Fatty acid and stable isotope biomarkers suggest microbe-induced differences in benthic food webs between depths

Benthic marine consumers inhabiting the subphotic zone rely on subsidies of energy synthesized by macrophytes and phytoplankton in the photic zone. The effects of this energy subsidy on the trophic ecology of deep invertebrates are generally unknown. We used fatty acids (FA) and multiple stable isotopes (MSI) as trophic biomarkers to compare tissues from conspecifics of primary and secondary consumers in photic and subphotic habitats (15 and 100 m depth) at three sites in the San Juan Archipelago, Washington. FA composition differed across depths for all five species and MSI differed across depths for six of seven species. We found a general pattern of enrichment in delta C-13 from shallow to deep for all consumers. delta N-15 was consistently enriched in deep herbivores and suspension feeders, but did not differ in predators. Total omega-3 FA were lower in deep primary consumers, whereas predator omega-3 FA did not differ between depths. Total bacterial marker FA were lower in deep suspension feeders but higher in deep predators. The results suggest a possible mechanism for the differences in FA and enrichment between habitats: deep consumers potentially ingest detritus that has been biochemically altered by microbes during transport. We found support for this hypothetical mechanism in an algal aging experiment. Aged algae colonized by microbes responded with increases in bacterial FA, and decreases in omega-3 FA. This study highlights the power of combining FA and MSI biomarkers, and provides evidence for the importance of organic matter degradation to food web studies.