PARENT SESSION
Oral Session #39: Invasions: Effects of invaders -- Aquatic, marsh, riparian. Presiding: C. Kolar.
Wednesday, August 8, 2001. 8:00 AM to 11:45 AM. Madison Ballroom C.

The effects of an invasive eelgrass, Zostera japonica, on species interactions in a Washington estuary: an experimental manipulation of native and invasive vegetation.

Hahn, Daniel¹, ¹

ABSTRACT- Invasive species can potentially alter community interactions and ecosystem processes, yet data that document these impacts remain scarce, especially for marine environments. In the Pacific Northwest, an exotic eelgrass, Zostera japonica, has invaded upper-intertidal mudflats previously unoccupied by macrophytes and may also displace a native congener, Z. marina, which occurs lower in the intertidal. In order to document the effects that Z. japonica has on community interactions and ecosystem processes, I transplanted the invader and its native congener to several tidal heights within Padilla Bay, Washington. Transplant locations corresponded to the tidal height of naturally occurring beds of native, invasive, and mixed eelgrass, all of which vary considerably throughout the bay. Interactions between native and invasive eelgrass varied substantially with tidal height. These experimental studies suggest that at very low tidal heights the invasive eelgrass has little effect on native eelgrass and at highest tidal heights only invasive eelgrass can survive. However, at intermediate tidal heights, interactions between the two species increased total above and belowground biomass relative to monocultures of either species. In addition, I found that different vegetation types (native, invasive, and mixed) had a large effect on the assemblage of organisms (invertebrates and microbes) associated with them. These results imply that Z. japonica increases productivity within the Padilla Bay estuary and alters the environment in such a way as to change local species assemblages.

KEY WORDS: Zostera japonica, Zostera marina, invasions, eelgrass