Ecological impacts, efficacy and economic feasibility of algal mat removal from temperate intertidal mudflats under blue nitrogen trading schemes

Driven by eutrophic conditions, AM (algal mat) proliferation is now ubiquitous in coastal areas generating significant ecological and economic impacts. The need to mitigate negative effects has prompted the exploration of removal methods, but neither the success nor the impacts on intertidal mudflats have been assessed. Limited success using a specially-adapted vessel, prompted a shift to manual removal by hand-rake at two UK (Portsmouth and Poole) and two French study sites (Brittany and Normandy). Significant reductions in AM biomass and percentage cover were only observed at one site (Portsmouth), in contrast to significant temporal effects throughout the 180 days at each site. Significant effects of removal on the benthos and birds were also limited to an increase in organic content at Brittany and a reduction in macrofaunal abundance at Poole but with all sites dominated by temporal effects. To assess if AM removal can be used to ameliorate excess nitrogen (N) we calculated the amount of N that could be removed from a site and its potential cost-effectiveness (price of N credit after subtraction of removal costs) within an NTS (Nutrient Trading Scheme). N export by AM removal is influenced by site and season, for example, 66 kg N ha⁻¹ yr⁻¹ (winter) to 95 kg N ha⁻¹ yr⁻¹ (summer) at Poole. N removal rates from some sites (Poole, all seasons; Brittany, autumn) are comparable to other Nature-Based Solutions (NBSs) such as clam aquaculture. However, a single annual AM harvest at these sites yields lower N removal rates compared to seaweed, mussel, and oyster aquaculture. Using a global mean N credit price, the removals at Poole and Portsmouth have medium/high cost-effectiveness across all seasons, potentially generating up to half a million pounds of N credits, which could be increased if post-harvesting value-chains were maximised e.g. biofuel production. Although, implementation at scale could rapidly reduce the many impacts of AMs and contribute to the blue-green bioeconomy revolution, to improve water quality, AM removal must be framed within a multifaceted management process.