Distribution and Performance of the Nonnative Seagrass Zostera japonica Across a Tidal Height Gradient on Shaw Island, Washington

Abstract

In the Northeast Pacific the nonnative seagrass Zostera japonica frequently exists at the same sites as the native seagrass Zostera marina. Although at some sites their vertical distributions overlap, at most sites in the Pacific Northwest there is a distinctive unvegetated zone between them. The objective of this study was to better understand why a gap between the lower limit of Z. japonica and the upper limit of Z. marina exists. To address this issue we carried out transplant experiments, conducted in situ monitoring of existing Z. japonica patches, and collected sediment samples at South Beach on Shaw Island, Washington, during the spring and summer of 2006. Transplant and in situ monitoring data indicate that survival and performance of Z. japonica are reduced lower in the intertidal zone. In addition, Z. japonica patches tended to be smaller and more spaced out at lower tidal heights. Although we found no Z. japonica seeds within or outside extant Z. japonica patches, high transplant mortality indicates that Z. japonica dispersal limitation is an unlikely cause of the unvegetated gap zone. Our field observations further suggest that herbivory, bioturbation, and epiphytes are unlikely causes of the gap pattern at our study site. Instead, we hypothesize that light limitation prevents Z. japonica from occurring lower in the intertidal. A review of published vertical distribution data for both Zostera species indicates that the lower limit of Z. japonica is relatively invariant among sites. In contrast, the upper limit of Z. marina is highly variable, ranging by more than 4 m within some subregions in Washington State. Consequently we hypothesize that intersite variability in the vertical distribution of Z. marina is the primary driver of spatial variability in the presence of the unvegetated gap.

Duration of temperature exposure controls growth of Zostera japonica: Implications for zonation and colonization

“…Within this range, Z. japonica sometimes cooccurs with Z. marina, although the species usually exhibit a spatially disjunct vertical zonation in which the Z. japonica bed is separated from the Z. marina bed by unstructured mudflat . Z. marina occupies the lower intertidal to upper subtidal zones, and Z. japonica typically occupies the mid-to upper intertidal zone (Britton-Simmons et al, 2010;. Competitive interactions with Z. marina have been hypothesized to play a role in determining the boundary between the lower limit of Z. japonica and the upper limit of Z. marina where both species co-exist (Bando, 2006;Hahn, 2003;Harrison, 1982aHarrison, , 1982b.…”

A Model Approach for Estimating Colony Size, Trends, and Habitat Associations of Burrow-Nesting Seabirds

“…Eelgrass beds provide habitat and food to a wide variety of marine organisms, protection for fry, and prevent beach erosion, thus being a critical component of the nearshore ecosystem. The invasive form of eelgrass typically does not coexist or compete with the native eelgrass (Z. marina) of Puget Sound but simply extends the eelgrass bed further into the upper intertidal zone (Britton-Simmons et al 2010). Within two decades of introduction, Z. japonica almost doubled the total eelgrass habitat in Boundary Bay, British Columbia (Williams 2007).…”

Native clams facilitate invasive species in an eelgrass bed

“…Despite Z. japonica frequently existing at the same sites as native Z. marina in the northeast Pacific (Britton-Simmons et al 2010), this is, to our knowledge, the first reported occurrence of Z. japonica in Barkley Sound.…”