Evaluating seaweed farming as an eco-engineering strategy for 'blue' shoreline infrastructure

Green spaces and agronomy have long been used in terrestrial urban environments to help mitigate the negative effects of urbanization on social-ecological systems. However, they represent an unexplored frontier in marine urban environments, which are also central to human-environment interactions in coastal cities. We evaluated the feasibility and potential environmental impacts of integrating small-scale seaweed farming into urban seawalls. We developed and tested the efficacy of a prototype "planter" unit for cultivating the red, agar-producing seaweed, Hydropuntia edulis, above its typical tidal range on urban seawalls in Singapore. Field trial results suggest that H. edulis can be successfully grown in the mid- and upper-intertidal zones on seawalls by using planters with water-retaining features. However, based on results from life cycle assessment (LCA) and integrated biological models of seaweed production, we found that the choice of materials for constructing planters and their positioning on the seawall were critical in determining net environmental impacts. Farmed seaweeds had an impressive capacity for carbon, nutrient, and metal uptake according to model estimates, but these benefits were outweighed by emissions from energy- and contaminant-intensive materials that are commonly used in seawall "eco-engineering." At low tidal heights, farmed seaweeds were also more readily dislodged, potentially leading to greater flux of detrital biomass to adjacent systems. This work highlights both the potential of seaweed farming for enhancing the multifunctionality of urban waterfronts, and the importance of objectively weighing overall environmental consequences of such approaches before advancing them as eco-engineering solutions.