Landscape Influences on Ecosystem Function: Local and Routing Control of Oxygen Dynamics in a Floodplain Aquifer

Abstract

Perspectives addressing either “local” or “routing” control over ecological form and function alternatively emphasize ambient conditions or recognize that form and processes at one location may be heavily influenced by other points in space. In their natural state, floodplains of montane gravel-bed rivers are organized laterally into parafluvial zones, where scour occurs annually, and orthofluvial environments, within which active (that is, early successional) and passive (that is, late successional) zones differ in regard to flooding influences. Vertically, these rivers interact with extensive alluvial aquifers throughout the length of their floodplains. We addressed how local and routing controls organize riverscape structure and function by investigating seasonal dynamics of interstitial dissolved oxygen (DO) vertically within the alluvial aquifer, longitudinally along flowpaths, and laterally across active and passive orthofluvial (PO) zones in the Nyack floodplain, Middle Fork Flathead River, MT. Groundwater DO was positively correlated to pH and negatively correlated electrical conductivity (EC) across all seasons. In some wells (7 of 19), DO was significantly lower and EC greater at shallow depths, and the magnitudes of vertical zonation were related to dissolved organic carbon concentrations. Longitudinal declines in DO were always and exclusively observed along flowpaths of the PO zone. At the same time, persistent hypoxic patches within the active orthofluvial environment suggest metabolic hotspots that contribute to local control of physicochemistry. In ecosystems heavily influenced by material transport, local and routing controls may be manifested simultaneously as distinct patterns of organization, which are expressed at different spatial and temporal scales.