Early papers made little mention of instream nutrient processes

Before the 1970s, there were a number of studies of chemical concentrations in streams and how these were related to sources. A notable example was Gibbs' (1970) study of major ions in world rivers and their relation to geological and precipitation sources and modification by instream evaporation and chemical precipitation. Bond's (1979) study of a Utah stream showed how terrestrial processes in an undisturbed watershed affected stream nutrient concentrations. Many studies illustrated how anthropogenic changes to watershed vegetation modified stream chemistry (e.g., Bormann et al. 1968, Brown et al. 1973, Johnson and Swank 1973, Vitousek and Reiners 1975). Also, many studies documented eutrophication of streams caused by nutrient inputs (summarized by Hynes 1960).

Various reviews at that time made almost no mention of instream biological processes. Hynes (1960, p. 68) showed a diagram with a decrease in nutrient concentrations downstream from a pollution source and wrote, “… if the river is long enough and receives enough extra water from tributaries and surface run-off, it can ‘re-purify’ itself.” However, in another chapter (p. 122), he noted that in addition to dilution, nutrients might be “used up,” clearly anticipating the significance of biological processes expressed in his later book (Hynes 1970). The summary by Owens et al. (1972) dealt mostly with sources of nutrients, although they did mention that low NO₃⁻ concentrations in summer could be because of plant uptake, denitrification, or NH₄⁺ volatilization. The book edited by Allen and Kramer (1972) was titled Nutrients in Natural Waters, but streams were only briefly mentioned in one chapter with no mention of nutrient processes in streams. Stream studies clearly lagged behind lake and marine research in this regard. Golterman (1975) made no mention of instream processes in his review of river chemistry and attributed seasonal and longitudinal chemical variations to terrestrial factors. This failure to mention instream nutrient processes was not an oversight on the part of these and other authors, but rather it reflected the lack of study of nutrient dynamics at that time. Hynes' (1970) chapter on chemical characteristics of flowing waters was primarily a discussion of chemical sources, distribution, and chemical reactions. He stated (p. 46), “To my knowledge nobody has ever demonstrated or suggested that any of them [K, N, or P] is a limiting factor of plant growth in a natural stream.” In his review of P in streams, Keup (1968) mentioned biological growth as a potentially important removal process. Similarly, Casey and Newton (1972, 1973) mentioned plant and microbial uptake of nutrients but concluded that N and P were generally in excess in English streams. They found evidence of significant plant uptake of P in only one stream. Westlake (1975) suggested that N and P and perhaps K were insufficient for optimum vascular plant growth in oligotrophic English streams. Hynes (1970, p. 52) clearly recognized the potential importance of instream biological processes and concluded his chapter on stream chemistry with, “A lifeless river would have a very different chemical regime.”

Instream processes modify nutrient concentrations

Four types of studies were primarily responsible for contributing to an early understanding of the role of instream processes: 1) evidence of increased plant production and decomposition in response to nutrient addition, 2) studies showing a downstream decrease in nutrient concentrations, 3) studies using radioisotopes, and 4) budget studies.

Other studies documenting instream nutrient dynamics

These 4 types of studies (nutrient addition, longitudinal decrease in nutrient concentrations, radioisotopes studies, and budget estimations) provided clear evidence that instream process can modify nutrient concentrations. A variety of other studies documented the importance of specific instream processes. Stake (1968) found that aquatic plants in a polluted stream in Sweden accumulated P, and he noted that rooted plants might get nutrients from the sediments. Kaushik and Hynes (1968; Fig. 1) and Mathews and Kowalczewski (1969) found heterotrophic immobilization of N during leaf decomposition. Gregory (1978) used a ³²P release to demonstrate heterotrophic immobilization in an Oregon stream. The importance of nutrient uptake by microbes associated with decaying leaves became recognized as one of the most significant nutrient processes in streams (e.g., Newbold et al. 1983b; Fig. 1).

Nutrient uptake by autotrophic and heterotrophic processes is now measured fairly routinely with low-concentration additions of nutrients, a technique first used by McColl (1974; Fig. 1), but measurement of the opposing process, mineralization, has been much more limited. Hynes (1975) noted that very little was known about N release back into the water. Leaf decomposition studies showed immobilization of nutrients but also suggested that, at some point, net mineralization of nutrients must occur.

Another valuable technique was the use of nutrient diffusing substrates (Pringle and Bowers 1984 [Fig. 1], Fairchild et al. 1985). This technique made it possible to compare potential nutrient limitation among various streams.

Woodall and Wallace (1975) noted the importance of detritivores to the release of chemicals from decomposing leaves, and various radiotracer studies have measured nutrient mineralization by consumers (e.g., Webster and Patten 1979). Detritivores also can influence nutrient uptake rates by heterotrophic microbes indirectly by their effects on detritus standing stocks. Mulholland et al. (1985) found that snails reduced nutrient uptake by accelerating leaf decomposition.

Another way by which consumers might contribute to stream nutrient dynamics is by translocation of nutrients. Juday et al. (1932) found that migrating salmon moved significant amounts of nutrients upstream. After their ³²P release into a stream, Ball et al. (1963; Fig. 1) found that some of the radiotracer moved upstream. They hypothesized that this upstream movement was probably the result of upstream movement by consumers. Fittkau (1970) suggested that caimans in Amazonian rivers might function like bears in Alaska by feeding on migrating fish and mineralizing the fish-carried nutrients. Hall (1972) suggested that migrating fish might influence stream P budgets, and Durbin et al. (1979) found that nutrient mineralization by alewife migrating upstream to spawn significantly increased respiration on leaves.

The processes of N fixation and denitrification are input and output processes that are unique to the N cycle. Denitrification was suggested to be important in steams by Hill (1979) but was not actually measured until Duff et al. (1984) used the acetylene block technique in large chambers containing undisrupted periphyton communities and found that denitrification during the night could be a considerable sink for NO₃⁻ in a N-rich stream. Gray (1951) noted the presence of N-fixing bacteria in an English chalk stream, and Horne and Carniggelt (1975) first measured N fixation by the cyanobacterium Nostoc in a California stream. Few other studies have demonstrated that stream N fixation is an important process (but see Marcarelli et al. 2008).

One study done before 1986 stands out as providing a comprehensive view of nutrient dynamics in streams. Using results of P radiotracer studies, Newbold et al. (1983a) developed and calibrated a quantitative, mass-balance model that included the simultaneous processes of cycling and transport of P in streams. This model was based on the spiraling concept of Webster and Patten (1979) and used the spiraling measurement techniques developed by Newbold et al. (1981; Fig. 1). Because suitable radiotracers for N do not exist, a similarly comprehensive understanding of stream N dynamics did not occur for another decade when routine and relatively inexpensive mass spectrophotometry made the use of ¹⁵N tracers possible.

Nutrient dynamics and further development of the spiraling concept

Two papers, both published in J-NABS and both products of workshops, were arguably the most significant papers on nutrient dynamics in streams since the introduction of the spiraling concept and the development of field indices to quantify spiraling. Meyer et al. (1988; Fig. 2) was a product of a meeting in April 1987 at the Flathead Lake Biological Station. The authors explored gaps in our understanding of elemental dynamics in streams and focused on landscape-level processes and the relative importance of upstream, riparian, and instream controls. Meyer et al. (1988) advocated a number of future research directions, including studying critical stream ecotones, such as biofilms, hyporheic zones, and floodplains, and emphasized the potential contributions of whole-stream manipulations supplemented by intensive studies of key processes and variables. Also, Meyer et al. (1988) advocated a network of experimental streams that laid the foundation for several large cross-site experiments (see Nutrient dynamics below).

Stream Solute Workshop (1990; Fig. 2) was the product of a meeting organized by Nick Aumen and held at the University of Mississippi in early 1989. The authors used the spiraling concept as a basis from which to produce a conceptual model for studies of stream nutrients and other solutes that integrated physical, chemical, and biological processes. They also identified advantages and limitations of various methods for studying solute dynamics and recommended short-term nutrient injection experiments as a more practical substitute for isotopic tracer approaches to quantify nutrient uptake lengths in streams. This paper showed how the new transient storage model approach to stream hydrodynamics developed by Bencala and others (e.g., Bencala and Walters 1983) could be used with the spiraling approach to develop a more holistic understanding of the physical controls on nutrient dynamics. Uptake velocity, sometimes called the mass transfer coefficient (in units of length/time), was introduced as a useful stream nutrient dynamics metric for relating uptake rate to nutrient concentration in water. In more recent years, this parameter has been used to scale nutrient dynamics to entire drainage networks (e.g., Wollheim et al. 2006, Poole 2010).

Several other significant papers have helped to refine application of the spiraling concept over the past 2 decades, although most of these papers have appeared in other journals. In an influential concept paper, Fisher et al. (1998; Fig. 2) considered the stream corridor as a hydrologically connected set of subsystems from riparian zones to the stream channel and showed how the spiraling concept could be applied to the broader stream corridor to quantify resistance to and recovery from flood disturbances. Doyle (2005; Fig. 2) incorporated hydrologic variability into spiraling theory by proposing a new metric, functionally equivalent discharge, which is “the single discharge that will reproduce the magnitude of nutrient retention generated by the full hydrologic frequency distribution when all discharge takes place at that rate” (p. G01003).

Several methodological advances have occurred recently. Payn et al. (2005) found a solution to the problem identified by Mulholland et al. (2002) when using the nutrient-addition approach rather than tracer additions to compare uptake lengths and rates among streams. Payn and colleagues showed how multiple nutrient-addition experiments could be used to determine uptake lengths comparable to those determined by tracer approaches. Runkel (2007) has suggested a transport-based approach for the analysis of time-series and steady-state data during tracer addition experiments that involves fitting a transient storage model that includes uptake terms to identify uptake rate coefficients for both the main channel and storage zones. This approach does not require steady-state tracer additions and might allow the use of pulse-type additions. Tank et al. (2008; Fig. 2) conducted a pulse-type N addition in a large river (Upper Snake River, Wyoming) and applied the transport-based approach recommended by Runkel (2007). Tank et al. (2008) reported that NH₄⁺ and NO₃⁻ uptake velocities were similar to those in smaller streams in the same drainage. This paper was important because it suggested that nutrient uptake in large rivers could be as high as in small streams and because it provided a method for determining nutrient uptake in streams and rivers that are too large to use the steady-state addition approach.

Nutrient limitation and uptake

Nutrient limitation of algae was thought to be less common in streams than in lakes or the ocean because the continuous flow of water in streams was expected to replenish nutrient supplies to biota (Hynes 1969). This view began to change in the late 1970s and early 1980s with publication of several pioneering papers on P limitation of stream algae. Nutrient limitation studies in streams became more common during the mid to late 1980s, and many of the most important papers on this topic were published in J-NABS. In fact, the very first paper in J-NABS was written by Grimm and Fisher (1986), who dealt with N limitation of algae in Sycamore Creek in the Sonoran Desert of Arizona. Their paper was one of the first to demonstrate N limitation in streams and showed that, in some regions, such as the southwestern US, N rather than P is often the limiting nutrient. Subsequently, Hill and Knight (1988) demonstrated N limitation of periphyton in a northern California stream, Lohman et al. (1991) reported N limitation of periphyton in an Ozark stream, and Wold and Hershey (1999) found colimitation by both N and P in a Lake Superior tributary stream. Together these papers expanded the regional extent of N limitation or colimitation. More recently, Francoeur (2001) did a meta-analysis of 237 stream nutrient-addition experiments and found that colimitation by N and P was more common than limitation by either nutrient alone. Rosemond (1994) showed how nutrients, light, and herbivory all limited algal production at different times of the year in a forested stream (see also Rosemond et al. 1993 [Fig. 2], 2000). Interaction between light and nutrient limitation also was demonstrated by Sabater et al. (2000), who reported that P uptake increased >2× in response to riparian deforestation along a Mediterranean stream, and by Hill and Fanta (2008), who showed that P and light could colimit stream periphyton growth simultaneously. Mulholland and Rosemond (1992) demonstrated the spatial effects of nutrient limitation in streams and showed that stream periphyton became increasingly P limited with distance downstream from nutrient inputs because of upstream uptake and retention of P.

Fine-scale nutrient recycling also is an important nutrient source for stream algae. Peterson and Grimm (1992), building from the seminal paper of Grimm (1987; Fig. 2) on algal succession, biomass development, and nutrient limitation, showed how internal N recycling became more important for meeting nutrient demand during the later stages of succession when large biomass accumulations reduced the availability of water-column nutrients. Similarly, Steinman et al. (1995), building on earlier work by Mulholland et al. (1991), showed that nutrient recycling within dense algal mats that developed in the absence of grazers was a significant nutrient source for P-limited stream periphyton.

In the last decade, work has focused on the relationship between nutrient uptake, algal biomass, and nutrient concentration. In a study of New Zealand streams, Biggs (2000) showed that variation in nutrient concentration explained up to ⅓ of the variation in benthic chlorophyll a across streams. Dodds and Welch (2000; Fig. 2) argued that nutrient criteria were the most effective way of preventing nuisance levels of algal biomass, and Dodds (2003) argued that total N and total P, rather than the dissolved inorganic forms of these nutrients, should be used to define nutrient status and establish nutrient criteria in streams. Dodds et al. (2002) showed that nutrient uptake was controlled by biological kinetics responding to nutrient concentration, but that nutrient uptake did not necessarily saturate at high concentration in streams. However, Newbold et al. (2006) found that uptake of NH₄⁺ and PO₄³⁻ in streams draining into New York City's drinking-water reservoirs could be described by Michaelis–Menten kinetics with half-saturation concentrations of ∼1 mg N/L and 12 µg P/L, respectively.

Nutrient uptake by heterotrophic organisms, particularly bacteria and fungi associated with decomposing leaves, also is important in streams (e.g., Tank et al. 2000, Webster et al. 2003, Mulholland 2004). At some times, particularly in autumn and winter in forested streams, it can be the dominant mechanism for nutrient uptake.

Role of hyporheic and riparian zones

Among the most significant new developments in stream nutrient dynamics during the past 25 y was identification of the importance of nutrient uptake and recycling in hyporheic and riparian zones. This development broadened the view of stream ecosystems to include more than surface water and benthic surfaces (reviewed in Boulton et al. 2010).

Among the most influential early papers on the role of the hyporheic zone in nutrient cycling was a series of studies by Triska and coworkers (Triska et al. 1989 [Fig. 2], 1990, 1993, Duff and Triska 1990). These papers showed that the hyporheic zone could be both a NO₃⁻ source (via nitrification) and sink (via denitrification), depending on the rate of hydrological exchange with surface water, which controlled NO₃⁻ supply and redox conditions in the hyporheic zone. Triska et al. (1989) were particularly influential in providing a hydrologic definition of the hyporheic zone (≥10% surface water) and a conceptual model of the role of the hyporheic zone in nutrient cycling. Two excellent short reviews in the mid-1990s (Findlay 1995, Jones and Holmes 1996) and a book shortly thereafter (Jones and Mulholland 2000) helped to stimulate additional work focusing on hydrologic exchange rate and residence time in hyporheic nutrient and organic C cycling and its role in stream nutrient dynamics.

J-NABS played an important role in our understanding of stream nutrient cycling and the hyporheic zone beginning with the paper of Valett et al. (1990; Fig. 2). Valett and colleagues showed that the hyporheic zone was a significant source of nutrients to surface water in Sycamore Creek, Arizona, as mediated by the spatial distribution of hydraulic head (upwelling vs downwelling), which controlled dissolved O₂ inputs and mineralization in the hyporheic zone. In the first issue of 1993, J-NABS published a series of papers on various hyporheic zone perspectives organized by Valett, Hakenkamp, and Boulton (Valett et al. 1993). This collection included a paper by Bencala (1993) that presented a catchment perspective on hyporheic zone hydrology and solute dynamics. Papers by Stanford and Ward (1993), Hendricks (1993), and Palmer (1993) touched on the topic of nutrient dynamics, but focused mostly on hydrology and organisms.

A series of 5 J-NABS papers focused specifically on hyporheic N dynamics. In a pair of studies in Sycamore Creek, Holmes et al. (1994) and Jones et al. (1995) showed that the parafluvial (hyporheic zone lateral to the stream channel) and hyporheic zones were sources of NO₃⁻ to surface water as a result of mineralization of organic N, nitrification, and flow paths that transported materials from these zones to surface water. Wondzell and Swanson (1996) reported similar findings for a stream in Oregon. Dent et al. (2001) showed that patchy geomorphic features produced characteristic spatial variability in hydrodynamics (upwelling and downwelling) and surface-water nutrient concentrations at scales ranging from a few meters to several kilometers in Sycamore Creek. Last, in an Antarctic stream study published in J-NABS, McKnight et al. (2004) reported that the hyporheic zone beneath seasonal glacial meltwater streams can act as either a NO₃⁻ source to benthic algal mats via mineralization or a NO₃⁻ sink via denitrification depending on the direction and rate of subsurface flow.

The effect of riparian zones on stream nutrient dynamics has not received much attention in J-NABS. A number of studies published in other journals have shown that the riparian zone can be an important sink for NO₃⁻ in groundwater before entering streams (e.g., Peterjohn and Correll 1984, Lowrance et al. 1984, McDowell et al. 1992, Hill 1996 [Fig. 2]). However, Schade et al. (2005b; Fig. 2) showed that the effect can be in the other direction, as well—riparian-zone vegetation can be a significant sink for streamwater N along water flowpaths from the stream to adjacent riparian zones.

N dynamics

In the past decade, studies on the rates and controls on N uptake and retention have become an important part of stream nutrient research. Concern about eutrophication of estuaries and coastal oceans (e.g., seasonal development of dead zones in the Gulf of Mexico, harmful algal blooms in North Carolina estuaries, loss of benthic macrophyte beds in the Chesapeake Bay) and the role of streams as N filters or sinks has driven much of this work.

Several studies in which traditional techniques, such as mass balance, uptake of added nutrients, and nitrapyrin addition (nitrification) or acetylene block (denitrification), were used to explore stream N dynamics were published in J-NABS. Kemp and Dodds (2001) showed how stream algae stimulated nitrification rates via dissolved O₂ production, and Strauss et al. (2004) showed that O₂ penetration into sediments controls nitrification rates in the Upper Mississippi River. In a study of N fluxes associated with epilithon communities in the River Garonne, France, Teissier et al. (2007) reported a biomass threshold between net N assimilation by autotrophs and net mineralization by heterotrophs at epilithon biomass levels of 23 g ash-free dry mass (AFDM)/m². In a recent paper, Hoellein et al. (2009) used substratum-specific and whole-stream NO₃⁻ addition experiments to show the importance of streambed composition on seasonal variability in reach-scale uptake and the important role of epilithic biofilms for NO₃⁻ uptake in forested streams.

A series of papers on denitrification using the acetylene block technique on sediments demonstrated the importance of NO₃⁻ concentration and, secondarily, temperature as controlling factors (Martin et al. 2001, Schaller et al. 2004, Strauss et al. 2006). Teissier et al. (2007) also showed that denitrification rates increased with biomass, but only in the dark when O₂ was not being produced by the autotrophic component of the biofilm. Ruehl et al. (2007) presented an interesting approach that combined measurements of longitudinal changes in NO₃⁻ concentration and stable N and O isotope enrichment factors to evaluate NO₃⁻ uptake rates and the role of denitrification (which results in isotopic enrichment of NO₃⁻) in the Pajaro River, California.

One of the most important advances in the study of N dynamics in streams was the development of a field tracer ¹⁵N addition approach. This approach was first used in a study of NH₄⁺ uptake in the Kuparuk River (Peterson et al. 1997; Fig. 2). Wollheim et al. (1999; Fig. 2) showed how data from field ¹⁵N experiments could be coupled with a model to provide a more synoptic view of N cycling in streams, and Hall et al. (1998; Fig. 2) showed how the ¹⁵N approach could be used to trace N cycling in stream food webs. Others then used the tracer ¹⁵N additions to examine foodweb linkages (Mulholland et al. 2000), preferential use of the upper layer of epilithon by grazers (Rezanka and Hershey 2003), and the relative importance of autochthonous versus allochthonous N in stream food webs (Hamilton et al. 2004).

The ¹⁵N addition approach was the basis of a large cross-site study known as the Lotic Intersite Nitrogen Experiment (LINX). In summary papers from the LINX study, Peterson et al. (2001; Fig. 2) and Webster et al. (2003) used experimentally measured average rates of N cycling (gross N uptake and remineralization) to show that instream uptake could reduce N concentrations and flux by an average of ⅔ over a 1-km distance in a 1^st-order stream. Mulholland et al. (2004) adapted the ¹⁵N addition approach to measure reach-scale NO₃⁻ uptake and denitrification in streams, and in a 2^nd cross-site study (LINX II), Mulholland and colleagues used it to compare NO₃⁻ dynamics in 72 streams draining different land uses across the US (Mulholland et al. 2008 [Fig. 2], 2009a, Hall et al. 2009).

In recent years, several studies have attempted to put stream N uptake into a landscape and even global context by defining the role of streams in N retention in large river basins. This work has resulted in somewhat conflicting views regarding where within the drainage network N retention is most important. Some authors suggest that small streams (generally ≤3^rd–4^th-order) might control N exports from river networks because of their high surface area to volume ratios and large contribution to total network stream length (Alexander et al. 2000 [Fig. 2], Peterson et al. 2001). Other studies indicate that larger streams and rivers might dominate N removal in drainage networks because of their longer water residence times and transport distances (Seitzinger et al. 2002, Wollheim et al. 2006, Ensign and Doyle 2006). Mulholland et al. (2008) offered an explanation for these conflicting views based on their observation that NO₃⁻ uptake and denitrification rates are strongly related to NO₃⁻ concentration across many biomes and landuse types. Mulholland et al. (2008) developed a model that used this relationship to predict N retention in a large river basin and showed that the relative significance of small vs large streams is a function of NO₃⁻ loading, with the importance of larger streams increasing at high loading rates as the capacity for uptake in smaller streams becomes saturated and a larger fraction of the NO₃⁻ load is exported downstream. More recently, Alexander et al. (2009) showed how interactions among N loading, instream denitrification rates, and hydrological factors controlled seasonal as well as spatial variation in N retention within river networks. These landscape or globally oriented stream and river N-uptake papers have not appeared in J-NABS, probably because of the attempt of the authors to reach a broader audience.

Role of consumers in stream nutrient dynamics

The role of anadromous fish migrations as nutrient subsidies enhancing nutrient availability and cycling in streams has been a valuable area of research. Early studies by Richey et al. (1975) and Sugai and Burrell (1984) and more recent studies (e.g., Wipfli et al. 1998, 1999, Johnston et al. 2004) have reported greater primary productivity or epilithic biomass in streams with spawning salmon runs or when salmon carcasses were experimentally added to streams than in streams without salmon, presumably because of the nutrient subsidy provided by salmon. Natural abundance ¹⁵N studies have shown that marine-derived N is incorporated into food webs and cycled within stream ecosystems (Kline et al. 1990, Bilby et al. 1996). However, some studies found no effect of spawning fish carcasses on periphyton (Minshall et al. 1991, Rand et al. 1992). A recent study in southeast Alaska streams indicated that salmon clearly increased nutrient concentrations, but responses by epilithon were variable because of factors, such as light limitation and hydrologic disturbance (Mitchell and Lamberti 2006). In a related study focusing on the aquatic insect communities of these Alaskan streams, Lessard and Merritt (2006) found that the positive effect of marine-derived N on abundance and biomass was limited to certain taxa, primarily chironomid midges.

J-NABS has played a modest role as an outlet for work in this area of research. Schuldt and Hershey (1995) used both comparative and experimental approaches to examine the effect of salmon carcass decomposition on Lake Superior tributary streams and found higher P concentrations and periphyton biomass with than without salmon. Schuldt and Hershey (1995) also reported ¹⁵N evidence that salmon-derived N cycled through the stream food web. Minakawa et al. (2002) reported increased biomass and growth rate of stream insects with experimentally added salmon carcasses in a Washington stream, primarily as a result of direct consumption of the carcasses by insects. In contrast, Ambrose et al. (2004) reported that salmon carcass additions to northern California streams had no effect on periphyton biomass or primary production, possibly because of light limitation. However, Peterson and Matthews (2009) reported higher periphyton biomass and significant uptake of salmon-derived nutrients by periphyton, bryophytes, and decomposing leaves when salmon carcasses were added to laboratory streams. Summarizing work to date, it appears that spawning runs of salmon and other anadromous fish can provide nutrient subsidies and enhance nutrient uptake in stream ecosystems through both direct feeding by consumers on carcasses and bottom-up enhancement of periphyton productivity by nutrients released during carcass decomposition, but the latter mechanism might be confined to those streams in which light levels are high and nutrients are strongly limiting.

Nutrient retention in urban streams

Recent interest in nutrient cycling and retention in urban streams appears to be driven largely by rapid urbanization and the need for better understanding of nutrient cycling and its controls in urban streams to mitigate or minimize effects of urbanization on stream ecosystem function and basin-scale nutrient retention. Historically, the focus of research on the effects of urbanization on stream nutrient dynamics has been on point-source nutrient inputs from wastewater treatment plants and other facilities. Some studies, including 2 in J-NABS (Meals et al. 1999, Haggard et al. 2005) examined nutrient uptake below wastewater treatment plant effluents.

Much of the recent work on nutrient dynamics in urban streams has been more comprehensive and has evaluated how structural and functional changes caused by urbanization affect nutrient uptake and cycling. In a review of urban streams, Paul and Meyer (2001) noted that ecosystem processes were understudied and nutrient uptake and retention largely ignored, and that urban streams provide opportunities to test stream concepts and to understand and manage ecosystems that include humans. Perhaps in response to the challenges laid out by Paul and Meyer (2001), a special series of papers from a symposium held in Melbourne, Australia, in December 2003 was published in J-NABS Volume 24, Issue 3 (Feminella and Walsh 2005). Several of these papers focused on nutrient dynamics in urban streams. Meyer et al. (2005; Fig. 2) showed that NH₄⁺ and soluble reactive P (SRP) uptake rates decreased with the proportion of urbanized area in the catchment and that this effect appeared to be related to declines in sediment organic matter and the biotic demand for nutrients associated with this material. Groffman et al. (2005) also reported that denitrification could be a significant sink for streamwater NO₃⁻ in urban streams, but that denitrification in these geomorphologically unstable systems is limited by the paucity of debris accumulations that provide the conditions necessary for denitrification. Grimm et al. (2005) suggested that some urban modifications to stream networks, such as detention basins and artificial lakes in the Phoenix, Arizona, area could enhance nutrient uptake and retention.

Last, several recent papers have compared N dynamics among urban, agricultural, and forested streams in the same region. O'Brien et al. (2007) used ¹⁵N additions to compare NO₃⁻uptake among 9 streams of contrasting land use in Kansas and found that NO₃⁻ concentration rather than land use per se was the most important factor controlling uptake and denitrification rates. Perhaps somewhat surprisingly, O'Brien and colleagues also reported that uptake rates did not appear to saturate with increasing NO₃⁻ concentration when comparing among streams, a finding confirmed in the full LINX II study (Mulholland et al. 2008). Arango and Tank (2008) compared nitrification and denitrification rates among 18 agricultural and urban streams in southwestern Michigan and found that both were positively related to sediment C content, which was not related to land use. However, denitrification was also positively related to NO₃⁻ concentration which tended to be higher in agricultural than in urban streams. In a whole-stream ¹⁵N addition study conducted in forested, agricultural, and urban streams in northeastern Spain, von Schiller et al. (2009) reported highest rates of NO₃⁻ uptake and denitrification in the agricultural stream and intermediate rates in the urban stream, a result that probably reflected the higher NO₃⁻ concentrations compared to the forested stream. Epilithon largely accounted for the higher NO₃⁻ uptake rates in the agricultural stream, probably because the streambed was largely cobble, and epilithon were more productive under higher NO₃⁻ concentration.

Importance of instream uptake for catchment nutrient budgets

An important advance in the understanding of stream nutrient dynamics was the appreciation that nutrient concentrations and flux in streams are not necessarily reflective of the biogeochemistry of the terrestrial ecosystems they drain. The development of the small-catchment approach to terrestrial biogeochemistry with the seminal studies of Likens and Bormann (e.g., Likens et al. 1977) used stream chemistry as the spatial integrator of the net effects of the forest in processing atmospheric inputs, internal cycling of nutrients, and the biogeochemical response to disturbances. Although never explicitly stated, and despite the early recognition by Hynes (1970) that streams affect nutrient concentrations, the underlying assumption (at least by some) was that instream processes would not appreciably alter the signals from terrestrial processes (i.e., streams were largely drainage pipes).

Gradually this view began to change, beginning with papers by Meyer and Likens (1979), Grimm (1987), Munn and Meyer (1990), Mulholland (1992; Fig. 2), and Burns (1998) that showed substantial uptake of N and P within streams. Mulholland used an inverse modeling approach to show that distinct seasonal patterns in NO₃⁻ and SRP concentrations were related to instream processes rather than to seasonal changes in the forest (Mulholland and Hill 1997, Mulholland 2004). Peterson et al. (2001) used a model of stream nutrient dynamics and data from cross-site field ¹⁵N addition experiments to show that instream processes could remove most of the N entering streams in groundwater within 1 km of its entry. Bernhardt et al. (2003; Fig. 2) showed that the increases in NO₃⁻ concentrations commonly observed in response to forest disturbance were highly dampened by instream uptake of NO₃⁻ after a severe ice storm that caused extensive forest damage at Hubbard Brook Experimental Forest. In a subsequent paper, Bernhardt et al. (2005) argued that the unexplained long-term decline in NO₃⁻ outputs from New England catchments might be caused, in part, by increased uptake in streams as forests have aged and debris dams and organic matter storage in streams has increased. While assimilatory uptake is probably not a long-term sink for N, it might enhance denitrification rates by providing the organic-rich sediments conducive to development of anoxic hotspots for tightly coupled processes of mineralization and denitrification (sensu Seitzinger et al. 2006).

Building on past work in Walker Branch, Roberts and Mulholland (2007) used a mass-balance approach to show that high rates of instream inorganic N retention are related to seasonal peaks of primary production during early spring and of heterotrophic respiration associated with leaf decomposition in autumn. Hall and Tank (2003) also showed tight coupling of NO₃⁻ uptake with primary production in streams (see also Tank et al. 2010). Goodale et al. (2009) reported very low stream NO₃⁻ concentrations after leaffall in autumn, a pattern increasingly being reported for streams draining deciduous forests and resulting from high rates of instream uptake by heterotrophic microbes during leaf decomposition. A revised concept of catchment nutrient dynamics that includes the active role of stream processes in controlling exports has been nicely summarized by Hall (2003). However, Brookshire et al. (2009) argued that most streams are in longitudinal steady state with no net uptake of nutrients, and thus, stream chemistry can be used as an integrated measure of terrestrial outputs. Clearly, more work is needed on this subject.