Burial of seeds and seedlings by the lugworm Arenicola marina hampers eelgrass (Zostera marina) recovery

Eelgrass (Zostera marina) used to dominate the vegetation in Odense Fjord, Denmark, and covered > 17 km2 of the shallow fjord in 1983. Decades of excessive nutrient loading has lead to decreased eelgrass distribution, and only ~ 2 km2 is covered at present. The state of low eelgrass coverage has not changed despite significant improvements of water quality in the past > 10 years, and lugworms, Arenicola marina, have colonized the former eelgrass areas (1–8 ind. m−2). It was hypothesized that the lack of eelgrass recovery was due to A. marina, which was investigated by a combined field and laboratory approach. At a study site where eelgrass used to dominate, a seasonal study of lugworm population dynamics and sediment reworking activity was performed. Additionally, density dependent burial of eelgrass seeds and seedlings due to sediment reworking by A. marina was investigated in mesocosm experiments. Our results indicate that A. marina may negatively impact eelgrass recovery, since sediment reworking lead to rapid burial of eelgrass seeds and seedlings; within 1–2 months, 95% of seeds and 75% of seedlings were buried below critical depth. Considerations based on empirical modeling suggest that negative impact occur even at low A. marina density (5–10 ind. m−2). Therefore the spread of A. marina into former eelgrass areas is critical, since eelgrass recovery may be severely impaired, even when water quality favors eelgrass recolonization.

► Eelgrass recolonization in Odense Fjord is hampered despite improved water quality.
► Former eelgrass areas have been invaded by the lugworm Arenicola marina.
► It was tested if A. marina was the reason for hampered eelgrass recolonization.
► In laboratory experiments A. marina lead to a rapid burial of seeds and seedlings.
► Conclusion; even low densities of A. marina may hamper eelgrass recolonization.