A sea-to-soil solution for the green tides threatening shellfish aquaculture: conceptual model and lessons learned...

Seaweeds of the genus Ulva form massive blooms known as green tides, which threaten coastal ecosystems and livelihoods. In the Pacific Northwest, seasonal Ulva blooms coat commercial shellfish beds, fouling gear and causing hypoxic conditions that can harm shellfish and other biota. Shellfish growers must regularly remove the seaweed, but lacking a disposal mechanism the biomass is typically left to decompose on tideflats where it releases carbon and nutrients back into the aquatic environment, exacerbating local ocean acidification and eutrophication. Deliberate harvesting of this 'nuisance' Ulva, however, presents a mechanism for removal of excess carbon and nutrients from the aquatic environment. Further, the utilization of harvested Ulva as an agricultural soil amendment holds potential for enhancing nutrient circularity and building soil carbon. Such a sea-to-soil solution could provide a combination of environmental, economic, and social benefits to both shellfish and land-based farms. Here, we describe a project piloting this practice, including network and logistics development and assessment of the environmental, economic, and social impacts of practice adoption. We present the project concept and structure, preliminary proof-of-concept accomplishments, logistical lessons learned, and considerations for future research and development. In particular, we highlight considerations and recommendations related to seaweed harvest and handling; the presence of undesirable substances; application method and timing; environmental, economic, and social benefits; transferability; and the human aspects of complex stakeholder-engaged applied research on this topic. The goal of this article is to provide a roadmap for continued innovation and advancement in the area of Ulva valorization as a soil amendment.