related to earlier breeding and higher breeding success. HBI biomarkers are a promising tool for estimating the contribution of organic matter derived from ice algae in pelagic consumers from Antarctica.     

Fatty acid and stable isotope characteristics of sea ice and pelagic particulate organic matter in the Bering Sea: tools for estimating sea ice algal contribution to Arctic food web production

We determined fatty acid (FA) profiles and carbon stable isotopic composition of individual FAs (δ¹³C_FA values) from sea ice particulate organic matter (i-POM) and pelagic POM (p-POM) in the Bering Sea during maximum ice extent, ice melt, and ice-free conditions in 2010. Based on FA biomarkers, differences in relative composition of diatoms, dinoflagellates, and bacteria were inferred for i-POM versus p-POM and for seasonal succession stages in p-POM. Proportions of diatom markers were higher in i-POM (16:4n-1, 6.6–8.7 %; 20:5n-3, 19.6–25.9 %) than in p-POM (16:4n-1, 1.2–4.0 %; 20:5n-3, 5.5–14.0 %). The dinoflagellate marker 22:6n-3/20:5n-3 was highest in p-POM. Bacterial FA concentration was higher in the bottom 1 cm of sea ice (14–245 μg L^?1) than in the water column (0.6–1.7 μg L^?1). Many i-POM δ¹³C_FA values were higher (up to ~10 ‰) than those of p-POM, and i-POM δ¹³C_FA values increased with day length. The higher i-POM δ¹³C_FA values are most likely related to the reduced dissolved inorganic carbon (DIC) availability within the semi-closed sea ice brine channel system. Based on a modified Rayleigh equation, the fraction of sea ice DIC fixed in i-POM ranged from 12 to 73 %, implying that carbon was not limiting for primary productivity in the sympagic habitat. These differences in FA composition and δ¹³C_FA values between i-POM and p-POM will aid efforts to track the proportional contribution of sea ice algal carbon to higher trophic levels in the Bering Sea and likely other Arctic seas.     

The sea-ice algal community of seasonal pack ice in the southwestern Kara Sea in late winter

The composition and abundance of ice algae have been studied in a 64-cm core of seasonal pack ice taken in the southwestern Kara Sea in February 1996. The cell numbers, biomass and taxonomic diversity of the ice microalgal assemblages were rather low, and there was a predominance of diatoms. The comparison of its composition with that of the pelagic microalgal community demonstrated a great difference between them. The ecological function of the seasonal ice habitat as a depository for seeding material of the early-spring pelagic diatom is suggested. Accepted: 7 August 2000     

Influence of sea ice on the composition of the spring phytoplankton bloom in the northern Baltic Sea

During the late winter and spring of 1994, the influence of sea ice on phytoplankton succession in the water was studied at a coastal station in the northern Baltic Sea. Ice cores were taken together with water samples from the underlying water and analysed for algal composition, chlorophyll a and nutrients. Sediment traps were placed under the ice and near the bottom, and the sedimented material was analysed for algal composition. The highest concentration of ice algae (4.1 mmol C m⁻²) was found shortly before ice break-up in the middle of April, coincidental with the onset of an under-ice phytoplankton bloom. The ice algae were dominated by the diatoms Chaetoceros wighamii Brightwell, Melosira arctica (Ehrenberg) Dickie and Nitzschia frigida Grunow. Under the ice the diatom Achnanthes taeniata Grunow and the dinoflagellate Peridiniella catenata (Levander) Balech were dominant. Calculations of sinking rates and residence times of the dominant ice algal species in the photic water column indicated that only one ice algal species (Chaetoceros wighamii) had a seeding effect on the water column: this diatom dominated the spring phytoplankton bloom in the water together with Achnanthes taeniata and Peridiniella catenata. Received: 9 May 1997 / Accepted: 15 February 1998     

Stable isotope composition of fatty acids in organisms of different trophic levels in the Yenisei River

We studied four-link food chain, periphytic microalgae and water moss (producers), trichopteran larvae (consumers I), gammarids (omnivorous--consumers II) and Siberian grayling (consumers III) at a littoral site of the Yenisei River on the basis of three years monthly sampling. Analysis of bulk carbon stable isotopes and compound specific isotope analysis of fatty acids (FA) were done. As found, there was a gradual depletion in (13)C contents of fatty acids, including essential FA upward the food chain. In all the trophic levels a parabolic dependence of δ(13)C values of fatty acids on their degree of unsaturation/chain length occurred, with 18:2n-6 and 18:3n-3 in its lowest point. The pattern in the δ(13)C differences between individual fatty acids was quite similar to that reported in literature for marine pelagic food webs. Hypotheses on isotope fractionation were suggested to explain the findings.     

Broad sampling and diverse biomarkers allow characterization of nearshore particulate organic matter

Suspended particulate organic matter (POM) is a primary food source for benthic and pelagic consumers in aquatic and marine ecosystems. POM is potentially composed of many sources including phytoplankton, bacteria, zooplankton and macrophyte (seaweed and seagrass) and terrestrial detritus. The relative importance of these sources to POM consumers is debated, in large part due to differing interpretations of stable isotope and fatty acid biomarkers. We investigated POM composition in a nearshore marine ecosystem using multiple methods including visual quantification of living and detrital components, multiple stable isotope (MSI) and fatty acid (FA) analyses. Sampling was conducted at multiple temporal and spatial scales to 1) determine the range of variability in POM biomarkers, 2) quantitatively evaluate δ¹³C, δ¹⁵N, δ³⁴S and FA biomarkers with proportional abundance of putative sources and 3) determine the availability of phytoplankton, macrophytes and terrestrial carbon in nearshore POM. Variation of total FA concentration and proportions, and δ¹³C and δ³⁴S were strongly correlated to phytoplankton abundance, at tidal and seasonal timescales. Using multivariate multiple regressions, MSI and FA explained 59.6% and 89.7% of the variation in POM composition, respectively. As phytoplankton abundance increased, total FA concentration increased concurrent to δ¹³C and δ³⁴S enrichment. In high detritus samples, bacterial FA and saturated FA were proportionally higher, corresponding to depletion of δ¹³C and δ³⁴S and enrichment of δ¹⁵N. We identify MSI and FA biomarkers that are good predictors of diatom, dinoflagellate and detrital contributions to the POM. The results of this multi‐scale study show that POM composition is highly dynamic and largely driven by phytoplankton abundance, with minor contributions from terrestrial or macrophyte subsidies. This quantitative approach provides novel and critical empirical information linking POM compositional dynamics to specific biomarkers that are commonly used for tracking energy subsidies and biogeochemical cycling in aquatic ecosystems.     

高浓度硅藻对桡足类繁殖的抑制作用

硅藻作为海洋浮游植物重要的组分 ,一直被看作是桡足类主要的食物来源。但近 10年来许多现场和实验室研究表明 :硅藻 ,特别是高浓度硅藻对桡足类的繁殖具有抑制作用。目前 ,对于抑制作用的机制存在着两种假设 :硅藻细胞缺乏某种关键营养物质或自身产生某种有毒物质阻碍了桡足类繁殖过程。大量的室内实验发现 :关键营养物缺乏使桡足类产卵率降低 ,对于硅藻细胞化学组成的分析表明这种物质可能是某些不饱和脂肪酸。而高浓度硅藻是否会产生毒性物质影响桡足类孵化率则存在很大的分歧。许多室内和野外实验证据显示当硅藻浓度很高时 ,桡足类孵化率显著降低 ,表现为大量未孵化的卵和畸形无节幼体。对硅藻溶出液的分析发现其中含有的不饱和醛类可能正是阻碍胚胎发育的物质。但也有一些室内和现场实验的结果表明并非所有的硅藻和桡足类之间都存在这种抑制作用。目前研究结果大多来自于室内实验 ,在自然海区中的作用机制和影响程度尚不清楚 ,但是如果这一现象在自然海区中普遍存在 ,传统上关于“硅藻——桡足类——鱼类”的海洋经典食物链观点势必存在极大的缺陷。文章针对这一问题分析了目前研究进展和将来工作前景     

Lipids and fatty acids in ice algae and phytoplankton from the Marginal Ice Zone in the Barents Sea

Samples of ice algae from the Marginal Ice Zone in the Barents Sea could be divided into two categories: one dominated by assemblages of Melosira arctica, and the other dominated by Nitzschia frigida and associated diatoms. Total lipid from the Melosira assemblages consisted of approximately equal amounts of polar lipids and triacylglycerols. Total lipid from the Nitzschia assemblages contained more triacylglycerols than polar lipids. Total lipid from the Melosira assemblages had higher percentages of C16 PUFA, especially 16:4(n-1) and 20:5(n-3), than that from the Nitzschia assemblages, this reflecting the higher percentages of both C16 PUFA and 20:5(n-3) in polar lipids than in triacylglycerols. Phytoplankton from the pelagic zone were␣richer in flagellates and contained less C16 PUFA and 20:5(n-3) but more C18 PUFA and 22:6(n-3). The dominance of diatoms in the ice-algae assemblages in the Marginal Ice Zone and their high nutritional value as a source of 20:5(n-3) for higher trophic levels are emphasised. Received: 24 November 1997 / Accepted: 8 February 1998     

Fatty acids as organic markers for the study of trophic relationships in littoral cladoceran communities of a pond

The fatty acid (FA) composition of zooplankton and paniculatematter (including phytoplankton) associated with the submergedvegetation of a pond shoreline was analysed by gas chromatography.The FA of paniculate matter was characterized by fluctuatingproportions of polyunsaturated FA (PUFA), comprising between31.7 and 51.1% of total FA and originating from phytoplankton.Important levels of bacterial FA (branched 15:0 and 17:0) anddetrital FA (17:0, 17:1) were also detected. Littoral zooplanktonwas dominated by two cladocerans: Eurycercus lametlatus andSimocephalus velulus. Their triacyglycerol fatty acids (TAG-FA)were characterized by PUFA (18:2(n-6), 18:3(n-3). 20:5(n-3))originating in phytoplankton and periphytic algae, and alsoby bacterial and detrital odd-numbered FA. An unusual unsaturatedFA (22:2(n-6)) was detected in the trophic web. It was incorporatedinto cladoceran polar lipids, together with long 22-carbon FAof (n-6) series. Unsaturation of polar lipids was inverselycorrelated with water temperature in both cladocerans. The influenceof dietary FA on the composition of polar lipid FA is also considered.     

Springtime coupling between ice algal and phytoplankton assemblages in southeastern Hudson Bay,Canadian Arctic

Microalgal assemblages from the bottom ice, the ice-water interface and the water column were systematically sampled from April to June 1986, in southeastern Hudson Bay (Canadian Arctic). The taxonomic similarity between samples from the three environments was assessed using a clustering procedure. There were two groups that comprised samples from both the ice-water interface and the water column, while five other groups were made of samples originating from a single environment. Taxonomic compositions of the two mixed groups suggest two types of connexion between the ice-water interface and the water column, i.e. before the phytoplankton bloom, there was seeding of the water column by ice algae and, during ice melt, interfacial algae contributed to the water column communities that were otherwise typically phytoplankton. Overall, the phytoplankton community underwent a succession from pennate to centric diatoms. Sinking rates of algae from the ice-water interface were estimated using settling columns (SETCOL). The sinking rates increased seasonally (0.4–2.7 m d^–1), which enhanced accessibility of ice-algal cells to the pelagic grazers. Ice algae contributed to water column production as they became accessible to the pelagic grazers, and also by seeding the water column before the phytoplankton bloom.Contribution to the programs of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec) and of the Maurice Lamontagne Institute (Department of Fisheries and Oceans)     

Sea ice phenology and timing of primary production pulses in the Arctic Ocean

Arctic organisms are adapted to the strong seasonality of environmental forcing. A small timing mismatch between biological processes and the environment could potentially have significant consequences for the entire food web. Climate warming causes shrinking ice coverage and earlier ice retreat in the Arctic, which is likely to change the timing of primary production. In this study, we test predictions on the interactions among sea ice phenology and production timing of ice algae and pelagic phytoplankton. We do so using the following (1) a synthesis of available satellite observation data; and (2) the application of a coupled ice‐ocean ecosystem model. The data and model results suggest that, over a large portion of the Arctic marginal seas, the timing variability in ice retreat at a specific location has a strong impact on the timing variability in pelagic phytoplankton peaks, but weak or no impact on the timing of ice‐algae peaks in those regions. The model predicts latitudinal and regional differences in the timing of ice algae biomass peak (varying from April to May) and the time lags between ice algae and pelagic phytoplankton peaks (varying from 45 to 90 days). The correlation between the time lag and ice retreat is significant in areas where ice retreat has no significant impact on ice‐algae peak timing, suggesting that changes in pelagic phytoplankton peak timing control the variability in time lags. Phenological variability in primary production is likely to have consequences for higher trophic levels, particularly for the zooplankton grazers, whose main food source is composed of the dually pulsed algae production of the Arctic.     

Benthic diatoms and phytoplankton to assess nutrients in a large lake: Complementarity of their use in Lake Geneva (France–Switzerland)

Lakes can be monitored using different bioindicators, among which phytoplankton and benthic diatoms. We compared these two indicators in Lake Geneva for nutrient assessment. Bimonthly samplings were carried out during one year in the euphotic layer of a pelagic site. In the same time four littoral sites were sampled at 40–50 cm depth. Species compositions and abundances of both bioindicators were analyzed following European standards. Water physico-chemical composition was analyzed at the same time. Seasonal succession of benthic diatom guilds was observed. The dynamics of benthic diatom communities were better correlated to the pelagic chemistry than to the local littoral chemistry. We also observed that in the sampling sites frequently exposed to winds and waves, benthic diatoms showed lower correlations to physico-chemical dynamics, because of an increase of pioneer diatoms abundance adapted to turbulent environment, such sites must be avoided for lake monitoring. Finally, biotic indices calculated with benthic diatoms in wind protected sites showed higher correlations with pelagic nutrient concentration (PO₄³⁻) than indices calculated with phytoplankton. This unexpected situation can be explained by differences of temporal variability of chemical and biological compartments. Littoral chemistry changed faster than pelagic chemistry because of rains, diffuse flow from watershed and rivers flowing in the littoral zone whereas pelagic chemistry has a much smoother evolution because it is situated 10 km from the coast. But phytoplankton showed a high temporal variability because of wind influence, which explained the low correlation with the smooth evolution of pelagic chemistry. On the other hand, benthic diatoms from sites protected from the dominant wind, showed a lower temporal variability and were more in synchrony with the smooth evolution of pelagic chemistry.Even if we show that diatom seem to be promising indicator of nutrient level of the lake, we also underline the complementarity of using both indicators: benthic diatom and phytoplankton. Each of them brings different information about temporal variability of the lake and about the functioning of different habitats. Comparing these two bioindicators only on the basis of nutrient correlation ability would be an over-simplification whom managers must be warned.     

Seasonal variation in the biochemical composition of particulate material collected by sediment traps at Signy Island, Antarctica

Particulate material recovered over an 18-month period from sediment traps deployed at a shallow-water nearshore Antarctic site was analysed for photosynthetic pigments, aliphatic hydrocarbons and fatty acids. All components showed a distinct seasonal variation, with high recovery rates during the summer open-water phytoplankton bloom and low rates under winter fast ice. The amount of trapped material differed between the two summers, indicating inter-annual variability of vertical flux associated with differences in the intensity of the summer phytoplankton bloom. Particulate material trapped in summer was dominated by that which originated in diatoms. High recoveries of chlorophyll a, fucoxanthin, n-C_21:6 hydrocarbon, 20:5(n-3) fatty acid and shorter chain (C₁₅–C₂₄) aliphatic hydrocarbons all pointed to a significant summer flux of ungrazed diatoms. There were, however, also signals of zooplankton grazing activity (notably pyrophaeophorbide a), and the presence of C18:4(n-3) and C22:6(n-3) fatty acids suggested a small flux of material from flagellates and other sources. Longer chain n-alkanes (C₂₅–C₃₄) indicative of nanoplankton were detected all year, but there was no significant deposition of zooplankton material in any sample. The major recovery rate of photosynthetic pigments was in late summer (February to April), and the major grazing signal occurred after the peak of the summer diatom bloom. Most of the diatom bloom appeared to settle out from the water column without being grazed. The major seasonal contrast in the biochemistry of the trapped material was the dominance of the diatom signature in summer, and in winter the predominance (but at much lower recovery rates) of material from nanoplankton. Received: 2 March 1998 / Accepted: 12 June 1998     

Seasonal changes on planktonic diatom communities along an inshore-offshore gradient in the Gulf of Gabes (Tunisia)

We investigated the composition, biomass and cell size dynamics of the marine planktonic diatom species along a coastal-open sea gradient in relation to the hydrological characteristics, during four oceanographic cruises in July 2005, May–June 2006, September 2006 and March 2007 in the Gulf of Gabes. The study of the marine planktonic diatoms throughout the sampling period showed the presence of 40 different species belonging to 22 pennate and 18 centric diatom species. Centric diatoms were more abundant than the pennate ones; 56% and 44% of total diatom abundance, respectively. Diatoms were very abundant, representing about 60% of the total phytoplankton abundance, with an exception in July 2005 (19%) during which Dictyochophyceae were the most dominant group (41% of the total phytoplankton abundance). Diatoms, which were dominant in the coastal samples, mainly proliferate in the semi-mixing conditions (May–June 2006), whereas they declined in the offshore area, most likely due to silicate shortage. In this period, in spite of the high abundance of diatom planktonic cells, only diatom biomass was correlated with silica amount, proving that biomass was a better ecological bio-indicator than abundance. The results suggest that the marine planktonic diatoms taxa were generally adapted to specific hydrological structure. In fact, the dominance index of diatoms showed that the biodiversity of diatoms increased gradually along the coastal-open sea gradient except in May–June 2006 during which a slight decrease along the inshore towards offshore areas was observed. This index increase depended on a coastal-open sea distance during thermal stratification (July 2005 and September 2006) and mixing periods (March 2007).     

Dependency of Antarctic zooplankton species on ice algae‐produced carbon suggests a sea ice‐driven pelagic ecosystem during winter

How the abundant pelagic life of the Southern Ocean survives winter darkness, when the sea is covered by pack ice and phytoplankton production is nearly zero, is poorly understood. Ice‐associated (“sympagic”) microalgae could serve as a high‐quality carbon source during winter, but their significance in the food web is so far unquantified. To better understand the importance of ice algae‐produced carbon for the overwintering of Antarctic organisms, we investigated fatty acid (FA) and stable isotope compositions of 10 zooplankton species, and their potential sympagic and pelagic carbon sources. FA‐specific carbon stable isotope compositions were used in stable isotope mixing models to quantify the contribution of ice algae‐produced carbon (α_Ice) to the body carbon of each species. Mean α_Ice estimates ranged from 4% to 67%, with large variations between species and depending on the FA used for the modelling. Integrating the α_Ice estimates from all models, the sympagic amphipod Eusirus laticarpus was the most dependent on ice algal carbon (α_Ice: 54%–67%), and the salp Salpa thompsoni showed the least dependency on ice algal carbon (α_Ice: 8%–40%). Differences in α_Ice estimates between FAs associated with short‐term vs. long‐term lipid pools suggested an increasing importance of ice algal carbon for many species as the winter season progressed. In the abundant winter‐active copepod Calanus propinquus, mean α_Ice reached more than 50% in late winter. The trophic carbon flux from ice algae into this copepod was between 3 and 5 mg C m^?2 day^?1. This indicates that copepods and other ice‐dependent zooplankton species transfer significant amounts of carbon from ice algae into the pelagic system, where it fuels the food web, the biological carbon pump and elemental cycling. Understanding the role of ice algae‐produced carbon in these processes will be the key to predictions of the impact of future sea ice decline on Antarctic ecosystem functioning.     

Spatial distribution of aquatic marine fungi across the western Arctic and sub‐arctic

Fungi are important parasites of primary producers and nutrient cyclers in aquatic ecosystems. In the Pacific‐Arctic domain, fungal parasitism is linked to light intensities and algal stress that can elevate disease incidence on algae and reduce diatom concentrations. Fungi are vastly understudied in the marine realm and knowledge of their function is constrained by the current understanding of fungal distribution and drivers on global scales. To investigate the spatial distribution of fungi in the western Arctic and sub‐Arctic, we used high throughput methods to sequence 18S rRNA, cloned and sequenced 28S rRNA and microscopically counted chytrid‐infected diatoms. We identified a broad distribution of fungal taxa predominated by Chytridiomycota and Dikarya. Phylogenetic analysis of our Chytridiomycota clones placed Arctic marine fungi sister to the order Lobulomycetales. This clade of fungi predominated in fungal communities under ice with low snowpack. Microscopic examination of fixed seawater and sea ice samples revealed chytrids parasitizing diatoms collected across the Arctic that notably infected 25% of a single diatom species in the Bering Sea. The Pezizomycotina comprised > 95% of eukaryotic sequence reads in Greenland, providing preliminary evidence for osmotrophs being a substitute for algae as the base of food webs.     

Intron features of key functional genes mediating nitrogen metabolism in marine phytoplankton

Introns are widespread and variable in eukaryotic genomes. Although their histories and functions, or even whether all of them have any function, remain largely unknown, analysis of intron sequences and genomic contexts may shed light on the evolutionary history of genes and organisms. The number and frequency of introns vary widely in the small number of published genomes of protists and algae suggesting that the same is true of the vast diversity of protists and algae that remain uncultivated. The objective of this study were to investigate introns in sequences of functional genes of phytoplankton, both in published genomes and in sequences obtained from environmental clone libraries. We examined the introns of the genes involved in nitrogen uptake and assimilation pathways in the genome sequences of cultivated phytoplankton as well as in environmental clone libraries of nitrate reductases (NR), nitrite reductase (NiR), nitrate transporter (Nrt2) and ammonium transporter (AMT) genes constructed from pelagic phytoplankton communities in Monterey Bay (CA, USA) and Onslow Bay (NC, USA). Here we describe the most extensive set to date of intron sequences from uncultivated marine algae and report important differences for diatom vs. non-diatom sequences. The majority of the introns in NR, NiR, Nrt2 and AMT from cultured phytoplankton and environmental libraries showed canonical splice patterns. Introns found in diatom-like NR environmental libraries had lower GC content than the respective exons. The green algal-like NR and Nrt2 environmental sequences had introns and exons of much more similar GC content, and both higher than in diatoms. These patterns suggest a different evolutionary history and recent acquisition of diatom introns compared to other algae.