Award: OCE-0962306

Suspension feeders, sometimes called filter feeders, sustain themselves by filtering particles of food from the water around them. Susupension feeding is a widespread strategy in the oceans, especially in shallow water where single-celled plants or phytoplankton are an abundant food source. Suspension feeders include mussels, oysters, and other bivalves as well as a multitude of other species from sea anemones to tunicates or sea squirts. Suspension feeders often dominate shallow marine communities in their biomass, and are economically important to many fisheries as well as ecologically important as links between plankton and larger predators that eat them such as marine mammals, fish and crustaceans. Since the 1980's, researchers have speculated that forests of kelp, in particular giant kelp, that are found in temperate waters may serve as an important food source for suspension feeders, and may in many cases supercede phytoplankton and other particles in their role sustaining suspension feeders. The evidence for this idea has been based almost entirely on carbon stable isotope data. Carbon stable isotopes can be used to trace sources of food through food webs, particularly if the sources are known and have significant differences in their stable isotope composition. Many studies have used the perceived isotope differences between kelp and phytoplankton to estimate the contribution of these sources to coastal marine food webs, particularly kelp forests. We found, however, that phytoplankton carbon isotope values vary substantially and often approach or overlap those of kelp. We tested the hypothesis that kelp is an important food source for suspension feeders in kelp forests off Santa Barbara, California, in several ways, and we also conducted some work with collaborators on kelp forests in new Zealand. We compared isotope values of suspension feeders on reefs that varied widely in kelp abundance, including areas where kelp was experimentally removed. We did experiments where we supplemented the diet of suspension feeders in the lab with naturally sloughing kelp detritus. We used essential fatty acids as an alternative tracer of kelp-derived food. In all this work, we found no support for the idea that kelp detritus is an important source of food for suspension feeders. Instead we found that phytoplankton is their main food source. However, we also found support for the hypothesis that through shading the bottom, kelp does increase the abundance of suspension feeders by inhibiting other macroalgae that compete with them for space. We also found that small kelp grazers in the canopy are an important source of food to canopy fish like kelp bass and kelp rockfish, which are important in recreational and commercial fisheries. Our results provide better information on the role of giant kelp in the kelp forest ecosystem, and can be used to inform managers who with to restore fish populations and kelp forests along the California coast. Last Modified: 07/17/2017 Submitted by: Robert J Miller