Environmental and Organismal Predictors of Intraspecific Variation in the Stoichiometry of a Neotropical Freshwater Fish

Rana W El-Sabaawi,Alexander S Flecker,Matthew Walsh,Joseph Travis,Steven A Thomas,James F Gilliam,Tyler J Kohler,Eugenia Zandoná,Michael C Marshall,Catherine Pringle,David N Reznick

doi:10.1371/journal.pone.0032713

Abstract

The elemental composition of animals, or their organismal stoichiometry, is thought to constrain their contribution to nutrient recycling, their interactions with other animals, and their demographic rates. Factors that affect organismal stoichiometry are generally poorly understood, but likely reflect elemental investments in morphological features and life history traits, acting in concert with the environmental availability of elements. We assessed the relative contribution of organismal traits and environmental variability to the stoichiometry of an insectivorous Neotropical stream fish, Rivulus hartii. We characterized the influence of body size, life history phenotype, stage of maturity, and environmental variability on organismal stoichiometry in 6 streams that differ in a broad suite of environmental variables. The elemental composition of R. hartii was variable, and overlapped with the wide range of elemental composition documented across freshwater fish taxa. Average %P composition was ∼3.2%(±0.6), average %N∼10.7%(±0.9), and average %C∼41.7%(±3.1). Streams were the strongest predictor of organismal stoichiometry, and explained up to 18% of the overall variance. This effect appeared to be largely explained by variability in quality of basal resources such as epilithon N∶P and benthic organic matter C∶N, along with variability in invertebrate standing stocks, an important food source for R. hartii. Organismal traits were weak predictors of organismal stoichiometry in this species, explaining when combined up to 7% of the overall variance in stoichiometry. Body size was significantly and positively correlated with %P, and negatively with N∶P, and C∶P, and life history phenotype was significantly correlated with %C, %P, C∶P and C∶N. Our study suggests that spatial variability in elemental availability is more strongly correlated with organismal stoichiometry than organismal traits, and suggests that the stoichiometry of carnivores may not be completely buffered from environmental variability. We discuss the relevance of these findings to ecological stoichiometry theory.

Highlights

Ecological stoichiometry expresses ecological interactions and biogeochemical processes as proportions of essential elements (C, N and P), whose flux is governed by mass-balance relationships
Our study suggests that environmental variables are stronger predictors of organismal stoichiometry than organismal traits such as body size, life history phenotype or stage of maturity
To the best of our knowledge, this is the first study to demonstrate a significant correlation between basal resource quantity or quality and the organismal stoichiometry of an insectivorous fish in the field

Summary

Introduction

Ecological stoichiometry expresses ecological interactions and biogeochemical processes as proportions of essential elements (C, N and P), whose flux is governed by mass-balance relationships. Mismatches between the elemental requirements of an animal and the supply of elements in its diet constrain growth and reproduction, which can have subsequent effects on a range of ecological, biogeochemical and physiological processes [2,3]. In this context the elemental composition of organisms, or their organismal stoichiometry, act as a proxy for their nutritional demand, and can constrain their ecosystem function, behavior and community interactions [4,5,6,7]. Investments in skeletal support as fish grow larger produce a positive relationship between P content and body size in many species [9]

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