Low Levels of Allochthony in Consumers Across Three High-Elevation Lake Types

The integration of lakes into watershed-scale energy budgets remains a major goal of aquatic ecosystem ecology. However, this enterprise has focused almost exclusively on temperate and boreal systems and on zooplankton as representatives of system-wide energy flow. We investigated the proportion of consumer biomass derived from terrestrial sources, allochthony, in three classes of high-elevation lakes—alpine, large and small montane—of varying geometry and watershed ecosystem development, and across five taxa, including macrobenthic invertebrates and fish. We used stable isotopes of carbon (¹³C:¹²C), nitrogen (¹⁵N:¹⁴N), and hydrogen (²H:¹H) to fit a modular Bayesian mixing model, which estimated proportional assimilation of phytoplankton, algal periphyton, and terrestrial organic matter by each consumer. The stable isotope analysis was supplemented with a comparison of fatty acid profiles between consumers and producers, and with a Daphnia magna rearing study involving aquatic and terrestrial nutrient sources. The most probable estimate of allochthony across consumer taxa was 41% in small montane lakes (< 0.1 ha) with high terrestrial C loading. For large montane (3–11 ha) and alpine lakes (0.8–3 ha), with substantially less terrestrial influence, allochthony was just 4 and 3%, respectively. Allochthony was also lower on average for benthic grazers than for pelagic consumers. Our results corroborate previous findings that lake size, depth, and light penetration are dominant physical controls on allochthony, but add that it sharply declines at high elevation due to changes in terrestrial primary production near or above tree line.