DISSERTATION RESEARCH: Thermal heterogeneity in river mainstems prolong a subsidy to tributary salmonids by migratory mayflies

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Natural rivers are comprised of many different environments. These environmental differences create variation in water temperatures, particularly in summer, when the water levels are low. Warmer temperatures cause many aquatic insects to grow faster and emerge earlier than cooler temperatures. Given the temperature variability which occurs within a given river, timing of insect emergence is not synchronized. Insects emerge from warmer habitats first, then cooler habitats. Variable aquatic insect emergence extends the overall emergence period of aquatic insects from the river, and supply food to predators for a longer period of time than if all insects emerged at once. This project investigates how the temperature variation in natural habitats sustains prolonged insect emergence from rivers, and effects of this on the growth of predators, such as fish, that eat emerging insects from the river. The importance of environmental variation in nature has been discussed with regard to its effects on biodiversity and species interactions, however interactions of variation in space and time are less well-studied. This project investigates how spatial variation translates into temporal variation through spatially asynchronizing life history events of a key prey species, and how that affects the transfer of energy to predators, including the steelhead trout, a game fish in California. This linkage of spatial heterogeneity with its temporal and trophic consequences is of general importance, but needs more attention from field ecologists in light river restoration efforts to support healthy fish populations.The mayfly Ephemerella maculata emerges from sunlit mainstem rivers, flies into dark, unproductive tributaries, oviposits, and dies. Their mass migration subsidizes tributary predators that would otherwise be food-limited. In this project, the hypothesis that spatial abiotic heterogeneity in the mainstem strengthens the effect of the E. maculata subsidy on the growth of juvenile steelhead trout in the tributaries, will be tested in three ways. 1] A field rearing experiment to see if thermal heterogeneity created by the geomorphological and hydrological heterogeneity in the mainstem river spatially varies the emergence timing of E. maculata; [2] Isotope analysis will address the question if asynchronous emergence and migration of adult E. maculata into tributaries prolong a trophic subsidy that would otherwise be only a brief pulse; [3] A field manipulative experiment to see if the prolonged resource subsidy by E. maculata increases the efficiency of trophic transfer, and consequently the total growth of juvenile steelhead trout, during their critical summer growth period.