Maximising the productivity of the attached cultivation of Ulva tepida in land-based systems

The viability of land-based seaweed cultivation is linked to high areal productivity of biomass and best suited for species that grow vegetatively and unattached. To develop optimised strategies for cultivation in land-based systems for attached species characterised by short growth cycles with periodic reproduction, ropes were seeded with zoids of the green seaweed Ulva tepida and stacked vertically in the water column from one to four layers deep. The layers spanned a culture depth from 50 to 350 mm below the surface of the water. After an initial grow-out period of 10 days, the biomass was continuously harvested on a rolling 4-day cycle which maintained the biomass in an actively growing state, over the 62-day cultivation period. Compared with the single layer treatment the vertical stacking of seeded ropes, in combination with frequent harvesting, increased the areal productivity of U. tepida by 40% with productivities > 20 g dw m(-2)day(-1). Notably, productivities were similar between stacking treatments with two, three, or four layers due to a decreasing availability of light with an increasing number of layers and an increasing depth in the water column. Consequently, the most effective method of cultivation was to stack two layers of ropes in combination with the vertical movement of the layers through the water column with frequent harvesting to reduce the onset of reproduction.