Horizons in stream biogeochemistry: Flowpaths to progress

Over the past 50 years, conceptual developments in stream ecology and ecosystem ecology have converged, thanks to biogeochemistry and the recognition that in situ processing on one hand and spatial translation of materials, processes, and influence along flowpaths on the other, unite to generate a holistic picture of ecosystem functioning at the landscape level. Early emphases in stream biogeochemistry involved organic carbon dynamics and whole-ecosystem budgets. These approaches were holistic but cumbersome and laborious and ignored several crucial issues, such as variation in organic matter quality. Nutrient-spiraling approaches rectified this shortcoming and provided a flowpath-specific technique for resolving the dynamics of both inorganic and organic materials and for comparing streams of different sizes and flow rates. The ability of nutrient-spiraling approaches to deal with multiple elements and fluctuating flows, including floods, remains elusive, however. There are several opportunities for stream ecology and biogeochemistry to continue cross-fertilization to the benefit of both disciplines. Chief among these is an emphasis on streams as spatially distributed heterogeneous systems, constantly shifting in shape and configuration through time. Hydrologic and material flowpaths integrate this heterogeneity as they alternately converge and diverge and perfuse the channel, hyporheic zone, riparian zone, and floodplain as they transport materials inexorably downstream. Conceptualizing streams as branched structures adds new insights to biogeochemical processing in space and raises several questions about network shape as an independent variable influencing biogeochemical dynamics such as nutrient retention efficiency. Junctions of streams emerge as potentially significant functional units when streams are viewed as networks. Ultimately, stream drainages finely dissect the land and receive water from soils in channelless upland regions which are themselves pervaded by convergent flowpaths, thus blurring the difference between streams and the landscapes they drain. Other lucrative future research opportunities are discussed including a plea for greater use of stoichiometric approaches and a consideration of geometry, disturbance, and legacies at multiple scales of space and time. We predict that future research in biogeochemistry will increasingly seek and find similarities among diverse ecosystem types by focusing on the spatially explicit flowpaths that integrate and connect them.