Seeding filamentous Ulva tepida on free-floating surfaces: A novel cultivation method

The selection of seaweed species for the integrated cultivation in land-based aquaculture systems is primarily based on the ability to grow in free-floating cultures thereby increasing stocking densities and areal biomass productivity. We assessed a novel approach to enable the free-floating cultivation of seaweed species that are dependent on the attachment to surface structures by seeding zoids of Ulva tepida onto small surfaces which float in the water column. In this study we firstly assessed how the density of settlement surfaces (hereafter referred to as 'bioballs') and density of zoids influenced the settlement onto bioballs, and secondly how the stocking density of these seeded bioballs affected the biomass yield in outdoor cultivation over a 35 day period. Settlement was not affected by the density of bioballs with zoids settling evenly across bioball treatments. The number of zoids that settled successfully increased with increasing density. However, both seeding factors (density of bioballs and zoids) had a minimal effect on growth rates, yield of biomass per bioball and total biomass harvested, which were primarily affected by stocking density. Low stocking density generally resulted in higher growth rates and yield of biomass per bioball, although the total yield was lower compared to higher stocking densities. Overall, growth rates decreased over time for all stocking densities with a sharp decrease from 27 days of outdoor cultivation and onwards due to reproductive events. Our study demonstrates that the rate of biomass production of U. tepida is primarily driven by the stocking density of seeded bioballs, and underlines the importance of short cultivation cycles with harvest prior to reproduction. This novel cultivation method enables the free-floating cultivation of species that normally depend on attachment to fixed surfaces and thereby expands the range of seaweed species for land-based cultivation.