Spatial subsidy in the subtidal zone: utilization of drift algae by a deep subtidal sea urchin

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Spatial subsidies link adjacent habitats and can have strong effects on recipient systems. Within temperate nearshore marine ecosystems, algae dominate rocky benthic communities in the shallow subtidal but are supplanted by invertebrates at deeper depths where light becomes limiting. Detrital macrophytes transported into deep subtidal habitats by hydrodynamic forces may subsidize deep organisms, but little is known about the availability or utilization of this organic mate- rial. We used descriptive field studies to study drift macrophyte utilization by the red urchin Strongy- locentrotus franciscanus, a commercially harvested species found in both shallow and deep habitats in Washington, USA. Our primary goals were to evaluate urchin diet, urchin fecundity, and drift macrophyte abundance in the deep subtidal zone (>20 m depth). Although seasonally variable, macrophyte detritus was present year-round in the deep subtidal (23 m depth) and diet data suggest it is the primary food resource for deep subtidal red urchins. Perennial kelps were the main contrib- utors to the detrital pool (54% of biomass), particularly the shallow subtidal species Saccharina sub- simplex (37% of biomass). At 4 of our 5 study sites, red urchins from deep habitats had gonad indices as large as sea urchins from shallow habitats, suggesting that drift algae are an abundant food resource in the deep subtidal zone of our study region. A comparison of our results with previous studies indicates that the availability of macroalgal detritus in the deep subtidal varies considerably in space and may be influenced by the hydrodynamic context. We suggest that successful manage- ment of subsidized species may require understanding subsidy sources and fluxes because changes in the production or delivery of organic material can have important consequences for recipient populations.