Natural-abundance stable C and N isotopes indicate weak upstream-downstream linkage of food webs in a grassland river
Huryn, Alexander; Riley, Ralph H.; Young, Roger G.; Arbuckle, Chris J.; Peacock, Kathi
Archiv für Hydrobiologie Volume 153 Number 2 (2002), p. 177 - 196
published: Feb 27, 2002
ArtNo. ESP141015302000, Price: 29.00 €
Current conceptual models of river ecosystems differ in their predictions of the dependence of food webs on organic matter from upstream sources versus that produced within the river channel or in the adjacent riparian zone. Model suitability may vary at the valley segment scale, however, due to the role of channel type in determining sources of organic matter. We analyzed stable isotopes to assess sources of organic matter for consumers in a grassland river (Taieri River, New Zealand) that flows through two distinct valley segments with contrasting channel types - bedrock confined (n = 2 upstream reaches) versus alluvial floodplain (n = 2 downstream reaches). We tested predictions that: (a) riverine consumer production is based on valley-segment scale sources of organic matter, and (b) upstream-downstream linkage of food webs via organic matter transported from bedrock confined to alluvial floodplain channels is weak. δl3C and δ15N values for consumers in confined reaches indicated that primary production from the channel was their primary food source during summer, but terrestrial vegetation was important during winter. In comparison, terrestrial vegetation was the major food source in one floodplain reach, but aquatic primary production appeared to be the major food source in the other floodplain reach. When considered simultaneously, δl3C and δ15N values for consumers and their food sources indicated little overlap between channel types and negligible linkage of food webs by organic matter transported from confined to floodplain valley segments. These results suggest that the floodplain may be the primary contributor of carbon and nitrogen to riverine food webs in floodplain valley segments. Local factors, rather than upstream-downstream linkage between confined and floodplain valley segments, appear to be determinants of trophic resources for consumers in floodplain reaches of this grassland river. This conclusion supports models viewing river channels traversing discrete valley segments as being independent with regard to food web dynamics.