Laiz-Carrion, R. L. | bioRxiv Science

bioRxiv2026

Comparative food-web analysis of bluefin tuna spawning habitats in the eastern Indian Ocean and Gulf of Mexico

Using linear inverse ecosystem modeling as a data assimilation tool, we compare spawning grounds of Atlantic and Southern Bluefin Tuna (ABT and SBT, respectively) based on results from field campaigns in the Gulf of Mexico (GoM) and eastern Indian Ocean off northwest Australia (Argo Basin). Both regions are warm, stratified, low-nutrient waters dominated by cyanobacteria (Prochlorococcus). Despite these similarities, the Argo Basin is more productive, with [~]1.5X higher net primary production and nearly 2X higher production of top trophic levels in the model (tuna larvae, planktivorous fish, and predatory gelatinous zooplankton). Higher primary production in the Argo Basin is mainly driven by higher N2 fixation and storm mixing of new nutrients in the upper and lower euphotic zone, respectively. Increased ecosystem efficiency (secondary production of top trophic levels / primary production) results from differences in plankton food web organization. In the GoM, protistan zooplankton are the direct consumers of nearly all phytoplankton production. In contrast, higher rates of herbivory by crustaceans feeding on nanophytoplankton combines with a higher impact of appendicularians on cyanobacteria to convert plankton production into larval tuna prey more efficiently in the Argo Basin. Despite similarities in the proportions of phytoplankton production mediated by cyanobacteria and other picoplankton in both systems, food web pathways to larval tuna and other planktivorous fish are substantially shorter in the Argo Basin. Our results highlight the impact of distinct zooplankton ecological niches on ecosystem efficiency and suggest a need for better inclusion of plankton food-web structure in models simulating climate impacts on fisheries production. HIGHLIGHTSO_LIDeveloped food web models of tuna spawning habitat (Indian Ocean & Gulf of Mexico) C_LIO_LISpawning habitats in the Argo Basin and Gulf of Mexico (GoM) are both oligotrophic C_LIO_LIArgo Basin had higher net primary production in part as a result of nitrogen fixation C_LIO_LIArgo Basin had higher rates of direct herbivory by metazoan zooplankton C_LIO_LIThis resulted in greater ecosystem efficiency in the Argo Basin. C_LI

ecology↗

bioRxiv2025

Analysis of environmental data during a lagrangian experiment: The influence of vertical movements.

Investigating the time evolution of physical and biochemical properties of the ocean with in situ sampling can follow two approaches: Eulerian and Lagrangian. In the Eulerian approach, repeated measurements are taken at fixed locations, whereas the sampling point moves with the displacement of a water parcel in the Lagrangian approach. During the BLOOFINZ-IO cruise off northwest Australia, four Lagrangian experiments ("cycles") were conducted with multidisciplinary sampling done at regular intervals for several days following a satellite tracked drifter with mixed-layer drogue. To test the Lagrangian nature of these experiments, we adapted the Bindoff and McDougall (1994) approach for decomposing observed changes between adjacent CTD profiles into components due to vertical movement (heaving) and those occurring along isopycnal surfaces. Profile depth variability was mainly driven by vertical displacements of isopycnals (internal waves), while temperature, chlorophyll, oxygen and salinity were relatively stable when observed on isopycnal surfaces across all casts within the same cycle, and different on average between cycles. Our analysis clearly indicated that density surface was a more appropriate vertical coordinate than physical depth for assessing real environmental variability during each cycle and confirmed the Lagrangian character of the experiments. While the BLOOFINZ cruise serves as a case study, the methodology can be readily extended to analyze other biochemical variables in different ocean regions.

ecology↗