Spatial drivers of ecosystem structure and function in a floodplain riverscape: Springbrook nutrient dynamics

On riverine flood plains, reorganization by fluvial processes creates and maintains a mosaic of aquatic and riparian landscape elements across a biophysical gradient of disturbance and succession. Across flood plains of gravel-bottom rivers, spring brooks emerge from points of groundwater discharge that may occur in distinct landscape positions. We investigated how ecosystem processes in spring brooks differ spatially across biophysical zones, reflecting how landscape position dictates severity of flood disturbance, allochthonous loading from riparian forests, and inputs from groundwater systems. Between July and October 2011, we quantified aspects of ecosystem structure and function among 6 spring brooks of the Nyack flood plain, Flathead River, Montana. Structural features varied predictably across near-channel (i.e., parafluvial) and late successional (i.e., orthofluvial) biophysical zones. Large wood standing stocks increased >40× (0.19-9.19 kg/m²), dominant particle size class differed by an order of magnitude (median particle size [D₅₀] = 2-27), and measures of vertical hydraulic gradient (-0.06 to +0.12 cm/cm) reflected differences in landscape position. We found fine sediment accumulation, stronger groundwater inputs, and greater benthic and large wood standing stocks in orthofluvial than in parafluvial spring brooks. Algal biomass was negatively correlated with insolation and positively related to vertical hydraulic gradient. Results from microcosm experiments showed increasing N 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.