Vulnerability of Zostera marina seedlings to physical stress

Abstract

Eelgrass coverage in Odense Fjord (Denmark) has declined by 90% since 1983, due to eutrophication and its associated pressures, and the state of low eelgrass coverage has remained sta- ble despite 10 to 15 yr of reduced nutrient loading and improved water quality. We hypothesize that the survival of eelgrass seedlings, and thus recolonization through reproductive dispersal, is nega- tively affected by physical disturbances. The 3 most likely physical mechanisms involved are uproot- ing or burial through drifting macroalgae, Arenicola marina sediment reworking and current-driven sediment resuspension. Our hypothesis was tested by field observations during the summer of 2009, when the mortality of seedlings was followed through time. The density of seedlings decreased dra- matically by 80% during the first month of observations, and no seedlings survived past August, cor- responding to an average seedling mortality of 1.5% d -1 . This was >3 times higher than the mortal- ity for seedlings protected from physical disturbance by enclosures (0.4% d -1 ), indicating that physical disturbance contributed to high seedling mortality. A significant correlation (p = 0.02) between macroalgal drift and seedling mortality suggested that ~40% of seedlings were lost due to the physical disturbance of drifting algae. In contrast, no correlations were found between A. marina reworking or resuspension and seedling mortality, despite a mobility of up to 400 cm 3 sediment m -2 d -1 by these mechanisms. Given the observed intensity of macroalgal drift, we speculate that this