Kelp and rivers subsidize rocky intertidal communities in the Pacific Northwest (USA)

Rocky intertidal filter feeders couple benthic and pelagic systems by consuming particles in the water column and transferring energy and nutrients to sediments and benthic predators. Less is known about their role as links between terrestrial and marine systems. I explored this link using a dual stable isotope approach to discern the diets and trophic position of 4 common sympatric rocky intertidal filter feeders (Mytilus californianus, Balanus glandula, Semibalanus cariosus, Pollicipes polymerus) living near river mouths in the Pacific Northwest (USA). Freshwater particulate organic matter (POM) was assimilated by all filter feeders, though at relatively low levels (5 to 41%). Intertidal macroalgae and seagrass also contributed to filter feeder diets, but the primary resource base was kelp in both summer and winter, accounting for 11 to 88%, in filter feeder diets, depending on species and season. Although these species shared a common resource base, they occupied distinct trophic positions. Natural and logging-induced increases in riverine inputs were related to higher trophic position for all species, indicating that riverine carbon may enter the rocky intertidal food web through the microbial loop. I show that filter feeder trophic positions were higher in winter when river discharge is highest, near river mouths (in winter) where river-borne inputs are highest, and downstream of more heavily logged watersheds where river loads of dissolved organic carbon are highest. Rocky intertidal filter feeders do couple land and sea, further emphasizing the need to manage these ecosystems jointly.