Re-introduction of seagrass in the Netherlands Wadden Sea

Summary

Two seagrass species, eelgrass (Zostera marina) and dwarf eelgrass (Zostera noltii), had declined sharply in the Netherlands Wadden Sea since the 1930s wasting disease and 1970s eutrophication and mechanical shellfishing, disappearing entirely from the western Wadden Sea despite water-quality improvements from the 1980s onward that made natural recovery still unlikely. Radboud University Nijmegen and Euroconsult Mott MacDonald ran mesocosm trials on donor stock from France, Germany, Denmark and the Netherlands to screen for suitable transplant sources, then conducted 39 transplantation events at 14 Wadden Sea locations between 1991 and 2004, using roughly 10,000 dwarf eelgrass shoots and 23,000 eelgrass shoots, mostly bare-root transplants completed within 48 hours of harvest to limit stress. Eelgrass transplants showed highly variable boom-and-bust performance and eventually disappeared everywhere, surviving at most eight years, hampered by premature die-off in muddy, algae-covered sites and poor seedling survival in sandy areas. Dwarf eelgrass transplants performed markedly better, with a Balgzand population still surviving after 13 years when monitoring ended, likely because the perennial species does not depend on annual seed production and germination. Major difficulties included logistical constraints on replanting speed, poorly understood optimal techniques that had to be learned mid-program, and highly variable, hard-to-interpret results. The project rates itself Partially Successful, since the annual eelgrass form ultimately failed everywhere while the perennial dwarf eelgrass established durably.