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  • Nutrient spiraling in streams and river networks

    Scott H. Ensign1 and Martin W. Doyle2

    Received 24 October 2005; revised 22 June 2006; accepted 25 July 2006; published 17 November 2006.

    [1] Over the past 3 decades, nutrient spiraling has become a unifying paradigm forstream biogeochemical research. This paper presents (1) a quantitative synthesis of thenutrient spiraling literature and (2) application of these data to elucidate trends innutrient spiraling within stream networks. Results are based on 404 individualexperiments on ammonium (NH4), nitrate (NO3), and phosphate (PO4) from 52 publishedstudies. Sixty-nine percent of the experiments were performed in first- and second-orderstreams, and 31%were performed in third- to fifth-order streams. Uptake lengths, Sw, of NH4(median = 86 m) and PO4 (median = 96 m) were significantly different (a = 0.05) thanNO3 (median = 236 m). Areal uptake rates of NH4 (median = 28 mg m

    2 min1) weresignificantly different than NO3 and PO4 (median = 15 and 14 mg m

    2 min1,respectively). There were significant differences among NH4, NO3, and PO4 uptakevelocity (median = 5, 1, and 2 mm min1, respectively). Correlation analysis resultswere equivocal on the effect of transient storage on nutrient spiraling. Application ofthese data to a stream network model showed that recycling (defined here as streamlength Sw) of NH4 and NO3 generally increased with stream order, while PO4recycling remained constant along a first- to fifth-order stream gradient. Within thishypothetical stream network, cumulative NH4 uptake decreased slightly with streamorder, while cumulative NO3 and PO4 uptake increased with stream order. Thesedata suggest the importance of larger rivers to nutrient spiraling and the need toconsider how stream networks affect nutrient flux between terrestrial and marineecosystems.

    Citation: Ensign, S. H., and M. W. Doyle (2006), Nutrient spiraling in streams and river networks, J. Geophys. Res., 111, G04009,

    doi:10.1029/2005JG000114.

    1. Introduction

    [2] Studies of nitrogen (N) and phosphorus (P) cyclingare a cornerstone of ecosystem biogeochemistry becauseall biota depend on these elements for critical cellularprocesses. Productivity of aquatic ecosystems is ofteninfluenced by the concentration, molecular form, andstoichiometry of N and P. In excess, however, N and Pcan enhance rates of photosynthetic and heterotrophicproductivity and result in fundamental changes to aquaticfood webs. Eutrophication, the process of acceleratedorganic carbon production in aquatic systems, negativelyaffects rivers, lakes, and estuaries worldwide, and nowaccounts for the foremost aquatic ecosystem managementproblem in the U.S. [Bricker et al., 1999]. Fundamentalto the management of eutrophication is an understandingof how lakes, estuaries, and the coastal ocean are coupled

    with nutrient export from their watersheds [Howarth etal., 2002]. Not only do streams and rivers provide thishydrologic coupling, they also play a central role inmodulating the concentrations and forms of nutrientsexported downstream.[3] The importance of in-stream uptake to watershed-

    scale nutrient loads has been demonstrated by studies inwhich nutrient export in streamflow has been found to beless than terrestrial inputs to the stream [e.g., Behrendtand Opitz, 2000; Stow et al., 2001; Mulholland, 2004;Williams et al., 2004]. While some in-stream uptakeprocesses are permanent (e.g., denitrification and burial)others are temporary (e.g., biotic sequestration) and resultin remineralization and subsequent downstream transport.Knowledge of the processes contributing to in-streamnutrient uptake is largely attributable to the concept ofnutrient spiraling, a conceptual and empirical model ofnutrient cycling in fluvial ecosystems. The current studyis intended to provide a review and quantitative summaryof data from studies of nutrient spiraling, with emphasison the spatial variability within stream networks. In equalparts, this study is a response to (1) the lack of aquantitative summary and synthesis of nutrient spiralingdata in the literature and (2) the call for increasedattention to the ecological connectivity within streamnetworks. We begin this paper with a review of the

    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, G04009, doi:10.1029/2005JG000114, 2006ClickHere

    for

    FullArticle

    1Curriculum in Ecology, University of North Carolina at Chapel Hill,Morehead City, North Carolina, USA.

    2Department of Geography, University of North Carolina at ChapelHill, Chapel Hill, North Carolina, USA.

    Copyright 2006 by the American Geophysical Union.0148-0227/06/2005JG000114$09.00

    G04009 1 of 13

    http://dx.doi.org/10.1029/2005JG000114

  • development and applications of the nutrient spiralingmodel.

    1.1. Nutrient Spiraling Model

    [4] Unlike other ecosystems, the spatial and temporaldimensions of nutrient cycling in streams and rivers areintimately coupled due to the continual movement of water-borne constituents downstream. This unique aspect offluvial environments (relative to other ecosystems) washighlighted by Webster [1975] wherein he coined the termnutrient spiraling to describe the cycling of nutrients asthey are assimilated from the water column into benthicbiomass, temporarily retained, and mineralized back intothe water column. Though this concept was mentionedbriefly by Wallace et al. [1977] and Fisher [1977], Websterand Patten [1979] were the first to apply the nutrientspiraling concept to compare nutrient recycling in differentstreams. Without explicitly referring to nutrient spiraling,Meyer and Likens [1979] brought increased attention to therole of in-stream processes in modifying P inputs toHubbard Brook, New Hampshire.[5] In 1981, the nutrient spiraling concept was formalized

    by Newbold and colleagues who presented a mathematicalframework to support a more robust conceptual model thanhad previously been presented [Newbold et al., 1981]. Theauthors introduced spiraling length (S) as the distancerequired for a nutrient atom to complete 1 cycle from itsdissolved inorganic form in the water column (this distancebeing Sw), through a particulate phase (Sp), and finallythrough a consumer phase (Sc) to be returned to the watercolumn in a dissolved inorganic form. In this model,spiraling length is an integrated measure of water flowvelocity and the influence of biochemical demand of thebenthos (though subsequent studies have included chemicalsorption by stream bed sediments as an uptake pathway).Further, the authors demonstrated how these spatial metrics(S, Sw, Sp, Sc) could be converted into flux measurements byincorporating data on the flux of dissolved and particulatefractions in the stream. This and subsequent publications[Elwood et al., 1982] paved the way for a burgeoning fieldin stream ecology in which nutrient spiraling could bequantitatively compared between different streams.[6] The first whole-ecosystem study of P spiraling was

    conducted in Walker Branch, Tennessee [Newbold et al.,1983] using 32P as an isotope tracer. This was followed by aseasonal investigation of nutrient spiraling in the samestream [Mulholland et al., 1985]. However, because of thedifficulties entailed by using radioactive P in surface waters,the only other whole ecosystem P spiraling study performedto date was also conducted in Walker Branch [Mulholland etal., 1990]. The dominant source of information regardingwhole-stream N-cycling came from the Lotic IntersiteNitrogen eXperiment (LINX). Beginning in the late1990s, the LINX project used 11 streams spanning a broadrange of ecosystem types across North America toinvestigate stream ecosystem processes of N retention andfood web transformation. Utilizing similar methods acrossall sites and experiments, the investigators quantified theuptake and trophic transfer of N between multiple streamcompartments. The LINX project advanced understandingof the many in-stream processes affecting nitrogen uptake(summarized by Webster et al. [2003]), and the contribution

    of trophic transfer and turnover to nutrient processing in awide variety of streams. At the time of the present research,a LINX II study is underway to examine NO3 uptake in adiverse array of stream ecosystems.[7] In contrast to these whole-ecosystem studies of

    nutrient spiraling using isotope tracers, the majority ofnutrient spiraling studies have been conducted using moresimplistic techniques. Bulk addition of inorganic N and Pto a stream allows measurement of nutrient uptake along astream reach. This technique has been applied since theearly 1970s prior to the genesis of the nutrient spiralingmodel [McColl, 1974], but gained widespread notorietyafter a review of the theory and empirical procedures waspublished [Stream Solute Workshop, 1990]. The StreamSolute Workshop publication reviewed the conceptual andempirical underpinnings of nutrient spiraling, anddiscussed the relative merits of using different metrics tocompare nutrient uptake in streams, as well as thelimitations and assumptions necessary when nutrientadditions are used.[8] There are three fundamental limitations to the nutrient

    addition approach. First, only one portion (Sw) of the totalspiraling length (S) is measured using nutrient additions,providing no information on specific mechanisms of uptakeor transfer between stream biota. Nutrient additions alterbiochemical nutrient uptake, subsequently affectingmeasurements of Sw [Mulholland et al., 1990, 2002] andareal uptake (U) [Dodds et al., 2002]. Second is variabilityin time: Despite wide recognition that spiraling variessubstantially with discharge, the approach currently usedlimits results to a snapshot view of stream biogeochemistry,and results are only representative of a particular time and aparticular discharge. Only recently have developments beenmade in how to account for

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