Feeding dynamics of the invasive copepod Pseudodiaptomus inopinus in two northeast Pacific estuaries, and potential competition with its invasive congener, P. forbesi

Background/Question/Methods

Northeast Pacific estuaries have been invaded by at least nine Asian copepod species, including the calanoid copepod Pseudodiaptomus inopinus. First observed in the Columbia River Estuary (CRE) in the early 1990s, it has since become rare or absent in this system. It is now the dominant zooplankton taxon in at least eleven other Northeast Pacific estuaries, yet little is known about P. inopinus feeding dynamics. The disappearance of P. inopinus from the CRE coincided with the introduction of a congener species, P. forbesi. This study aimed to: 1) determine the feeding dynamics of P. inopinus in two estuaries where it is dominant, and 2) compare feeding rates and selectivity of P. inopinus with rates previously measured for P. forbesi in the CRE, to explore the potential for resource competition between these two species. We conducted incubation experiments with samples collected from the Chehalis River estuary (Washington, USA) and the Yaquina River estuary (Oregon, USA) during autumn of 2019 and 2020, when P. inopinus was at peak abundance.

Results/Conclusions

Preliminary results show that P. inopinus feeds preferentially on ciliates (in Chehalis) and diatoms (in Yaquina), whereas P. forbesi was found to feed non-preferentially in the CRE. Based on the feeding behaviors of these two species, and the composition of the prey assemblage in the CRE, we conclude that P. inopinus prefers prey taxa not readily available and thus was ill-suited to this environment, whereas P. forbesi was likely better able to thrive on the available prey in the CRE. From this, we now have a clearer picture of the trophic impacts of these planktonic species on their invaded ecosystems, as well as the potential for competition between the two.