White‐tailed deer consumption of emergent macrophytes mediates aquatic‐to‐terrestrial nutrient flows

1. Trophic interactions between mobile animals and their food sources often vector resource flows across ecosystem boundaries. However, the quality and quantity of such ecological subsidies may be altered by indirect interactions between seemingly unconnected taxa. We studied whether emergent macrophytes growing at the aquatic–terrestrial interface facilitate multi‐step aquatic‐to‐terrestrial resource flows between streams and terrestrial herbivores. We also explored whether aquatic animal aggregations indirectly promote such resource flows by creating biogeochemical hotspots of nutrient cycling and availability. 2. We tested whether white‐tailed deer (Odocoileus virginianus) in eastern North America vector nutrient fluxes from streams to terrestrial ecosystems by consuming emergent macrophytes (Justicia americana) using isotope and nutrient analyses of fecal samples and motion‐sensing cameras. We also tested whether mussel‐generated biogeochemical hotspots might promote such fluxes by surveying the density and nutrient stoichiometry of J. americana beds growing in association with variable densities of freshwater mussels (Bivalvia: Unionoida). 3. Fecal samples from riparian deer had 3% lower C:N and 20% lower C:P ratios than those in upland habitats. C and N isotopes suggested riparian deer ate both terrestrial and aquatic (J. americana) vegetation, whereas upland deer ate more terrestrial foods. Motion‐sensing cameras showed deer eating J. americana more than twice as frequently at mussel‐generated hotspots than non‐mussel sites. However, mussels were not associated with variation in J. americana growth or N and P content—although N isotopes in J. americana leaves did suggest assimilation of animal‐derived nutrients. 4. Our findings suggest that white‐tailed deer may conduct significant transfers of aquatic‐derived nutrients into terrestrial habitats when they feed on macrophytes and defecate on land. Whether aquatic animal aggregations promote such resource flows by creating biogeochemical hotspots remains unresolved, but the nearly global distributions of the deer family (Cervidae) and of macrophytes suggest that cervid‐driven aquatic‐to‐terrestrial nutrient flows may be widespread and ecologically important.