Grantee Research Project Results
Final Report: Nonlinear and Threshold Responses to Environmental Stressors in Land-river Networks at Regional to Continental Scales
EPA Grant Number: R833261Title: Nonlinear and Threshold Responses to Environmental Stressors in Land-river Networks at Regional to Continental Scales
Investigators: Melillo, Jerry , Peterson, Bruce J. , Vörösmarty, Charles J. , Felzer, Benjamin S. , Kicklighter, David Wesley , McClelland, James , Wollheim, Wil
Institution: Marine Biological Laboratory , University of New Hampshire
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 2007 through August 31, 2010 (Extended to August 31, 2011)
Project Amount: $899,191
RFA: Nonlinear Responses to Global Change in Linked Aquatic and Terrestrial Ecosystems and Effects of Multiple Factors on Terrestrial Ecosystems: A Joint Research Solicitation- EPA, DOE (2005) RFA Text | Recipients Lists
Research Category: Climate Change , Aquatic Ecosystems
Objective:
Water is an essential building block of the Earth system and is critical to human prosperity. At the same time, humans are rapidly embedding themselves in the basic character of the water cycle without full knowledge of the consequences. Major pathways of water-system change are complex and often involve multiple stressors. These include resource mismanagement and overuse, river-flow distortion, pollution, watershed disturbance, invasive species introductions, and greenhouse warming. Definition of the collective significance of such changes remains, in our view, a grand challenge for the natural and social sciences alike.
Our objective in this research has been to explore the relationships among environmental stresses, the nonlinear and threshold behaviors they cause within freshwater ecosystems of large drainage basins, and the changes they cause in ecosystem services provided by the streams and rivers of the basins. To do this, we have refined our process-based aquatic model, the Aquatic Ecosystem Model (AEM), and tested its ability to simulate documented nonlinear and threshold responses to environmental stresses at a variety of spatial scales, from the river reach to the entire river network within a drainage basin. We have coupled the AEM with our improved terrestrial biogeochemistry model, the Terrestrial Ecosystem Model (TEM), thereby creating a new version of our Drainage Basin Model, which we are using for regional and global analyses of nonlinear and threshold behaviors in freshwater systems at large scales.
Our research had four focus areas:
- Expand and refine our existing data archives with new high resolution data that are relevant for Aquatic Ecosystem analysis;
- Revise our Framework for Aquatic Modeling of the Earth Systems (FrAMES) to better serve the AEM both in terms of providing interfaces to our data archive and enable coupling of AEM components;
- Improve the representation of constituent processes in the coupled AEM and link it with an improved version of TEM; and
- Apply AEM and the linked AEM-TEM to identify nonlinear threshold behaviors in freshwater ecosystems.
Summary/Accomplishments (Outputs/Outcomes):
The major accomplishments are summarized in the following bullets.
- Developed regional, continental and global data sets to allow us to explore nonlinear and threshold responses to environmental stresses in land-river networks (http://csdms.colorado.edu).
- Refined our Aquatic Ecosystem Model (AEM) to improve simulations of biological and physical processes in river networks (Wollheim et al. 2008a,b).
- Tested AEM and have begun to explore emerging nonlinear and threshold responses (Wollheim et al. 2011).
- Refined our Terrestrial Ecosystem Model (TEM) to improve land-river linkages (Felzer et al. 2009, 2011; Kicklighter et al. 2011a).
- Integrated TEM and AEM into our updated Drainage Basin Model (Kicklighter et al. 2011b).
Database Development
A considerable amount of effort was dedicated to refining our data archive to serve a variety of Earth System studies. Various data from outside sources were incorporated into the data archive in addition to the development and inclusion of new data products associated with this project. We put special emphasis on collecting a wide range of precipitation data products, both global and regional, from different sources based on in situ and satellite observations as this is one of the most important climate variables for our AEM in addition to air temperature. These and related data products are now held in the CCNY data archive (http://csdms.colorado.edu).
Perhaps the most interesting data set that we developed was a mapping of point source loading of total nitrogen (TN) from wastewater treatment plants across the United States. To develop this data set, we consolidated and resolved the differences between the 1984 and 2004 Clean Water Needs Surveys (CWNS) database. We also analyzed the improvement of wastewater treatment in terms of total nitrogen release to freshwater. The majority of the treatment plants (12,737 out of 15,317 in 1984 and 14,037 out of 16,583 in 2004) release their outflow to surface waters. For those treatment plants where the removal efficiency (the ratio of the influent and effluent TN concentration) was not available, we estimated efficiency from reporting facilities as a function of treatment level. Assigning removal efficiency to all treatment plants allowed us to estimate the TN loading in 1984 and 2004. While significant regional differences exist, our most important finding was that the nation-wide TN loading only decreased slightly (5.6%) despite the cumulative public and private sector capital expenditure of $202.5 billion (indexed to constant 2004 dollars). We have had to make large investments in wastewater management in recent decades. These large investments have allowed treatment of 26% more influent with a modest improvement in removal efficiency (67% to 78%) with the upgrade to mostly secondary or advanced treatment levels. This suggests that a major financial challenge will be associated with water management in the future.
Land-Water Linkages
Overall, we found that the direction of future runoff changes from terrestrial ecosystems is largely dependent upon precipitation changes based on our simulations with the Terrestrial Ecosystem Model. However, other environmental factors moderated these runoff changes to cause nonlinear and sometimes threshold responses to past and future climate change.
In forested watersheds of the eastern United States, we predicted that elevated CO2 on ecosystem function (stomatal closure and CO2 fertilization) would increase runoff by 3-7%, as compared to the effects of climate alone based on the projections of two climate models (NCAR CCSM3 and DOE PCM) and two emissions scenarios (SRES B1and A2) (Felzer et al., 2009). We also predicted that progressive nitrogen limitation and ozone damage to photosynthesis of these forests would increase runoff by a further 6–11%. Our results suggest that failure to consider the effects of the interactions among nitrogen, ozone, and elevated CO2 may lead to significant regional underestimates of future runoff.
In the drier ecosystems of the western United States, we found that warming projected by the NCAR CCSM3 A2 scenario not only directly increases nitrogen mineralization rates but also indirectly decreases them by reducing soil moisture, so the net effect of warming is highly dependent on climatic conditions within each biome (Felzer et al., 2011). Increased soil moisture resulting from larger water use efficiency from the elevated CO2 leads to more net nitrogen mineralization in forests and grasslands, which reduces N-limiting conditions. The effect of CO2 on stomatal conductance is therefore enhanced because of its effect on reducing nitrogen limiting conditions. The ET increases in both forests and grasslands, but decreases in shrublands, largely due to the smaller amount of precipitation. Soil moisture and runoff decrease in all three biomes, but especially in shrublands and grasslands. Our results suggest that moisture limitation is therefore a crucial regulator of nitrogen limitation, which determines the future productivity and water availability in the Western United States.
In relation to water quantity in pan-arctic ecosystems, we found a threshold was reached in simulated river discharges from the pan-arctic watershed during the mid-1960s such that discharge increases between 1900 and 1963, but then began to decrease after 1964 (Kicklighter et al., 2011). The change in river discharge trend was related to changes in the trends of both precipitation and evapotranspiration, which in turn, was related to changes in the warming trend. Increases in precipitation after 1964 were only about one-half the rate observed before 1963. As a result, the larger increase in the rate of evapotranspiration, which more than doubled after 1964 compared to the earlier time period, caused less moisture to be available for runoff during the later time period. The changes in evapotranspiration rates were related to changes in air temperature, which quadrupled the pre-1963 warming rates after 1964. While the simulated trends in pan-arctic river discharge between 1964 and 2006 appeared to be consistent with similar decreases in observed river discharge from northern Canada over this time period, they were not consistent with the increasing discharge observed for the Eurasian rivers. Similar to other studies, we find that the precipitation data sets used as inputs for our study prescribe trends that are similar to the trends in observed river discharge for northern Canada, but opposite those trends observed for some Eurasian Rivers. Thus, the representation of precipitation trends may largely account for the inability of TEM to capture the observed increases in discharge of Eurasian rivers during the latter part of the 20th century.
In relation to water quality, we estimated that the rate of terrestrial DOC loading from the pan-arctic watershed has been increasing over the 20th century primarily as a result of increases in air temperatures and precipitation (Kicklighter et al, 2011). The difference in trends between terrestrial DOC loading and discharge suggested that DOC became continually more concentrated in the runoff to the river networks after 1964. This increase in DOC concentrations in the runoff was caused in part by the enhanced rate of warming after 1964, but also was caused by a deepening of the active layer allowing more organic matter to be exposed to decomposition.
Aquatic Processes
Within river networks, we found that the biota plays a large role in the removal of nitrogen from these aquatic ecosystems through denitrification and is largely responsible for the nonlinear responses of water quality to environmental stressors. Model simulations suggested that 53% of the nitrogen inputs to aquatic systems are removed by these systems (Wollheim et al., 2008b). The rate of removal varies across different aquatic communities with the highest rates of denitrification and lowest residence times occurring in rivers and streams, and the lowest denitrification rates and highest residence times occurring in ground water (Green et al., 2009). The simulations by Wollheim et al. (2008b) indicated that the integrated removal of nitrogen from aquatic systems is similar between small rivers (16.5% of inputs), large rivers (13.6%), and lakes (15.2%), while large reservoirs are less important (5.2%). Small rivers are disproportionately important because they first intercept nonpoint inputs that dominate nitrogen loading globally, and the relatively high surface to volume ratio in these ecosystems makes the nitrogen more available for uptake and removal by the benthic community (Wollheim et al., 2008b). Similarly, we have found that small lakes and reservoirs are disproportionately important in nitrogen removal from lentic ecosystems because of higher drainage ratios (catchment surface:lake or reservoir surface area), higher apparent settling velocities for nitrogen, and greater average nitrogen loading rates to reservoirs (Harrison et al., 2009). In another analysis (Wollheim et al., 2008b), we found that the removal of nitrogen by aquatic biota reduced the length of river reach needed to attain a water quality standard of 10 mg N per liter from a pollutant source by 46% than when only considering dilution from conservative mixing.
The removal efficiency of nitrogen by aquatic biota varies over the year and depends on runoff, water temperature, land use, nitrate inputs and river nitrate concentrations. We found that the nitrate removal efficiency of streams is appreciably reduced during months of high discharge and nitrate flux, but increases during months of low discharge and low nitrate flux (Alexander et al., 2009). We also found that streams in highly nitrogen-enriched watersheds had disproportionately lower nitrate removal efficiency as compared to similarly sized streams in the less nitrate-enriched watershed. In addition, we found that nitrogen removal from a suburban river network approach 100% removal efficiency during low flows, even with the relatively low and saturating uptake velocities typical of surface water denitrification (Wollheim et al., 2008a). However, annual removal percentages were moderate because most nitrogen inputs occurred during high flow periods when hydraulic conditions and temperatures were less favorable for removal.
Within river and stream ecosystems, empirical studies indicated that denitrification is a nonlinear function of concentration. We explored the impact that this nonlinearity has had on the flux of dissolved inorganic nitrogen (DIN) to the coastal zone between preindustrial and contemporary periods with the AEM (Wollheim et al., 2011). The nonlinearity leads to a weakened capacity of entire river systems to control nutrient exports. On average, each order of magnitude increase in loading to a watershed results in a 24% decline in the proportion of aquatic loading that is removed. However, the construction of reservoirs can offset some, though not all of this decline. Our analysis indicates that efficiency loss contributes 25% of the increase in nutrient exports to the coastal zone in the contemporary era due to anthropogenic activities (Wollheim et al., 2011). That is, the nonlinearity in denitrification rates results in a positive feedback in DIN export, so that exports to the coastal increase by a greater factor than do inputs to the river system. Our analyses also suggested that freshwater aquatic systems can account for 10% of global anthropogenic N2O emissions (Beaulieu et al., 2011).
Conclusions:
- Threshold responses in water quantity. Our analyses suggest that tradeoffs between future increases in evapotranspiration (caused by increases in air temperature) and precipitation may have already caused threshold responses in runoff and river discharge in some locations in the pan-arctic region or may cause such thresholds to occur in the future. However, verification of these threshold responses will require better information on the spatial distribution of precipitation and its temporal trends and resolving discrepancies between observed river discharges and precipitation in the associated watershed.
- Threshold responses in water quality. Our analyses also suggest that the denitrification capabilities of aquatic biota have already been overloaded in many river networks such that 25% of the contemporary increase in global riverine DIN export to the ocean due to anthropogenic activities is a result of losses in removal efficiencies.
- Impacts on human water security and aquatic biodiversity. Most of the world’s population (~80%) is exposed to high levels of threat to water security (Vörösmarty et al., 2010). In addition, habitats associated with 65% of the continental river discharge have been classified as moderately to highly threatened. The nonlinear and threshold responses of river watersheds to land-use change and climate change described in our study could have major impacts on both the quantity and quality of water to sustain a growing human population and maintain biodiversity in the future and should be taken into consideration in future water management decisions.
Journal Articles on this Report : 13 Displayed | Download in RIS Format
Other project views: | All 28 publications | 13 publications in selected types | All 13 journal articles |
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Alexander RB, Bohlke JK, Boyer EW, David MB, Harvey JW, Mulholland PJ, Seitzinger SP, Tobias CR, Tonitto C, Wollheim WM. Dynamic modeling of nitrogen losses in river networks unravels the coupled effects of hydrological and biogeochemical processes. Biogeochemistry 2009;93(1-2):91-116. |
R833261 (2008) R833261 (2009) R833261 (2010) R833261 (Final) R834187 (2010) R834187 (2011) R834187 (2012) R834187 (2013) R834187 (Final) |
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Beaulieu JJ, Tank JL, Hamilton SK, Wollheim WM, Hall Jr. RO, Mulholland PJ, Peterson BJ, Ashkenas LR, Cooper LW, Dahm CN, Dodds WK, Grimm NB, Johnson SL, McDowell WH, Poole GC, Valett HM, Arango CP, Bernot MJ, Burgin AJ, Crenshaw CL, Helton AM, Johnson LT, O'Brien JM, Potter JD, Sheibley RW, Sobota DJ, Thomas SM. Nitrous oxide emission from denitrification in stream and river networks. Proceedings of the National Academy of Sciences of the United States of America 2011;108(1):214-219. |
R833261 (2010) R833261 (Final) R834187 (2010) R834187 (2011) R834187 (2012) R834187 (2013) R834187 (Final) |
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Felzer BS, Cronin TW, Melillo JM, Kicklighter DW, Schlosser CA. Importance of carbon-nitrogen interactions and ozone on ecosystem hydrology during the 21st century. Journal of Geophysical Research-Biogeosciences 2009;114(G1):G01020 (10 pp.). |
R833261 (2008) R833261 (2009) R833261 (2010) R833261 (Final) |
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Felzer BS, Cronin TW, Melillo JM, Kicklighter DW, Schlosser CA, Dangal SRS. Nitrogen effect on carbon-water coupling in forests, grasslands, and shrublands in the arid western United States. Journal of Geophysical Research-Biogeosciences 2011;116(G3):G03023 (23 pp.). |
R833261 (Final) |
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Green MB, Wollheim WM, Basu NB, Gettel G, Rao PS, Morse N, Stewart R. Effective denitrification scales predictably with water residence time across diverse systems. Nature Precedings 2009;3520.1. |
R833261 (2009) R833261 (2010) R833261 (Final) R834187 (2010) R834187 (2011) R834187 (2012) R834187 (2013) R834187 (Final) |
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Harrison JA, Maranger RJ, Alexander RB, Giblin AE, Jacinthe P-A, Mayorga E, Seitzinger SP, Sobota DJ, Wollheim WM. The regional and global significance of nitrogen removal in lakes and reservoirs. Biogeochemistry 2009;93(1-2):143-157. |
R833261 (2008) R833261 (2009) R833261 (2010) R833261 (Final) R834187 (2010) R834187 (2011) R834187 (2012) R834187 (2013) R834187 (Final) |
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Helton AM, Poole GC, Meyer JL, Wollheim WM, Peterson BJ, Mulholland PJ, Bernhardt ES, Stanford JA, Arango C, Ashkenas LR, Cooper LW, Dodds WK, Gregory SV, Hall Jr. RO, Hamilton SK, Johnson SL, McDowell WH, Potter JD, Tank JL, Thomas SM, Valett HM, Webster JR, Zeglin L. Thinking outside the channel: modeling nitrogen cycling in networked river ecosystems. Frontiers in Ecology and the Environment 2011;9(4):229-238. |
R833261 (2010) R833261 (Final) R834187 (2010) R834187 (2011) R834187 (2012) R834187 (2013) R834187 (Final) |
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Kicklighter DW, Hayes DJ, McClelland JW, Peterson BJ, McGuire AD, Melillo JM. Insights and issues with simulating terrestrial DOC loading of Arctic river networks. Ecological Applications 2013;23(8):1817-1836. |
R833261 (Final) |
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McGuire AD, Hayes DJ, Kicklighter DW, Manizza M, Zhuang Q, Chen M, Follows MJ, Gurney KR, McClelland JW, Melillo JM, Peterson BJ, Prinn RG. An analysis of the carbon balance of the Arctic Basin from 1997 to 2006. Tellus, Series B:Chemical and Physical Meteorology 2010;62(5):455-474. |
R833261 (Final) |
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Vorosmarty CJ, McIntyre PB, Gessner MO, Dudgeon D, Prusevich A, Green P, Glidden S, Bunn SE, Sullivan CA, Liermann CR, Davies PM. Global threats to human water security and river biodiversity. Nature 2010;467(7315):555-561. |
R833261 (Final) |
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Wollheim WM, Vorosmarty CJ, Bouwman AF, Green P, Harrison J, Linder E, Peterson BJ, Seitzinger SP, Syvitski JPM. Global N removal by freshwater aquatic systems using a spatially distributed, within-basin approach. Global Biogeochemical Cycles 2008;22(2):GB2026. |
R833261 (2008) R833261 (2010) R833261 (Final) |
Exit Exit |
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Wollheim WM, Peterson BJ, Thomas SM, Hopkinson CH, Vorosmarty CJ. Dynamics of N removal over annual time periods in a suburban river network. Journal of Geophysical Research-Biogeosciences 2008;113(G3):G03038. |
R833261 (2008) R833261 (2010) R833261 (Final) |
Exit Exit |
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Xiao J, Zhuang Q, Liang E, Shao X, McGuire AD, Moody A, Kicklighter DW, Melillo JM. Twentieth century droughts and their impacts on terrestrial carbon cycling in China. Earth Interactions 2009;13(10):1-31. |
R833261 (Final) |
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Supplemental Keywords:
Global change, nonlinear ecosystem response, land cover change, hydrologic regime, hydrological modeling, multiple scalesRelevant Websites:
Gridded data sets and the Framework for Aquatic Modeling of the Earth Systems (FrAMES): http://csdms.colorado.edu Exit
Terrestrial Ecosystem Model: http://ecosystems.mbl.edu/tem Exit
Aquatic Ecosystems Model: http://www.wsag.unh.edu Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.