Spatial overlap between lugworm (Arenicola marina) and eelgrass (Zostera marina) distribution in coastal waters: The role of environmental stressors

This study investigates the role of external physical and chemical stressors on the distribution of lugworms and eelgrass in sandy sediments at three shallow locations along the Danish coasts. The density of and spatial overlap between lugworm and eelgrass distribution were quantified during May–October in 2017 and 2018 and related to the potential role of two environmental stressors: Physical disturbance by waves and high levels of sulfide in the sediment. The inverse relationship between the density of lugworms and eelgrass shoots at the study sites suggests a general antagonistic distribution. The spatial overlap between these species was quantified by Levins’ index. The overlap was asymmetrical and much higher for lugworms than for eelgrass with the smallest overlap at the two locations mostly impacted by physical and chemical stress. Almost no eelgrass was present at these two impacted locations when lugworms exceeded 30 ind m−2, while the critical density approached 70 ind m−2 at the less impacted location. These results suggest that eelgrass is less resistant than lugworms to biological interactions when exposed to environmental stress. Eelgrass apparently require large and coherent meadows with high shoot density to tolerate multiple biological and environmental stressors. The knowledge acquired in this study improves the general understanding of species interactions in coastal ecosystems and is important for appropriate planning and management of future eelgrass restoration ventures.