Kelp forest size alters microbial community structure and function on Vancouver Island, Canada

Bacteria are ubiquitous and important components of marine ecosystems, yet the interaction between bacteria and higher trophic levels remain poorly understood. The trophic cascade involving sea otters, urchins, and kelp in the North Pacific is a classic case of altered ecosystem states; however, its impacts on microbial communities are unknown. We investigated the response of microbial communities to variation in kelp abundance between regions with and without sea otter populations along the west coast of Vancouver Island, British Columbia, Canada. We compared bacterial community structure and function between regions with large and small kelp forests, including an subset of the bacterial community that produces alginate lyase, which allows direct utilization of kelp carbon. The abundance and activity of alginate-lyase-producing bacteria were 3.2 and 1.4 times higher, respectively, in the region with large kelp forests, and declined rapidly with increasing distance from kelp. Total bacterial abundance was 2.7 times greater, and bacteria grew faster and experienced more zooplankton grazing and viral-mediated mortality in the presence of large kelp forests. These patterns suggest that larger kelp forests produce more detritus and dissolved organic matter, which stimulate microbial activity. Our results indicate that variation in kelp forest size alters the community structure and productivity of microbes and contributes to the growing evidence that top predators interact with microbes and ecosystem processes through a cascade of indirect effects.