Trampling in a seagrass assemblage: direct effects, response of associated fauna, and the role of substrate characteristics

Human wading in shallow coastal waters is a common activity that inherently involves trampling of the substrate. An experiment was conducted in Thalassia testudinum seagrass beds in Puerto Rico to determine how seagrass and associated mobile fauna respond to this anthropogenic disturbance. Three trampling intensities were applied to replicate seagrass beds throughout a 4 mo period. Seagrass biomass was inversely related to trampling intensity and duration. There was moderate recovery in the trampled areas 7 mo after the last trampling event. Intense levels of trampling resulted in decreased shrimp abundances, especially for Thor manningi. Fish abundances and composition of shrimp and fish assemblages did not change significantly after 4 mo of trampling. T. testudinum beds with softer substrates lost more seagrass biomass as a result of trampling than seagrass beds with firm substrates, suggesting that substrate firmness can modify disturbance effects. Educators and resource managers should limit trampling by large groups, or confine it to small areas with firm substrates, and researchers should be mindful of artifacts arising from trampling in and around sampling areas.