Table of Contents

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

This dataset is derived from mesozooplankton net tows in the Indian Ocean Argo Basin off Northwestern Australia carried out from the R/V Roger Revelle cruise in January to March 2022, which were part of an NSF-OCE project (aka: BLOOFINZ-IO) led by Prof. Michael R. Landry to investigate the epipelagic marine nitrogen cycle, plankton dynamics, and impacts on growth and survival of larval Southern Bluefin Tuna (SBT). The cruise report can be found at http://hdl.handle.net/1834/43464. These data are meant to be used in inter-species, interregional comparisons to data from the BLOOFIN-GoM study of larval Atlantic Bluefin Tuna (ABT) in the Gulf of Mexico spawning region.

Oblique net tows were taken to obtain estimates of mesozooplankton standing stocks and grazing over the depth range of the upper mixed layer and the full euphotic zone. Generally, we sampled during midday (1000-1200 h) and midnight (2100-0300 h) hours repeatedly over 3-4 days following a drogued drifter (termed "Cycles"), allowing estimates of diel vertical migrant biomass by difference within the same water parcel. Additional transect stations were also sampled at variable times of the day. We used a 1-meter (m) diameter ring net with a 202-micrometer (µm) mesh size to sample the full euphotic zone, and a 60-centimeter (cm) diameter Bongo (aka: Bongo-60) net with one net having a 202-µm mesh size and the other a 50-µm mesh size to sample the upper mixed layer. All nets were Nitex mesh and fitted with General Oceanics flow meters in the center of the opening to measure volume filtered. Depth of tow was controlled by a depth sensor on the hydrowire and exact depths recorded by a depth logger fitted to the net frames. Net tow contents from the Ring net and Bongo-60 202-µm mesh sized nets were anesthetized with ice-cold carbonated water and split with a Folsom splitter, with half preserved in 4% buffered formalin and half further split into two quarters. Each quarter was size-fractionated using nested sieves into five size classes: 0.2-0.5, 0.5-1, 1-2, 2-5, and >5 millimeters (mm). In the quarter dedicated for measuring biomass, each size fraction was concentrated on a preweighed 202-µm Nitex filter, rinsed with isotonic ammonium formate to remove sea salt, dried at 60 degrees Celsius (°C), and stored dry in sealed containers with desiccant. The other quarter was dedicated to measuring gut pigments for estimating mesozooplankton grazing. Net content from the Bongo-60 50-µm mesh sized net was treated similarly except that half was preserved in formalin, and half split into 0.05-0.1, 0.1-0.2, and >0.2 mm size fractions using nested sieves and dried for measuring biomass.

In the laboratory, dry filters were weighed on an analytical balance and filter preweights subtracted to calculate zooplankton dry weight for each size fraction. Zooplankton dry weight in milligrams per square meter (mg m-2) was calculated from the measured volume, depth of tow, and fraction of sample analyzed. The dried sample was subsequently scraped off the filter, ground to a powder with a mortar and pestle, and subsampled by weight for carbon (C), nitrogen (N), and stable isotope (13C and 15N) analyses.

CN subsamples were weighed in small tin boats, packed into pellets, and analyzed by a standard elemental analyzer, isotope ratio mass spectrometry (EA-IRMS) at the Stable Isotope Facility at University of California, Davis (https://stableisotopefacility.ucdavis.edu/). The system consists of a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). Acetanilide, USGS41, and facility internal standards was used for instrument stability and corrections for both elemental and isotopic measurements on every run. C and N biomass estimates (mg m-2) were computed for each size fraction from C:DW and N:DW ratios. Stable isotope values are reported in standard delta (‰) notation relative to atmospheric N2 and Vienna Pee Dee Belemnite for carbon.

Data processing was carried out in Microsoft Excel and Access.

Sample % carbon (C), % nitrogen (N), C:N ratio, delta 13C and 15N were only measured in two night and two day net tows on Cycles, and not on any transect stations. For this reason, biomass in C and N per square meter is only calculated for Cycles. For each Cycle, averages in % C and % N were calculated for each net type, in each size fraction, and for day and night, respectively (n=2). These averages were then applied to estimate C and N per square meter from all Cycle net tows.

- Imported original file "BCO_DOM_BLOOFINZ-IO_mesozooplankton_size_fractionated_biomass_data.xlsx" into the BCO-DMO system.
- Marked "nd" as a missing data value (missing data are empty/blank in the final CSV file).
- Renamed fields to comply with BCO-DMO naming conventions.
- Converted date-time fields to ISO 8601 format.
- Rounded "Depth" column to 1 decimal place and all other numeric columns to 4 deicmal places.
- Saved final file as "956590_v1_bloofinz-io_mesozooplankton_biomass_and_isotopes.csv".

NSF Award Abstract:
The small area between NW Australia and Indonesia in the eastern Indian Ocean (IO) is the only known spawning ground of Southern Bluefin Tuna (SBT), a critically endangered top marine predator. Adult SBT migrate thousands of miles each year from high latitude feeding areas to lay their eggs in these tropical waters, where food concentrations on average are below levels that can support optimal feeding and growth of their larvae. Many critical aspects of this habitat are poorly known, such as the main source of nitrogen nutrient that sustains system productivity, how the planktonic food web operates to produce the unusual types of zooplankton prey that tuna larvae prefer, and how environmental differences in habitat quality associated with ocean fronts and eddies might be utilized by adult spawning tuna to give their larvae a greater chance for rapid growth and survival success. This project investigates these questions on a 38-day expedition in early 2021, during the peak time of SBT spawning. This project is a US contribution to the 2nd International Indian Ocean Expedition (IIOE-2) that advances understanding of biogeochemical and ecological dynamics in the poorly studied eastern IO. This is the first detailed study of nitrogen and carbon cycling in the region linking Pacific and IO waters. The shared dietary preferences of SBT larvae with those of other large tuna and billfish species may also make the insights gained broadly applicable to understanding larval recruitment issues for top consumers in other marine ecosystems. New information from the study will enhance international management efforts for SBT. The shared larval dietary preferences of large tuna and billfish species may also extend the insights gained broadly to many other marine top consumers, including Atlantic bluefin tuna that spawn in US waters of the Gulf of Mexico. The end-to-end study approach, highlights connections among physical environmental variability, biogeochemistry, and plankton food webs leading to charismatic and economically valuable fish production, is the theme for developing educational tools and modules through the "scientists-in-the-schools" program of the Center for Ocean-Atmospheric Prediction Studies at Florida State University, through a program for enhancing STEM learning pathways for underrepresented students in Hawaii, and through public outreach products for display at the Birch Aquarium in San Diego. The study also aims to support an immersive field experience to introduce talented high school students to marine research, with the goal of developing a sustainable marine-related educational program for underrepresented students in rural northwestern Florida.

Southern Bluefin Tuna (SBT) migrate long distances from high-latitude feeding grounds to spawn exclusively in a small oligotrophic area of the tropical eastern Indian Ocean (IO) that is rich in mesoscale structures, driven by complex currents and seasonally reversing monsoonal winds. To survive, SBT larvae must feed and grow rapidly under environmental conditions that challenge conventional understanding of food-web structure and functional relationships in poor open-ocean systems. The preferred prey of SBT larvae, cladocerans and Corycaeidae copepods, are poorly studied and have widely different implications for trophic transfer efficiencies to larvae. Differences in nitrogen sources - N fixation vs deep nitrate of Pacific origin - to sustain new production in the region also has implications for conditions that may select for prey types (notably cladocerans) that enhance transfer efficiency and growth rates of SBT larvae. The relative importance of these N sources for the IO ecosystem may affect SBT resiliency to projected increased ocean stratification. This research expedition investigates how mesoscale variability in new production, food-web structure and trophic fluxes affects feeding and growth conditions for SBT larvae. Sampling across mesoscale features tests hypothesized relationships linking variability in SBT larval feeding and prey preferences (gut contents), growth rates (otolith analyses) and trophic positions (TP) to the environmental conditions of waters selected by adult spawners. Trophic Positions of larvae and their prey are determined using Compound-Specific Isotope Analyses of Amino Acids (CSIA-AA). Lagrangian experiments investigate underlying process rates and relationships through measurements of water-column 14C productivity, N2 fixation, 15NO3- uptake and nitrification; community biomass and composition (flow cytometry, pigments, microscopy, in situ imaging, genetic analyses); and trophic fluxes through micro- and mesozooplankton grazing, remineralization and export. Biogeochemical and food web elements of the study are linked by CSIA-AA (N source, TP), 15N-constrained budgets and modeling. The project elements comprise an end-to-end coupled biogeochemistry-trophic study as has not been done previously for any pelagic ecosystem.

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.

Description from the program website:
The Second International Indian Ocean Expedition (IIOE-2) is a major global scientific program which will engage the international scientific community in collaborative oceanographic and atmospheric research from coastal environments to the deep sea over the period 2015-2020, revealing new information on the Indian Ocean (i.e. its currents, its influence upon the climate, its marine ecosystems) which is fundamental for future sustainable development and expansion of the Indian Ocean's blue economy. A large number of scientists from research institutions from around the Indian Ocean and beyond are planning their involvement in IIOE-2 in accordance with the overarching six scientific themes of the program. Already some large collaborative research projects are under development, and it is anticipated that by the time these projects are underway, many more will be in planning or about to commence as the scope and global engagement in IIOE-2 grows.

Focused research on the Indian Ocean has a number of benefits for all nations. The Indian Ocean is complex and drives the region's climate including extreme events (e.g. cyclones, droughts, severe rains, waves and storm surges). It is the source of important socio-economic resources (e.g. fisheries, oil and gas exploration/extraction, eco-tourism, and food and energy security) and is the background and focus of many of the region's human populations around its margins. Research and observations supported through IIOE-2 will result in an improved understanding of the ocean's physical and biological oceanography, and related air-ocean climate interactions (both in the short-term and long-term). The IIOE-2's program will complement and harmonise with other regional programs underway and collectively the outcomes of IIOE-2 will be of huge benefit to individual and regional sustainable development as the information is a critical component of improved decision making in areas such as maritime services and safety, environmental management, climate monitoring and prediction, food and energy security.

IIOE-2 activities will also include a significant focus on building the capacity of all nations around the Indian Ocean to understand and apply observational data or research outputs for their own socio-economic requirements and decisions. IIOE-2 capacity building programs will therefore be focused on the translation of the science and information outputs for societal benefit and training of relevant individuals from surrounding nations in these areas.

A Steering Committee was established to support U.S. participation in IIOE-2. More information is available on their website at https://web.whoi.edu/iioe2/.