Young, Old, and Weathered Carbon—Part 2: Using Radiocarbon and Stable Isotopes to Identify Terrestrial Carbon Support of the Food Web in an Alkaline, Humic Lake

Carbon (C) and nitrogen (N) stable isotope analysis (SIA) has been used to identify the terrestrial subsidy of freshwater food webs. However, SIA fails to differentiate between the contributions of old and recently fixed terrestrial C and consequently cannot fully determine the source, age, and biochemical quality of terrestrial carbon. Natural abundance radiocarbon (Δ 14 C) was used to examine the age and origin of carbon in Lower Lough Erne, Northern Ireland. 14 C and stable isotope values were obtained from invertebrate, algae, and fish samples, and the results indicate that terrestrial organic C is evident at all trophic levels. High winter δ 15 N values in calanoid zooplankton (δ 15 N = 24‰) relative to phytoplankton and particulate organic matter (δ 15 N = 6‰ and 12‰, respectively) may reflect several microbial trophic levels between terrestrial C and calanoid invertebrates. Winter and summer calanoid Δ 14 C values show a seasonal switch between autochthonous and terrestrial carbon sources. Fish Δ 14 C values indicate terrestrial support at the highest trophic levels in littoral and pelagic food webs. 14 C therefore is useful in attributing the source of carbon in freshwater in addition to tracing the pathway of terrestrial carbon through the food web.