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

Evelyn M Keaveney,Robert H Foy,Paula J Reimer

doi:10.2458/azu_rc.57.18355

Evelyn M Keaveney, Robert H Foy + Show 1 more

Open Access

https://doi.org/10.2458/azu_rc.57.18355

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Abstract

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 (Δ14C) was used to examine the age and origin of carbon in Lower Lough Erne, Northern Ireland.14C 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 δ15N values in calanoid zooplankton (δ15N = 24‰) relative to phytoplankton and particulate organic matter (δ15N = 6‰ and 12‰, respectively) may reflect several microbial trophic levels between terrestrial C and calanoid invertebrates. Winter and summer calanoid Δ14C values show a seasonal switch between autochthonous and terrestrial carbon sources. Fish Δ14C values indicate terrestrial support at the highest trophic levels in littoral and pelagic food webs.14C therefore is useful in attributing the source of carbon in freshwater in addition to tracing the pathway of terrestrial carbon through the food web.

Highlights

The investigation of trophic levels in a food web is complex yet fundamental when understanding the dynamics of a freshwater system
Perch (Perca fluviatilis, n= 16) collected in the same location had elevated δ 15N, yet perch were more 13C -enriched than pollan and trout
The use of the radiocarbon method in analysing the Lower Lough Erne food web has elucidated the sources of carbon utilised in the lake

Summary

Introduction

The investigation of trophic levels in a food web is complex yet fundamental when understanding the dynamics of a freshwater system. Identification of carbon sources can be problematic due to large variation of stable carbon isotope values in freshwater systems, and becomes even more difficult when there is a significant terrestrial subsidy of the food web as terrestrial and freshwater isotope values often overlap. Lakes with higher trophic status may be supported by catchment inputs; the terrestrial support of pelagic food webs in such lakes is difficult to ascertain and can be underestimated. Terrestrial carbon can supplant autochthonous carbon in systems with low algal abundance. The availability of this carbon to lake heterotrophs (organisms that cannot fix carbon but utilise organic carbon for growth) is related to its age, which in turn is related to its biochemical composition and nutrient quality (biochemical quality). Bacteria have been shown to metabolise both autochthonous carbon and labile and decayed terrestrial carbon (Guillemette et al 2013) but preference is linked to nutrient content which decreases over time leading to preferential selection of autochthonous and labile terrestrial carbon sources (Taipale et al 2007; McCallister and Giorgio 2008; Taipale et al 2008; de Kluijver et al 2012; Zigah et al 2012)

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