Seaweed Mariculture versus Afforestation – what is the difference?

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Article by Jacqueline Boivin with assistance from Yue Liu

Seaweed serves vital roles in mariculture and afforestation. Understanding the distinction between these terms is crucial for informed decision-making in sustainable practices.

Currently, seaweed is being used extensively for seaweed mariculture, which is a more widely known practice than the concept of afforestation[1]. Seaweed mariculture involves the cultivation of seaweed in controlled environments, such as underwater farms or on suspended ropes[2]. It is a sustainable practice with several benefits, one of which is carbon dioxide sequestration – the extent to which, is currently being measured and studied by researchers Yue Liu, Ling Cao, William Cheung and Rashid Sumaila.[3]

Seaweed may also contribute to the biodiversity of surrounding species, although this is still being studied by PhD student Yue Liu and Dr. Ling Cao. Seaweed can also help filter water of excess nutrients and metals and thereby improve water quality. Another benefit is economic, such as with the extraction of carrageenan, which is used in commercial applications (see “Environmental impacts and implications of tropical carrageenophyte seaweed farming”).[4] In a subsequent web article, some potential environmental downsides to seaweed farming will be explored.

Image Credit: Adobe Stock.

Additional tangible products include biofuel, paper, food, animal feed, beauty and pharmaceutical products.  As suggested by Liu, its structure could also potentially be used as floating breakwater. In the paper entitled “Reducing global land-use pressures with seaweed farming,” the authors discuss the possibilities and potential challenges involved with shifting farming practices from terrestrial to ocean. They explore the possible food and other products that can be produced.[5] Challenges, for example with food products, include increasing the global consumption of seaweed. In this case, there may be slower consumer uptake in countries where this food source is not as common.

Seaweed mariculture could also be used for carbon neutralization, as some Chinese studies have explored. For example, a 2021 paper by Gao, Jiang, Jian and He looked at “the potential of seaweed cultivation to achieve carbon neutrality of China by 2060 using a new method that included lost POC and excreted DOC.” This paper states that lost POC (particulate organic carbon) and excreted DOC (dissolved organic carbon) during seaweed farming have previously been overlooked. Although, they account for considerable proportions of net primary productivity (NPP).[6]

Another concept in addition to seaweed mariculture is seaweed afforestation. Afforestation involves planting and growing macroalgae and other marine vegetation in open waters, such as in offshore deep waters. Ideally, the macroalgae would be brought there on rafts made of biodegradable materials.[7] “[Afforestation] requires purposeful occupation of offshore waters with rafts of coastally derived macroalgae, as only they have sufficient depth to result in successful carbon sequestration needed for this marine CDR (carbon dioxide removal) approach.”[8]

The process of afforestation may have a much larger impact on offsetting carbon dioxide, however much less is currently known. Of importance, is to find out and study further risks such as invasive species, and an unpredictable future of the marine environment. For example, the 2021 paper by Gao looks at both indirect and direct consequences of afforestation.[9]

In summary, as stated above there are many potential solutions that both seaweed mariculture and afforestation can offer to the global food, climate and biodiversity crisis. However, both require further research into biodiversity and the ecological impacts.