Top Down Control of Canopy-Forming Kelp by Crustacean Consumers

Bull kelp (Nereocystis luetkeana) plays an important ecological role in nearshore subtidal ecosystems around the Salish Sea, including primary production, supply of detrital material, and creation of three-dimensional habitat for other species. Large crustacean consumers such as the northern kelp crab (Pugettia producta) may influence kelp distribution and abundance. Exclusion cages at a subtidal field site near Point Caution on San Juan Island were used to assess herbivory on juvenile N. luetkeana. The four treatments using mesh and concrete blocks with attached juvenile bull kelp, were: unenclosed, partially mesh enclosed, fully mesh enclosed, and fully mesh enclosed with P. producta inside. The only treatment in which juvenile bull kelp increased in mass or blade length was the fully caged treatment. To assess crabs’ potential to exert top-down control on kelp bed species, we quantified feeding electivities and preferences of kelp crabs (P. producta) in laboratory feeding trials on a range of prey items, including several macroalgal species and snails. In these trials, kelp crabs were starved for 12 hours and then allowed to feed for 12 hours. In all trials, kelp crabs elected to eat more N. luetkeana than any other macroalga tested; in the kelp vs. snail trial, they showed no significant preference. In a follow-up laboratory feeding trial, kelp crabs were starved for 12 hours, then allowed to feed for 12 hours on two similar-sized pieces of N. luetkeana. One of the pieces was freshly collected and the other aged in a dark, flowing seawater tank for one week. On average, P. producta consumed 6.0 g of fresh blade and 1.5 g of aged blade. The results of these experiments indicate that kelp crabs frequently elect to eat fresh N. luetkeana over other macroalgae, in addition to their role as detritivores, and may exert top-down control on bull kelp distribution and abundance when kelp sporophytes are small and therefore most vulnerable to catastrophic herbivory