Abstract
Hydrogen stable isotopes (δ2H) have recently been used to complement δ13C and δ15N in food web studies due to their potentially greater power to separate sources of organic matter in aquatic food webs. However, uncertainties remain regarding the use of δ2H, since little is known about the potential variation in the amount of exchangeable hydrogen (Hex) among common sample materials or the patterns of δ2H when entire food webs are considered. We assessed differences in Hex among the typical sample materials in freshwater studies and used δ2H, δ13C and δ15N to compare their effectiveness in tracing allochthonous matter in food webs of two small temperate lakes. Our results showed higher average amounts of Hex in animal tissues (27% in fish and macroinvertebrates, 19% in zooplankton) compared to most plant material (15% in terrestrial plants and 8% in seston/periphyton), with the exception of aquatic vascular plants (23%, referred to as macrophytes). The amount of Hex correlated strongly with sample lipid content (inferred from C:N ratios) in fish and zooplankton samples. Overall, the three isotopes provided good separation of sources (seston, periphyton, macrophytes and allochthonous organic matter), particularly the δ2H followed by δ13C. Aquatic macrophytes revealed unexpectedly high δ2H values, having more elevated δ2H values than terrestrial organic matter with direct implications for estimating consumer allochthony. Organic matter from macrophytes significantly contributed to the food webs in both lakes highlighting the need to include macrophytes as a potential source when using stable isotopes to estimate trophic structures and contributions from allochthonous sources.
Highlights
Stable isotope analysis (SIA) is routinely used to study food webs and energy flow within aquatic ecosystems
Many studies are faced with substantial difficulties in separating potential sources with sufficient precision due to their overlapping δ13C and δ15N values and/or too many sources to be separated with only two isotopes
Fish and macroinvertebrates had consistently more Hex in their tissues compared to zooplankton (F5,141 = 37.7, p < 0.001), while the amount in plant material was highly variable between aquatic and terrestrial plants (Table 1)
Summary
Stable isotope analysis (SIA) is routinely used to study food webs and energy flow within aquatic ecosystems. Enrichment studies, using for example 13C-bicarbonate [4, 11,12] or C4 plant material such as cane sugar or maize (Zea mays) [13,14,15], are a useful way to alter the naturally occurring isotope ratios within the sources to enhance separation. These methods are not always applicable due to high costs and effort, and alternative measures are needed
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