Acartia arbruta (previously A. tonsa) in British Columbia: a bioindicator of climate-driven ecosystem variability in the northeast Pacific Ocean

Climate change is driving biogeographic shifts in marine zooplankton. In the northeast (NE) Pacific, the distribution of the estuarine copepod, Acartia tonsa (Dana, 1849), is generally limited to California except during marine warming events where it is found as far north as British Columbia (BC). Documentation of such events relies on accurate species identification. Phylogenetic reconstruction of the genus Acartia using 768 mitochondrial cytochrome oxidase subunit I Deoxyribonucleic Acid (DNA) sequences collected worldwide demonstrates that NE Pacific A. tonsa is distinct from all conspecifics and congeners. Males of NE Pacific A. tonsa are larger with slight differences in the fifth pair of legs as compared to geographically isolated conspecifics. Accordingly, we propose NE Pacific A. tonsa be reclassified to Acartia arbruta. Analysis of mitochondrial 16S ribosomal RNA DNA sequences from 154 specimens of A. arbruta collected from California, Oregon and BC suggests that A. arbruta detected in BC is a mixture of southern populations. Southern populations are likely driven north during "warm" phases of the oceanographic processes indexed by the Pacific Decadal Oscillation, which is positively correlated with A. arbruta biomass anomalies on the continental shelf of Vancouver Island, BC. The presence of A. arbruta in BC waters is a useful bioindicator of marine warming events.