Trophic resources to subtidal suspension feeders

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Kelps are highly productive, yet little is known about the fates of kelp primary production. Traditionally, kelps have been assumed to form the base of kelp forest food webs. This assumption extended naturally to suspension feeders, which can comprise up to 80% of the animal biomass in kelp forests and have been hypothesized to consume small particles of kelp sloughing off the extensive surface canopy. Subtidal suspension feeders graze on abundant phytoplankton, but if kelp detritus is a viable food source for suspension feeders, it could be particularly significant during seasonal declines in nutrient concentrations and consequently phytoplankton production. However, direct evidence of kelp detritus assimilation by suspension feeders is scarce:  most inference has been based on stable isotope analyses which are often based on untested assumptions. To more directly address this problem, we measured  kelp detrital sloughing rates and ran experiments to assess kelp particle availability and suspension feeder utilization of naturally occurring kelp detritus when phytoplankton are either abundant or scarce. These studies were conducted with two kelp species: Ecklonia radiata in northeastern New Zealand and with Macrocystis pyrifera in Santa Barbara, California. The results of these studies indicate that phytoplankton rather than kelp detritus are the primary food source for subtidal suspension feeders.