Use of eDNA to understand spatiotemporal variability in phytoplankton diversity available as potential food for Crown-of-Thorns Seastar larvae in the Great Barrier Reef.

Maria Gomez Cabrera, Frances  Patel¹, Jason  R Doyle¹, Sven  Uthicke¹

Acanthaster spp. or Crown-of-Thorns Seastar (CoTS) is a major predator of coral in the Indo-Pacific, contributing to widespread degradation of coral reefs. The Great Barrier Reef (GBR) is experiencing the 4th outbreak of this predator since the 1960s. Irruptions of CoTS appear to originate in mid-shelf reefs in the north-central region of the GBR and then move southward through larval dispersion, but little is known about the triggers for high densities in CoTS larvae. The nutrient hypothesis has been proposed to explain CoTS outbreaks, arguing that changes in land use and management have increased sediment and nutrient runoff in the GBR basin triggering phytoplankton blooms. Following, these blooms create a hyperabundance of food for CoTS larvae, promoting fast development and survivorship in large numbers. There is very limited information on phytoplankton diversity in the GBR and their interaction with CoTS larvae. For the current study, CoTS larval density was assessed using eDNA from plankton samples collected over two years (30 stations). At the same sites, we collected water quality (Chl, PN, POC, Salinity) and, for the first time on the GBR, eDNA was extracted from parallel seawater samples to study the phytoplankton community. Here we describe the phytoplankton biodiversity along a latitudinal gradient on the GBR including temporal changes as well as the relationship with presence of CoTS larvae. We use statistical modelling to test whether water quality and phytoplankton communities differ in samples with CoTS larvae present and those without larvae.

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