Temporal variability of benthic algal δ13C signatures influences assessments of carbon flows in stream food webs

Abstract

Stream food web function is often assessed using carbon stable isotope assessments of the relative contribution of autochthonous and allochthonous sources of organic matter to consumer diets. As a result, variability in source signatures can strongly influence the assessment of carbon flows. To examine the implications of temporal source variability on food web interpretations, benthic algal d 13 C signatures were measured over 8 weeks in five streams in subtropical Queensland, Australia. All food webs were largely driven by benthic algal carbon; however, substantial week-to-week variation in benthic algal d 13 C signatures modified the calculated contributions of algae to consumer diets, with differences in autochthonous contributions of up to 11% between weeks. In addition, variable algal signatures led to many occasions in which the d 13 C signatures of some consumers was beyond the range of available sources, meaning the mixing model analyses did not have a valid solution. Together, these findings suggest that temporal variability in algal d 13 C signatures can strongly influence the interpretation of carbon flows in stream food webs. Future food web studies should assess the temporal variability of sources prior to sampling consumers, in order to characterise end member signatures and their relevance to consumers at the time of collection.

The effects of dams on longitudinal variation in river food webs

“…13 C in our rivers may have been due to high variability in the d 13 C values among species of algae and other biota that make up the biofilm (Hadwen et al 2010b). An alternative explanation is that the factors that influence fractionation of d 13 C by algae, such as carbon supply and photosynthetic rates, may be too spatially or temporally (sampling took place over several months) variable across our rivers to generate consistent longitudinal patterns.…”

Consequences of employing amino acid vs. bulk‐tissue, stable isotope analysis: a laboratory trophic position experiment

“…From an ecological perspective, both d 15 N and d 13 C can vary spatially and temporally in even pristine ecosystems-a problem which seems especially acute in rivers. Variations in flow within small to large rivers can alter d 13 C ratios laterally over distances of a few meters in some cases and vertically within millimeters in benthic algal layers , Hayes 1993, Herman et al 2000, Hadwen et al 2010, Woodland et al 2012, Jardine et al 2013. Likewise, d 15 N ratios can vary from areas in the main channel with its mostly continuous flows to lateral backwater areas with minimal to zero flows, as both sources and N-processing pathways change (e.g., variable abundance of denitrifying bacteria) (Thorp et al 2008).…”

“…However, the use of bulk-tissue analysis has potential limitations in aquatic systems. Field conditions often restrict its use because: (1) autotrophic sources may be unknown; (2) it is difficult to obtain clean epilithic and epiphytic algae (uncontaminated with other food items, including host vascular tissue) or suspended algae uncontaminated with dead organic matter (but see colloidal silica separation techniques in ); and (3) algal signatures are often highly variable in time and space , Hayes 1993, Herman et al 2000, Hadwen et al 2010, Woodland et al 2012, Jardine et al 2013. In an attempt to circumvent this problem, some ecologists have used the isotopic signatures of primary (herbivorous) consumers.…”

Influence of substratum on the variability of benthic biofilm stable isotope signatures: implications for energy flow to a primary consumer

“…Stable isotope analysis (SIA) is a frequently used technique, which has facilitated the tracing of allochthonous and autochthonous carbon through freshwater food webs as it measures assimilation rather than simply ingestion (e.g. Huryn et al, 2001;Reid et al, 2008;Hadwen et al, 2010). However, biofilm d 13 C signatures are typically quite spatially and temporally variable in freshwater ecosystems (France, 1995;Finlay, 2001;Hadwen et al, 2010;Hladyz et al, 2010).…”