- Understanding food webs is fundamental in conserving endangered species and maintaining healthy ecosystem function, particularly in desert spring systems. We identified dominant energy sources in the Muddy River Warm Springs area, Clark County, NV using carbon and nitrogen natural abundance stable isotope analyses. We examined isotopic signatures of specific macroinvertebrate food resources for the endangered, drift-feeding Moapa dace (Moapa coriacea) and looked for changes in isotopic ratios over the stream gradient. Our results indicate the Muddy River Warm Springs area is supported primarily by allochthonous energy sources, and Moapa dace did not focus on specific macroinvertebrate taxa. We characterized a pattern of downstream δ¹³C enrichment, which is seemingly common in groundwater-fed systems but discussed very little in the literature. We conclude that ground water carbon of marine origin, heterotrophic respiration, and fractionation and depletion of the δ¹³C pool by autotrophs contribute to a wide range δ¹³C values in primary producers and to the downstream enrichment pattern in the Muddy River Warm Springs. We then quantified macroinvertebrate drift throughout the Moapa dace's current and historic range within the Muddy River Warm Springs. We examined the relationship of total drift biomass and temperature and associations between specific macroinvertebrate orders and Moapa dace abundance. We also explored habitat characteristics associated with Moapa dace and their prey. In 2008 and 2009, total drift biomass/time and the biomass of most orders was negatively correlated with temperature and positively correlated with discharge. Only Lepidoptera and Neotaenioglossa had positive associations with temperature in 2008. In 2009, total biomass/volume had a negative linear relationship with temperature and a positive linear relationship with discharge, though these relationships were absent in 2008. Moapa dace were associated with high temperatures, low stream discharge, shallow depth, and low drift biomass both years. They were positively associated with thermally tolerant Lepidoptera and Neotaenioglossa in 2008 and no orders in 2009. Macroinvertebrate drift may be greater in cooler, downstream reaches because temperatures in those reaches are favorable to more taxa, there is more cumulative benthic area contributing to production, and there are fewer drift feeding insectivores.
Lastly, we examined Moapa dace feeding flexibility by experimentally manipulating drift (0, 34, 64, 100, and 200% of the natural drift level) and quantified their feeding attempts and success. We found that Moapa dace are obligate drift feeders, selecting drift 40 times more frequently than benthos. They did not adaptively shift foraging modes from drift to benthic when drift was experimentally reduced. Drift feeding rate increased in response to increasing drift, but drift feeding success only increased from the 0% to the 64% drift level and did not change in the 100% and 200% levels. No relationship between fish size or food availability and aggression was detected. Moapa dace may share an interspecific foraging association with sympatric White River springfish (Crenichthys baileyi). Moapa dace are currently excluded from lower, more productive reaches by invasive species and fish barriers. Maintaining spring flows, eradicating non-natives, and restoring system connectivity will likely benefit Moapa dace and other Muddy River Warm Springs endemics.