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On the Pacific coast of North America two seagrass species in the genus Zostera co-exist; the native species Zostera marina, and an introduced species, Zostera japonica. These two species typically occupy separate tidal elevations, with Z. marina occupying the lower intertidal and shallow subtidal zones, and Z. japonica occupying the mid- to upper intertidal zone. This study was designed to compare the photosynthetic characteristics of Z. japonica and Z. marina after exposure to high and low light. Nursery pots containing Z. japonica and Z. marina were grown intermixed in replicate mesocosm tanks at two different light levels (50 and 150 mu mol photons m(-2) s(-1)). We measured photosynthetic parameters for Z. japonica and Z. marina leaf segments and whole plants (WP). Z. japonica leaf segment photosynthetic efficiency (alpha) was greater than that of Z. marina and based on the high photosynthetic rate, alpha and saturating irradiance, we suggest that Z. japonica is high light adapted. Whole plant (WP) photosynthetic rates were similar 123 +/- 11 vs. 155 +/- 21 mu mol O-2 gDW(-1) h(-1) for Z. marina and Z. japonica respectively. However, the WP respiration rate of Z. japonica was 2 fold greater than Z. marina. Consequently, Z. marina would be expected to acquire and store more carbon than Z. japonica. We suggest that light limitation does not explain the observed disjunct vertical distribution of these two species and that other factors (e.g. rhizome growth, branching frequency and seed germination, etc.) likely play a large role in controlling Z. japonica colonization. Published by Elsevier B.V.