Challenges for interpreting stable isotope fractionation of carbon and nitrogen in tropical aquatic ecosystems.
Abstract: Food
web studies have greatly benefited from the use of stable isotope
analyses, especially of carbon and nitrogen (FRY 2006). Understanding
the isotope fractionation between consumers and prey is vital to
constructing food webs, but this is not a simple relationship. Earlier
field studies on pelagic temperate freshwater systems (VANDERZANDEN
& RASMUSSEN 1999, 2001, POST 2002) indicated that fractionation of
δ13C was ca 0.5‰ and of δ15N ca. 3.4‰ per
trophic step. [Notation for trophic fractionation is Δδ13C
and Δδ15N.] In their meta-analysis, VANDERZANDEN &
RASMUSSEN (2001) found differences for Δδ15N between field
and laboratory studies (3.4‰ vs 2.7‰ δ15N,
respectively) and noted large variations for herbivores, although this
was not observed by POST (2002). However, studies in tropical streams
in Costa Rica and Puerto Rico (KILHAM & PRINGLE 2000) and in Panama
(HUNTE-BROWN 2006, VERBURG et al. 2007) indicated that
Δδ15N was much lower (ca. 1.6‰) and of
Δδ13C much higher (ca. 1.4‰) per trophic step (Table
1).
One of the difficulties in comparing isotope fractionation in lakes and streams is that lake food webs tend to have distinct trophic categories, especially in pelagic food webs, but it is often difficult to discern clear trophic levels in an examination of a biplot of δ13C and δ15N for stream food webs. This may be due to a greater degree of omnivory in streams, or simply because there is a much broader range of basal resources available, which creates a number of simultaneous food chains that overlap. This can also be true for littoral food webs in lakes (POST 2002). Basal resources in streams are often complex mixtures of materials with varying sources of both carbon and nitrogen. For example, biofilms on stream substrates, which are a dominant benthic food source, are generally mixtures of autotrophs and heterotrophs (STOCK & WARD 1989). Furthermore, patchiness of resources is greater in streams than lakes and flow variation is also greater.
If a greater understanding can be achieved of the important processes leading to this variation in stable isotope fractionation, we may be able to extract more information about interactions within food webs. This paper will explore some of these sources of variation in stream food webs. The sources of variation fall into several categories: turnover rates, trophic level, taxon, and environmental conditions.
One of the difficulties in comparing isotope fractionation in lakes and streams is that lake food webs tend to have distinct trophic categories, especially in pelagic food webs, but it is often difficult to discern clear trophic levels in an examination of a biplot of δ13C and δ15N for stream food webs. This may be due to a greater degree of omnivory in streams, or simply because there is a much broader range of basal resources available, which creates a number of simultaneous food chains that overlap. This can also be true for littoral food webs in lakes (POST 2002). Basal resources in streams are often complex mixtures of materials with varying sources of both carbon and nitrogen. For example, biofilms on stream substrates, which are a dominant benthic food source, are generally mixtures of autotrophs and heterotrophs (STOCK & WARD 1989). Furthermore, patchiness of resources is greater in streams than lakes and flow variation is also greater.
If a greater understanding can be achieved of the important processes leading to this variation in stable isotope fractionation, we may be able to extract more information about interactions within food webs. This paper will explore some of these sources of variation in stream food webs. The sources of variation fall into several categories: turnover rates, trophic level, taxon, and environmental conditions.
Kilham, S., M. Hunte-Brown, P. Verburg, C. M. Pringle, M. R. Whiles, K. R.Lips, and E. Zandona. 2008. Challenges for interpreting stable isotope fractionation of carbon and nitrogen in tropical aquatic ecosystems. Verhandlungen Internationale Verein. Limnol. 30: xx-xx.
Back to Literature Page
| Pringle's Home Page | University of Georgia | Odum School of Ecology |