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In headwater streams that are heavily shaded by surrounding forests, primary production is often low and inputs of particulate and dissolved organic carbon from terrestrial environments are important resources in food webs. Stable isotope methods are useful for quantifying food web fluxes of different carbon sources but data are frequently difficult to interpret. Here, I developed novel stable isotope techniques to study the role of terrestrial energy subsidies in stream food webs in Trinidad and Tobago and the Adirondack region of New York. Specifically, I sought to determine the importance of carbon sources from terrestrial environments versus in-stream primary production in streams with varying light availability and fish communities. I compared natural canopy streams to streams with experimentally thinned canopies using a novel carbon and nitrogen dual isotope tracer technique. Comparing data from 13 C-acetate tracers, which are assimilated only by heterotrophic bacteria, and 15N- ammonium tracers, which are assimilated by both heterotrophs and autotrophs, allowed me to evaluate the relative importance of terrestrial carbon inputs and heterotrophic pathways. I also compared food webs in Trinidad and Adirondack streams along a natural gradient of canopy cover using stable isotopes of hydrogen, which indicated how much terrestrial organic matter was assimilated by consumers. In the Adirondacks, terrestrial carbon subsidies and heterotrophic bacteria were important resources for consumers, but the amount of bacterial carbon assimilated by invertebrates declined when light availability and primary production were experimentally increased. Fish introductions and light availability both influenced food webs in Trinidad streams. There were increased fluxes of carbon and nitrogen to consumers in streams with high light. Effects of fish introduction varied by consumer taxon, with strong positive effects of fish on filter-feeding invertebrates, and weaker positive effects of fish on grazer invertebrates. In comparative studies across light gradients, most consumer taxa were flexible in the amount of terrestrial carbon they assimilated, with higher reliance on terrestrial subsidies in streams with high canopy cover than in larger streams with less canopy cover. Overall, these results suggest that the light environment can have a strong influence on the role of terrestrial subsidies in streams.

Terrestrial inputs may play a substantial role in subsidizing aquatic food webs. The detritivorous gizzard shad (Dorosoma cepedianum) is a dominant fish in the eastern USA, particularly where terrestrial inputs of detritus and inorganic nutrients are high, as from agricultural watersheds. The extent to which gizzard shad consume settled phytodetritus versus detritus from terrestrial sources is unknown.This study used deuterium to quantify the contribution of terrestrial detritus to gizzard shad across 11 reservoirs in Ohio, USA with land use ranging from 6 to 86% agricultural. Deuterium values for gizzard shad were similar among all 11 reservoirs, with means ranging from -145.9‰ to -179.4‰. Allochthony estimates ranged from 27 to 48%. We found that allochthony was related to both land use and watershed area. Our results suggest that gizzard shad rely heavily on phytodetritus, while several factors may influence the availability and quality of terrestrial subsidies.