Congener comparison of native (Zostera marina) and introduced (Z. japonica) eelgrass at multiple scales within a Pacific Northwest estuary
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A congener comparison of native (Zostera marina) and introduced (Zostera japonica) eelgrasses was conducted in Willapa Bay, Washington, USA. Along intertidal transects, Z. japonica (0.1–1.5 m above mean lower low water [MLLW]) occurred above Z. marina (<0.6 m MLLW). Both species declined in shoot length at higher elevation, but Z. japonica was always shorter (20 vs. 100 cm) and occurred at higher shoot density (>3,800 vs. <360 m−2 in Z. marina). Z. japonica exhibited greater seasonal variation in biomass, with increases supported by both sustained asexual reproduction (rhizome branching) and recruitment from seeds (30 vs. <5% in Z. marina). Z. japonica’s successful invasion appears related to small size and high reproductive output, allowing it to spread quickly in a variable and stressful intertidal environment where competitive effects are low. Based on interannual changes in abundance, the native eelgrass has also recently increased in Willapa Bay, and one hypothesis involves “engineering” of suitable habitat at higher tidal elevations by Z. japonica.
KeywordsSeagrasses Invasion Life history Estuary Congener Tidal elevation Zostera marina Zostera japonica Willapa Bay Washington Darwin's naturalization hypothesis
We thank M. Briya, H. Ibrahim, T. Jones, M. Kavanaugh, J. Shaefers, R. Craig, S. Bradley, S. DeAmicis, and L. McCoy. Washington Department of Fish and Wildlife provided temperature data and laboratory space for processing samples. The manuscript benefited from comments by an anonymous reviewer, J. Kaldy and P. Eldridge. The work reported in this publication was supported in part by the Western Regional Aquaculture Center through Grant No. 2003-38500-13198 from the United States Department of Agriculture, Cooperative State Research, Education, and Extension Service. JLR and ACT were funded by the Andrew W. Mellon Foundation.
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