Macroalgae δ 15 N values in well-mixed estuaries: indicator of anthropogenic nitrogen input or macroalgae metabolism?

International audienceAlthough nitrogen stable isotope ratio (d15N) in macroalgae is widely used as a bioindicator of anthropogenic nitrogen inputs to the coastal zone, recent studies suggest the possible role of macroalgae metabolism in d15N variability. Simultaneous determinations of d15N of dissolved inorganic nitrogen(DIN) along the landesea continuum, inter-species variability of d15N and its sensitivity to environmental factors are necessary to confirm the efficiency of macroalgae d15N in monitoring nitrogen origin in mixed-use watersheds. In this study, d15N of annual and perennial macroalgae (Ulva sp., Enteromorpha sp., Fucus vesiculosus and Fucus serratus) are compared to d15N-DIN along the Charente Estuary, after characterizing d15N of the three main DIN sources (i.e. cultivated area, pasture, sewage treatment plant outlet). During late winter and spring, when human activities produce high DIN inputs, DIN sources exhibit distinct d15N signals in nitrate (NO3-) and ammonium (NH4+): cultivated area (+6.5 ± 0.6 ‰ and +9.0 ± 11.0 ‰), pasture (+9.2 ± 1.8 ‰ and +12.4 ‰) and sewage treatment plant discharge (+16.9 ± 8.7 ‰ and +25.4 ± 5.9 ‰). While sources show distinct d15N - NO3- in this multiple source catchment, the overall mixture of NO3- sources - generally >95% DIN - leads to low variations of d15N - NO3- at the mouth of the estuary (+7.7 to +8.4 ‰). Even if estuarine d15N - NO3- values are not significantly different from pristine continental and oceanic site (+7.3 ‰ and +7.4 ‰), macroalgae d15N values are generally higher at the mouth of the estuary. This highlights high anthropogenic DIN inputs in the estuary, and enhanced contribution of 15N-depleted NH4+ in oceanic waters. Although seasonal variations in d15N - NO3- are low, the same temporal trends in macroalgae d15N values at estuarine and oceanic sites, and inter-species differences in d15N values, suggest that macroalgae d15N values might bemodified by the metabolic response of macroalgae to environmental parameters (e.g., temperature, light, DIN concentrations). Differences between annual and perennial macroalgae indicate both a higher integration time of perennial compared to annual macroalgae and the possible role of passive versus active uptake mechanisms. Further studies are required to characterize the sensitivity of macroalgae fractionation to variable environmental conditions and uptake mechanisms

Dynamics and sources of suspended particulate organic matter in the Marennes-Oléron oyster farming bay: Insights from stable isotopes and microalgae ecology

International audienceThe aim of this study was to distinguish between sources of the complex variety of Marennes-Oléron Bay suspended particulate organic matter (SPOM) contributing to the tropho-dynamics of the Marennes-Oléron oyster farming bay. Basic biomarkers (Chl a, C/N and POC/Chl a ratios), carbon and nitrogen stable isotopes from SPOM were analyzed and the microalgae community was characterized. The sampling strategy was bimonthly from March 2002 to December 2003; samples were taken from an intertidal mudflat. Four main sources contributed to the SPOM pool: terrigenous input from rivers, neritic phytoplankton, resuspended microphytobenthos and periodic inputs from intertidal Zostera noltii meadows. Seasonal fluctuations were observed in both years of the study period: (1) SPOM collected in the spring of 2002 (δ13C = −25‰ to −23‰) was mainly composed of fresh estuarine inputs; (2) SPOM from the summer and fall of 2002 and 2003 was predominantly neritic phytoplankton (δ13C = − 22‰ to −19‰); (3) SPOM from the winter of 2002, spring of 2003 and winter of 2003 (δ13C = −21 to −23‰) was composed of a mixture of decayed terrigenous river inputs and pelagic phytoplankton, which was predominantly resuspended microphytobenthos. In the summer of 2003—the warmest summer on record in southern France and Europe—SPOM was particularly enriched for 13C, with δ13C values ranging from −14‰ to −12‰. Pulses in δ13C values, indicative of 13C-enriched decaying materials, extended into the fall. These were attributed to benthic intertidal inputs, including both resuspended microphytobenthos and Z. noltii detritus. Changes in SPOM sources in Marennes-Oléron Bay may lead to differences in the quality of the trophic environment available for reared oysters

Stable isotope compositions (δ¹³C and δ¹⁵N) of seston measured on water bottle samples from CTD/Large volume Water-sampler-system during POLARSTERN cruise PS100 (ARK-XXX/2)

Seston samples collected with Niskin bottles; 2 L filtered onto precombusted 25mm (Ø) GF/F filters. δ¹³C and δ¹⁵N stable isotopes analyzed with elemental analyzer (Thermo Flash 2000) coupled to an isotope-ratio mass spectrometer (Thermo, Delta V Plus). Sample preparation for IRMS analysis (BSIA) were done according to the LIENSs SOPs. (see: https://lienss.univ-larochelle.fr/IMG/pdf/lienss_sif_appendix_2_protocols_v3-8.pdf) Devices (LIENSs Stable Isotope Facility, La Rochelle, France): Elemental analyzer: Flash 2000, Thermo Scientific, Milan, Italy. Isotope ratio mass spectrometer: Delta V Plus with a Conflo IV interface, Thermo Scientific, Bremen, Germany. Reference materials for calibration: δ¹³C: USGS-24, IAEA-CH6, IAEA-600, USGS-61, USGS-62. δ¹⁵N: IAEA-N2, IAEA-NO-3, IAEA-600, USGS-61, USGS-62. Working standards (two-point calibration): %C, %N, δ¹³C and δ¹⁵N: USGS-61 (Caffeine) and USGS-62 (Caffeine) Analytical precision: δ¹³C: <0.10 ‰ (Thermo Scientific specifications). δ¹⁵N: <0.10 ‰ (Thermo Scientific specifications). Results are expressed in the δ unit notation as deviations from standards (Vienna Pee Dee Belemnite for δ¹³C and N₂ in air for δ¹⁵N) following the formula: δ¹³C or δ¹⁵N = [(Rsample/Rstandard) - 1] x 10³, where R is ¹³C/¹²C or ¹⁵N/¹⁴N, respectively

Effect of seasonal variation in trophic conditions and the gametogenic cycle on δ13C and δ15N levels of diploid and triploid Pacific oysters <I>Crassostrea gigas</I>

International audienceCarbon and nitrogen stable isotopes were investigated in separate organs of diploid and sterile triploid Pacific oysters Crassostrea gigas for 13 mo, together with changes in chemical and isotope composition of suspended matter sampled from an intertidal mudflat within Marennes-Oléron Bay, France. Particulate organic matter (POM) was a mixture of pelagic and benthic material with a predominance of neritic phytoplankton in spring, and resuspended microphytobenthos in summer and autumn. A remarkable shift of +3‰ in δ13C was reflected in both diploids and triploids from spring to summer, and further temporal differences were observed amongst their tissues. Seasonal changes in POM δ15N were also reflected in oyster tissues, with digestive gland and muscle tissues showing the largest and the least variability, respectively. Use of δ13C and C:N ratio relationships in separate tissues allowed for an assessment of the influences of trophic condition, seasonal changes, and gametogenic cycle on tissue δ13C. Diploid digestive gland δ13C matched those of gonads, and differences between diploids and triploids in digestive gland and mantle δ13C were less than –1‰ during gametogenesis. The reproductive and rest periods were easily distinguished in these tissues and were characterised by enriched δ13C values in summer–autumn compared with spring, which is consistent with POM δ13C seasonal changes. A similar trend was observed in muscle, with a preferential incorporation of 13C-enriched carbon during the summer–autumn growing season. However, despite the similar roles of mantle and digestive gland in lipid synthesis in both diploids and triploids, the correlation of δ13C with the C:N ratio highlighted the transfer of lipids to gonads in diploids and their differential allocation to growing tissues in sterile triploids

Major Sources of Organic Matter in a Complex Coral Reef Lagoon: Identification from Isotopic Signatures (δ13C and δ15N)

International audienceA wide investigation was conducted into the main organic matter (OM) sources supporting coral reef trophic networks in the lagoon of New Caledonia. Sampling included different reef locations (fringing, intermediate and barrier reef), different associated ecosystems (mangroves and seagrass beds) and rivers. In total, 30 taxa of macrophytes, plus pools of particulate and sedimentary OM (POM and SOM) were sampled. Isotopic signatures (C and N) of each OM sources was characterized and the composition of OM pools assessed. In addition, spatial and seasonal variations of reef OM sources were examined. Mangroves isotopic signatures were the most C-depleted (-30.17 ± 0.41 ‰) and seagrass signatures were the most C-enriched (-4.36 ± 0.72 ‰). Trichodesmium spp. had the most N-depleted signatures (-0.14 ± 0.03 ‰) whereas mangroves had the most N-enriched signatures (6.47 ± 0.41 ‰). The composition of POM and SOM varied along a coast-to-barrier reef gradient. River POM and marine POM contributed equally to coastal POM, whereas marine POM represented 90% of the POM on barrier reefs, compared to 10% river POM. The relative importance of river POM, marine POM and mangroves to the SOM pool decreased from fringing to barrier reefs. Conversely, the relative importance of seagrass, Trichodesmium spp. and macroalgae increased along this gradient. Overall, spatial fluctuations in POM and SOM were much greater than in primary producers. Seasonal fluctuations were low for all OM sources. Our results demonstrated that a large variety of OM sources sustain coral reefs, varying in their origin, composition and role and suggest that δ13C was a more useful fingerprint than δ15N in this endeavour. This study also suggested substantial OM exchanges and trophic connections between coral reefs and surrounding ecosystems. Finally, the importance of accounting for environmental characteristics at small temporal and spatial scales before drawing general patterns is highlighted

Trophic niche overlap between sympatric harbour seals(Phoca vitulina) and grey seals (Halichoerus grypus) at thesouthern limit of their European range (Eastern EnglishChannel)

International audienceSympatric harbour (Phoca vitulina) and grey seals (Halichoerus grypus) are increasinglyconsidered potential competitors, especially since recent local declines in harbourseal numbers while grey seal numbers remained stable or increased at their Europeancore distributions. A better understanding of the interactions between these speciesis critical for conservation efforts. This study aimed to identify the trophic nicheoverlap between harbour and grey seals at the southern limit of their Europeanrange, in the Baie de Somme (BDS, Eastern English Channel, France), where numbersof resident harbour seals and visiting grey seals are increasing exponentially. Dietaryoverlap was identified from scat contents using hierarchical clustering. Isotopic nicheoverlap was quantified using δ13C and δ15N isotopic values from whiskers of 18 individuals,by estimating isotopic standard ellipses with a novel hierarchical modeldeveloped in a Bayesian framework to consider both intraindividual variability andinterindividual variability. Foraging areas of these individuals were identified fromtelemetry data. The three independent approaches provided converging results, revealinga high trophic niche overlap due to consumption of benthic flatfish. Two dietclusters were dominated by either small or large benthic flatfish; these comprised85.5% [CI95%: 80.3%–90.2%]of harbour seal scats and 46.8% [35.1%–58.4%]of greyseal scats. The narrower isotopic niche of harbour seals was nested within that ofgrey seals (58.2% [22.7%–100%]overlap). Grey seals with isotopic values similar toharbour seals foraged in coastal waters close to the BDS alike harbour seals did, suggestingthe niche overlap may be due to individual grey seal strategies. Our findingstherefore provide the basis for potential competition between both species (foragingon benthic flatfish close to the BDS). We suggest that a continued increase in sealnumbers and/or a decrease in flatfish supply in this area could cause/amplify competitiveinteractions and have deleterious effects on harbour seal colonies

Hg-Stable Isotope Variations in Marine Top Predators of the Western Arctic Ocean

International audienceRecent studies on mercury (Hg)-stable isotopes in Alaskan seabird eggs and ringed seal livers illustrated the control of sea ice cover on marine methyl-Hg photochemistry. Here, complementary marine mammal tissues have been analyzed to document variations in Hg-, carbon (C)-, and nitrogen (N)-stable isotope compositions of Arctic marine food webs. Hg-stable isotope ratios were measured in liver samples of 55 beluga whales (Delphinapterus leucas) and 15 polar bears (Ursus maritimus) collected since 1990. Large variations in δ202Hg (≈2.1‰) and Δ199Hg (≈1.7‰) are observed between species and within species stocks covering the Gulf of Alaska-Bering Sea-Arctic Ocean regions. Polar bears, mainly feeding on ringed seal (δ15N shift of 4.2‰), show identical liver Δ199Hg of 0.5‰, confirming the absence of metabolic mass-independent fractionation, and 0.33 ± 0.11‰ enrichment in heavy Hg isotopes. Beluga whale liver total Hg concentrations increase with age, reflecting lifetime bioaccumulation, while Hg speciation shifts to inorganic Hg with age as a result of hepatic methyl-Hg breakdown. Δ200Hg variations in biota show a small, 0.1‰ decrease from North Pacific Ocean to Arctic Ocean habitats, suggesting atmospheric Hg deposition to be important in the Pacific and terrestrial Hg inputs to dominate in the Arctic Ocean. Similar to seabird eggs, a consistent south to north gradient in Δ199Hg baseline is seen in mammal liver tissues, confirming sea ice cover as a control factor on marine Hg photoreduction and Δ199Hg. Arctic Ocean beluga whales have near zero Δ199Hg, indicating that terrestrial Hg and in-situ-produced methyl-Hg are not measurably photoreduced in the Arctic Ocean before entering the marine food we