PARENT SESSION
Symposium 4: Ecosystem ecology at the watershed scale: Linking biogeochemical cycles across the terrestrial - aquatic divide
Organized by: ES Bernhardt and MH Valett
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 517 A, Level 5, Palais des congrès de Montréal
Fighting gravity: Bidirectional exchange of nutrients and water in watershed ecosystems.
Sabo, John1, McCluney, Kevin1, Keller, Andrew 1, Soykan, Candan 1, Roemer, James1, 1 Faculty of Ecology, Evolution and Environmental Science, Tempe, AZ, USA
ABSTRACT- Groundwater depletion threatens native ecosystems across the continental U.S. Here we report results from large-scale experiments and the measurement of stable isotopes of hydrogen (2H) and oxygen (18O) quantifying the impact of water fluxes between below- and above-ground compartments on riparian food webs. In large-scale experiments, removal of litter from replicated (625 m2) plots led to relatively small changes in the abundance of a conspicuous detritivore. By contrast, litter removal had much stronger effects on the predators of this detritivore. Spiders were significantly more abundant in ambient litter plots whereas lizards were more frequently observed on cleared plots. Spiders likely track litter as a source of structural cover from predators (lizards and wasps) while lizards may track higher daytime surface temperatures offered by surface soils in litter-free plots. In situ cafeteria experiments provided further evidence that low quality, dry litter is neither a limiting resource nor a significant structural refuge from predators for detritivores. Instead, detritivores sought out freshly fallen (water-laden) leaves, while ignoring dry leaves altogether. Thus large riparian trees that obtain much of their water from groundwater may alleviate water stress experienced by detritivores during the dry season via green fall. In this way belowground water may indirectly enhance secondary productivity in aboveground terrestrial habitats. Finally, the effects of litter quantity on detritivores may counteract those of litter quality (as measured by water content) by providing habitat for their predators. In watersheds, gravity directs nutrients, energy and water downslope and downstream. These data provide one example of counter-gravitational flow of a limiting resource. We conclude with a conceptual summary of the many pathways by which organisms transport nutrients, energy and water across watersheds. This transport significantly blurs the boundary between aquatic and terrestrial ecosystems.
Key words: ground water, riparian, food web, leaf litter