Sulphur stable isotopes can distinguish trophic dependence on sediments and plankton in boreal lakes

Abstract

1. Stable isotopes of carbon are useful for differentiating between freshwater food chains based on planktonic algae or benthic algae, but are reported to be of limited use for identifying food chains based on sedimentary detritus. Because data from marine systems suggest that stable isotopes of sulphur (d 34 S values) have potential in this regard, we tested their utility in freshwater lakes. 2. We found that sulphate in the water column of four boreal lakes was enriched in 34 S compared to the sulphur in bulk sediments from these lakes. Furthermore, within a given lake, insects known to feed on sediment (directly or via predation) had d 34 S values similar to those of sediment, whereas planktonic and benthic invertebrates known to feed on suspended particles had d 34 S values similar to those of sulphate in the water column. 3. Using the stable S isotope values of invertebrates that obtain their S from either the sediment or the water column as end members in a two-source mixing model, we show that two fish species obtain their food from both planktonic and sedimentary sources. Furthermore, model results suggest that, as expected, the more benthic-feeding fish species obtains more of its S from the sediment compartment than does the species that feeds in the water-column. 4. Our results suggest that measurements of stable sulphur isotopes provide a means of distinguishing between members of food chains that are based in the water column from those based on sedimentary detritus. As such, they would be a useful complement to stable C isotopes that are used to distinguish between food chains based on planktonic or benthic algae.

Using sulfur stable isotopes to assess mercury bioaccumulation and biomagnification in temperate lake food webs

“…In the lakes in the present study, most d 34 S measures of aqueous SO 4 2-(median 10.5‰), sediments (11.2‰), zooplankton (10.3‰), and all littoral invertebrate taxa (from 10.8‰ in Aeshnidae dragonflies to 12.0‰ in Limnephilidae caddis flies) were >10‰, a benchmark indicating dietary reliance on marine sources of S for birds (e.g., Ofukany et al [24] and Lavoie et al [46]). In fact, fish d 34 S values from our temperate systems were higher and more consistent with values from estuarine and marine fishes [26,28] than values in freshwater ecosystems in other parts of North America [12,14,25,27,44,45,47]. In the present study, the similarity of d 34 S composition in aquatic ecosystems in this region to estimates from marine systems may be related to their proximity to the coast.…”

“…Values of d 34 S in bulk sediments and aqueous SO 4 2from our study lakes fell within the range typical for precipitation SO 4 2- [43], and these d 34 S data were more positive than results from other freshwater food webs [12,14,25,27,44,45]. In the lakes in the present study, most d 34 S measures of aqueous SO 4 2-(median 10.5‰), sediments (11.2‰), zooplankton (10.3‰), and all littoral invertebrate taxa (from 10.8‰ in Aeshnidae dragonflies to 12.0‰ in Limnephilidae caddis flies) were >10‰, a benchmark indicating dietary reliance on marine sources of S for birds (e.g., Ofukany et al [24] and Lavoie et al [46]).…”

Bioaccumulation and Biomagnification of Mercury through Food Webs

“…Indeed, Ethier et al (2008) report correlations between δ 34 S and Hg concentrations in pumpkinseed sunfish from lakes in Ontario, Canada, suggesting a link between the cycling of these two elements. Another recent Canadian study of boreal lakes found that analysis of δ 34 S can distinguish between planktonic (sulfate in the water column) and benthic (sedimentary) sources of sulfur (Croisetière et al, 2009); thus, S isotopes may complement studies of N and C isotopes by adding greater resolution to our understanding of food web structure and Hg and nutrient sources.…”

Sulfur stable isotope indicators of residency in estuarine fish

“…The synthesis of organic sulfur in plants proceeds with little fractionation and plants typically have high d 34 S values near those of sulfate (Trust and Fry 1992). However, there are larger fractionations in the production of sulfides in benthic sediments, and entry of sulfides into benthic food webs leads to lower faunal d 34 S values Croisetiere et al 2009). These food web fractionations proved important in this study, but showed a consistent pattern across the salinity gradient so that d 34 S could still be used to track fish movement within salinity zones.…”