Terrestrial subsidies that enter aquatic systems from the surrounding watershed can play a crucial role in energy flow in aquatic ecosystems. Thus, quantifying the inputs of terrestrial organic matter to these systems is vital to understanding how aquatic food webs function. Deuterium (δD), a stable isotope of hydrogen, has recently been shown to discriminate between terrestrial and aquatic sources of primary production. We employed δD to quantify the amount of terrestrial subsidies incorporated into the zooplankton of alpine and subalpine lakes in the Beartooth Mountains, MT and WY (BT) (7 lakes; 2464-3143m elevation), and in Lake Tahoe and the Desolation Wilderness, CA (TD) (7 lakes; 1898-2128m elevation).
Copepods in the genus Diaptomus are consistently more depleted in δD than cladocerans in both the BT (-248 ± 9‰, -234 ± 13‰) and TD (-198 ± 3‰, -186 ± 6‰) lakes, indicative of strong reliance on autochthonous carbon. The patterns in δD are also consistent with δ13C of Diaptomus sp. and seston collected from offshore sites in Lake Tahoe; however the discrimination of food sources using δ13C was not as clear. In contrast, the δD of seston in both lake regions is enriched (BT -204 ± 8‰, TD -171 ± 4‰) compared to Diaptomus sp. and cladocerans and seston δD were more variable across lakes. This indicates that seston provides a combined source of terrestrial and aquatic energy in these low-productive lakes. These results differ from a river food web study suggesting combined reliance of primary consumers on both aquatic and terrestrial energy sources. Our results suggest that both Diaptomus sp. and cladocerans in alpine and subalpine lakes are utilizing a food source that is isotopically lighter than seston and may indicate that these herbivores feed selectively on autochthonous carbon in these systems. Even though there are methodological limitations of using δD, it may prove a useful tool in quantifying energy sources in aquatic ecosystems.