Spatial heterogeneity in water temperature asynchronizes aquatic insect emergence and prolongs a trophic supply to juvenile steelhead trout

Background/Question/Methods

When a mobile organism transports cross-habitat subsidies, its phenology will affect the temporal window of the trophic transfer. Spatial heterogeneity in subsidizing organisms’ source habitat may asynchronize their phenology, and prolong a resource pulse into the recipient habitat.  This spatial asynchrony and temporal extension can then strengthen trophic transfers to recipient consumers.  Natural rivers typically feature considerable spatial heterogeneity in patch- or reach-scale microhabitats, particularly in water temperature. Thermal variation may asynchronize timing of aquatic insect emergence, with strong potential consequences for consumers of this emergence.

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.  We measured growth rates and emergence timing of groups of 100 locally-collected E.maculata nymphs reared in flow-through enclosures, distributed over an 8.3 km reach of mainstem channel.  Simultaneously, the durations of the natural migratory mayfly subsidy was monitored at recipient tributaries of this mainstem reach.  This research complements another large scale field experiment planned to investigate the effect of the E.maculata subsidy period on juvenile steelhead growth in recipient tributaries. 

Results/Conclusions

Experimentally reared E.maculata nymphs emerged earlier at warmer temperatures.  Mayflies in nature emerged 1-3 weeks earlier from warmer sunlit mainstem habitats and later from cooler upstream reaches. While the emergence from each thermally distinct mainstem habitat lasts only two weeks, overall, the collective emergence from spatially separated E.maculata groups lasted four weeks, corresponding to the observed 4-week adult flight period in tributaries. Preliminary ³⁴S isotope analyses supported the observation that adults that arrived earlier in tributaries originated from warmer mainstem habitats, and later arrivals came from cooler upstream.  We predict that the temporal extension of the subsidy period increases the efficiency of trophic transfer to juvenile steelhead trout and other recipient consumers in tributaries.

The linkage of the spatial heterogeneity to the temporal duration of subsidies by the phenology of mobile organisms is infrequently quantified, but is likely of general importance. Multiple scales of spatial heterogeneities: source and sink habitats for the subsidy, and spatial heterogeneity affecting rates of growth and development in the source habitat, may interact, and play important roles in natural landscapes. Dynamic consequences of multi-scale spatial heterogeneity in temperature and other environmental controls over ecological interactions in natural landscapes need more attention in a changing world.