ABSTRACT: The hypothesis that sediment organic content is limiting growth and distribution of the seagrass Zostera marina was tested in Chincoteague Bay, Maryland, and in a controlled mesocosm experiment. In the field, Z. marina was usually absent from areas with sediment organic content > 4%, especially compared with areas with sediment organic content < 4%. In contrast, in a mesocosm experiment, Z. marina thrived in organic rich (4 to 6%) sediment, developing long leaves and disproportionately short roots. Such plants have high drag and low anchoring capacity. As a result, Z. marina plants grown in organic rich sediment are more likely to be dislodged than are plants grown in organic poor sand. We hypothesize that when organic rich sediments are found in hydrodynamically active areas, a mismatch occurs between plant morphology and the physical environment, leading to the loss of seagrasses due to uprooting. Therefore, sediment organic content limitations in seagrass habitats need to be evaluated within the local hydrodynamic settings. Fine organic sediment may be less limiting to seagrasses in quiescent waters while sand with low organic content may be required for seagrass survival in hydrodynamically active areas.