Occurrence Oftheasian Calanoid CopepodPseudodiaptomus Inopinus in the Zooplankton of the Columbia River Estuary

Abstract

Five species of introduced planktonic copepods from Asia, including two species of the genus Pseudodiaptomus, have been reported from coastal bays of California. Recently we have found a third species of Pseudodiaptomus endemic to the Indo-Pacific, P. inopinus, to be established in the Columbia River estuary. Comprehensive studies of both pelagic and epibenthic zooplankton in the Columbia River estuary in 1979 and 1980 did not record P. inopinus (see Simenstad and Cordell, 1985; Jones et al., 1990). In September 1990, we sampled zooplankton at three depths across several tide cycles. These samples contained high densities and all life-history stages of P. inopinus. It now appears that this species has become established and is prominent in the estuarine turbidity maxima (ETM) region. Pseudodiaptomus inopinus cooccurs in the ETM region with extensive populations of the calanoid copepod Eurytemora affinis and the harpacticoid copepod Scottolana canadensis. Preliminary results suggest that the distribution and abundance of these three species vary in response to the physical processes of the ETM. These differences may reduce potential competitive interactions between the preexisting and exotic species. Introduction was probably via ballast water from ships arriving from Asia. However, the establishment of P. inopinus may have been encouraged by a synergism between increased ballast dumping, decrease in maximum flows due to regulation of the river, and the attenuation of extreme low temperatures in the estuary during the last decade.