Structure of tropical river food webs revealed by stable isotope ratios

Abstract

2002. Structure of tropical river food webs revealed by stable isotope ratios. -Oikos 96: 46 -55.Fish assemblages in tropical river food webs are characterized by high taxonomic diversity, diverse foraging modes, omnivory, and an abundance of detritivores. Feeding links are complex and modified by hydrologic seasonality and system productivity. These properties make it difficult to generalize about feeding relationships and to identify dominant linkages of energy flow. We analyzed the stable carbon and nitrogen isotope ratios of 276 fishes and other food web components living in four Venezuelan rivers that differed in basal food resources to determine 1) whether fish trophic guilds integrated food resources in a predictable fashion, thereby providing similar trophic resolution as individual species, 2) whether food chain length differed with system productivity, and 3) how omnivory and detritivory influenced trophic structure within these food webs. Fishes were grouped into four trophic guilds (herbivores, detritivores/algivores, omnivores, piscivores) based on literature reports and external morphological characteristics. Results of discriminant function analyses showed that isotope data were effective at reclassifying individual fish into their pre-identified trophic category. Nutrient-poor, black-water rivers showed greater compartmentalization in isotope values than more productive rivers, leading to greater reclassification success. In three out of four food webs, omnivores were more often misclassified than other trophic groups, reflecting the diverse food sources they assimilated. When fish d 15 N values were used to estimate species position in the trophic hierarchy, top piscivores in nutrient-poor rivers had higher trophic positions than those in more productive rivers. This was in contrast to our expectation that productive systems would promote longer food chains. Although isotope ratios could not resolve species-level feeding pathways, they did reveal how top consumers integrate isotopic variability occurring lower in the food web. Top piscivores, regardless of species, had carbon and nitrogen profiles less variable than other trophic groups.

The trophic position of the Atlantic blue crab Callinectes sapidus Rathbun 1896 in the food web of Parila Lagoon (South Eastern Adriatic, Croatia): a first assessment using stable isotopes.

“…Isotopic signatures were expressed in conventional ä notation (as parts per mil) in relation to international standards (PeeDee Belemnite for carbon and atmospheric N 2 for nitrogen). The formula is a generalization of that presented in Jepsen and Winemiller (2002), where ä 15 N Consumer is the nitrogen isotopic signature of the blue crab, Δ 15 N is the trophic level fractionation of ä 15 N, while ä 15 N Baseline and λ are the nitrogen isotopic signature and the trophic level of the baseline indicator, respectively. ä 15 N Baseline and the associated standard deviation SD Baseline was calculated using the mean isotopic signature of the filter feeder bivalve Mytilus galloprovincialis, expected to be one of the blue crab preferred prey (e.g., Hines, 2007).…”

Changes in Fish Diets and Food Web Mercury Bioaccumulation Induced by an Invasive Planktivorous Fish

“…Gut content analyses are biased toward diet items recently consumed and provide only point-in-time analysis of prey consumed but not necessarily assimilated. Carbon and nitrogen stable isotopes provide a means for overcoming some of these limitations and have been used as a robust way to model complex trophic dynamics (Jeppesen and Winemiller 2002). Stable isotopes provide an integrated index of diet assimilated over a much longer time span (Maruyama et al 2001), and each isotope offers different information regarding energy input.…”

Mercury Trophic Transfer in a Eutrophic Lake: The Importance of Habitat‐specific Foraging

“…Carbon and nitrogen stable isotopes overcome some of these limitations and have been used as a robust way to model complex trophic dynamics (Jepesen and Winemiller 2002). Stable isotopes, however, rarely provide taxon-specific information about prey items.…”