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
02:30 PM
QUANTIFYING MULTITROPHIC CARBON FLUX NETWORKS OF THE BERMUDA ATLANTIC TIME-SERIES (9271)
Primary Presenter: Falk Eigemann, Technical University Berlin (falkeigemann@gmail.com)
Modern sampling and analytical advances have produced a wealth of high-resolution data on the composition of microbial ecosystems. For the Bermuda Atlantic Time-series Study (BATS), this includes DNA sequences, cell counts, and metabolite concentrations. However, direct observation of microbial interactions, essential for a quantitative, mechanistic understanding, remains challenging and generally requires inference. To address this, we integrated BATS data of a 4-year time series into the mechanistic ecosystem model FluxNet, that quantifies interactions, specifically carbon fluxes, across different trophic levels. We simulate the ~40 most abundant species across key biological compartments (heterotrophic bacteria, phytoplankton, zooplankton, heterotrophic protists, and particulate and dissolved organic matter), and quantify complex multitrophic, multispecies interactions by estimating carbon fluxes between all species and compartments. By dividing the upper 200 m of the water column into three dynamically-varying vertical layers - surface, deep chlorophyll maximum, and upper mesopelagic - the model provides insights into carbon transfer between layers and carbon export to the deep ocean. This three-layer, mass-balanced, mechanistically-constrained model generates quantitative flux estimates for each interaction and time point across species and vertical layers, making it a powerful tool for unraveling the complex dynamics of microbial ecosystems in the surface ocean.
02:45 PM
Zooplankton trophic processes in the Eastern Indian Ocean Southern Bluefin Tuna spawning region (9108)
Primary Presenter: Moira Decima, University of California San Diego (mdecima@ucsd.edu)
The region off northwestern Australia in the Eastern Indian Ocean is the spawning ground for Southern bluefin tuna. We conducted four multi-day Lagrangian experiments as well as transect sampling in this area during peak spawning season (Jan-Feb 2022) to investigate plankton flows and trophic structure within bluefin tuna habitat. These cycles were carried out after a recent mixing event and subsequent warming and stratification of the water column, emulating processes expected as a result of global climate warming. We quantified zooplankton size-fractionated biomass and grazing for the upper 30m and 150m, targeting processes that coincide with bluefin tuna larval habitat (30m) and the full euphotic zone (150m). We find higher zooplankton biomass in the upper 30m at the start of our study, coinciding with high abundances of pre-flexion larvae, suggesting spawning locations coincide with areas where processes enhance zooplankton prey biomass. Euphotic-zone integrated grazing peaked later in our sampling period, coinciding with higher stratification and increasing abundances of small diatoms at depth. This study highlights the role of metazoan zooplankton in the warm, oligotrophic waters of the Eastern Tropical Pacific, under conditions that emulate climate change scenarios related to warming and stratification, with flow-on effects to the rest of the food chain. We highlight unexpected outcomes that suggest complex trophic interactions could compensate for the impacts of warming and trophic transfer efficiency can be sustained or enhanced despite increasing stratification.
03:00 PM
Feeding ecology of larval Bluefin Tuna in oligotrophic nurseries – selection for short efficient food chains (8958)
Primary Presenter: Rasmus Swalethorp, Scripps Institution of Oceanography (rswalethorp@ucsd.edu)
Food web architectures that support abundant high-quality prey within nursery habitats can be important for larval fishes to grow rapidly and successfully navigate their early life of fishes. Bluefin tuna species spawn in several oligotrophic regions of the world’s oceans that would seem poor choices for nursing larvae towards adulthood. So how do they fend off starvation and what mechanisms are in place to ensure that sufficient energy reaches the larval population? In this study, we investigated the diets, prey selections and trophic characteristics of larval Southern Bluefin Tuna (Thunnus maccoyii) and Atlantic Bluefin Tuna (Thunnus thynnus) in their Western Indian Ocean (IO) and Gulf of Mexico (GoM) spawning areas. Two cruises were carried out in the GoM (2017-18) and one in the IO (2022) where larvae and zooplankton were collected, identified in situ and in larval stomachs, and analyzed for stable isotopes. We found that larvae are highly selective for specific types of zooplankton prey, principally Appendicularia in the IO and Cladocera in the GoM. The characteristics of both prey types are high catchability and low trophic position relative to other zooplankton groups. High dietary contributions of these prey translate to faster growth and higher efficiency in energy transfer from the base of the food chain to the larvae. Our findings suggest a mechanism for maximizing growth and survival potential in low productivity spawning environments.
03:15 PM
REVISITING DAILY GROWTH OF SOUTHERN BLUEFIN TUNA (THUNNUS MACCOYII) LARVAE FROM THE EASTERN INDIAN OCEAN (9551)
Primary Presenter: Estrella Malca, University of Miami (estrella.malcaphd@gmail.com)
Southern bluefin tuna (Thunnus maccoyii, SBT) is a commercially important highly migratory predator whose adults migrate to their only known reproductive hot spot, the nutrient-depleted eastern Indian Ocean (IO). SBT larvae were collected during January to March 2022 during the peak spawning season aboard the interdisciplinary BLOOFINZ-IO survey (part of the 2nd International IO Expedition). Taxonomic alongside genetic identifications distinguished SBT-2022 from co-occurring scombrid species, and most of the larval tuna were identified as SBT larvae. Somatic variables, otolith microstructure, and daily growth patterns were determined and compared to previous SBT growth estimates for similar sized SBT from 1987 (SBT-1987) as well as to Atlantic bluefin tuna larvae (Thunnus thynnus, ABFT) from the Gulf of Mexico and western Mediterranean Sea. Overall, during the 2022 survey, sea surface temperatures were approximately 2°C warmer, and SBT-2022 grew faster than SBT-1987. In addition, SBT-2022 had larger otoliths and wider increments than in 1987, while otolith growth was strikingly similar to ABFT from the Gulf of Mexico spawning habitat. Complementary studies analyzing environmental conditions and plankton food web structure during the BLOOFINZ-IO survey will be presented in this session and will further our understanding of larval tuna trophodynamics in this climate-sensitive oligotrophic region.
03:30 PM
TROPHIC INTERACTIONS AND GROWTH VARIABILITY OF TUNA LARVAE IN THE EASTERN INDIAN OCEAN BY STABLE ISOTOPE ANALYSIS (8829)
Primary Presenter: RAUL LAIZ-CARRION, Instituto Español de Oceanografia Ieo-Csic (raul.laiz@ieo.csic.es)
We examined early life trophic interactions of larval southern bluefin tuna (Thunnus maccoyii, SBT) together with the co-occurring albacore tuna (T. alalunga, ALB), and skipjack (Katsuwonus pelamis, SKJ) using stable isotope analysis. Larvae were collected during the BLOOFINZ-IO oceanographic survey (Jan–Mar 2022, part of the 2nd International Indian Ocean Expedition), from the only known SBT spawning ground. Species identification was based on morphology, meristic characters, pigmentation, and genetics. We determined species trophic positions, isotopic niche widths and overlaps. Trophodynamic influences on larvae were assessed by contrasting environmental variables, daily growth, otolith biometry, and stable isotopes characteristics of post-flexion subpopulations of deficient vs. optimal growers. Trophic positions were similar among species. However, species differences in isotopic niche widths and overlaps suggest trophic resource partitioning with a mixture of competition and trophic differentiation that allow these species to share resources during their early life history. The results are discussed in terms of larval daily growth variability and compared with the larvae of other top predators in the region to evaluate species-specific distinctions. Understanding the growth-trophodynamics relationship is essential for identifying quality of the SBT nursery habitat in the oligotrophic spawning habitat of the Indian Ocean. This study was funded by INDITUN project PID2021/122862NB/100 (UE-FEDER) and BLOOFINZ-IO (USA-NSF).
03:45 PM
LONG-TERM INCREASE IN MESOPELAGIC FISH LARVAE IN THE SUBTROPICAL NORTH ATLANTIC OCEAN AND WESTERN ANTARCTIC PENINSULA (9207)
Primary Presenter: Tor Mowatt-Larssen, Virginia Institute of Marine Science (tmowattlarssen@vims.edu)
The high global biomass, broad distribution, and diel vertical migration behavior of mesopelagic fishes suggest they are important mediators of deep-sea carbon storage and open-ocean food webs. However, their abundance patterns and sensitivity to environmental change remain poorly understood. We investigated long-term patterns in the abundance of mesopelagic fish larvae incidentally collected in epipelagic zooplankton net tows from two long-term biogeochemical and ecological time series: the Bermuda Atlantic Time-series Study (BATS) in the North Atlantic subtropical gyre and the Palmer Long-Term Ecological Research (PAL LTER) program in the western Antarctic Peninsula (WAP) from 1994-2021 and 1993-2024, respectively. For the first time, we report long-term increases in the abundance of mesopelagic fish larvae in both regions. At BATS, we posit long-term change in the structure of the plankton food web has led to enhanced zooplankton biomass and increased abundance of fish larvae (e.g., bristlemouths Cyclothone spp.). Along the WAP, warming and changing physical dynamics could lead to habitat expansion for pelagic species, explaining recent increases in mesopelagic fish larvae (e.g., lanternfish Electrona antarctica) abundance at slope stations and modest increases at inshore (coastal) stations. Because these earliest stages of pelagic fishes are thought to represent an index of adult spawning biomass, long-term increases in fish larvae may signal changes in micronekton communities in these ecosystems, with implications for pelagic food web dynamics and carbon cycling.
SS24B - 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: 2:30 PM
Date: 27/3/2025
Room: W201CD