Inland Waters (2014) 4, pp. 233-242 © International Society of Limnology 2014 DOI: 10.5268/IW-4.2.700 233 Article Assigning hydrogen, carbon, and nitrogen isotope values for phytoplankton and terrestrial detritus in aquatic food web studies Carol Yang 1 *, Grace M. Wilkinson 1 , Jonathan J. Cole 2 , Stephen A. Macko 1 , and Michael L. Pace 1 1 University of Virginia, Charlottesville, VA USA 2 Cary Institute of Ecosystem Studies, Millbrook, NY USA * Corresponding author: cy5n@virginia.edu Received 30 September 2013; accepted 22 January 2014; published 29 April 2014 Abstract Studies designed to assess the resources supporting aquatic consumers using stable isotope analysis require measure- ments of the potential end members (basal resources). While some basal resources are easily measured, it is often difficult to physically separate phytoplankton (one potential end member) from other components in seston. Further, terrestrial materials entering aquatic ecosystems undergo diagenetic change, potentially altering isotope composition and making it difficult to assign end member values. We tested techniques for determining the isotopic hydrogen (δ 2 H), carbon (δ 13 C), and nitrogen (δ 15 N) values of terrestrial and phytoplankton end members in seston. Long term in situ leaf decomposition experiments were performed. No appreciable change was found in the isotope values of degraded material (mean change 3.6‰ for δ 2 H, 0.0‰ for δ 13 C, and −0.1‰ for δ 15 N). We conclude that the isotope values of terrestrial plant material can be used to assign end members for terrestrial detritus. Using samples collected from 10 lakes with phytoplankton-dominated seston, we compared 3 published methods for estimating the δ 13 C and δ 15 N of phytoplankton. One method, which corrected bulk particulate organic matter (POM) isotope values based on a δ 2 H mixing model, accurately predicted measured phytoplankton δ 13 C. Another method, which used a C:N mixing model to correct bulk POM, also performed well. A new method, proposed here, modified seston isotope values using the difference in C:N of phytoplankton and terrestrial material in a δ 2 H mixing model and correctly predicted measured phytoplankton δ 15 N. We recommend estimating phytoplankton δ 13 C and δ 15 N by correcting bulk POM using a δ 2 H mixing model, with the C:N modification proposed here for δ 15 N. Key words: food web, mixing models, phytoplankton, stable isotope analysis, terrestrial organic matter Introduction Stable isotope analysis is a common tool for evaluating resource availability, trophic structure, and consumer basal resource use. In aquatic ecosystems, stable isotope values of carbon ( 13 C/ 12 C: δ 13 C), nitrogen ( 15 N/ 14 N: δ 15 N), and hydrogen ( 2 H/ 1 H: δ 2 H) can be employed to quantify the use of terrestrial (allochthonous) organic matter by aquatic consumers (Marcarelli et al. 2011). Methodologi- cal difficulties arise when quantifying consumer basal resources using mixing models, however, because assigning appropriate end member values for some source materials is complicated. For example, phytoplankton are often difficult to physically separate from bulk seston (Hamilton et al. 2005). In addition, some materials with long residence times (e.g., pelagic and benthic detritus) may undergo changes in isotope ratios over time. Although H isotopes may present an advantage over C and N isotopes owing to the large end member separation (Doucett et al. 2007), uncertainties remain in estimating the incorporation of environmental water in consumer tissues (Solomon et al. 2009), diagenetic changes of end member isotope values (Macko et al. 1983), and the photosynthetic discrimination relative to water that results in the depletion of the H isotope deuterium in phytoplankton.