The relation between environmental factors and leaf morphology of Zostera marina L. have long been unclear, primarily because the species is intrinsically variable. The common-garden method was used to determine the genetic, environmental, and interaction components of leaf size variation. Zostera marina consists of several ecotypes with a wide range of phenotypic plasticity. Variation in the morphology of Z. marina was of three types: genetic, accounting for 14% across the localities studied; environmental (phenotypic plasticity along temporal and spatial gradients), accounting for 32%; and interaction between genotype and environmental, acounting for 35%. Five ecotypes were described for the North American Pacific coast: Z. marina L. var. izembekensis Backman, Z. marina L. var. typica Setchell, Z. marina L. var. phillipsii Backman, Z. marina L. var. latifolia Morong, Z. marina L. var. atàm Backman. Temporal variation was due to seasonal phenotypic changes in ecotypes. Zostera marina var. izembekensis showed little seasonal morphological changes; Z. marina var. typica demonstrated minor increase in leaf size in spring and summer. Zostera marina var. phillipsii and Z. marina var. latifolia behaved similarly in that leaf size of both increased markedly in spring through early summer. Zostera marina var. phillipsii is adapted to Hood Canal and Puget Sound while Z. marina var. latifolia occupies the outer coast. Zostera marina var. atàm exhibits sexual reproduction exclusively and is specifically adapted to the Gulf of California.