Upstream and Upslope Translocation of River-Borne Materials by Aquatic and Riparian Organisms: Contrasts in Spatial Fluxes along Mainstems and at Tributary Confluences

(Funded 2002-2003)

Principal Investigator: Mary E. Power
Integrative Biology
UC Berkeley
(510) 643.7776
mepower@socrates.berkeley.edu

Executive Summary:
Statement of the critical state water research problem: Nutrients and contaminants exported from watersheds to rivers can be incorporated back into terrestrial food webs via aquatic insect emergence. Emerging insects from contaminated area deliver contaminants to terrestrial insectivores, which may be disproportionately concentrated in riparian habitats. Contaminants derived from mainstem may penetrate further upslope along tributary confluences, but the impacts of tributaries on the abundances, activities, and landscape movements predators of aquatic emergence are poorly known, as are river-to-watershed fluxes in general. To evaluate backflows of contaminants across the landscape, we must evaluate how aquatic and riparian organisms move and interact across landscape boundaries in river networks.

Research approach: We propose to examine these river-to-watershed fluxes by sampling organisms and surveying their stable carbon and nitrogen isotopic composition. We will compare mainstem and tributary associated fluxes in a protected forested watershed (the South Fork Eel River in the Angelo Coast Range Reserve), and in an impacted river (the Truckee River north of Lake Tahoe). We have preliminary data from both watersheds indicating that carbon and nitrogen originating in mainstem, tributary, and adjacent terrestrial habitats are isotopically distinct. Therefore, mixing models will be informative for assessing upslope and upstream backflows mediated by insect emergence and its consumption by riparian predators.

Anticipated outcome of the study: We expect that movements of mainstem derived carbon and nitrogen into terrestrial watersheds will be detectable, and that it will move further upslope in the vicinity of tributaries, for one or more possible reasons. Tributaries enrich mainstems by loading pulses of drifting organisms, nutrients and organic matter, so that aquatic emergence and its tracking by predators may be higher near confluences than along isolated reaches of mainstems. Tributaries may also provide physical conditions that protect certain invertebrate predators from desiccation or other stresses or hazards of mainstem environments, allowing them to remain more abundant, active, and wide-ranging in these habitats.

Potential benefits: This study will reveal the spatial scales of movements of nutrients and other substances from rivers back into watersheds, and evaluate the effect on these fluxes of tributaries. This information will allow us to evaluate positive benefits of these fluxes (e.g. nutritional subsidies from mainstem rivers to riparian consumers we value, such as bats, or song birds) as well as the spatial extent of adverse effects (contaminant transport into upland areas) that arise where rivers have been polluted.