Trends in stream nitrogen concentrations for forested reference catchments across the USA

Human alteration of the nitrogen (N) cycle is a major environmental issue that crosses spatial scales from the catchment to the globe. Energy and food production have led to a ten-fold increase in reactive forms of N in terrestrial ecosystems from 1860 to 2005 (Galloway et al 2008). Reactive N is routed to freshwaters through surface runoff and groundwater discharge or atmospheric deposition, resulting in alteration of the forms and concentrations of N species, which may result in aquatic eutrophication and altered stream functions (Stoddard 1994). Despite the implementation of the Clean Water Act (1972) and the Clean Air Act (1970), stream and groundwater N concentrations have continued to increase in large areas of the USA during recent decades (Smith et al 1987, Lettenmaier et al 1991, Rupert 2008, Sprague et al 2011) and have been predicted to further increase in the future (Howarth et al 2002). However, much of what is known about temporal changes in stream water quality originates from studies where catchments have been altered by land use and land cover change.

In contrast, we know little about temporal trends in N dynamics at forested, headwater streams with minimal human impacts, even though they serve as a benchmark against which we can evaluate more heavily human-modified catchments. Trends in stream-water nutrients in the relative absence of changes in land use or land cover can be evaluated using long-term data from unmanaged forested headwater (i.e. reference) catchments, such as those in the USDA Forest Service Experimental Forests and Ranges (EFR) network. These datasets represent the most complete information available on stream N concentration for reference sites because of their length (e.g., Hubbard Brook Experimental Forest, HJ Andrews and Coweeta Hydrologic Laboratory have been analyzing stream chemistry since 1963, 1969, and 1972 respectively), because of their sampling frequency (i.e., at least biweekly for all sites except HJ Andrews where they collect three-weekly composite samples), and finally because of their spatial coverage (EFRs encompass a suite of climates and forest types across the USA). Data from these reference catchments are frequently used in paired watershed comparisons to evaluate effects of land use treatments, assuming that in the absence of the perturbation under study, both the reference and treatment catchments would behave similarly.

Although stream chemistry trends in reference catchments have been investigated at individual EFRs, a comparison of trends in reference sites across the country has not occurred. Researchers at both Coweeta Hydrologic Laboratory in North Carolina (Swank and Vose 1997) and Fernow Experimental Forest in West Virginia (Peterjohn et al 1996) observed increased stream N over the last decades in reference catchments and attributed the trend to higher N deposition, and changes in nutrient demand and forest succession within catchments. At the Hubbard Brook Experimental Forest in New Hampshire, stream nitrate concentrations have been declining in recent decades after increasing from 1963 into the 1970s (Likens and Bormann 1995, Campbell et al 2007). These trends in stream nitrate at Hubbard Brook have not been explained by changes in atmospheric deposition and are thought to be due in part to the long-lasting effect of forest cutting in the early 1900s combined with effects of changing climate (Bernal et al 2012).

Further, little is known about the synchrony of trends among adjacent catchments within EFRs. Our study fills this gap by analyzing trends in stream nitrate (NO₃–N) and ammonium (NH₄–N) concentrations from forested reference catchments in multiple EFRs across the USA. To evaluate if stream N trends were synchronous with trends in likely drivers, we examined correlations between trends in stream N, streamflow, and ammonium and nitrate concentration in atmospheric wet deposition.

We analyzed stream inorganic N from 22 independent forested reference catchments in seven EFRs across the continental USA and Puerto Rico (figure 1(a)); these span a wide range of climatic, hydrologic and vegetation conditions (table S1 available at stacks.iop.org/ERL/8/014039/mmedia). Each catchment had a minimum of 12 yr of consistent, high frequency stream chemistry data, daily streamflow data, and weekly wet deposition chemistry collected nearby. These catchments are considered reference because they have not experienced direct anthropogenic disturbances other than atmospheric deposition during the last 60 yr. A total of 559 yr of stream nitrate and 523 yr of stream ammonium data collected at least biweekly were analyzed.

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Trends were analyzed using the Seasonal Kendall test (Hirsch et al 1982). This non-parametric, rank test has been proven robust in evaluating trends in time series that have strong seasonality. We selected this test because, in comparison to other trend analysis methods (e.g., linear regression, time series analysis, etc), the Seasonal Kendall test does not make assumptions about the distribution of the data and allows missing values and censored data without biasing the analysis (Helsel 2005). The Seasonal Kendall test is an extension of the Mann–Kendall test for monotonic trends (Mann 1945). If (x₁,y₁),(x₂,y₂),...,(x_n,y_n) are observations where X is time and Y is the object variable, the Kendall S statistic can be computed from each data pair as:

$$\begin{eqnarray*} &&S=P-N \end{eqnarray*}$$

where P is the number of Y_i < Y_j for all i < j and N is the number of Y_i > Y_j for i < j.

S has a mean of zero and variance:

$$\begin{eqnarray*} &&{\sigma }^{2}=[n(n-1)(2 n+5)-\Sigma t(t-1)(2 t-5)]/1 8 \end{eqnarray*}$$

where t is the number of data pairs involved at any given time. All observations below the detection limit are considered tied, and the differences of all tied pairs are zero. The Seasonal Kendall accounts for the effects of seasonality on trends by combining the Mann–Kendall test computed on each of the seasons separately (Hirsch et al 1982). Seasonal Kendall tau ranges between −1 and +1 and is the ratio of the number of positive differences minus the number of negative differences to the number of pairs (discounted for ties). If there is enough disparity between the number of positive and negative differences, then tau is statistically significant.

We analyzed trends in monthly flow-weighted concentrations of nitrate and ammonium in streams, monthly nitrate and ammonium concentration in wet deposition, and monthly streamflow (see table S2 available at stacks.iop.org/ERL/8/014039/mmedia for more details about sampling frequency, flow-weighted concentration calculations, and detection limits). Prior to the analysis, stream concentrations for each site were censored so that the highest detection limit during the analysis period was used consistently. The magnitude of the trend was assessed using the Sen slope (Sen 1968) which report the median change in value versus time (slope) of all the possible pairs in the dataset including zero differences. The existence of a significant trend with a Sen slope of zero is possible when a large number of observations are below the detection limit and therefore considered tied. The analyses were performed in R v2.13 using the 'Wq' and 'Kendall' packages with a threshold for significance set to p < 0.05. To standardize analyses among catchments and sites, trends were calculated for three different periods (calendar years from 1996 to 2007, 1987 to 2007, and 1972 to 2007), except for wet deposition. The longest period analyzed for trends in wet deposition began in 1980 for the Andrews, 1985 for Luquillo, and 1978 for the rest of the sites.

To evaluate more fully the influence of the length and period of the data record on trend detection, we conducted additional analyses on stream N concentrations from HJ Andrews and Hubbard Brook catchments. After analyzing trends for a minimum period (1996–2007), we iteratively reanalyzed trends after increasing the length of record in 1 yr increments for the full data record (1969–2007 for HJ Andrews, 1964–2007 for Hubbard Brook).

To understand how trends in stream N relate to trends in streamflow or to trends in N in wet deposition at a national level, we performed correlation analyses (Kendall's tau) between the Sen slopes for stream nitrate concentrations versus the Sen slopes for the potential drivers (i.e., streamflow, ammonium and nitrate concentration in atmospheric wet deposition) as well as stream ammonium versus potential drivers.

From 1996 to 2007, mean stream nitrate concentrations at all EFR study catchments except Fernow were ≤0.16 mg NO₃–N l⁻¹; average nitrate at Fernow was 0.75 mg NO₃–N l⁻¹ (figure 1(b)). Ammonium concentrations in streams were equal to or below 0.01 mg NH₄–N l⁻¹ at 19 of the 21 study catchments; concentrations higher than 0.01 mg NH₄–N l⁻¹ were observed at Marcell (0.14 mg NH₄–N l⁻¹ at S2 and 0.05 mg NH₄–N l⁻¹ at S5). Most of the ammonium values were below detection at Fernow and Hubbard Brook (figure 1(d)). Catchments showed a wide range of mean annual streamflow, ranging from a low of 10.7 cm at Marcell-S5 to a high of 269.2 cm at Luquillo-Q3 (figure 1(c)). Mean annual wet deposition of dissolved inorganic N was highest at Fernow (5.6 kg ha⁻¹) and lowest at HJ Andrews (0.9 kg ha⁻¹; table S1 available at stacks.iop.org/ERL/8/014039/mmedia).

3.1. Trends over time in stream N concentrations

Stream nitrate concentrations, during the 1996–2007 period, significantly decreased in 11 of 22 reference catchments, increased in six, and showed no trend in five (figure 2). From 1987 to 2007, nitrate concentrations significantly decreased in seven of 17 catchments and significantly increased in four catchments. Over the 36 yr period between 1972 and 2007, the four Coweeta catchments showed significant increasing trends and the two catchments at Hubbard Brook and the two at HJ Andrews showed significant decreasing trends (figure 2). The slopes of the observed significant trends ranged between −10 and 8.2 μg NO₃–N l⁻¹ yr⁻¹ (figure S1 available at stacks.iop.org/ERL/8/014039/mmedia).

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Stream ammonium concentrations, during the 1996–2007 period, significantly decreased in nine of 22 catchments and increased in four. From 1987 to 2007, six of 13 catchments showed significant decreasing trends and four showed significant increasing trends. During the 36 yr period between 1972 and 2007, five of eight catchments (the four Coweeta catchments and HJ Andrews-WS9) showed significant increasing trends, and the two catchments at Hubbard Brook showed decreasing trends (figure 2). The slopes of the observed significant trends ranged between −0.7 and 0.8 μg NH₄–N l⁻¹ yr⁻¹ (figure S1 available at stacks.iop.org/ERL/8/014039/mmedia).

Within an EFR site, some reference catchments that were close to each other displayed opposite trends in stream N during the same time periods. For example, during the shortest time period evaluated (1996–2007), stream nitrate decreased at Coweeta-WS2 and Coweeta-WS18 but increased at Coweeta-WS27 and Coweeta-WS36 (figure 2). Similarly, during the 1983 and 1984–2007 period, stream ammonium decreased at HJ Andrews-WS8 and increased for HJ Andrews-Mack and HJ Andrews-WS2 (figures 3(g)–(j)).

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Trends at some individual catchments were consistent over the entire period of collection (e.g., negative nitrate trends at HJ Andrews-WS9; figure 3(f)), whereas trends at other catchments changed direction depending upon the length of record analyzed. For example, HJ Andrews-Mack showed increasing stream ammonium concentrations when analyzing 25, 24, 23, 22 or 21 yr of record prior to 2007; no significant changes in stream ammonium when considering 20–13 yr of record; and decreasing stream ammonium concentration when analyzing 12 yr of record (figure 3(g)). In addition, the shift between positive and negative trends did not occur simultaneously among catchments or in nitrate and ammonium concentrations.

3.2. Associated trends and relationships between them

Long-term data from reference forested catchments provide a unique opportunity to evaluate complex patterns of stream N concentrations over more than four decades across the USA. Through synthesis of data from 22 reference catchments at seven EFRs, we find that there are trends in stream N concentrations even at these minimally disturbed reference sites and that they present considerable spatial and temporal variability both among catchments within sites and among sites.

4.1. Spatial and temporal variability in trends

4.2. N concentrations and magnitude of trends

4.3. Possible drivers of changes in stream N

Understanding whether nutrient concentrations are changing over time in reference streams is vital for good management and protection of water resources. The data presented here provide a unique opportunity to document changes in N concentrations in streams in the absence of changes in land use or other anthropogenic impacts except atmospheric deposition. Trends in stream N concentrations show high spatial variability both within and among sites, and our results demonstrate the transient nature of trends. The direction and significance of trends varied with record length at some catchments, a finding that reinforces the value of long data records, the need of properly pairing record lengths for catchment comparisons and the importance of caution when extrapolating trends from short time periods to longer periods.

Synthesis of long-term stream chemistry data from multiple catchments is valuable for understanding trends and for determining spatial variation across the USA, while showing some perils of broadly extrapolating information from individual catchment studies or short data records. Local factors including catchment characteristics and natural disturbance events influence trends within a site. Differences of trends within and among EFRs highlight a need for considering multiple reference catchments at both site and national levels to serve as benchmarks against which we can evaluate more heavily human-modified ecosystems. Reference catchments are also essential to improve our basic understanding of patterns and processes governing element cycles within intact ecosystems. Both of these functions can inform the management of N-pollution effects (e.g. through establishment of water quality standards or total maximum daily loads). These results also emphasize the importance of site-specific strategies that are relevant to choice of catchments and sampling schemes; such information is vital when considering trends, the refinement of existing programs, and establishment of new monitoring sites.

This synthesis was facilitated by funding from USDA Forest Service Research and Development, the National Council on Air and Stream Improvement, and the American Reinvestment and Recovery Act (PNW/OSU 10-JV-11260489-026). We acknowledge helpful comments from Brian Lutz, Don Buso, Fred Swanson, Evan Kane and two anonymous reviewers. Collection and archiving of long-term data at these EFRs have been funded by the USDA Forest Service, National Science Foundation's Long-Term Ecological Research (LTER) and Long-Term Research in Environmental Biology (LTREB) programs, and other site-specific grants. For a detailed description of site acknowledgments and datasets, see the supplementary material S5 (available at stacks.iop.org/ERL/8/014039/mmedia).