Citation:

Young, D., Pat Clinton, D. Specht, AND TChris MochonCollura. Comparison of non-indigenous dwarf eelgrass (Zostera japonica) and native eelgrass (Z. marina) distributions in a northeast Pacific estuary: 1997-2014. BOTANICA MARINA. Walter de Gruyter GmbH, Berlin, Germany, 58:239-250, (2015).

Impact/Purpose:

This study demonstrated that the major (~1500%) expansion of an invasive eelgrass in Yaquina estuary over the last 17 years had no detectable effect on the extent of the native eelgrass, considered to provide a critical habitat for estuarine fish, shellfish and other benthic/ epibenthic organisms. Eleven aerial photography surveys taken in the spring or summer between 1997 and 2007 showed no significant change in the overall extent of the native eelgrass, or in its average distribution either from the estuary mouth or across the intertidal zone of the lower estuary. In contrast, the invasive species underwent an exponential expansion in aerial distribution over the 10 year period of photo surveys, with the average elevation decreasing significantly from 1.6 m to 1.3 meters relative to the Mean Lower Low Water (MLLW) datum. However, in Yaquina estuary the distributions of the two eelgrass congeners almost never overlap; the invasive eelgrass occupies the upper intertidal zone, while the native species is centered on the zero meter elevation (MLLW). This study provides important baseline distributions of the two eelgrass species in this coastal estuary, and may provide guidance regarding the principal factors controlling their estuarine distributions in the Pacific Northwest.

Description:

This study addressed the following question: In a coastal estuary of the northeastern Pacific Ocean with a relatively large areal extent of the native eelgrass Zostera marina, is an expanding distribution of the non-indigenous dwarf eelgrass Z. japonica accompanied by a measurable change in the distribution of the native eelgrass? The question was addressed by monitoring distributions of the congeners between 1997 and 2014 in lower Yaquina estuary on the central Oregon coast, USA. Digital classifications of color infrared aerial photographs and ground surveys were used to obtain annual areal distributions of the two congeners. Correction factors for seasonal variations in cover were obtained to normalize the annual survey results to a common date (mid-August). Major expansions in areal distributions of Z. japonica meadows over most of the 17 year study period were observed, accompanied by a significant down-slope expansion overall. However, there was no indication that this large (~ 1500 percent) increase in areal extent of Z. japonica in the lower estuary was accompanied by a change either in average elevation or total areal extent of the native Z. marina in this system.

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