Senior Thesis Bachelor of Science Degree April 2006 Major professor=Sylvia YamadaBiological introductions may considerably alter the composition and community dynamics of invaded areas thereby changing many ecosystem processes including productivity, decomposition, nutrient cycling and hydrology. The effects on native species may be obscured by a lack of information on distribution and abundance patterns. The Japanese eelgrass, Zostera japonica, was introduced to Pacific Northwest estuaries in the 1950’s when used as packing material for industry seed oysters. To assess and further track the impacts of Z. japonica in PNW estuaries, the current distribution and abundance of Z. japonica was documented in Netarts Bay, South Slough National Estuarine Research Reserve (SSNERR), and Bandon Marsh National Wildlife Refuge (BMNWR). Transects were established through the center of contiguous eelgrass beds. The percent cover of Z. japonica was taken at 10-m intervals using a 50x50-cm quadrat. In Netarts Bay, Z. japonica beds were intermingled with native eelgrass, Zostera marina, throughout most of its range with Z. japonica covering on average 44% of the substrate. Within the SSNERR, Z. japonica and Z. marina beds occupied different intertidal zones, seldom overlapping. Where present, Z. japonica occupied 17% of the substrate in March and 64% cover by August. Within BMNWR, Z. japonica was documented in small isolated patches at less than 25% cover. This study can serve as a baseline for documenting future range expansions and changes in abundance of Z. japonica within these estuaries. In November 2005 and in April of 2006, control experiments were initiated at Bandon Marsh National Wildlife Refuge. Future researchers will visit the site and evaluate the efficacy of manually removing patches of Z. japonica (see Appendix).