WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2023 19 DSW and 60 percent shade cover. Macroalgae are phenotypically plastic and have the capacity to change shape based on environmental conditions, such as attachment methods or tank shape— round vs. square vs. conical bottom. When spliced into blue steel line, H. hawaiiana thalli tend to develop new branches with multiple growing tips, compared to longer, less branched thalli in tumble culture. Flume Tank Experiments/Off-Shore The goal of the MARINER projects is to develop offshore macroalgae production in US waters. Initially, Ocean Era planned to use a wave-generated pump to bring DSW from a depth of 150m to fertilize a farm array located 10m below the surface. However, further investigation indicated that this method could result in a significant carbon footprint by bringing up the DSW supersaturated with CO2 (Chambers 2022). Consequently, the revised plan involves engineering an array that will lower the farm below the chemocline to depths of 150 - 250m at night for nutrient uptake and bring it back up near the surface during the day for photosynthesis. This depth cycling approach, similar to the kelp elevator (Navarrete 2021), shows promise for macroalgae farming in oligotrophic waters. Additionally, pulsing DSW at night could potentially prevent the growth of epiphytic diatoms without negatively affecting macroalgae growth, as DSW is high in silica. Flume tank trials powered by pumps examined the interaction between current speed, DSW concentration, and macroalgae growth rate. Six partially recirculating flume tanks, each with a volume of 1,200L, had current velocities ranging from 0 cm/s to 30 cm/s (Figure 1). Higher current speeds theoretically reduce the boundary layer and enhance nutrient availability. Optimum growth for Caulerpa and Ulva was observed at 12-16 cm/s (Figure 7). Future Direction/Prospects for the Future Our research results will support the development of tropical mariculture operations in Hawai’i and other Pacific Island nations, reducing their reliance on imported seafood. We aim to make healthy local seafood, including seaweed, more accessible to FIGURE 4. Gracilaria parvispora grown in tumble culture in finfish effluent. FIGURE 5. Halymenia hawaiiana attached to a blue-steel line. (CONTINUED ON PAGE 20) FIGURE 6. Caulerpa lentillifera grown attached to mesh.
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