The potential role of climate in the distribution and zonation of the introduced seagrass Zostera japonica in North America

Abstract The current distribution of the introduced seagrass Zostera japonica is restricted to the mid- to upper intertidal zone in the coastal Pacific Northwest region of North America. The climate in this region is cool and wet, becoming hotter and dryer with increasing distance southward. Since temperature is likely to be an important factor affecting distribution of this species, growth of two populations located near the northern and southern limits of its established range along the Pacific Coast of North America were measured in an experimental setting across a range of temperatures typical of those in the field during the growing season (10, 20, and 30 °C). The effects of temperature and population were both significant. Leaf elongation, growth, and areal productivity rates of the northern population were consistently lower than those of the southern population. Across the range of temperatures, mean leaf elongation rates ranged from 0.47 to 1.40 cm 2  shoot −1  d −1 ; mean growth rates ranged from 0.19 to 0.52 mg dry wt shoot −1  d −1 . Mean areal productivity ranged from 0.54 to 1.92 g dry wt m −2  d −1 . Maximum rates of leaf elongation, growth, and areal productivity for both populations were observed at 20 °C. However, leaf elongation, growth, and areal productivity of the northern population declined markedly at 30 °C, whereas no comparable declines were observed for the southern population. This suggests that Z. japonica populations near the southern limits of its established range may be better adapted to warmer temperatures than populations near the northern range limits and further range extensions southward along the California coast may be likely. These differences could be important in predicting the outcome of competitive interactions between native and introduced seagrass species, and in determining future patterns of distribution and zonation of Pacific Coast seagrasses.