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Table 1 Environmental and societal risk associated with terrestrial biofuels (after Koh and Ghazoul,[16]) and macroalgae biofuels

From: Biogas from Macroalgae: is it time to revisit the idea?

Environmental and societal advantages of macroalgae production for biofuels
Net GHG emissions from land-use change The culture of macroalgae for biofuel would be entirely marine based and would not have the associated GHG emissions associated with land use change.
Threats to biodiversity Macroalgae cultivation takes place in the water column above the seabed. Impacts of large scale macroalgae production on benthic biodiversity are currently unquantified. Likely impacts will include shading and competition for nutrients. However, most production will be in waters where the seabed is deeper than the photic zone, and where terrestrial nutrient run off creates hypernutrified water. It is likely that biodiversity would increase in the vicinity of macroalgae farms as a result of increased habitat structural complexity.
Impacts on food prices Currently most macroalgae cultivation is for human consumption. Large scale production of macroalgae for biofuels is bound to distort this market. However the impacts on the supply of macroalgae to human food chain is likely to be small due to a clear market segregation and the far higher value of macroalgae as food compared to the price of energy.
Competition for water resources Mass cultivation of macroalgae has a zero freshwater requirement and only modest amounts are required in anaerobic digestion