Changes in within-canopy environmental conditions and the population structure of the speckled scallop associated to localized loss of above-ground seagrass cover

The three-dimensional structure provided by seagrasses facilitates diverse communities through the provision of refuge from predators, foraging opportunities, and the amelioration of physical stress. The effects of seagrass structural complexity on associated communities have been linked to seagrass shoot biomass and density, often using mesocosm experiments that cannot incorporate naturally occurring physical conditions. Here, we manipulate seagrass cover in the field to identify the short-term and local effects of the loss of Zostera marina (eelgrass) on the physical conditions under the canopy and on the size distribution and mortality of a common bivalve, the scallop Argopecten circularis. Compared to intact eelgrass (100% cover), plots with 50% cover or no cover (100% bare sediment) experienced significant increases of average daily light and temperature. Changes in scallop relative mortality were associated to the reduction in eelgrass cover, while the decrease in scallop size was associated to changes in light and temperature after eelgrass trimming. These results emphasize the importance of seagrass cover for mediating the environmental conditions under the canopy and potentially modulating biological interactions of associated invertebrates.