Trophic ecology and vertical patterns of carbon and nitrogen stable isotopes in zooplankton from oxygen minimum zone regions

The unique physical and biogeochemical characteristics of oxygen minimum zones (OMZs) influence plankton ecology, including zooplankton trophic webs. Using carbon and nitrogen stable isotopes, this study examined zooplankton trophic webs in the Eastern Tropical North Pacific (ETNP) OMZ. δ¹³C values were used to indicate zooplankton food sources, and δ¹⁵N values were used to indicate zooplankton trophic position and nitrogen cycle pathways. Vertically stratified MOCNESS net tows collected zooplankton from 0 to 1000 m at two stations along a north-south transect in the ETNP during 2007 and 2008, the Tehuantepec Bowl and the Costa Rica Dome. Zooplankton samples were separated into four size fractions for stable isotope analyses. Particulate organic matter (POM), assumed to represent a primary food source for zooplankton, was collected with McLane large volume in situ pumps.The isotopic composition and trophic ecology of the ETNP zooplankton community had distinct spatial and vertical patterns influenced by OMZ structure. The most pronounced vertical isotope gradients occurred near the upper and lower OMZ oxyclines. Material with lower δ¹³C values was apparently produced in the upper oxycline, possibly by chemoautotrophic microbes, and was subsequently consumed by zooplankton. Between-station differences in δ¹⁵N values suggested that different nitrogen cycle processes were dominant at the two locations, which influenced the isotopic characteristics of the zooplankton community. A strong depth gradient in zooplankton δ¹⁵N values in the lower oxycline suggested an increase in trophic cycling just below the core of the OMZ. Shallow POM (0–110 m) was likely the most important food source for mixed layer, upper oxycline, and OMZ core zooplankton, while deep POM was an important food source for most lower oxycline zooplankton (except for samples dominated by the seasonally migrating copepod Eucalanus inermis). There was no consistent isotopic progression among the four zooplankton size classes for these bulk mixed assemblage samples, implying overlapping trophic webs within the total size range considered.     

Basin-scale distributions of stable nitrogen isotopes in the subtropical North Atlantic Ocean: Contribution of diazotroph nitrogen to particulate organic matter and mesozooplankton

New N inputs via biological N₂-fixation play a critical role in supporting oceanic primary production and influencing global biogeochemical cycles. Numerous studies have documented significant N₂-fixation rates in the North Atlantic, but relatively little is known of the pathways and fluxes of new N through planktonic food webs. Here, we report the natural abundance of ¹⁵N in, and contribution of diazotroph N (N_D) inputs to, suspended particles and mesozooplankton collected along two transects in the subtropical North Atlantic Ocean (STNA). Samples were collected in April-May 2000 along the two main transects to evaluate spatial trends of ¹⁵N within the Western Atlantic and across the basin. We found that N₂-fixation is a key component in supporting both primary and secondary productions throughout the STNA. N_D contribution was generally higher for suspended particles than for mesozooplankton, and we observed a high N_D contribution to suspended particles over large spatial scales in the western and central STNA. Mesozooplankton N_D incorporation suggests that diazotroph production supports oceanic food webs over longer timescales (e.g., weeks to months) than that of particle turnover (e.g., days). Larger mesozooplankton (1000-4000 ??m) generally incorporated more N_D than smaller mesozooplankton (250-1000 ??m), and thus may exert a stronger influence on an N_D movement within the water column. Spatial and vertical patterns of variation in mesozooplankton ??¹⁵N also suggest either broad geographical differences in an N_D contribution to secondary production, or temporal variations in N_D incorporation via mesozooplankton communities.     

Lipid and 13C signatures of submicron and suspended particulate organic matter in the Eastern Tropical North Pacific: Implications for the contribution of Bacteria

The contribution of bacterial biomass to total particulate organic matter (POM) in the ocean, including exported material, is poorly constrained. To examine potential signatures for the presence and export of bacterioplankton and their detrital remains, here we provide a detailed compound-specific ¹³C characterization of fatty acids from membrane polar lipids obtained from a water column profile in the Eastern Tropical North Pacific. POM of submicron size (0.2–0.7 μm; “X-POM”) was sampled and analyzed separately from the size class typically collected as “suspended” POM (0.7–53 μm; “L-POM”). The distributions of polar head group classes, specific fatty acid side chains, and natural ¹³C contents all vary, both between particle size classes and with depth in the water column. In general, the polar lipids in submicron material – and by inference, lipids of bacterial origin – have higher ¹³C content than polar lipids from larger POM and are equally abundant. Lipid signatures from the photic zone appear to be partially conserved in the suspended pool during transit down the water column. However, bacterial heterotrophy and possibly chemoautotrophy partially overprint these surface signatures. In addition, active metabolisms in the oxygen minimum zone (OMZ) appear to mediate the disaggregation of POM transported from the surface, thus adding complexity to the pathways of mid-water carbon flux and providing additional organic substrates to the OMZ and below. This “substrate injection” may provide important fuel for the denitrification and anammox reactions. Finally, examination of ¹³C content in polar lipids provides a basis for new interpretation of depth-related variations in δ¹³C values of bulk suspended POM.     

Feeding ecology of five commercial shark species of the Celtic Sea through stable isotope and trace metal analysis

In order to trace their feeding habits, stable carbon and nitrogen isotope ratios (delta15N and delta13C), as well as trace metal concentrations (Zn, Cd, Fe, Cu, Se and Hg) were analysed in the tissues of five commercial shark species from the Celtic Sea: the tope shark Galeorhinus galeus, the black-mouthed catshark Galeus melastomus, the starry smooth hound Mustelus asterias, the spiny dogfish Squalus acanthias and the lesser-spotted dogfish Scyliorhinus canicula. Our results were compared to previously described stomach contents and isotopic composition of potential preys. Isotopic ratio delta15N suggested that tope sharks fed at a higher trophic level (16.7 per thousand in the muscle) than the other species, reflecting its piscivorous diet. The lower values of spiny dogfish (11.6 per thousand in the muscle) might be explained, amongst other things, by either its migratory behaviour or its preference for preys from lower trophic levels. Cd and Hg were correlated with isotopic ratios delta13C and delta15N, and were shown to be diet-related whereas Zn, Fe and Cu seemed much more linked to species-specific metabolism. Although this multidisciplinary approach is revealed as a useful tool for the study of shark ecology, the lack of known trophic fractionation suggests that isotopic data be compared to traditional diet analyses.     

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

The 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.