Different kelp species support unique macroinvertebrate assemblages, suggesting the potential community-wide impacts of kelp harvesting along the Humboldt Current System

Kelp forests provide habitat for myriad species yet remain poorly studied in some ecoregions, including where there is increasing interest in commercial kelp harvesting for the lucrative alginate industry. To identify indicators for monitoring the impacts of kelp harvesting, the structure and composition of macrobenthic invertebrate assemblages associated with large brown macroalgae, Eisenia cokeri, Lessonia trabeculata and Macrocystis pyrifera, from the Humboldt Current System of Peru were determined. Within kelp habitats two distinct habitats were sampled: (a) adult kelp holdfasts; and (b) the benthos adjacent to the kelp sporophyte (hereafter under-canopy rock). For each sample, organisms were identified, enumerated and weighed. Totals of 108, 102 and 113 different species were found associated with Eisenia cokeri, M. pyrifera and L. trabeculata, respectively comprising distinct assemblages associated with each kelp forest. Both habitat types (holdfasts and under-canopy rock) supported diverse, but different, macroinvertebrate assemblages, with richness, abundance and functional trophic groups in general higher in holdfasts compared with on under-canopy rock. Lastly, macroinvertebrate abundance, biomass, and species richness significantly increased with holdfast size. Results confirm that each kelp species is unique in terms of associated assemblage composition and indicate that kelp harvesting for the alginate industry may have large impacts. We suggest that these effects on associated biodiversity should be incorporated within kelp harvesting management plans. We suggest that holdfast diameter is a good ecological indicator as reductions in holdfast size across the population will indicate a decline in the diversity of macroinvertebrates that are important food sources for higher order organisms. We therefore suggest that monitoring studies include morphological measurements as well as measurement of kelp density.