Spatial Drivers of Ecosystem Structure and Function in a Floodplain Riverscape: Springbrook Nutrient Dynamics

Samantha Kate Caldwell, The University of Montana

Abstract

On riverine floodplains, reorganization by fluvial processes creates and maintains a mosaic of aquatic and riparian landscape elements across a biophysical gradient of disturbance and succession. I hypothesized that ecosystem processes in spring brooks would differ spatially across biophysical zones because landscape position dictates severity of flood disturbance and allochthonous inputs from contiguous terrestrial and groundwater systems. Between July and October 2011, I quantified aspects of ecosystem structure and function among six streams (i.e., spring brooks) originating on the Nyack floodplain, Flathead River, Montana. Among sites large wood standing stocks differed over 300-fold (0.04 – 13.5 kg m-2), dominant particle size class varied by an order of magnitude (< 2 – 64 mm), and measures of vertical hydraulic gradient (-0.14 to +0.20 cm cm-1) reflected landscape position (i.e., parafluvial and orthofluvial zones). I found fine sediment accumulation, stronger groundwater inputs, and greater benthic and large wood standing stocks in orthofluvial compared to parafluvial spring brooks. Algal biomass was negatively correlated with insolation and positively related to vertical hydraulic gradient. Solute injections were used to address biogeochemical cycling of nitrogen at the reach scale. Limited uptake of nitrate in spring brooks suggested abiotic interference or strong co-limitation by nitrogen and phosphorus. However, results from microcosm experiments showed increasing nitrogen uptake across the gradient from parafluvial to orthofluvial spring brooks. Functional response to landscape-scale organization of springbrook structure underscores the need for a spatially-explicit model of floodplain ecology.