Non-indigenous copepods and plankton dynamics of the lower Columbia River Estuary

Background/Question/Methods

As part of a multi-year field study to investigate plankton dynamics in the lower Columbia River estuary, we conducted monthly sampling of chlorophyll, nanoplankton, microplankton, and mesozooplankton.  In addition to the description of seasonal variation in the plankton community and relationships to environmental predictors, we sought to better understand the linkages between different taxonomic groups and the role of native vs. non-indigenous copepods. To do this we performed group-specific ordinations and cluster analyses that related community composition to both environmental and biological variables. 

Results/Conclusions

While all plankton communities displayed strong seasonal cycles and were correlated to freshwater discharge and salinity, analyses also suggested large inter-group differences. Unlike heterotrophic communities, the diatom community was not strongly associated with a temperature gradient. Both diatom and microzooplankton communities were related to predator forcings. Conversely, the mesozooplankton community lacked strong relationships with potential prey groups, with the exception of total nanoplankton.  The copepod community was numerically dominated by Eurytemora affinis, Coullana canadensis, and Pseudodiaptomus forbesi.  During autumn, the non-indigenous P. forbesi dominated and we also saw peak abundances of Limnoithona tetraspina, another non-indigenous copepod. These results indicate that plankton communities in the lower Columbia River estuary are strongly influenced by both biological and physical processes, with the relative importance of these processes varying between different groups of plankton.  On-going analyses will include examination of potential relationships between specific size classes of nano- and microplankton and mesozooplankton community composition, particularly with respect to native vs. non-indigenous copepods.