Horizons in Stream Biogeochemistry: Flowpaths to Progress

Abstract

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.

Maintenance of terrestrial nutrient loss signatures during in‐stream transport

“…The chemistry of small streams integrates biogeochemical cycling in stream channels as well as the terrestrial systems they drain, and has played a key role in the development of ecosystem ecology (Fisher et al 2004). The small-watershed concept (Bormann and Likens 1967) advanced the study of terrestrial nutrient cycles with the idea that stream chemical loads represent nutrient losses from the wider ecosystem, and is today perhaps the most common example of forest biogeochemistry in introductory ecology textbooks.…”

“…3 holds and U ¼ R or U ¼ R ¼ 0 (e.g., during storm flows), then this must be true. Critically, however, high levels of nutrient accumulation and gaseous loss may occur along soil-to-stream flow paths (Mulholland 1992, Hedin et al 1998, Fisher et al 2004) and therefore stream losses may not equal losses from rooting zones of upland ecosystems but rather from ground-streamwater interfaces. Our results also point to conditions under which stream nutrient loads may not equal terrestrial losses: single-source nutrient inputs, low ground-water inflow, periods of high light and net biomass growth, and high rates of denitrification or burial.…”

Quantifying phosphorus uptake using pulse and steady‐state approaches in streams

“…However, many situations with significance to riverine nutrient transport and processing at large temporal and spatial scales are outside the scope of the steady-state approach. These situations include conditions of fluctuating background flows and fluxes (Fisher et al 2004), high water residence time (e.g., long flowpaths and those containing wetlands, ponds, or large backwaters), and high discharge (e.g., large rivers, floods). Research in these biogeochemical frontiers would facilitate identifying the Quantifying phosphorus uptake using pulse and steady-state approaches in streams places and times at which nutrient removal mechanisms are meaningful to long-term dynamics, and would be valuable within the context of continuing global change.…”

“…Although the general approach has been cited as laborious (Fisher et al 2004), strengths relate to its methodological and mathematical elegance and simplicity. These characteristics have, fortuitously, facilitated widespread application of the model (Ensign and Doyle 2006).…”

Temporal dynamics of seston: A recurring nighttime peak and seasonal shifts in composition in a stream ecosystem

“…Peaks in seston concentration found in a deciduous European forest stream at midmorning and evening were attributed to diurnal rhythms in algal drift (Pozo et al 1994), but no diel patterns were found in either boreal (Naiman 1982) or glacier-fed alpine streams (Hieber et al 2003). Information about temporal patterns can help evaluate the sources of POC (Kendall et al 2001), quality of the energetic subsidy to downstream biota (RosiMarshall 2004), controls on transport (Pozo et al 1994), and lead to better estimates of baseflow POC fluxes, especially in streams with pulsed inputs of organic matter (Fisher et al 2004).…”