Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

This dataset contains amino acid compound specific concentrations in micronekton collected during R/V Kilo Moana cruises around the ALOHA observatory (KM1407, KM1418, KM2011, and a few other undocumented cruises).

For more information about the ALOHA observatory see: http://aco-ssds.soest.hawaii.edu/

These data were published in Gloeckler et al (2018), Supporting Information file lno10762-sup-0001-suppinfo1.xlsx

Micronekton were collected using a 10 m2 multiple opening-closing net and environmental sensing system (MOCNESS) at Station ALOHA (22.75˚N, 158˚W) in March and August of 2011 and in February and September of 2014 with a few samples from other locations around Oahu in 2011 (Choy et al 2015). Micronekton were collected over five depth zones between the surface and 1500 m: 0 – 100 m, 100 – 500 m, 500 – 700 m, 700 – 1000 m and 1000 – 1500 m. At sea, micronekton were sorted and identified to the most specific taxonomic level, then measured and photographed. Standard length measurements were taken for fish, carapace length and total length were taken for crustaceans and both mantle length and total length were taken for cephalopods. For most fishes, white muscle tissue was removed and frozen in a cryovial in liquid nitrogen. Small fishes, crustaceans and gelatinous organisms were frozen whole or individuals were pooled for sufficient tissue required for stable isotope analysis. Specimens were transferred to a ‑80˚C freezer until the samples could be prepared for stable isotope analysis.

Eighty-three samples (individual specimens or small groups of conspecifics) were selected for stable isotope analysis. Samples selected for stable isotope analysis represented different combinations of trophic strategies (suspension feeding, zooplanktivores, micronektonivores), depth guilds (epipelagic, mesopelagic, bathypelagic) and migrating behaviors based on available ecological information (e.g., Clarke 1973, Maynard 1982). Each sample was freeze-dried and ground using a ceramic mortar and pestle. For bulk tissue carbon and nitrogen isotope analysis approximately 0.5 mg of each sample was weighed and placed into a tin boat. Carbon and nitrogen isotopic compositions were determined using an isotope ratio mass spectrometer (DeltaPlusXP) coupled to an elemental analyzer (Costech Model 4010). Isotopic ratios are given in δ-notation relative to the international standards VPDB and atmospheric N2. Accuracy and precision were 0.2‰ based on glycine and homogenized fish tissue reference materials analyzed every ten samples. The isotopic compositions of the reference materials have been extensively characterized using NIST certified reference materials in the UH laboratory and verified independently in other isotope laboratories.

Amino acid-specific stable N isotope composition was determined on approximately 15 mg (dry weight) of each sample underwent acid hydrolysis and derivatization yielding trifluoroacetic (TFA) amino acid esters following the methods of Popp et al. (2007) and Hannides et al. (2009b).; The nitrogen isotope composition of the trifluoroacetic amino acid esters were determined using an isotope ratio mass spectrometer (Thermo Scientific Delta V Plus or Thermo Scientific MAT 253 IRMS) interfaced with a Thermo Finnigan GC-C III. Samples were injected onto a BPx5 forte capillary column (60m x 0.32 mm x 1.0 µm film thickness) at an injector temperature of 180˚C with a constant helium flow rate of 1.4 mL/min. The column was initially held at 50˚C for two minutes and then increased at a rate of 15˚C/min to 120˚C. Temperature was then increased at a rate of 4˚C/min to 195˚C, then to 255˚C at a rate of 5˚C/min and finally to 300˚C at a rate of 15˚C/min, holding at the final temperature for eight minutes. Each sample was analyzed in triplicate and co-injected with the reference compounds norleucine (Nor) and aminoadipic acid (AAA) of known isotopic composition. A suite of pure amino acids of known nitrogen isotopic composition (Ala, Thr, Ile, Pro, Glu, and Phe) was also injected every three runs as an extra measure of accuracy for the instrument. Reference compounds Nor and AAA, as well as the suite of amino acids, were used to normalize the measured isotope values. Standard deviation for all amino acids averaged ±0.4‰ (range 0.0-3.1‰).

BCO-DMO Processing Notes:
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions

NSF award abstract:
Mercury is a pervasive trace element that exists in several states in the marine environment, including monomethylmercury (MMHg), a neurotoxin that bioaccumulates in marine organisms and poses a human health threat. Understanding the fate of mercury in the ocean and resulting impacts on ocean food webs requires understanding the mechanisms controlling the depths at which mercury chemical transformations occur. Preliminary mercury analyses on nine species of marine fish from the North Pacific Ocean indicated that intermediate waters are an important entry point for MMHg into open ocean food webs. To elucidate the process controlling this, researchers will examine mercury dynamics in regions with differing vertical dissolved oxygen profiles, which should influence depths of mercury transformation. Results of the study will aid in a better understanding of the pathways by which mercury enters the marine food chain and can ultimately impact humans. This project will provide training for graduate and undergraduate students, and spread awareness on oceanic mercury through public outreach and informal science programs.

Mercury isotopic variations can provide insight into a wide variety of environmental processes. Isotopic compositions of mercury display mass-dependent fractionation (MDF) during most biotic and abiotic chemical reactions and mass-independent fractionation (MIF) during photochemical radical pair reactions. The unusual combination of MDF and MIF can provide information on reaction pathways and the biogeochemical history of mercury. Results from preliminary research provide strong evidence that net MMHg formation occurred below the surface mixed layer in the pycnocline and suggested that MMHg in low oxygen intermediate waters is an important entry point for mercury into open ocean food webs. These findings highlight the critical need to understand how MMHg levels in marine biota will respond to changes in atmospheric mercury emissions, deposition of inorganic mercury to the surface ocean, and hypothesized future expansion of oxygen minimum zones. Using field collections across ecosystems with contrasting biogeochemistry and mercury isotope fractionation experiments researchers will fill key knowledge gaps in mercury biogeochemistry. Results of the proposed research will enable scientists to assess the biogeochemical controls on where in the water column mercury methylation and demethylation likely occur.

Related background publication with supplemental data section:
Joel D. Blum, Brian N. Popp, Jeffrey C. Drazen, C. Anela Choy & Marcus W. Johnson. 2013. Methylmercury production below the mixed layer in the North Pacific Ocean. Nature Geoscience 6, 879–884. doi:10.1038/ngeo1918