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

1. We used stable isotopes to study the temporal (early summer versus autum n) pattern of use of terrestrial and aquatic sources of organic carbon by c onsumers in two bedrock-confined reaches of a grassland river in New Zealan d. 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 delta C -13 between summer and autumn. Leaf litter (mean delta C-13 < - 26.25) was depleted in C-13 compared to biofilm (mean delta(13)C > - 19.92). 3. In contrast to leaf litter and biofilm, the delta C-13 Of consumers chan ged over time, being enriched in C-13 in the autumn compared with early sum mer. Both the magnitude (> 5 parts per thousand in some cases) and rapidity of this shift 1 (less-than or equal to 3 months) was surprising. 4. A two-source mixing model indicated that, during early summer, terrestri al carbon comprised > 50% of tissue carbon for 15 of the 17 taxa of aquatic consumers analysed. During autumn, terrestrial carbon comprised > 50% of t he tissue carbon of only five of 25 taxa. Because the mean delta C-13 of pu tative food sources was consistent over time, the shift in delta C-13 value s for consumers is attributed to a change in relative amounts of terrestria l 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 delta C-13 values for seston showed a significant shift toward C-13 enrich ment. This indicated that the relative availability of terrestrial carbon d ecreased 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) pro bably occurs during early summer as a consequence of rising temperature and high quality food in the form of biofilm.