Archived DNA reveals marine heatwave-associated shifts infish assemblages

Abstract

Marine heatwaves can drive large-scale shifts in marine ecosystems, but studyingtheir impacts on whole species assemblages is difficult. Analysis combining micro-scopic observations with environmental DNA (eDNA) metabarcoding of the ethanolpreservative of an ichthyoplankton biorepository spanning a 23 years time series cap-tures major and sometimes unexpected changes to fish assemblages in the CaliforniaCurrent Large Marine Ecosystem during and after the 2014–2016 Pacific MarineHeatwave. Joint modeling efforts reveal patterns of tropicalization with increasesin southern, mesopelagic species and associated declines in commercially importanttemperate fish species (e.g., North Pacific Hake [Merluccius productus] and PacificSardine [Sardinops sagax]). Data show shifts in fisheries assemblages (e.g., NorthernAnchovy, Engraulis mordax) even after the return to average water temperatures,corroborating ecosystem impacts found through multiple traditional surveys of thisstudy area. Our innovative approach of metabarcoding preservative eDNA coupledwith quantitative modeling leverages the taxonomic breadth and resolution of DNAsequences combined with microscopy-derived ichthyoplankton identification toyield higher-resolution, species-specific quantitative abundance estimates. This workopens the door to economically reconstruct the historical dynamics of assemblagesfrom modern and archived samples worldwide.