The oceans need to be cooler and to hold much less carbon. To do this humanity must reverse its carbon emissions as soon as possible. Transitioning towards net zero economies is both essential and urgent if we are to have even a chance at limiting global warming to below 2 degrees celsius^[4].

However, achieving global net zero will not go far enough. We will still be left with the task of drawing down an additional 100 – 900 gigatons of CO2 from the atmosphere by 2100^[5]. What is required is industrial scale solutions to draw down carbon on a gigatonne scale.

Seaweed is the unlikely organism that has huge potential as a solution to the greatest challenge currently facing humanity. 

Oxygen generating system under threat

Phytoplankton form the base of the marine food web and are an essential organism for the survival of life on Earth. Every second breath we take is oxygen produced mostly by phytoplankton^[6]. These microscopic organisms are responsible for cycling most of the globe’s carbon dioxide between the oceans and the atmosphere^[7].

Phytoplankton rely on upwelling for nutrients however evidence shows significant disruption to phytoplankton populations through the reduction of upwelling, and associated nutrient loss in many open oceans^[8]. That loss of upwelling stems from increasing water temperatures due to climate change^[9]. Global Oceans have continuously warmed since 1970 and have taken up more than 90% of the excess heat in the climate system^[10].

The world’s oceans have acted as a buffer to rising CO2 levels in the atmosphere by absorbing excess CO2 into the water. Around 25% of the CO2 released by human activity is taken up by the oceans^[11]. However that absorption has come at the cost of increased ocean acidification and a loss of oxygen from the surface down to a depth of 1000m^[12].

Ocean deoxygenation is another side effect of climate change that is creating significant damage^[13], particularly along coastlines which are vulnerable to hypoxic events^[14].

SOLUTION:
Seaweed agriculture provides oxygen-rich habitats that give refuge to marine organisms ^[15][16] offering a real environmental benefit by near-shore seaweed farms.

Seaweed farms provide habitat for herbaceous fish species that support higher order species in the food chain. These dense habitats support commercial fishing on the seaweed farms that reduces the pressure on wild catch. Research by the Wageningen University in the Netherlands has found that 180,000 square kilometers of seaweed farms could produce enough protein to feed the entire world^[17]. Conducting agriculture in this way has the potential to substantially reduce the pressure humankind is exerting on natural ecosystems across land and water.

Climate change is killing sea-life, and destroying habitats

Coral reefs are essential habitat for the maintenance of marine ecosystems. Around 25% of the ocean’s fish rely on healthy coral reefs with many species depending on coral habitats for shelter, food and reproduction^[18]. However increasing acidification, caused by excess CO2 in the oceans, is leading to dissolving coral reefs^[19]. One study by Nature Journal finds that 25% of 36 reefs studied around the world were significantly degraded and that reefs may become net-dissolving between 2031 and 2082^[20].

Climate change is also heating up our oceans. Rapidly accelerating sea surface temperatures are increasing the frequency and intensity of coral bleaching events globally ^[21][22][23]. Bleaching events are deadly to coral,  decreasing the amount of coral cover and devastating essential coral-reef-fish communities that has flow on effects up through the food chain^[24][25].

Like corals, seaweeds are key habitat structures that harbour magnificent biodiversity. In many parts of the world seaweed forests are declining due to climate change, overfishing and wild harvesting^[26][27]. Climate change has a significant indirect effect through sea urchins.

Sea urchins are major grazers of kelp world wide preferring to sit immobile feeding off drift kelp that falls off in giant stands of healthy kelp. However increasing storm intensity associated with climate change creates large scale disturbances that remove vast amounts of kelp forest. Sea urchins mobilise in these conditions grazing on young kelp. They also climb and feed on adult kelp, removing kelp strands and preventing the reestablishment of the forest^[28] after extreme weather events, which is leading to barren seascapes.

This scenario has played out in the California Current Large Marine Ecosystem where bull kelp has lost over 90% of its mass along the coast line^[29]

SOLUTION:
Seaweed is a remarkable macroalgae with numerous benefits. As the fastest growing plant in the world, 100m2 of seaweed can capture 16t of carbon per year^[30]. Many have advocated for large scale seaweed harvesting as one of the most promising carbon drawdown methods currently available^[31][32]. As seaweed draws out carbon dioxide it de-acidifies the water increasing the carbon absorption capacity of the oceans^[33].

Plastics are suffocating the ocean, and poisoning our food

The rise in ocean plastic pollution is increasing exponentially worldwide^[34]. Research by Ostle, et al. (2019), published in Nature Journal reveals the extent of macroplastic accumulation as presented below in figure 1.

Figure 1. Cumulative Sum of Macroplastics.

Light blue bars = annual counts of macroplastic, grey bars = natural items and green = cumulative sum of total counts of macroplastics plotted on the second y-axis^[35].

Plastics are wreaking havoc on our oceans. The most common documented impact stems from entanglement and ingestion of macroplastic debris. Studies have shown that the portion of mammals, turtles and bird species that ingest plastic are 40%, 100% and 46% respectively and that become entangled in it are 46%, 100%, and 26% respectively. Each successive review finds more species are being affected by macroplastic debris^[36], some with secondary effects.

For example, Zooplankton consumption of microplastic could decrease water column oxygen by 10% in the North Pacific and accelerate global oxygen loss by an extra 0.2 – 0.5%^[37].

Toxic substances can be released upon ingestion and accumulate in fatty tissues. Once in the food chain these microplastics find their way into humans witch could potentially be quite harmful to our health^[38]. Ocean plastics are harmful to economies as well. Research by Beaumont et al., has discovered that marine plastics come at a cost of between $3300 to $3300 USD per tonne when considering negative effects on ocean based services^[39].

SOLUTION:
Seaweed bioplastics are a viable substitute for fossil fuel plastics and other biomass plastics as they are 100% biodegradable^[40], do not require pesticide or land use,  grow rapidly and are easy to harvest^[41]. The future success of seaweed bioplastics will be dependent on a reliable, industrial scale supply of seaweed feedstock at a price competitive with oil and thus first dependent on the development of seaweed farms.

Seaweed is just one part of the solution

Seaweed farming will not be a silver bullet to our environmental problems but likely part of a much broader solution. More work needs to be done in researching the carbon sequestration potential of seaweed^[42]. Prototypes of large scale ocean systems need to be built, studied and iterated. Noting of course, an area 4 times the size of Australia would be required just to absorb the planets current annual CO2 emissions^[43], so it’s no silver bullet.

We recognise that seaweed farms could be an essential part of a much broader strategy on carbon drawdown, while also significantly contributing to the sustainability of ocean ecosystems.

References:

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