Comparing changes in community structure and ecosystem function in San Francisco, CA and Williapa Bay, WA resulting from Spartina invasion.

Background/Question/Methods

Plant introductions in estuarine systems can produce substantial changes ranging from species replacement to broad-scale alteration of ecosystem properties.  Here we examine the changes produced by the invasion of Atlantic smooth cordgrass Spartina alterniflora in two Pacific estuaries, San Francisco Bay, CA and Willapa Bay, WA.  We compare and contrast benthic invertebrate foodwebs inhabiting invasive Spartina meadows relative to naturally unvegetated muflats (SF Bay) or relative to mudflats colonized by introduced Zostera japonica (Willapa Bay).  In both bays, we use dual isotopic tracer experiments with δ ¹⁵N labeled Spartina detritus and δ ¹³C labeled microalgae as well as δ ¹⁵N labeled Zostera japonica detritus in Willapa Bay.  

Results/Conclusions

At both sites, our results showed similar and substantial shifts in benthic communities including reductions in larger, surface-feeding taxa concurrent with increases in smaller, subsurface detritivores with potentially negative impacts for higher trophic level consumers including crabs, fishes and birds.  We also found that the uptake of labeled Zostera was significantly greater than for Spartina suggesting greater palatability and usage by benthic consumers.  We discuss these results in the context of introduced plant effects in other estuaries to help develop a predictive framework future estuarine plant invasions.