Use of Stable Isotopes to Test Literature-Based Trophic Classifications of Small-Bodied Stream Fishes

By Franssen, Nathan R.; Gido, Keith B. | The American Midland Naturalist, July 2006 | Go to article overview

Use of Stable Isotopes to Test Literature-Based Trophic Classifications of Small-Bodied Stream Fishes


Franssen, Nathan R., Gido, Keith B., The American Midland Naturalist


ABSTRACT.

Defining the trophic position of stream organisms is a first step in understanding the ecology of lotic systems. Whereas trophic positions of stream fishes have been traditionally assigned based on dietary analysis, stable isotope ratios may provide additional information on the validity of this approach and may be used to verify energy acquisition assumed from dietary studies. In this study, we assessed the concordance of literature-based trophic classifications and isotopic δ^sup 15^N signatures for small-bodied fishes from four streams in Kansas, Oklahoma and New Mexico. ANOVA results revealed no significant difference (F^sub 2,27^ = 1.71, P = 0.201 ) in trophic position based on δ^sup 15^N values among three broad trophic classifications derived from literature sources (algivores/detritivores, omnivores and invertivores). Both the prevalence of omnivory in stream fishes and potential biases associated with isotope fractionation at different trophic levels poses potential problems when classifying lotic fish into trophic positions.

INTRODUCTION

Trophic classification of fishes aids our understanding of interactions among aquatic organisms by providing a general index of diet overlap and a basis for the construction of food webs. This information can be used to make inferences on the direct or indirect influences of species within or among trophic groups. Trophic groupings also provide insight into the functional roles (e.g., top down versus bottom up effects) that fishes play in aquatic systems (Power, 1992; Gelwick et al., 1997; Matthews, 1998). Understanding these food web interactions is often necessary for conservation. For example, resource use by introduced species might help predict which species are successful invaders as well as their potential effects on native fish assemblages (Moyle and Light, 1996; Kolar and Lodge, 2002). Moreover, trophic classification is central to indices of biotic integrity that are commonly used to evaluate stream health (Karr, 1981). Given the overwhelming importance of trophic classification in the field of aquatic ecology, it is necessary to evaluate classification schemes across species and aquatic habitats.

Two common methods for classifying fishes into trophic groups are diet and stable isotope analysis. Diet analysis, based on an evaluation of ingested material, only reflects recently consumed items (<24 h), and does not discriminate between ingested and assimilated material. Gut contents might indicate the structural affects of fishes in ecosystems, as most materials ingested (assimilated or not) are killed or at least temporarily removed from the system. More recently, the ratio of ^sup 15^N: ^sup 14^N has been used as an indicator of trophic position and the ratio of ^sup 13^C: ^sup 12^C as an indicator of carbon source. Stable isotope ratios differ from traditional gut content analysis by providing a time-integrated measure of trophic position that only reflects items that were assimilated by the organism. However, the use of isotopes is limited by system specific variability in baseline isotope ratios (Vander Zanden et al., 1997; Vander Zanden and Rasmussen, 1999) and trophic shifts may not immediately be reflected in the isotopic signatures of fishes due to tissue turnover time lags, which are dependant on growth rates and the metabolic rates of specific tissues (Hesslein et al., 1993; Sakano et al., 2005). Assuming these biases are controlled, differences between diet and stable isotope analysis should reflect differential assimilation of ingested material.

Our primary objective was to use the N isotope signature of muscle tissues to test trophic classifications of small-bodied stream fishes based on literature-based diets. Whereas isotopic ratios of N should be notably different among trophic specialists from different trophic levels, classifying omnivorous fishes into trophic positions may be problematic (Gu et al., 1997; Pinnegar and Polunin, 2000; Ponsard and Arditi, 2001; Jepsen and Winemiller, 2002). …

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