LAB SCALE FEASIBILITY STUDY INVESTIGATING THE POTENTIAL USE OF THE SHELLFISH TOWER DESIGN FOR OPEN OCEAN
Low trophic aquatic food provides the potential to reduce food and nutrition insecurity and tackle malnutrition , while simultaneously posing little stress on the climate and environment , thereby delivering essential ecosystem services and enabling the achievement of the UN SDGs . The Shellfish Tower is an innovative system design for open ocean aquaculture of bivalves, another low trophic aquatic food . In the current study, we examined the feasibility of using the Shellfish Tower for cultivation of Ulva compressa. Ulva compressa is widespread in the Baltic and North Sea and can exhibit two different morphologies: a tubular form, most common in the Baltic, and a foliose form, most common in the North Sea. Therefore, we tested the hypotheses that the Shellfish Tower could be used to cultivate U. compressa in different salinity environments, and that the morphology, physiology and biochemical properties of U. compressa would differ in the Baltic and North Sea salinity scenarios. Nets were seeded with U. compressa gametes after fertility induction and incubated for one month at 15°C, 30 PSU and 100 µmol photons m-2 s-1 light intensity. Once the U. compressa reached 2-3 cm in size, the nets were wrapped around the outer frame of 20 miniature Shellfish Towers, which were constructed based on the original structure design. The frames with seeded U. compressa were distributed to four different salinities: 10, 15, 30 and 35 PSU. Growth rates, photosynthetic efficiency and cell size were monitored weekly for 5 weeks. Biochemical properties (protein, carbohydrate, fiber, ash and chlorophyll and carotenoid content) were measured at the end of the experiment. Significant differences in growth rates and photosynthetic efficiency were observed between the low and high salinities (Figure 1), but U. compressa successfully grew on the Shellfish Tower models at all salinities (Figure 2) . Differences in cell size and biochemical properties will be discussed, but we did not observe the foliose morphology of U. compressa in any salinity treatment. This study shows that the Shellfish Tower can be used to cultivate U. compressa in different salinity environments, but the success in an open ocean environment must still be evaluated, as well as the impact of the vertical distribution of light and its impact on seaweed growth.