Living kelp versus plankton as food sources for suspension feeders

Large amounts of primary production are routed into detrital food webs, but the importance of detritus is poorly understood in many ecosystems. As one of the most conspicuous primary producers in temperate coastal ecosystems, kelps have long been hypothesized to provide detrital food to benthic suspension feeders through sloughing of canopy laminar tissue. Evidence for this phenomenon has largely rested on interpretation of carbon stable isotope values, and most direct evidence shows that phytoplankton, and to some extent zooplankton and bacteria, are the major sources of nutrition for marine suspension feeders. In this study, we experimentally tested the effects of kelp and naturally occurring phytoplankton on growth and survival of benthic suspension feeders in 2 kelp forest systems, southern California and New Zealand. To do so, we maintained suspension feeders in flow-through tanks with presence or absence of phytoplankton via raw and filtered seawater and presence or absence of mechanically agitated kelp laminae. Suspension feeder growth was significantly increased by phytoplankton but not kelp availability. Large amounts of kelp added to tanks, moreover, did not significantly contribute to the particulate organic matter (POM) pool, suggesting that sloughing from the eroding margins of kelp tissue is not a meaningful mechanism for small detrital particle production. Kelp forests are diverse and important ecosystems, but our results do not support the idea that kelps themselves trophically support subtidal and intertidal suspension feeders through detrital inputs.