Biodiversity-Ecosystem Functioning: Increased species diversity does not increase the productivity of cultivated kelp

Biodiversity is often assumed to underpin rates of ecosystem functioning and their associated ecosystem services. Manipulative experiments have demonstrated the influence of diversity on the productivity of primary producers where multi-species assemblages tend to outperform single-species monocultures in terms of total biomass production through complementarity or selection effects. In a marine context, applying these ecological principles has the potential to increase the yield of cultivated macroalgae.

This study tested experimentally whether more diverse mixtures of kelp species are more productive than monocultures, independent of density, when cultivated and whether these effects differed among species. Kelps were cultivated in a 3-species mixture (Saccharina latissima, Alaria esculenta and Laminaria digitata), 2-species mixtures (S. latissima and A. esculenta; S. latissima and L. digitata; A. esculenta and L. digitata) and monocultures of each species (S. latissima; A. esculenta; L. digitata) at two sites in Co. Cork, Ireland. A second experiment tested for density effects on the growth and productivity (density, biomass, length and width) of two kelp species (S. latissima and A. esculenta).

During the experiment many of multi-species treatments became monocultures spontaneously, and we found that the most productive treatments were dominated by A. esculenta, suggesting that selection effects and inter-specific competition rather than complementarity play a key role. The results were density-independent, probably driven by intra-specific competition and self-thinning at the gametophyte or young-sporophyte stage.

This study demonstrates that biodiversity-ecosystem functioning relationships do not readily translate into increased productivity in kelp cultivation but highlight the potential for multi-species assemblages to provide stability for biomass yields under future environmental change. We also demonstrate the importance of site selection for ensuring consistency and reliability of ecosystem services provision by seaweed farms and its key role in the sustainable development of macroalgal aquaculture systems and regenerative ocean farming.