Islands in the stream: kelp detritus as faunal magnets

Biomass from nearshore primary producers can be an important subsidy to both pelagic and benthic communities, which are disconnected in space from sources of production. We examine the role of this macrophyte biomass in two habitats (pelagic and nearshore benthic) in terms of both trophic support and spatial refugia. Experimental benthic “islands” of seaweed biomass created nearshore subtidal habitats (at 10–30 m depth) which were colonized by diverse fauna, ranging from abundant harpacticoid copepods, to shrimp and snails, to small fishes. The abundance and species composition of colonizers varied somewhat among kelp species and with the degree of degradation of the kelp. Use of plastic strips as kelp mimics allowed us to decouple primary trophic and refugia effects; plastic attracted some macrofauna but far fewer organisms in the <1.5 mm size class. Observational data taken from floating pelagic detrital islands showed that these algal rafts similarly attract many small crustaceans and other invertebrates, which then can serve as prey for consumers such as juvenile rockfish. The fauna of the pelagic drift islands was dominated by harpacticoid copepods and gammarid amphipods. Stomach contents of associated juvenile splitnose rockfish were dominated by gammarids and calanoid copepods.