This dataset includes the environmental variables and the abundance of fish larvae by species and by develoment stage from eight MOCNESS samples taken during the SR2114 expedition to the Eastern Tropical North Pacific Oxygen Minimum Zone (OMZ), onboard the R/V Sally Ride from December 2021 to January 2022. The MOCNESS tows were performed at locations of particular biological interest, such as the CRTD (M1), San Jose submarine canyon (M2), the center (M3, M5) and borders (M4, M6) of two anticyclonic eddies (abbreviated ACE-1 and ACE-2), and across the northern portion of the OMZ (M7 and M8). The MOCNESS tows were performed at 1.5-2 knots net speed and at 40-50° net angle. A horizontal net tow strategy was followed. Fish larvae and juveniles were separated and counted under a stereoscope. Fish larvae were identified to the most specific taxonomic level possible, using a specialized bibliography. The larval stages (preflexion, flexion, postflexion, and transformation) were defined according to Moser (1996). Larval abundances were standardized and were considered absolute abundances. Adult abundances were standardized and are considered relative.

Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Data were collected on R/V Sally Ride cruise SR2114 in the Eastern Tropical North Pacific from December 2021 to January 2022.

The MOCNESS was equipped with 10 nets of 1 square meter (m^2) mouth opening and 333 micrometer (µm) mesh size, a SeaBird SBE9+ CTD, a SeaBird SBE 43 dissolved oxygen (DO) sensor, and flow meter and angle sensors. The MOCNESS tows were performed at 1.5-2 knots net speed and at a 40-50° net angle. A horizontal net tow strategy was followed to sample selected DO concentrations (oxypleths) in five sampling levels: 1) the oxic level (~200 micromoles per kilogram (μmol/kg)­­­­, near surface), 2) the hypoxic (~100 μmol/kg­­­­) and 3) suboxic (~10 μmol/kg­­­­) levels in the upper core boundary, 4) the anoxic core level (<1 μmol/kg­­­­, at the center of the anoxic core), and 5) the deep level (~10 μmol/kg in the lower boundary below the anoxic core). The remaining 5 MONCESS nets were opened during transitions between target depths and were not used for this study. This horizontal sampling protocol allows for discrete, punctual sampling events, but cannot provide continuous, vertically-integrated abundances as oblique tows would (Wiebe et al. 2015).

Fish larvae and juveniles were separated and counted under a stereoscope using metal tweezers. Fish larvae were identified to the most specific taxonomic level possible using a specialized bibliography. The larval stages (preflexion, flexion, postflexion, and transformation) were defined according to Moser (1996). Preflexion and flexion larvae were considered "early larval stages" as they both lack fully-developed fins. Larval abundances were standardized to larvae per 1000 cubic meters and were considered absolute abundances. Juveniles and adults were separated, counted, and identified to family level, except for the Gonostomatidae, that were represented entirely by the genus Cyclothone. Adult abundances were standardized to fish per 1000 cubic meters. Because the increased swimming ability of adult fish could affect fishing efficiency, abundances were considered relative, and should not be compared with the absolute abundances of fish larvae.

Using manufacturer software (SBE Data Processing), CTD data were filtered and aligned. CTD-rosette data were binned to 1 meter depth and MOCNESS data were binned to 10 seconds. Conservative Temperature and Absolute Salinity were calculated. CTD-rosette data were used to construct hydrographic sections, while MOCNESS sensor data was used to describe the environmental conditions of zooplankton samples.

Zooplankton biovolume was measured by the displacement method (Steedman 1976) and standardized to milliliters per 1000 cubic meters (mL/1000 m^3) by dividing the zooplankton displacement volume (mL) by the volume of water filtered by the net (m^3).

Larval abundances were standardized to larvae per 1000 cubic meters (larvae/1000 m^3) and were considered absolute abundances.

Adult abundances were standardized to fish per 1000 cubic meters (fish/1000 m^3). Because the increased swimming ability of adult fish could affect fishing efficiency, abundances were considered relative, and should not be compared with the absolute abundances of fish larvae.

- Imported original file "SR2114_FishLarvae__dev stages.xlsx" into the BCO-DMO system.

- Loaded sheets containing the standard data (sheet numbers 2,4,7,9,11,13) (named with "std"; omitted the combined pre+flex sheet as instructed by the data submitter).

- Concatenated all sheets into one dataset.

- Renamed fields to comply with BCO-DMO naming conventions.

- Converted Date column to YYYY-MM-DD format.

- Replaced the following in the Gpo_Dom column to remove special characters:
-- Copépodos with Copepods
-- Anfípodos with Amphipods
-- Eufáusidos with Euphausiids

-Renamed the following species names that contained typos, identified using WoRMS taxa match tool:
-- Acanthocybium solandrii changed to Acanthocybium solandri
-- Ceratoscopelus townsendii changed to Ceratoscopelus townsendi
-- Citarichthys platophrys changed to Citharichthys platophrys
-- Evermanella ahlstromi changed to Evermannella ahlstromi
-- Lampadaena urophaos changed to Lampadena urophaos
-- Lepidocybium flavobrunnei changed to Lepidocybium flavobrunneum
-- Muraenida sp2 changed to Muraenidae sp2
-- Ophichthus cf zophochyr changed to Ophichthus cf zophochir
-- Unnamed column "zzz" changed to Myctophidae
-- Ophichthus sp1 (zophochir?) changed to to Ophichthus cf zophochir
-- Scopelogados buspinnosus changed to Scopelogadus bispinosus
-- Scopelogadus buspinosus changed to Scopelogadus bispinosus

- Saved the final file as "930172_v1_s2r114_fish_larvae_abundances.csv".

Abrupt vertical oxygen gradients and the lagged depth control of the MOCNESS tow caused DO values to be less than half or more than double the desired values in 7 out of 40 nets (M1-Oxic, M1-Hypoxic, M6-Hypoxic, M3-Suboxic, M6-Suboxic, M7-Suboxic, and M8-Anoxic).

NSF Award Abstract:
Several regions of the deep ocean naturally contain almost no oxygen. Because of this lack of oxygen, microbes living in these regions live in ways that differ from those in oxygenated waters consuming nitrate ions instead of oxygen for respiration. Use of nitrate for microbial respiration results in the production of nitrogen gas which is called denitrification. The resulting removal of nitrate has consequences for the whole ocean as nitrogen is an important nutrient controlling plant growth; however, whereas plants can use nitrogen in the form of nitrate, they cannot, with a few exceptions, use nitrogen gas. There remains a number of uncertainties regarding how much denitrification occurs in the ocean, what controls it, and how it varies in time and space. Traditional studies of ocean denitrification have been limited by the time ships can be at sea and the relatively small proportion of the ocean they can observe. Our project plans to remedy this problem by using vehicles called floats that can operate autonomously in the ocean for three years or more as they drift with currents over hundreds of kilometers. We will outfit ten floats with sensors to measure oxygen and nitrogen gas which will be placed throughout the oxygen-depleted region of the Pacific Ocean to the west of Mexico. This is the largest such region in the ocean from which we have two years of results from a prototype float which validated our approach. This study may well transform our understanding of ocean denitrification and ultimately benefit society as a whole through greater confidence in predictions of the ocean's nitrogen cycle and capacity to fix carbon dioxide under current and future conditions. Application and further development of float systems using commercially available technology will directly benefit successor studies, and more broadly showcase the use of water-following platforms to tackle difficult oceanographic problems. Advances from this study are expected to carry over to other disciplines including ocean biogeochemical modeling. Outreach activities, support for an early career scientist, and student training are included in the project. For the outreach activities, the investigators plan to tie into well-established after-school programs serving underrepresented populations in Massachusetts and established opportunities for public presentations using float related display materials at the University of Washington.

Oxygen deficient zones (ODZs), despite constituting a small fraction of total oceanic volume, play important roles in regulating global ocean carbon and nitrogen cycles including hosting 30 to 50% of the global loss of fixed nitrogen. Unfortunately, current uncertainty in ODZ nitrogen loss derives from substantial temporal and spatial variability in rates that remain under-sampled by ship-based measurements. While local regulation of nitrogen loss by oxygen and organic matter availability are well accepted, temporal/spatial variability in the nitrogen flux is likely a result of the influence of physical forcings such as remote ventilation, seasonal variability, and mesoscale eddies. Understanding how the impact of physical forcings on nitrogen loss as mediated through oxygen and organic flux will be required to fully understand the causes and consequences of any future ODZ expansion. To improve our understanding of ODZ nitrogen loss, we will carry out a multiyear, autonomous float-based observational program to address outstanding questions regarding bioavailable nitrogen loss in ODZs. As the largest ODZ and region of our pilot deployments, our operation area will be the Eastern Tropical N. Pacific (ETNP) where our study will determine over a multi-year period, in-situ nM-level oxygen and biogenic nitrogen on float profiles spanning geographic gradients in oxygen and surface productivity. For the first time, our study will also determine in situ nitrogen loss rates from changes in nitrogen concentration during 1 to 2 week Lagrangian float drifts along a constant density surface. A pilot 2 yr float deployment in the ETNP documents our ability to do so. Critically, our float-based approach more closely matches the frequency and distribution of observations to the expected variability in biogenic nitrogen production as compared to prior work and will dramatically increase the data density for this region by acquiring >500 profiles/drifts for nitrogen and >1000 profiles for nM oxygen.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.