Ecological Effects of Reed Canarygrass in the Lower Columbia River

Invasive plants can impact ecosystem services, such as by reducing availability of nutritional resources for detritivorous arthropods that are valued for juvenile fish production. We compared invasive reed canarygrass (Phalaris arundinacea) and native Lyngbye's sedge (Carex lyngbyei) stands along the lower Columbia River, focusing on their contributions to juvenile salmon prey resources. Controlling for environmental factors such as river reach, elevation, and hydrology, we sampled invertebrates from fallout traps, emergence traps, benthic cores, and litter bags in emergent wetland habitats dominated by either P. arundinacea or C. lyngbyei. In litter bags macrodetritus quantity, quality, and decay rates were also quantified. Detritus quality measured by the carbon to nitrogen ratio, was higher from C. lyngbyei suggesting that it produces more and higher quality detritus during the time when juvenile salmonids occupy the area. The abundance and biomass of combined invertebrates in fallout and emergence traps were similar between the vegetation types, but in benthic cores overall invertebrate abundance was greater in C. lyngbyei. Densities of the salmon prey groups total dipteran insects and Chironomidae from fallout traps and benthic cores, and the biomass from fallout traps were greater in C. lyngbyei. Emergent Diptera and Chironomidae abundance and biomass were similar between the vegetation types. Overall macroinvertebrate assemblage and diversity was not affected by P. arundinacea, but the salmon prey taxa Diptera and Chironomidae were reduced in P. arundinacea. It is unknown whether the difference between the two vegetation types is of a magnitude that affects juvenile Chinook salmon trophic function. Additional studies including a comparison to juvenile salmon insect consumption would contribute to reducing uncertainty.