Effects of chemical preservation on bulk and amino acid isotope ratios of zooplankton, fish, and squid tissues.

RationaleIt is imperative to understand how chemical preservation alters tissue isotopic compositions before using historical samples in ecological studies. Specifically, although compound-specific isotope analysis of amino acids (CSIA-AA) is becoming a widely used tool, there is little information on how preservation techniques affect amino acid δ15 N values.MethodsWe evaluated the effects of chemical preservatives on bulk tissue δ13 C and δ15 N and amino acid δ15 N values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna (Thunnus albacares) and squid (Dosidicus gigas) muscle tissues that were fixed in formaldehyde and stored in ethanol for 2 years and (b) two copepod species, Calanus pacificus and Eucalanus californicus, which were preserved in formaldehyde for 24-25 years.ResultsTissues in formaldehyde-ethanol had higher bulk δ15 N values (+1.4, D. gigas; +1.6‰, T. albacares), higher δ13 C values for D. gigas (+0.5‰), and lower δ13 C values for T. albacares (-0.8‰) than frozen samples. The bulk δ15 N values from copepods were not different those from frozen samples, although the δ13 C values from both species were lower (-1.0‰ for E. californicus and -2.2‰ for C. pacificus) than those from frozen samples. The mean amino acid δ15 N values from chemically preserved tissues were largely within 1‰ of those of frozen tissues, but the phenylalanine δ15 N values were altered to a larger extent (range: 0.5-4.5‰).ConclusionsThe effects of preservation on bulk δ13 C values were variable, where the direction and magnitude of change varied among taxa. The changes in bulk δ15 N values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation of δ15 N values used in ecological studies. The preservation effects on amino acid δ15 N values were also mostly minimal, mirroring bulk δ15 N trends, which is promising for future CSIA-AA studies of archived specimens. However, there were substantial differences in phenylalanine and valine δ15 N values, which we speculate resulted from interference in the chromatographic resolution of unknown compounds rather than alteration of tissue isotopic composition due to chemical preservation

Quantifying mercury isotope dynamics in captive Pacific Bluefin tuna (Thunnus orientalis)

Analyses of mercury (Hg) isotope ratios in fish tissues are used increasingly to infer sources and biogeochemical processes of Hg in natural aquatic ecosystems. Controlled experiments that can couple internal Hg isotope behavior with traditional isotope tracers (delta C-13, delta N-15) can improve the applicability of Hg isotopes as natural ecological tracers. In this study, we investigated changes in Hg isotope ratios (delta Hg-202, Delta Hg-199) during bioaccumulation of natural diets in the pelagic Pacific bluefin tuna (Thunnus orientalis; PBFT). Juvenile PBFT were fed a mixture of natural prey and a dietary supplement (60% Loligo opalescens, 31% Sardinops sagax, 9% gel supplement) in captivity for 2914 days, and white muscle tissues were analyzed for Hg isotope ratios and compared to time in captivity and internal turnover of delta C-13 and delta N-15. PBFT muscle tissues equilibrated to Hg isotope ratios of the dietary mixture within similar to 700 days, after which we observed a cessation in further shifts in Delta Hg-199, and small but significant negative delta Hg-202 shifts from the dietary mixture. The internal behavior of Delta Hg-199 is consistent with previous fish studies, which showed an absence of Delta Hg-199 fractionation during Hg bioaccumulation. The negative delta Hg-202 shifts can be attributed to either preferential excretion of Hg with higher delta Hg-202 values or individual variability in captive PBFT feeding preferences and/or consumption rates. The overall internal behavior of Hg isotopes is similar to that described for delta C-13 and delta N-15, though observed Hg turnover was slower compared to carbon and nitrogen. This improved understanding of internal dynamics of Hg isotopes in relation to delta C-13 and delta N-15 enhances the applicability of Hg isotope ratios in fish tissues for tracing Hg sources in natural ecosystems.118Nsciescopu

Trophic interactions of megafauna in the Mariana and Kermadec trenches inferred from stable isotope analysis

Hadal trenches house distinct ecosystems but we know little about their sources of nutrition or trophic structures. We evaluated megafaunal food web structure and nutritional sources in the Kermadec and Mariana trenches using carbon and nitrogen stable isotope analysis (δ15N and δ13C values) of bulk tissues and proteinaceous individual amino acids (AAs). In the Kermadec Trench, bulk δ15N values ranged from 5.8‰ in trench sediment to 17.5‰ in tissues of the supergiant amphipod, Allicela gigantea. δ15N values of detritivores were much higher than those of sediments (by 7.5‰ more). The δ13C values ranged from −21.4‰ in sediments to −17.3‰ in the brittle star, Ophiolimna sp., and did not co-vary with δ15N values. In the Mariana Trench, only bait-attending fauna and surface sediments were available for analysis. Mariana Trench fishes, amphipods, and sediments had slightly lower δ15N values than those from the Kermadec Trench, possibly because the Mariana Trench lies under more oligotrophic surface waters. We found evidence for multiple food inputs to the system in each trench, namely substantially higher δ15N values in detritivores relative to sediment and high variability in δ13C values. Trophic levels determined from isotopic analysis of individual AAs in the Kermadec Trench ranged from three for detritivores to five for fishes. Source AA δ15N values were variable (range of ~7.0‰ in average δ15N source AA values), with much of this variation occurring in small amphipods. For the other fauna sampled, there was a significant increase in δ15N source AA values with increasing collection depth. This increase could reflect larger amounts of highly microbially reworked organic matter with increasing depth or sporadic input from turbidity flows. Although further sampling across a broader faunal diversity will be required to understand these food webs, our results provide new insights into hadal trophic interactions and suggest that trench food webs are very dynamic

Long-term trends in the foraging ecology and habitat use of an endangered species: an isotopic perspective.

Evaluating long-term drivers of foraging ecology and population productivity is crucial for providing ecological baselines and forecasting species responses to future environmental conditions. Here, we examine the trophic ecology and habitat use of North Atlantic leatherback turtles (St. Croix nesting population) and investigate the effects of large-scale oceanographic conditions on leatherback foraging dynamics. We used bulk and compound-specific nitrogen isotope analysis of amino acids (CSIA-AA) to estimate leatherback trophic position (TP) over an 18-year period, compare these estimates with TP estimates from a Pacific leatherback population, and elucidate the pre-nesting habitat use patterns of leatherbacks. Our secondary objective was to use oceanographic indices and nesting information from St. Croix leatherbacks to evaluate relationships between trophic ecology, nesting parameters, and regional environmental conditions measured by the North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation. We found no change in leatherback TP over time and no difference in TP between Atlantic and Pacific leatherbacks, indicating that differences in trophic ecology between populations are an unlikely driver of the population dichotomy between Pacific and Atlantic leatherbacks. Isotope data suggested that St. Croix leatherbacks inhabit multiple oceanic regions prior to nesting, although, like their conspecifics in the Pacific, individuals exhibit fidelity to specific foraging regions. Leatherback nesting parameters were weakly related to the NAO, which may suggest that positive NAO phases benefit St. Croix leatherbacks, potentially through increases in resource availability in their foraging areas. Our data contribute to the understanding of leatherback turtle ecology and potential mechanistic drivers of the dichotomy between populations of this protected species

Seasonal patterns of alkenone production in the subtropical oligotrophic North Pacific

Seasonal alkenone concentrations, production rates, and unsaturation patterns (U₃₇ᴷ') were measured at station ALOHA in the oligotrophic subtropical North Pacific. Highest alkenone concentration and production rates were found in (winter and fall) or just below (summer) the surface mixed layer. Lowest alkenone concentration and production rates were found within the deep chlorophyll maximum layer (DCML). In the DCML, which occurs at 80–120 m throughout the year, U₃₇ᴷ' temperatures overestimated water temperatures by ∼2°–4°C. This result probably reflected the effect of light limitation on the physiology of alkenone‐producing algae. At the depth of maximum alkenone production, U₃₇ᴷ' temperatures underestimated water temperature by ∼2°–4°C in summer and fall but overestimated in situ temperatures by ∼1°–2°C in winter. The underestimate of measured water temperature in summer and fall most likely reflected a physiological response to limited nutrient availability. The U₃₇ᴷ' temperature overestimate in winter was best explained by a change in the ecology of alkenone‐producing algae

Small phytoplankton drive high summertime carbon and nutrient export in the Gulf of California and Eastern Tropical North Pacific

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 1309–1332, doi:10.1002/2015GB005134.Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including <50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method.NSF Grant Numbers: OCE-0726290, OCF-0962362, OCE-0726543, OCE-0726422; EU Grant Number: FP7-MC-IIF-220485; MEC Grant Number: CTM2007-31241-E/MAR; ICREA Academia; MERS Grant Number: 2014 SGR – 1356; Spain's Ministerio de Educación y Ciencia Grant Numbers: AP-2009-4733, BES-2004-3348; NASA New Investigator Award Grant Number: NNX10AQ81G; Sloan Research Fellowship2016-02-2

Small Phytoplankton Drive High Summertime Carbonand Nutrient Export in the Gulf of California and Eastern Tropical North Pacific

Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom‐dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton‐dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton‐ and diazotroph‐dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton‐dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including \u3c50 \u3eµm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method

Diet of the prehistoric population of Rapa Nui (Easter Island, Chile) shows environmental adaptation and resilience

Objectives: The Rapa Nui “ecocide” narrative questions whether the prehistoric population caused an avoidable ecological disaster through rapid deforestation and over-exploitation of natural resources. The objective of this study was to characterize prehistoric human diets to shed light on human adaptability and land use in an island environment with limited resources. Materials and methods: Materials for this study included human, faunal, and botanical remains from the archaeological sites Anakena and Ahu Tepeu on Rapa Nui, dating from c. 1400 AD to the historic period, and modern reference material. We used bulk carbon and nitrogen isotope analy- ses and amino acid compound specific isotope analyses (AA-CSIA) of collagen isolated from prehistoric human and faunal bone, to assess the use of marine versus terrestrial resources and to investigate the underlying baseline values. Similar isotope analyses of archaeological and modern botanical and marine samples were used to characterize the local environment. Results: Results of carbon and nitrogen AA-CSIA independently show that around half the protein in diets from the humans measured came from marine sources; markedly higher than previous estimates. We also observed higher d15N values in human collagen than could be expected from the local environment. Discussion: Our results suggest highly elevated d15N values could only have come from consump- tion of crops grown in substantially manipulated soils. These findings strongly suggest that the prehistoric population adapted and exhibited astute environmental awareness in a harsh environ- ment with nutrient poor soils. Our results also have implications for evaluating marine reservoir corrections of radiocarbon dates

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

The oceans are an important global reservoir for mercury (Hg), and marine fish consumption is the dominant human exposure pathway for its toxic methylated form. A more thorough understanding of the global biogeochemical cycle of Hg requires additional information on the mechanisms that control Hg cycling in pelagic marine waters. In this study, Hg isotope ratios and total Hg concentrations are used to explore Hg biogeochemistry in oligotrophic marine environments north of Hawaii. We present the first measurements of the vertical water column distribution of Hg concentrations and the Hg isotopic composition in precipitation, marine particles, and zooplankton near Station ALOHA (22°45â ²N, 158°W). Our results reveal production and demethylation of methylmercury in both the euphotic (0â 175 m) and mesopelagic zones (200â 1,000 m). We document a strong relationship between Hg isotopic composition and depth in particles, zooplankton, and fish in the water column and diurnal variations in Î 199Hg values in zooplankton sampled near the surface (25 m). Based on these observations and stable Hg isotope relationships in the marine food web, we suggest that the Hg found in large pelagic fish at Station ALOHA was originally deposited largely by precipitation, transformed into methylâ Hg, and bioaccumulated in situ in the water column. Our results highlight how Hg isotopic compositions reflect abiotic and biotic production and degradation of methylâ Hg throughout the water column and the importance of particles and zooplankton in the vertical transport of Hg.Key PointsMMHg bioaccumulated in fish is derived primarily from Hg (II) deposited in atmospheric precipitationMarine particles host the majority of Hg available for production of MMHg in the open oceanMethylation and demethylation of Hg occurs throughout the euphotic and mesopelagic zones in the North Pacific Subtropical GyrePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/150543/1/gbc20883.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150543/2/gbc20883_am.pd