Trophic connections of productive mainstems and unproductive tributaries in river networks by migratory aquatic insects

Background/Question/Methods

Migratory animals connect spatially distinct ecosystems as “mobile links” by transporting resources or as mobile consumers. In river networks, productivity increases from headwaters to larger mainstems as channels widen and receive more sunlight. At confluences of small tributaries and large mainstems, the dark, cool and unproductive habitat, and the sunlit, warm and productive habitat are adjacent to each other.  This proximity makes it possible for emergence of aquatic insects produced in productive tributaries to spill over, or to migrate, into tributary habitats.  We have discovered a migratory mayfly in California, Ephemerella maculata (Ephemerellidae) that rears and emerges from productive mainstem rivers.  Females mate, then fly up adjacent small tributaries to oviposit.  Subsequently, their dead bodies drift and collect in pools, where they support aquatic and riparian predators in otherwise unproductive dark tributary habitats. To examine and compare the effect of the trophic subsidy by the migratory mayfly and terrestrial fallen insects on predators in tributaries, I experimentally manipulated the presence and the absence of mayflies and the terrestrial insects that jump into fenced sections of a tributary, and compared the growth of juvenile steelhead trout among treatments. 

Results/Conclusions

The results of the field manipulative experiment showed that the juvenile steelhead trout that were subsidized by the migratory mayflies grew significantly more than the ones that were not, while the elimination of inputs of terrestrial  insects did not change the growth of juvenile steelhead trout. The biological backflow mediated by this mayfly migration delivers critical trophic support for predators rearing in small tributaries. This trophic supply in tributaries may become increasingly important for salmonid juveniles in the warming rivers in California. Spatial variation in the water temperature in the mainstem channel induced at lower flows by geomorphic complexity can asynchronize mayfly emergence, extending the period of the subsidy, and increasing the trophic benefit for predators in the tributaries.