Seaweed aquaculture has been posited as a multifaceted solution to addressing global food and energy demands without using arable land or freshwater, and working to transform carbon, nitrogen, and phosphorous pollution in our atmosphere and marine environment. While the majority of the US macroalgae production occurs in temperate zones, the US Pacific Islands’ exclusive economic zones, including the state of Hawai’i, constitutes 513 million hectares, roughly half of the total US EEZ.
The offshore environment surrounding the US Pacific Islands offers opportunities and challenges for macroalgae cultivation. High rates of insolation and low levels of turbidity offer high levels of photosynthetically active radiation (PAR) to great depths. While arrays will be present in nutrient poor surface waters during day light hours, deep cycling of arrays to nutrient rich waters may permit nutrient acquisition for the macroalgae during nocturnal hours.
Five Hawaiian macroalgae species (Caulerpa lentillifera, Gracilaria parvispora, Halymenia hawaiiana, Ulva ohnoi, and Ulva lactuca) were grown in varying levels of current velocity, and deep-sea water (DSW) pulsing concentrations (Figure 1). Six 1,200L partially recirculating flume-tanks exposed macroalgae to current velocities ranging from 0 cm/s to 30 cm/s. DSW was nocturnally pulsed at concentrations between 0% and 10% for 12 hour intervals. All trials were conducted in 60% shade coverage to mimic PAR at 10m depth. The feasibility of nutrient pulsing was determined by weekly analysis of specific growth rates (SGR) for each trial.
Results indicate that Ulva ohnoi grown in 1% and 10% DSW exhibited both higher SGRs and higher average mass in current speeds of 20 cm/s versus no current treatments. Likewise, C. lentillifera showed average SGRs of 3.5% and 1% in 20 cm/s and 0 cm/s current respectively.
This study assesses the feasibility of deep-cycling macroalgae arrays in oligotrophic environments for nutrient acquisition. Developing new technologies for offshore production of macroalgae has the potential to expand the aquaculture production of the U.S. Pacific Islands EEZs, while enhancing food security and providing valuable ecosystem services.