Distribution of Zostera japonica 2004 to 2013 in Padilla Bay, Washington

In Padilla Bay, Washington, Zostera japonica, an introduced eelgrass species grows both intermixed with the native Zostera marina and in homogeneous patches of the upper intertidal. Competition mechanisms between Z. japonica and Z. marina are not well understood. In light of the recent declaration of Z. japonica as a Class C noxious weed by the Washington State Noxious Weed Control Board we took the opportunity to update the status of Z. japonica in Padilla Bay by revisiting nearshore groundtruth sites that had been recorded in 2004 for a mapping project (the extensive eelgrass beds of Padilla Bay were mapped in 2004 from aerial photos interpreted using more than 1300 ground truth sites). The groundtruth data consisted of GPS site locations where species type, estimates of species composition, and visual estimate of percent cover were recorded for a 10m2 area. During the summer of 2013 we revisited nearly 500 of the sites where Z. japonica would be likely to occur (mid to upper intertidal) to determine if there were changes in species distribution. Preliminary results show 296 of all the sites had Z. Japonica present in 2004 and/or in 2013. At these sites Z. japonica was lost at 49% of the sites and Z. japonica gained at 51% of the sites. There were a of total of 263 sites where Z. japonica changed to or from Z. marina, at 53% of the sites Z. japonica was lost to Z. marina and at 47% of the sites Z. japonica gained at the expense of Z. marina. This data would support the accepted paradigm that while there is great annual variability at a given site, the eelgrass populations of Padilla Bay as a whole are reasonably stable.