Biophysical and eDNA sampling reveals upwelling-enhanced food availability for larvae of the corallivorous crown-of-thorns seastar, Acanthaster cf. solaris

Crown-of-thorns seastar (CoTS) predation is a major driver of coral loss on Indo-Pacific reefs, yet the causes of their episodic outbreaks remain debated. One hypothesis suggests that increased nutrient inputs enhance phytoplankton availability, removing food limitation for planktotrophic CoTS larvae. However, field data on larval distribution and associated water quality (WQ) remain limited. We present the first comprehensive dataset on CoTS larvae and WQ parameters-temperature, salinity, chlorophyll, particulate organic carbon (POC), particulate nitrogen (PN), and phytoplankton cell counts-collected at 258 stations during eight spawning seasons (2014-2024) on the Great Barrier Reef. Using species-specific eDNA, CoTS larvae were detected at 20% of stations. Surface waters (5 m) were consistently oligotrophic, with chlorophyll and cell densities below experimentally derived thresholds for accelerated larval development. In contrast, deeper waters (∼ > 20 m) often exceeded these thresholds, likely due to upwelling intrusions. Differences in WQ between larval-positive and negative stations were minimal, and WQ explained little variance in larval eDNA density. To resolve depth distribution, we deployed a plankton pump at discrete depths (5-30 m), revealing larvae across the entire range without clear preference, suggesting intermittent access to nutritious deeper layers. These findings indicate that, while terrestrial runoff may modulate outbreaks, natural processes such as upwelling provide an important mechanism for larvae to overcome food limitation. A coincidence of upwelling and runoff could create conditions favourable for primary outbreaks-a "perfect storm" scenario. Our results highlight the need to reassess nutrient hypotheses and consider depth-related processes in CoTS outbreak dynamics.