Zooplankton trophic processes in the Eastern Indian Ocean Southern Bluefin Tuna spawning region

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.

Presentation Preference: Oral

Primary Presenter: Moira Decima, University of California San Diego (mdecima@ucsd.edu)

Authors:

Rasmus Swalethorpe, University of California San Diego  (rswalethorp@ucsd.edu)

Grace Cawley, University of California San Diego  (gcawley@ucsd.edu)

Claudia Traboni, Stazione Zoologica Anton Dohrn  (claudia.traboni@szn.it)

Claire Davies, CSIRO  (Claire.Davies@csiro.au)

Moira Decima, University of California San Diego  (mdecima@ucsd.edu)