Desiccation is a limiting factor for eelgrass (Zostera marina L.) distribution in the intertidal zone of a northeastern Pacific (USA) estuary

Intertidal irradiance, temperature, and aerial exposure were measured for two years in intertidal Zostera marina beds located in Yaquina Bay (Newport, OR, USA). These physical data were correlated with plant growth and other metrics measured at intervals during the study. Photosynthesis vs. irradiance (P vs. I) curves were determined by PAM fluorometry on plants along an intertidal gradient, and were used to estimate saturating irradiance (I-k) and daily hours of saturating irradiance (H-sat). Mean I-k values were similar to 64 mu mol photons m(-2) s(-1) and H-sat increased with increasing tidal position. Measurements taken during the second winter showed that H-aat values in the low intertidal zone were within a critical range requiring eelgrass to use rhizome reserves to maintain carbon balance. However, plants continued to grow. Temperature differences across tidal gradients were not found to be physiologically important. These results suggested that temperature and irradiance were not limiting factors for intertidal Z. marina growth. High intertidal eelgrass, exposed to aerial conditions more frequently and for longer durations, exhibited signs of desiccation damage and appeared to have faster blade turn-over rates in spring and summer than their low intertidal neighbors. Thus, we concluded that the major factor limiting Z. marina in the upper intertidal zone was desiccation stress, which was acute and episodic rather than chronic.