Temporal shift in contribution of terrestrial organic matter to consumer production in a grassland river

• Published in: Freshwater biology Vol. 46; no. 2; pp. 213 - 226
• Main Authors: Huryn, Alexander D., Riley, Ralph H., Young, Roger G., Arbuckle, Chris J., Peacock, Kathi, Lyon, Graeme
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01-02-2001; Blackwell Science; Wiley Subscription Services, Inc
• Subjects: algae; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; detritus; food webs; Fresh water ecosystems; Freshwater; Fundamental and applied biological sciences. Psychology; New Zealand; seston; stable isotope analysis; Synecology; New Zealand; Oceania; New Zealand, South I., Taieri R; Terrestrial environment; Grassland; Isotopic analysis; Organic carbon; Organic matter; Litter; Material flow; Algae; Aquatic environment; Phytophagous; Food web; Secondary productivity; Seston; Stream; Invertebrata; Thallophyta
• ISSN: 0046-5070; 1365-2427
• DOI: 10.1046/j.1365-2427.2001.00648.x

1. We used stable isotopes to study the temporal (early summer versus autumn) pattern of use of terrestrial and aquatic sources of organic carbon by consumers in two bedrock‐confined reaches of a grassland river in New Zealand. 
2. The major sources of organic carbon available to primary consumers were expected to be terrestrial leaf‐litter and biofilm from the stream channel. These putative carbon sources showed no significant change in mean δ13C between summer and autumn. Leaf litter (mean δ13C<−26.25) was depleted in 13C compared to biofilm (mean δ13C>−19.92). 
3. In contrast to leaf litter and biofilm, the δ13C of consumers changed over time, being enriched in 13C in the autumn compared with early summer. Both the magnitude (>5‰ in some cases) and rapidity of this shift (<3 months) was surprising. 
4. A two‐source mixing model indicated that, during early summer, terrestrial carbon comprised> 50% of tissue carbon for 15 of the 17 taxa of aquatic consumers analysed. During autumn, terrestrial carbon comprised> 50% of the tissue carbon of only five of 25 taxa. Because the mean δ13C of putative food sources was consistent over time, the shift in δ13C values for consumers is attributed to a change in relative amounts of terrestrial and aquatic carbon available for consumption. 
5. Because seston consists of a mixture of many particles of diverse origin, it may provide an integrated measure of catchment‐wide sources of organic matter entering a stream channel. Like the tissues of most consumers, mean δ13C values for seston showed a significant shift toward 13C enrichment. This indicated that the relative availability of terrestrial carbon decreased from summer to autumn. 
6. The actual quantity of carbon contributed to the stream food‐web by this potential terrestrial–aquatic link is unknown. Although terrestrial carbon may comprise a high proportion of the tissue carbon of consumers prior to summer, the majority of secondary production (and carbon sequestration) probably occurs during early summer as a consequence of rising temperature and high quality food in the form of biofilm.