Resource subsidies can strongly affect food web structure and dynamics. Along with habitat shape and boundary permeability, relative productivities of donor and recipient habitats have been suggested to influence when and where subsidies are important. Yet, empirical support for this spatial framework is lacking. We suggest that efforts to test these predictions are hindered in part by the absence of a universally used definition of resource subsidy that clearly assigns the source of production. In particular, when whole organisms comprise the resource subsidy the habitat where the organic molecules that comprise their bodies were produced (i.e. "fixed") is not necessarily the same as the habitat where their tissues were formed. Here we measure the source and biomass of the carbon and nitrogen atoms in the bodies of emerging aquatic insects to estimate the flow of aquatically vs. terrestrially derived resources from small experimental pools to surrounding terrestrial food webs. We manipulated terrestrial leaf inputs to pools and light levels to determine the independent and combined influence of these two factors on the relationship between insect emergence and flux of aquatically derived resources to land in a naturally assembling community.

Results/Conclusions

We found that doubling terrestrial inputs resulted in a two- to four-fold increase in the number of Culex mosquito eggs laid in pools. However, the abundance and biomass of emerging mosquitoes showed a bell-shaped curve: three times more mosquitoes emerged from pools receiving medium amounts of terrestrial input vs. high or low. Primary productivity increased with terrestrial inputs in both high and low light treatments. Stable isotopes were used to distinguish carbon and nitrogen derived from aquatic vs. terrestrial sources in emerging insects (Culex mosquitoes). We found that filter-feeding larvae primarily assimilated terrestrial carbon, but that the proportion of their diets coming from terrestrial resources increased then decreased with terrestrial plant inputs. As a result, we found that total biomass of terrestrial resources leaving the aquatic system was highest in pools receiving medium amounts of terrestrial leaf inputs, where emerging insects were the most numerous and their diet was most terrestrial. Determining the source of organismal subsidies may be particularly crucial in systems where reciprocal subsidies and animals with complex life histories are common, such as wooded ponds and headwater streams. Establishing the true source of production is essential for predicting when and where resources subsidies will be important in food web dynamics.