While climate change impacts on high-latitude ecosystems are already substantial and being widely investigated to predict future consequences, the effects on already warm and nutritionally poor regions of the oceans are less clearly understood and will be more difficult to quantify relative to background variability. For such regions, predicting the consequences of future changes will also challenge our understanding of how these systems and the populations dependent on them function in the contemporary ocean. One specific concern is the nature of food webs that underlie successful recruitment of some the ocean’s top predators and economically important stocks, such as tunas, billfish and similar large pelagics. Adults of such species migrate freely, forage over long distances and have other capabilities, such as deep diving and thermoregulation, that minimize direct impacts of shifting ocean conditions on them. Many, however, spawn in restricted remote areas of tropical and subtropical oceans, leaving their larvae to feed, grow and survive (or not) in surface waters that are becoming warmer, more stratified, less productive and more acidic. Different from adults, larvae have limited thermal range, mobility and resistance to starvation leaving them vulnerable to changing ocean conditions, a likely recruitment bottleneck for large pelagics in the future. This session will provide a forum for biological, biogeochemical and fisheries oceanographers to share their complementary perspectives on assessing change in oligotrophic seas and its implications for larval habitat quality. We invite abstracts on topics including biogeochemical and lower-level ecosystem studies in oligotrophic seas that link to processes impacting larval feeding, growth or survival; the controls and limitations on productivity and trophic flows; new approaches that distinguish optimally and deficiently growing larval subpopulations and their defining characteristics; inter-species or inter-system comparative ecology of spawning habitats; and ecosystem-level modeling studies that relate to larval recruitment issues for large pelagics.
Lead Organizer: Michael Landry, University of California, San Diego (mlandry@ucsd.edu)
Co-organizers:
Michael Stukel, Florida State University (mstukel@ucsd.edu)
Sven Kranz, Rice University (sk235@rice.edu)
Presentations
09:00 AM
BLOOFINZ-INDITUN: EVIDENCE FOR A HABITAT QUALITY SHIFT FOR LARVAE OF SOUTHERN BLUEFIN TUNA IN THEIR EASTERN INDIAN OCEAN SPAWNING REGION (9476)
Primary Presenter: Michael Landry, University of California, San Diego (mlandry@ucsd.edu)
Southern bluefin tuna (Thunnus maccoyii, SBT) range broadly in high latitudes of the southern hemisphere but spawn only in a small tropical area off northwestern Australia. Their larvae, restricted to the upper 30 m, feed and grow under oligotrophic conditions and unavoidably experience the surface-ocean warming, stratification and acidification associated with climate change. BLOOFINZ and INDITUN Programs are a collaborative effort to understand the determinants of larval tuna habitat quality and climate recruitment vulnerabilities in the SBT spawning region. In Jan-Feb 2022 (peak spawning season), four multi-day Lagrangian experiments and transect sampling were conducted to assess controls of primary production, nitrogen budgets, plankton structure, grazing pathways and food web fluxes that support larval feeding, growth and survival in the spawning habitat. This presentation highlights one element from the study that compares 2022 results to those from the same region in 1987. Surface waters were warmer, many variables were similar, but larval feeding incidence, prey number per stomach and growth rates were significantly higher in 2022. The main change was a larval dietary shift from copepods in 1987 to appendicularians in 2022, which improves transfer efficiency from the microbially dominated food web. While not necessarily indicative of a positive long-term outcome for SBT, this result illustrates that climate-change impacts can involve complex trophic interactions that are difficult to predict from general warming and stratification trends in oligotrophic systems.
09:15 AM
NITROGEN SOURCES AND EXPORT PRODUCTION IN THE OLIGOTROPHIC EASTERN INDIAN OCEAN: INSIGHTS FROM SEDIMENT TRAP AND NITROGEN UPTAKE DATA (9172)
Primary Presenter: Natalia Yingling, Florida State University (ny18b@fsu.edu)
Southern bluefin tuna only spawn in a small low-nutrient, oligotrophic region of the eastern Indian Ocean off northwestern Australia. The nitrogen (N) sources fueling this ecosystem and the processes connecting the upper 25 m where larvae reside to the full euphotic zone are poorly understood. Potential N sources include recycled ammonium, lateral advection of particulate organic matter, upwelled nitrate, and N2 fixation. In this study, we conducted in-situ and deckboard incubations to quantify N uptake, focusing on nitrate and ammonium assimilation. We used a combination of flow cytometry and epifluorescence microscopy to assess phytoplankton community distributions and sediment traps to quantify vertical fluxes of organic matter and pigments within and out of the euphotic zone. Our results indicate that this region relies heavily on recycled N, particularly ammonium, and while lateral advection and N2 fixation contribute to N supply, upwelled nitrate is the dominant source of new N. Analysis of the euphotic zone vertical structure suggests that the deep chlorophyll maximum layer is primarily a site of remineralization, rather than net particle production. Comparative ecosystem analysis with the Atlantic bluefin tuna spawning site in the Gulf of Mexico explore these processes further. Understanding N dynamics and export in these oligotrophic waters is critical for improving biogeochemical models, to assess ecosystem impacts caused by climate change, and to improve our understanding of the recruitment issues for the tuna fisheries that rely on this ecosystem.
09:30 AM
SOURCES AND INVENTORIES OF BIOACTIVE TRACE ELEMENTS ALONG THE INDONESIAN THROUGHFLOW (9606)
Primary Presenter: Peter Morton, Texas A&M University (pete.morton@tamu.edu)
The Indonesian Throughflow (ITF) travels through southeast island Asia from the equatorial Pacific Ocean to the Indian Ocean, with the potential to become enriched in bioactive trace elements. To determine the distributions and inventories of trace metals in this region, as part of the BLOOFINZ-Indian Ocean expedition (January-March 2022), uncontaminated samples of dissolved and particulate trace elements were collected from the surface waters (~2 m) using a towed “fish” sampling system and from the upper ocean (30-1000 m) using Teflon-coated Niskin-X bottles. In addition, aerosol (3-4 day integrations) and episodic rain samples were collected to determine the atmospheric input of trace metals to the surface ocean, and sediment trap samples (3-4 day integrations) were collected using acid-washed tubes filled with low-trace metal brine to estimate the settling flux of these same elements at four depths from the mixed layer through the nutricline (~440 m). Our presentation will summarize the ongoing analyses and interpretation of these data, with the ultimate goal of quantifying inputs from atmospheric and lateral sources, water column export fluxes, and the "nutritional value" (i.e, chemical composition) of the ITF before it enters the Indian Ocean.
09:45 AM
MARINE PRODUCTIVITY AND N2 FIXATION IN THE OLIGOTROPHIC INDONESIAN THROUGHFLOW: BOTTOM-UP SUPPORT FOR SOUTHERN BLUEFIN TUNA LARVAE (9006)
Primary Presenter: Sven Kranz, Rice Univeristy (skranz@rice.edu)
Economically important Southern Bluefin Tuna (SBT) spawn exclusively in oligotrophic waters downstream of the Indonesian Throughflow (ITF). Understanding phytoplankton community composition, primary productivity and nitrogen sources in this poorly studied region is critical for evaluating the relative importance of various drivers and mechanisms that support feeding and growth of SBT larvae under low-nutrient conditions. During the 2022 BLOOFINZ research cruise, we conducted multi-day Lagrangian experiments to quantify primary productivity and N2 fixation in ITF waters by multiple methods. Despite very low nitrate but significant phosphorus in surface waters, N2 fixation contributed only 10-15% to marine productivity during the peak spawning season, likely constraining overall productivity. The phytoplankton community, primarily Prochlorochoccus in the upper euphotic zone and picoeukaryotes in the deep chlorophyl maximum, relied primarily on recycled N. Fast Repetition Rate fluorometry and Advanced Laser Fluorescence analysis further identified areas with limited potential for primary production, suggesting deficiency of an essential factor, such as iron. Our data suggest a contained system, with nitrate, and possibly iron constraining productivity potential of the phytoplankton assemblages. Feeding and growth success of SBT larvae appears to depend more on selecting prey that can streamline the transfer inefficiencies of a picophytoplankton-dominated system rather than the magnitude of primary production itself.
10:00 AM
COASTAL HOTSPOTS OF BIOLOGICAL NITROGEN FIXATION IN UNDERSAMPLED REGIONS OF THE INDIAN OCEAN (9248)
Primary Presenter: Ariana de Souza, Duke University (ariana.desouza@duke.edu)
Biological nitrogen fixation (BNF), the process by which dinitrogen (N2) is converted into a bioavailable form for the uptake by organisms, is crucial for determining ecological fertility and carbon sequestration in oceans and terrestrial landscapes. BNF in the oligotrophic Indonesian Throughflow (ITF) is especially under-researched, and is an important spawning site for Southern Bluefin Tuna, an economically important species in the area. In 2022, the BLOOFINZ cruise conducted a range of experiments in this region, including the first deployment of the Flow-Through Incubation Acetylene Reduction Assays by Cavity Ring Down Laser Absorption Spectroscopy (FARACAS) method in the ITF region. This method is one of the first to conduct continuous measurements of surface BNF, allowing the spatial heterogeneity in BNF to be observed. Overall rates ranged from below detectable levels to over 20 nmol N/L/day, indicating that BNF may have been a minor source of nitrogen overall. A significant hotspot occurred near the coast of Australia, in the Timor Sea, of over 49 nmol N/L/day. These hotspots support previous claims of higher BNF rates in coastal regions, going against conceptions of BNF being prevalent exclusively in open-ocean oligotrophic regions. Our findings suggest an oligotrophic region of low BNF activity, with the exception of local Australian coastal hotspots. Understanding the nutrient constraints and BNF hotspots in the region is essential for being able to predict mechanisms supporting the growth of larval tuna, as well as the system’s vulnerabilities to climate change.
10:15 AM
SEASONAL CHANGES IN TRICHODESMIUM CONCENTRATIONS AND SIZE IN THE NORTH PACIFIC SUBTROPICAL GYRE (9540)
Primary Presenter: Andrew Hirzel, University of Hawaiʻi at Mānoa (drewhirzel@gmail.com)
Trichodesmium is considered one of the largest suppliers of bioavailable nitrogen in the oligotrophic gyres, potentially accounting for significant fractions of total nitrogen fixation and new production when abundant. This study examines seasonal changes in Trichodesmium abundance and size via near-monthly sampling of the North Pacific Subtropical Gyre in coordination with the Hawaiian Ocean Time-series. Multiple instruments were used to capture free filament and colony morphologies, which collectively span from tens of microns to up to a millimeter in size. Surface seawater was sampled with an Imaging Flow CytoBot (IFCB), imaging primarily single Trichodesmium filaments. A towed Digital Autonomous Video Plankton Recorder (VPR) imaged colonies (‘puffs’, ‘tufts’, and ‘bowties’) while undulating through the upper 100 m of the water column. Trichodesmium free filament and colony abundance increased yearly during summer months. Seasonal enhancements varied in magnitude and duration between years. Colony abundance appeared to persist longer than free filaments in some years, increasing even after free filament abundance began to decline, perhaps suggesting aggregation. When Trichodesmium abundance was low, filament length was decreased and colonies were largely absent. Understanding the factors that influence Trichodesmium abundance and morphology is critical for understanding the impact of Trichodesmium on pelagic productivity and nutrient cycling, particularly in quantifying new production.
SS24A - Biogeochemistry and food webs of oligotrophic ocean regions and potential climate-change impacts on habitat quality for the larvae of large pelagic fishes
Description
Time: 9:00 AM
Date: 27/3/2025
Room: W201CD