Zooplankton in Columbia-Snake River system reservoirs, with special emphasis on the invasive copepod Pseudodiaptomus forbesi

Background/Question/Methods

Non-native zooplankton species have been introduced to many estuaries of the northeast Pacific Ocean as a result of ballast water discharge. The Asian copepod Pseudodiaptomus forbesi has recently established in the Columbia River Estuary, and has been found as far upstream as the lower reaches of the Snake River. However, little is known about its distribution and abundance, or how it might interact with native zooplankton species. Our objectives are to determine the seasonal variation in distribution and abundance of P. forbesi in the Columbia-Snake River system, and to elucidate the species-environment relationships that exist within the mesozooplankton community of the Columbia-Snake River system. Here we present results from two years of sampling (July 2009-June 2011). Plankton tows were collected by vertical hauls of a 0.5 m diameter, 73 µm mesh net in the Bonneville, John Day, Priest Rapids, and Ice Harbor dam reservoirs of the Middle Columbia and Lower Snake Rivers. The invasive Asian copepod, Pseudodiaptomus forbesi, was present in high abundances in three of four reservoirs, with the zooplankton community of the fourth reservoir (Priest Rapids) being made up entirely of native species. Zooplankton communities were similar in the three invaded reservoirs. Species were grouped into 9 categories, in which several cladocerans, cyclopoid copepods, native calanoid copepods, and the invasive copepod P. forbesi were most abundant. 

Results/Conclusions

A consistent pattern of seasonal succession was evident. Peak abundances of native cyclopoid and cladoceran species occurred in the summer, the invasive P. forbesi was most abundant in the late summer and fall, and a distinct bloom of rotifers occurred in the winter and spring. This pattern of succession appeared in all three reservoirs that have been invaded by P. forbesi. In the fourth reservoir, Priest Rapids, total zooplankton abundance was very low year-round. Future efforts will include multivariate analyses of zooplankton community structure in relation to environmental variables, and explicit comparisons of native versus invasive copepod population dynamics. Our results are providing basic insights into the biology and ecology of Columbia River zooplankton, which are necessary for understanding food web dynamics related to higher trophic level productivity (e.g., salmon) and ecosystem conservation.