Temporal and spatial trends in marine carbon isotopes in the Arctic Ocean and implications for food web studies

The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ¹³C) signatures in dissolved inorganic carbon (δ¹³C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ¹³C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ¹³C values in consumers can change as a result of environmentally driven variation in the δ¹³C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ¹³C baseline, with spatial gradients in the δ¹³C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ¹³C‐DIC values (?0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO₂) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ¹³C‐POC values and δ¹³C in arctic marine mammals reflects the anthropogenic CO₂ signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ¹³C‐POC values in the Arctic Ocean and partially explain the δ¹³C values in marine mammals with consideration of time‐varying integration of δ¹³C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean.     

Effect of lipid removal on carbon and nitrogen stable isotope ratios in crustacean tissues

The analysis of tissue's naturally occurring stable carbon and nitrogen isotope ratios is a useful tool to delineate trophic relationships. However, the interpretation of δ¹³C and δ¹⁵N is complicated by the influence of multiple factors such as the tissue-specific lipid content. The aim of this work was to evaluate the effects of lipid extraction on δ¹³C and δ¹⁵N compositions in muscle, hepatopancreas and gonads of a marine decapod crustacean, the spider crab Maja brachydactyla. Samples were analyzed for stable isotopes before and after lipid removal, using a derived Soxhlet extraction method. Differences in δ¹³C and δ¹⁵N were measured among tissues before and after treatment. Lipid extraction of muscle did not have a significant effect on either δ¹³C or δ¹⁵N. By contrast, ecologically significant shifts for both carbon and nitrogen stable isotopes ratios (+ 2.9 ± 0.8‰ for δ¹³C, and + 1.2 ± 0.7‰ for δ¹⁵N) were noticed in the hepatopancreas. In regard to gonads, lipid extraction led to a shift only on δ¹³C (+ 1.3 ± 0.3‰). Finally, the derived Soxhlet extraction method removed the lipid influence for δ¹³C, and had an effect on δ¹⁵N composition for lipid-rich samples. We recommend this treatment for carbon stable isotope studies on decapod crustacean lipid-rich tissues.     

Synergistic effects of ocean acidification and warming on overwintering pteropods in the Arctic

Ocean acidification and warming will be most pronounced in the Arctic Ocean. Aragonite shell‐bearing pteropods in the Arctic are expected to be among the first species to suffer from ocean acidification. Carbonate undersaturation in the Arctic will first occur in winter and because this period is also characterized by low food availability, the overwintering stages of polar pteropods may develop into a bottleneck in their life cycle. The impacts of ocean acidification and warming on growth, shell degradation (dissolution), and mortality of two thecosome pteropods, the polar Limacina helicina and the boreal L. retroversa, were studied for the first time during the Arctic winter in the Kongsfjord (Svalbard). The abundance of L. helicina and L. retroversa varied from 23.5 to 120 ind m^?2 and 12 to 38 ind m^?2, and the mean shell size ranged from 920 to 981 μm and 810 to 823 μm, respectively. Seawater was aragonite‐undersaturated at the overwintering depths of pteropods on two out of ten days of our observations. A 7‐day experiment [temperature levels: 2 and 7 °C, pCO₂ levels: 350, 650 (only for L. helicina) and 880 μatm] revealed a significant pCO₂ effect on shell degradation in both species, and synergistic effects between temperature and pCO₂ for L. helicina. A comparison of live and dead specimens kept under the same experimental conditions indicated that both species were capable of actively reducing the impacts of acidification on shell dissolution. A higher vulnerability to increasing pCO₂ and temperature during the winter season is indicated compared with a similar study from fall 2009. Considering the species winter phenology and the seasonal changes in carbonate chemistry in Arctic waters, negative climate change effects on Arctic thecosomes are likely to show up first during winter, possibly well before ocean acidification effects become detectable during the summer season.     

Temporal variation in body composition (C : N) helps explain seasonal patterns of zooplankton δ13C

1. The stable carbon isotope ratio δ¹³C is a useful tracer of energy flow in lake food webs, and the zooplankton signature is commonly used to establish a baseline for the pelagic habitat. However, sources of temporal variability in the δ¹³C of different zooplankton taxa are rarely considered. 2. Here, we investigate to what extent temporal variation in the δ¹³C of particulate organic matter (POM) (<41 μm) and the C : N of zooplankton can explain the temporal variability in δ¹³C of freshwater zooplankton. We compare temporal patterns of δ¹³C and C : N for Daphnia, Hesperodiaptomus franciscanus and Leptodiaptomus tyrelli over a 6‐month period at four sites in two oligotrophic lakes. 3. In all three taxa, seasonal variation in zooplankton C : N explained more of the variation in zooplankton δ¹³C than did the δ¹³C of POM. This suggests that variation in the lipid content of zooplankton can strongly influence temporal variation of δ¹³C in zooplankton. 4. Using these data, we evaluate procedures that estimate the δ¹³C of only the non‐lipid component of zooplankton. If zooplankton lipids are primarily dietary in origin, than extracting lipids or ‘normalising’δ¹³C based on C : N will exclude a major dietary source, and therefore may be inappropriate. 5. We conclude that temporal variation in body composition (C : N) of zooplankton can significantly influence the temporal variation of zooplankton δ¹³C signatures.     

Complex and interactive effects of ocean acidification and warming on the life span of a marine trematode parasite

Human activities have caused an increase in atmospheric CO₂ over the last 250 years, leading to unprecedented rates of change in seawater pH and temperature. These global scale processes are now commonly referred to as ocean acidification and warming, and have the potential to substantially alter the physiological performance of many marine organisms. It is vital that the effects of ocean acidification and warming on marine organisms are explored so that we can predict how marine communities may change in future. In particular, the effect of ocean acidification and warming on host-parasite dynamics is poorly understood, despite the ecological importance of these relationships. Here, we explore the response of one himasthlid trematode, Himasthla sp., an abundant and broadly distributed species of marine parasite, to combinations of elevated temperature and pCO₂ that represent physiological extremes, pre-industrial conditions, and end of century predictions. Specifically, we quantified the life span of the free-living cercarial stage under elevated temperature and pCO₂, focussing our research on functional life span (the time cercariae spend actively swimming) and absolute life span (the period before death). We found that the effects of temperature and pCO₂ were complex and interactive. Overall, increased temperature negatively affected functional and absolute life span, e.g. across all pCO₂ treatments the average time to 50% cessation of active swimming was approximately 8 h at 5 °C, 6 h at 15 °C, 4 h at 25 °C, and 2 h at 40 °C. The effect of pCO₂, which significantly affected absolute life span, was highly variable across temperature treatments. These results strongly suggest that ocean acidification and warming may alter the transmission success of trematode cercariae, and potentially reduce the input of cercariae to marine zooplankton. Either outcome could substantially alter the community structure of coastal marine systems.     

Trophodynamics and functional feeding groups of North Sea fauna: a combined stable isotope and fatty acid approach

The trophodynamics of pelagic and benthic animals of the North Sea, North Atlantic shelf, were assessed using stable isotope analysis (SIA) of natural abundance carbon and nitrogen isotopes, lipid fingerprinting and compound-specific SIA (CSIA) of phospholipid-derived fatty acids (PLFAs). Zooplankton (z), epi- and supra-benthic macrofauna were collected in the Southern Bight, at the Oyster Grounds and at North Dogger, 111 km north of the Dogger Bank. The study included 22 taxonomic groups with particular reference to Mollusca (Bivalvia and Gastropoda) and Crustacea. Primary consumers (Bivalvia) were overall most ¹⁵N enriched in the southern North Sea (6.1‰) and more depleted in the Oyster Grounds (5.5‰) and at North Dogger (2.8‰) demonstrating differences in isotopic baselines for bivalve fauna between the study sites. Higher trophic levels also followed this trend. Over an annual cycle, consumers tended to exhibit ¹⁵N depletion during spring followed by ¹⁵N enriched signatures in autumn and winter. The observed seasonal changes of δ ¹⁵N were more pronounced for suspension feeders and deposit feeders (dfs) than for filter feeders (ffs). The position of animals in plots of δ ¹³C and δ ¹⁵N largely concurred with the expected position according to literature-based functional feeding groups. PLFA fingerprints of groups such as z were distinct from benthic groups, e.g. benthic ffs and dfs, and predatory macrobenthos. δ ¹³C_PLFA signatures indicated similarities in ¹³C moiety sources that constituted δ ¹³C_PLFA. Although functional groups of pelagic zooplankton and (supra-) benthic animals represented phylogenetically distinct consumer groups, δ ¹³C_PLFA demonstrated that both groups were supported by pelagic primary production and relied on the same macronutrients such as PLFAs. Errors related to the static categorization of small invertebrates into fixed trophic positions defined by phylogenetic groupings rather than by functional feeding groups, and information on seasonal trophodynamic variability, may have implications for the reliability of numerical marine ecosystem models.     

Identifying critical habitat of the endangered vaquita (Phocoena sinus) with regional δ13C and δ15N isoscapes of the Upper Gulf of California,Mexico

The vaquita (Phocoena sinus) is the world's most endangered cetacean and has experienced a 60% reduction in the size of its population in the past decade. Knowledge of its basic ecology is essential for developing successful management plans to protect and conserve this species. In this study, we identified vaquita foraging areas by creating an isoscape of the Upper Gulf of California (UGC) based on sediment and zooplankton carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values. Our results confirm that this species is confined to the western region of the UGC, which is characterized by relatively high δ¹⁵N values (sediments: 10.2‰ ± 2.0‰, zooplankton: 15.8‰ ± 1.3‰), higher sea surface temperatures (～16°C–25°C), higher concentrations of silt in sediments, and the highest turbidity. In contrast, the eastern region of the UGC had relatively low sediment (7.7‰ ± 2.4‰) and zooplankton (14.6‰ ± 1.0‰) δ¹⁵N values, and the highest concentrations of sand in sediments. Our approach is an effective use of marine isoscapes over a small spatial scale (<200 km) to identify the environmental characteristics that define the critical habitat for an extremely endangered marine mammal.     

Ocean acidification and hypoxia alter organic carbon fluxes in marine soft sediments

Anthropogenic stressors can alter the structure and functioning of infaunal communities, which are key drivers of the carbon cycle in marine soft sediments. Nonetheless, the compounded effects of anthropogenic stressors on carbon fluxes in soft benthic systems remain largely unknown. Here, we investigated the cumulative effects of ocean acidification (OA) and hypoxia on the organic carbon fate in marine sediments, through a mesocosm experiment. Isotopically labelled macroalgal detritus (¹³C) was used as a tracer to assess carbon incorporation in faunal tissue and in sediments under different experimental conditions. In addition, labelled macroalgae (¹³C), previously exposed to elevated CO₂, were also used to assess the organic carbon uptake by fauna and sediments, when both sources and consumers were exposed to elevated CO₂. At elevated CO₂, infauna increased the uptake of carbon, likely as compensatory response to the higher energetic costs faced under adverse environmental conditions. By contrast, there was no increase in carbon uptake by fauna exposed to both stressors in combination, indicating that even a short‐term hypoxic event may weaken the ability of marine invertebrates to withstand elevated CO₂ conditions. In addition, both hypoxia and elevated CO₂ increased organic carbon burial in the sediment, potentially affecting sediment biogeochemical processes. Since hypoxia and OA are predicted to increase in the face of climate change, our results suggest that local reduction of hypoxic events may mitigate the impacts of global climate change on marine soft‐sediment systems.     

Poles Apart: The “Bipolar” Pteropod Species Limacina helicina Is Genetically Distinct Between the Arctic and Antarctic Oceans

The shelled pteropod (sea butterfly) Limacina helicina is currently recognised as a species complex comprising two sub-species and at least five “forma”. However, at the species level it is considered to be bipolar, occurring in both the Arctic and Antarctic oceans. Due to its aragonite shell and polar distribution L. helicina is particularly vulnerable to ocean acidification. As a key indicator of the acidification process, and a major component of polar ecosystems, L. helicina has become a focus for acidification research. New observations that taxonomic groups may respond quite differently to acidification prompted us to reassess the taxonomic status of this important species. We found a 33.56% (±0.09) difference in cytochrome c oxidase subunit I (COI) gene sequences between L. helicina collected from the Arctic and Antarctic oceans. This degree of separation is sufficient for ordinal level taxonomic separation in other organisms and provides strong evidence for the Arctic and Antarctic populations of L. helicina differing at least at the species level. Recent research has highlighted substantial physiological differences between the poles for another supposedly bipolar pteropod species, Clione limacina. Given the large genetic divergence between Arctic and Antarctic L. helicina populations shown here, similarly large physiological differences may exist between the poles for the L. helicina species group. Therefore, in addition to indicating that L. helicina is in fact not bipolar, our study demonstrates the need for acidification research to take into account the possibility that the L. helicina species group may not respond in the same way to ocean acidification in Arctic and Antarctic ecosystems.     

Climate change and ocean acidification effects on seagrasses and marine macroalgae

Although seagrasses and marine macroalgae (macro‐autotrophs) play critical ecological roles in reef, lagoon, coastal and open‐water ecosystems, their response to ocean acidification (OA) and climate change is not well understood. In this review, we examine marine macro‐autotroph biochemistry and physiology relevant to their response to elevated dissolved inorganic carbon [DIC], carbon dioxide [CO₂], and lower carbonate [CO₃^2?] and pH. We also explore the effects of increasing temperature under climate change and the interactions of elevated temperature and [CO₂]. Finally, recommendations are made for future research based on this synthesis. A literature review of >100 species revealed that marine macro‐autotroph photosynthesis is overwhelmingly C₃ (≥ 85%) with most species capable of utilizing HCO₃^?; however, most are not saturated at current ocean [DIC]. These results, and the presence of CO₂‐only users, lead us to conclude that photosynthetic and growth rates of marine macro‐autotrophs are likely to increase under elevated [CO₂] similar to terrestrial C₃ species. In the tropics, many species live close to their thermal limits and will have to up‐regulate stress‐response systems to tolerate sublethal temperature exposures with climate change, whereas elevated [CO₂] effects on thermal acclimation are unknown. Fundamental linkages between elevated [CO₂] and temperature on photorespiration, enzyme systems, carbohydrate production, and calcification dictate the need to consider these two parameters simultaneously. Relevant to calcifiers, elevated [CO₂] lowers net calcification and this effect is amplified by high temperature. Although the mechanisms are not clear, OA likely disrupts diffusion and transport systems of H⁺ and DIC. These fluxes control micro‐environments that promote calcification over dissolution and may be more important than CaCO₃ mineralogy in predicting macroalgal responses to OA. Calcareous macroalgae are highly vulnerable to OA, and it is likely that fleshy macroalgae will dominate in a higher CO₂ ocean; therefore, it is critical to elucidate the research gaps identified in this review.     

Methanogenesis in Eocene Arctic soils inferred from δC of tree fossil carbonates

We report on the carbon and oxygen stable isotope composition of fossil tree material collected at the White Mountain locality of the Buchanan Lake Formation on Axel Heiberg Island in the High Arctic of Canada. The fossils are Middle Eocene in age and have been permineralized with carbonate. Microscopic examination of fossils revealed them to be the remains of Metasequoia stems, composed of secondary carbonate (calcite) and original wood intermingled at the cellular level. Because the specimens show little compression, crushing, or tissue degradation, we believe that carbonate permineralization occurred soon after burial, and therefore provides insight into Eocene carbon cycling at the locality. The carbon isotope signature of the carbonate suggests that methanogenesis resulted in a ¹³C-enriched CO₂ pool that equilibrated with soil water and gave rise to unusually ¹³C-enriched CaCO₃. Tree fossil carbonate exhibited strikingly high δ¹³C values (+4.0 to +7.4‰) compared to published Phanerozoic pedogenic carbonate δ¹³C values. These δ¹³C values, in conjunction with fractionation factors (α) previously determined for carbonate precipitation and methanogenic pathways, indicate an Eocene soil CO₂ pool containing 80-95% CO₂ produced as a by-product of acetate-fermentation methanogenesis. Because methane in the atmosphere is a powerful greenhouse gas, we suggest that methane emissions from Axel Heiberg soils contributed to the relatively warm Arctic climate during the Middle Eocene.     

Terrestrial and marine trophic pathways support young-of-year growth in a nearshore Arctic fish

River discharge supplies nearshore communities with a terrestrial carbon source that is often reflected in invertebrate and fish consumers. Recent studies in the Beaufort Sea have documented widespread terrestrial carbon use among invertebrates, but only limited use among nearshore fish consumers. Here, we examine the carbon source and diet of rapidly growing young-of-year Arctic cisco (Coregonus autumnalis) using stable isotope values (δ¹³C and δ¹⁵N) from muscle and diet analysis (stomach contents) during a critical and previously unsampled life stage. Stable isotope values (δ¹⁵N and δ¹³C) may differentiate between terrestrial and marine sources and integrate over longer time frames (weeks). Diet analysis provides species-specific information, but only from recent foraging (days). Average δ¹³C for all individuals was ?25.7 ‰, with the smallest individuals possessing significantly depleted δ¹³C values indicative of a stronger reliance of terrestrial carbon sources as compared to larger individuals. Average δ¹⁵N for all individuals was 10.4 ‰, with little variation among individuals. As fish length increased, the proportion of offshore Calanus prey and neritic Mysis prey increased. Rapid young-of-year growth in Arctic cisco appears to use terrestrial carbon sources obtained by consuming a mixture of neritic and offshore zooplankton. Shifts in the magnitude or phenology of river discharge and the delivery of terrestrial carbon may alter the ecology of nearshore fish consumers.     

Variation in bulk tissue, fatty acid and monosaccharide δ13C values between autotrophic and heterotrophic plant organs

The flowers of 23 species of grass and herb plants were collected from a mesotrophic grassland to assess natural variability in bulk, monosaccharide and fatty acid δ¹³C values from one plant community and were compared with previous analyses of leaves from the same species. The total mean bulk δ¹³C value of flower tissues was −28.1‰, and there was no significant difference between the mean δ¹³C_flower values for grass (−27.8‰) and herb (−28.2‰) species. On average bulk δ¹³C_flower values were 1.1‰ higher than bulk δ¹³C_leaf values, however, the δ¹³C_flower and δ¹³C_leaf values of grasses did not differ between organs suggesting that carbon isotope discrimination is different in grass and herb species. The abundance of different monosaccharides abundance varied between plant types, i.e. xylose concentrations in the grass flowers were as high as 40%, compared with up to 15% in the herb species, but the general relationship δ¹³C_arabinose > δ¹³C_xylose > δ¹³C_glucose > δ¹³C_galactose which had been observed in leaves was similar in flowers (total mean δ¹³C values = −25.9‰, −27.2‰, −28.8‰ and −28.1‰, respectively). However, the average 5.4‰ depletion in the δ¹³C values of the C_16:0, C_18:2 and C_18:3 fatty acids in flowers compared to bulk tissue was significantly greater than observed for leaves. The trend C_16:0 < C_18:2 < C_18:3 previously observed in leaves was also observed in grass flowers (δ¹³C_C16:0 = −33.8‰; δ¹³C_C18:2 = −33.1‰; δ¹³C_C18:3 = −34.2‰) but not herb flowers (δ¹³C_C16:0 = −34.1‰; δ¹³C_C18:2 = −32.4‰; δ¹³C_C18:3 = −34.5‰). We conclude: (i) that the biological processes influencing carbon isotope discrimination in grass flowers are different from herbs flowers; and, (ii) that a range of post-photosynthetic fractionation effects caused the observed differences between flower and leaf δ¹³C values, especially the significant ¹³C-depletion in flower fatty acid δ¹³C values.     

Effects of ocean acidification on the growth and biochemical composition of a green alga (Ulva fasciata) and its associated microbiota

In marine ecosystems, fluctuations in surface-seawater carbon dioxide (CO₂), significantly influence the whole metabolism of marine algae, especially during the early stages of macroalgal development. In this study, the response of the green alga Ulva fasciata for elevating ocean acidification was investigated using four levels of pCO₂ ~ 280, 550, 750 and 1050 µatm. Maximum growth rate (6.6% day⁻¹), protein (32.43 %DW) and pigment (2.9 mg/g) accumulation were observed at pCO₂-550 with an increase of ~2-fold compared to control. On the other hand, lipid and carbohydrate contents recorded their maximum production (4.23 and 46.96 %DW, respectively) at pCO₂-750 while control showed 3.70 and 42.37 %DW, respectively. SDS-PAGE showed the presence of unique bands in response to pCO_2, especially at 550 µatm. Dominant associated bacteria was shifted from Halomonas hydrothermalis of control to Vibrio toranzoniae at pCO₂-1050. These findings suggest that ocean acidification at 550 µatm might impose noticeable effects on growth, protein, pigments, and protein profile of U. fasciata, which could be a good source for fish farming. While, pCO₂-750 was recommended for energetic purpose, due to its high lipid and carbohydrate contents.     

Stable isotope composition of Late Cretaceous benthic foraminifera from the southern South Atlantic: Biological and environmental effects

The stable carbon and oxygen isotope composition of different benthic foraminiferal species of the latest Campanian and earliest Maastrichtian from Ocean Drilling Project Hole 690C (Weddell Sea, southern South Atlantic, ∼1800 m paleowater depth) have been investigated. The total range of measured isotope values of all samples exceeds ∼4‰ for δ¹³C and 1.1‰ for δ¹⁸O. Carbon isotope values of proposed deep infaunal species are generally similar or only slightly lower when compared to proposed epifaunal to shallow infaunal species. Interspecific differences vary between samples probably reflecting temporal changes in organic carbon fluxes to the sea floor. Constantly lower δ¹³C values for Pullenia marssoni and Pullenia reussi suggest the deepest habitat for these species. The strong depletion of δ¹³C values by up to 3‰ within lenticulinids may be attributed to a deep infaunal microhabitat, strong vital effects, or different feeding strategy when compared to other species or modern lenticulinids. The mean δ¹⁸O values reveal a strong separation of epifaunal to shallow infaunal and deep infaunal species. Epifaunal to shallow infaunal species are characterized by low δ¹⁸O values, deep infaunal species by higher values. This result possibly reflects lower metabolic rates and longer life cycles of deep infaunal species or the operating of a pore water [CO₃²⁻] effect on the benthic foraminiferal stable isotopes.Pyramidina szajnochae shows an enrichment of oxygen isotopes with test size comprising a total of 0.6‰ between 250 and 1250 μm shell size. Although δ¹³C lacks a corresponding trend these data likely represent the presence of changes in metabolic rates during ontogenesis. These results demonstrate the general applicability of multi-species stable isotope measurements of pristine Cretaceous benthic foraminifera to reconstruct past microhabitats and to evaluate biological and environmental effects on the stable isotope composition.     

Ocean acidification impairs crab foraging behaviour

Anthropogenic elevation of atmospheric CO₂ is driving global-scale ocean acidification, which consequently influences calcification rates of many marine invertebrates and potentially alters their susceptibility to predation. Ocean acidification may also impair an organism''s ability to process environmental and biological cues. These counteracting impacts make it challenging to predict how acidification will alter species interactions and community structure. To examine effects of acidification on consumptive and behavioural interactions between mud crabs (Panopeus herbstii) and oysters (Crassostrea virginica), oysters were reared with and without caged crabs for 71 days at three pCO₂ levels. During subsequent predation trials, acidification reduced prey consumption, handling time and duration of unsuccessful predation attempt. These negative effects of ocean acidification on crab foraging behaviour more than offset any benefit to crabs resulting from a reduction in the net rate of oyster calcification. These findings reveal that efforts to evaluate how acidification will alter marine food webs should include quantifying impacts on both calcification rates and animal behaviour.     

Tracking seasonal changes in North Sea zooplankton trophic dynamics using stable isotopes

Trophodynamics of meso-zooplankton in the North Sea (NS) were assessed at a site in the southern NS, and at a shallow and a deep site in the central NS. Offshore and neritic species from different ecological niches, including Calanus spp., Temora spp. and Sagitta spp., were collected during seven cruises over 14 months from 2007 to 2008. Bulk stable isotope (SI) analysis, phospholipid-derived fatty acid (PLFA) compositions, and δ ¹³C_PLFA data of meso-zooplankton and particulate organic matter (POM) were used to describe changes in zooplankton relative trophic positions (RTPs) and trophodynamics. The aim of the study was to test the hypothesis that the RTPs of zooplankton in the North Sea vary spatially and seasonally, in response to hydrographic variability, with the microbial food web playing an important role at times. Zooplankton RTPs tended to be higher during winter and lower during the phytoplankton bloom in spring. RTPs were highest for predators such as Sagitta sp. and Calanus helgolandicus and lowest for small copepods such as Pseudocalanus elongatus and zoea larvae (Brachyura). δ ¹⁵N_POM-based RTPs were only moderate surrogates for animals’ ecological niches, because of the plasticity in source materials from the herbivorous and the microbial loop food web. Common (16:0) and essential (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA) structural lipids showed relatively constant abundances. This could be explained by incorporation of PLFAs with δ ¹³C signatures which followed seasonal changes in bulk δ ¹³C_POM and PLFA δ ¹³C_POM signatures. This study highlighted the complementarity of three biogeochemical approaches for trophodynamic studies and substantiated conceptual views of size-based food web analysis, in which small individuals of large species may be functionally equivalent to large individuals of small species. Seasonal and spatial variability was also important in altering the relative importance of the herbivorous and microbial food webs.     

Evaluation of commercial and natural food in experimental cultures of white shrimp based on stable carbon isotopes

The farming of shrimp is developing quickly worldwide, and recently, ingredients such as seaweeds in low proportion (25 to 4 %), incorporated into the commercial food, have been shown to improve the shrimp productive variables. The change of commercial foods to commercial feed with a proportion of natural food, and finally, to natural food has been little and simultaneously evaluated. The aim of our study was to determine the relative contribution of dietary carbon to the growth of Litopenaeus vannamei fed with a proportion of 4 % Sargassum (δ¹³C = ?20.9?±?0.05?‰), 4 % Ulva (δ¹³C = ?20.6?±?0.6?‰) meal, and a control diet (δ¹³C?=??22.6?±?0.2?‰) in 60-L tanks for 30 days, and finally, with the green seaweed Ulva spp. (δ¹³C = ?13.2?±?0.25?‰) and Ulva meal (δ¹³C = ?14.5?±?0.6?‰) in open-air ponds for 120 days, by measuring δ¹³C for each of the foods and in the muscle of shrimp. After 15 days, the rates of metabolic turnover (Δ¹³C = δ¹³C_shrimp ? δ¹³C_food) were constant until the end of the experiment in the tanks. The muscle of shrimp assimilated carbon from all diets, which demonstrated the potential use of combined diets and the optimization of their use in diets containing seaweed. Our data will be useful in future interpretations of field and laboratory isotopic values for this species.     

Ocean acidification refugia in variable environments

Climate change refugia in the terrestrial biosphere are areas where species are protected from global environmental change and arise from natural heterogeneity in landscapes and climate. Within the marine realm, ocean acidification, or the global decline in seawater pH, remains a pervasive threat to organisms and ecosystems. Natural variability in seawater carbon dioxide (CO₂) chemistry, however, presents an opportunity to identify ocean acidification refugia (OAR) for marine species. Here, we review the literature to examine the impacts of variable CO₂ chemistry on biological responses to ocean acidification and develop a framework of definitions and criteria that connects current OAR research to management goals. Under the concept of managing vulnerability, the most likely mechanisms by which OAR can mitigate ocean acidification impacts are by reducing exposure to harmful conditions or enhancing adaptive capacity. While local management options, such as OAR, show some promise, they present unique challenges, and reducing global anthropogenic CO₂ emissions must remain a priority.     

Does Drought Influence the Relationship Between Biodiversity and Ecosystem Functioning in Boreal Forests?

In mixed forests, interactions among species influence ecosystem functioning but environmental conditions also play an important role in shaping relationships between biodiversity and ecosystem functioning. In the context of climate change, the carbon and water balance in pure versus mixed forest stands may be differentially influenced by changing soil water availability. To test this hypothesis, we compared the influence of biodiversity on stand water use efficiency (WUE_S) in boreal forests between wet and dry years. We assessed the carbon isotope composition (δ ¹³C) of tree rings in Betula pendula, Pinus sylvestris, and Picea abies growing in pure versus mixed stands. In addition, we tested whether differences in WUE_S affected patterns of stand basal area increment (BAI_S). No biodiversity effect was found for stand δ ¹³C (δ ¹³C_S) during the wet year. However, there was a significant increase in δ ¹³C_S between the wet and the dry year and a significant effect of biodiversity on δ ¹³C_S in the dry year. The increase in δ ¹³C_S in mixed stands was associated with both selection and complementarity effects. Although BAI_S decreased significantly in the dry year, changes in δ ¹³C_S did not translate into variations in BAI_S along the biodiversity gradient. Our results confirmed that the physiological response of boreal forest ecosystems to changing soil water conditions is influenced by species interactions and that during dry growing seasons, species interactions in mixed stands can lead to lower soil moisture availability. This illustrates that biodiversity effects can also be negative in mixed stands in the sense that soil resources can be more intensively exhausted. Overall, our results confirm that in boreal forests, the biodiversity–ecosystem functioning relationship depends on local environmental conditions.