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Has the rapidly expanding invasive dwarf eelgrass Zostera japonica in Yaquina estuary, Oregon impacted the distribution of native eelgrass Zostera marina – a critical intertidal habitat?
Young, D., Pat Clinton, D. Specht, AND TChris MochonCollura. Has the rapidly expanding invasive dwarf eelgrass Zostera japonica in Yaquina estuary, Oregon impacted the distribution of native eelgrass Zostera marina – a critical intertidal habitat? Presented at HMSC Seminar Series, Newport, OR, June 06, 2013.
This research project has demonstrated that, despite the exponential increase in the areal distribution of the invasive dwarf eelgrass Zostera japonica in lower Yaquina estuary between 1997 and 2007, there was no significant change in the areal distribution of the native eelgrass Z. marina there. Based upon classification of six annual color-infrared aerial photography surveys during that period, the Z. japonica habitat increased from about 10 to 70 hectars while the Z. marina habitat averaged about 90 hectars. However, an overlay of these distributions with modeled bathymetry of the lower estuary showed little overlap between the two congeners, with their abundance distributions peaking at about 1.5 m and 0.0 m, respectively (relative to the Mean Lower Low Water datum). Extensive field observations between 1997 and 2012 have shown that Z. japonica and Z. marina almost never co-occur in Yaquina estuary.
Native eelgrass, Zostera marina, occupies a significant portion of marine-dominated intertidal and near-subtidal sectors of many coastal estuaries. In recent decades an invasive congener, Z. japonica, has become established in many Pacific Northwest estuaries. We measured the horizontal and vertical distributions of intertidal native and invasive eelgrass between 1997 and 2007 in Yaquina estuary using color-infrared aerial photography and digital classification to assess the impact, if any, of Z. japonica on Z. marina’s distribution. A bathymetric model was used to characterize the distribution of Z. marina and Z. japonica with intertidal elevation. Relative to Mean Lower Low Water, peak abundances of Z. marina and Z. japonica occurred at about 0.0 m and 1.5 m, respectively. Moreover, the two species seldom occupied the same bathymetric zone of a tidal flat. Although the areal extent of Z. japonica increased exponentially, and it now occupies a similar percentage of the intertidal zone as Z. marina, there has been no significant change in the areal extent of Z. marina over the study period.