Harnessing ecological theory to enhance kelp cultivationMacroalgae and Ecosystem Services (sponsored by Annals of Botany)

Nessa O’Connor¹, Francis Spellman¹, Abby Gilson¹

¹The University of Dublin, Ireland

Global food security, improving human health and reducing our reliance on fossil-fuel based energy sources are among the largest global challenges we face. Seaweed aquaculture, including cultivation of large brown macroalgae such as kelp, offers vast opportunities to address these challenges while also potentially mitigating impacts of climate change and possibly enhancing local biodiversity. Currently, most macroalgal cultivation is based on monocultures (similar to terrestrial agriculture), which may lack resistance to disease and fluctuations in environmental conditions and ultimately require further manipulations to artificially enhance their resilience. We applied general ecological principles, based on biodiversity-ecosystem functioning theory, to developing kelp cultivation practices in the NE Atlantic. We performed a series of large-scale experiments to test whether mixtures of multiple species of kelp cultivated together are more productive and resilient than monocultures. Our results suggest that interspecific competition among juvenile sporophytes was strong on kelp cultivation lines at sea and that this superseded any potential positive effects of species richness on production. We identified that Alaria esculenta (winged kelp) outperformed all other species in mixtures, while Saccharina latissima (sugar kelp) thrived but only in the absence of A. esculenta. We show that Laminaria digitata (oar weed) was outcompeted by all other species and failed to thrive during our experiments even when grown as a monoculture. We also found that an opportunistic kelp species (Saccorhiza polyschides) can perform as well as pre-seeded species and that these inter- and intra-species interactions were independent of initial seeding density. Understanding the mechanisms underpinning kelp biomass production and fundamental temperate reef ecology is essential to ensure sustainability in the emerging field of regenerative ocean farming.