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

Abstract

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.

Comparison of coarse particulate organic matter retention in meandering and straightened sections of a third-order New Zealand stream

“…Small, forest stream ecosystems are generally based on terrestrially derived carbon (e.g. Rounick et al, 1982;Hicks, 1997) but allochthonous carbon sources can be important even in large open rivers (Winterbourn et al, 1984), unshaded pasture (Hicks, 1997) or tussock grassland streams (Rounick et al, 1982, Huryn et al, 2001. In streams where allochthonous CPOM inputs are a significant energy source, channel straightening could lower ecosystem productivity and alter consumer communities and their predators.…”

Trophic importance of benthic microalgae to macrozoobenthos in coastal bay systems in Korea: dual stable C and N isotope analyses

“…For example, a remarkable decrease in the δ 13 C of oysters Crassostrea gigas occurred 1 to 2 mo after the incorporation of terrestrialorigin light carbon at the Charente estuary of Marennes-Oléron Bay (Riera & Richard 1997). By contrast, the δ 13 C values of most taxa of aquatic consumers in a grassland river in New Zealand showed strong increases from summer to autumn (≤ 3 mo) due to a decrease in the relative availability of terrestrial carbon (Huryn et al 2001). A previous study in Kwangyang Bay (Kang et al 2001) and our monthly monitoring data (this study and our unpubl.…”

Dissolved organic carbon in New Zealand peatlands

“…There is an absence of knowledge on DoC concentrations and chemistry in wetlands, particularly peatlands, of New Zealand, though work has been done on upland systems draining into streams (e.g., Findlay et al 2001;Huryn et al 2001;Riley et al 2003). Collier et al (1989) and Moore & Jackson (1989) examined DoC concentrations and fluxes in Westland (South Island) catchments containing Pakihi poorly drained soils, noting high DoC concentrations and export (30 to 38 mg litre -1 and 31 to 44 g m -2 yr -1 , respectively) and an increase in export with catchment drainage for forestry, in contrast to export of 8 to 21 g m -2 yr -1 in nearby upland forested and harvested sites (Moore 1989).…”