A FRAMEWORK FOR UNDERSTANDING WATER QUALITY AT RIVER NETWORK SCALES

• Sponsoring Institution: National Institute of Food and Agriculture
• Project Status: COMPLETE
• Funding Source: HATCH
• Reporting Frequency: Annual
• Accession No.: 1013433
• Project Start Date: Oct 1, 2017
• Project End Date: Sep 30, 2020
• Program Code: [(N/A)]- (N/A)

Recipient Organization
UNIVERSITY OF NEW HAMPSHIRE
51 COLLEGE RD SERVICE BLDG 107
DURHAM,NH 03824

Performing Department
Natural Resources and the Environment

Non Technical Summary
Water quality problems are known to result from a variety of human activities such as agriculture and urbanization, distributed throughout the landscape. However, many water quality problems are reduced because of processes in water bodies themselves. As pollutants travel through river systems, the probability that pollutant loads are at least somewhat reduced by freshwater ecosystems increases. The factors that control this function are poorly understood. The role of river networks in attenuating water quality problems is also under appreciated by many policy makers and land managers that are trying to address water quality problems. This project provides a framework for understanding this ecosystem service in New England, with an emphasis on non-point sources, including both agriculture and urban sources. Our approach is to isolate the amount of non-point pollutants entering into water bodies through sampling of headwater streams draining different land uses, while simultaneously sampling large rivers to quantify how these pollutants have declined. Models will be used to make these connections. Findings will help to prioritize management activities that are cost effective.

Animal Health Component

70%

Research Effort Categories

Basic

20%

Applied

70%

Developmental

10%

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
112	0210	1070	70%
133	0210	1070	30%

Knowledge Area
133 - Pollution Prevention and Mitigation; 112 - Watershed Protection and Management;

Subject Of Investigation
0210 - Water resources;

Field Of Science
1070 - Ecology;

Keywords

agriculture

water quality

river network

ecosystem function

loading

transformations

Goals / Objectives
The major goals are to 1) Develop and apply a general conceptual model for supply and demand at river network scales that integrates point and non-point sources (supply) from agriculture and urban areas, and aquatic sinks (demand). 2) Continue measurements of different water quality metrics including conductivity, dissolved oxygen, nitrogen, phosphorus, sediments, dissolved organic carbon and greenhouse gases across seasons and flow conditions in headwater streams and along larger rivers. 3) Develop models for each of these variables to understand their variation in time and space throughout river networks and the role of aquatic processes in defining these distributions. 4) Quantify the relative importance of different factors that control supply and demand of different variables in river networks.

Project Methods
Overarching Approach. We will undertake a number of activities to meet our goals of understanding the supply and demand at river network scales for multiple water quality variables. Measured and modeled variables will include phosphate, nitrate, chloride, dissolved organic carbon, sediments, pathogens, and greenhouse gases. First, we will develop a synthesis of the conceptual model with case study examples for each of the major water quality variables. Second, we will sample in headwaters of differing land use (range of agriculture and urban percentage) over time, to understand non-point source inputs from land, while simultaneously sampling in larger tributaries and the mainstem of rivers. Downstream samples, compared to headwaters, provide a means to evaluate the strength of river network sinks (Wollheim et al. 2015). Third, we will conduct process studies to improve model parameterization and model testing. Fourth, we will continue to develop and apply modules within the Framework for Aquatic Modeling of the Earth System (FrAMES, Wollheim et al. 2008b) to be able to look at all of these variables simultaneously at river network scales. We will use FrAMES to quantify the distribution of supply and demand for each variable in a series of different watersheds, including their relationship to agricultural and suburban sources.6.1. Quantify nitrogen, sediment, bacteria, and greenhouse gas concentrations and fluxes in headwater streams across a range of land usesWe will sample a series of headwater watersheds for water chemistry including dissolved nutrients, dissolved gases, conductivity, sediments, and bacteria (Wollheim et al. 2005, 2015). Sampling is designed characterize supply of materials across a range of conditions, accounting for different seasons and flow conditions. Headwater catchments will range in their percent urban, agriculture, wetland, and forest. Standard sampling will be monthly. To improve estimates of sources during rapidly changing storm event flow conditions, we will sample at higher frequency using two potential approaches: first, using high frequency in situ sensors and second using sigma autosamplers that collect regular samples during storms as programed by users (Carey et al. 2014, Morse and Wollheim 2014).In situ sensors can sample for nitrate, conductivity (~ chloride), dissolved oxygen (to derive gross primary production and respiration), fluorescent organic matter (terrestrial-derived dissolved organic carbon), turbidity ( ~ sediments), water temperature, and stage (~ discharge). We will deploy new, open source pCO2 sensors to gain time series of this greenhouse gas (Hunt et al. In Review). Grab samples will be collected and analyzed for nitrate, ammonium, phosphate, dissolved organic carbon, dissolved organic nitrogen, particulate organic nitrogen, conductivity, pH, sediments, and greenhouse gases (CO2, CH4, N2O). Results from grab samples will be used to either develop proxies between sensor data and variable of interest (e.g. chloride from conductivity), or to validate sensor measurements (e.g. nitrate, Carey et al. 2014). Fluxes will be estimated by combining discharge measurements and nutrient concentrations. Statistical analysis will include multiple regression of concentration vs. land use, flow condition, season, etc. to derive loading relationships (Wollheim et al. 2008a, 2015).6.2. . Quantify nitrogen, sediment, bacteria, and greenhouse gas concentrations and fluxes in tributaries and larger rivers.The same variables described above will be sampled in larger tributaries and rivers. Again, more frequent samples will be collected during storm events to characterize fluxes over relatively short time scales. We will measure basin mouths regularly (on the same schedule as the headwater sites described above), and periodically measure a wider array of tributary and mainstem locations to identify patterns of concentration and flux that can be used to identify hotspots of processing.6.3. Quantify process rates within different water bodies to parameterize models of aquatic demand under different conditions. Among the rates we will need to measure to improve model performance include: gas exchange rates, gross primary production, respiration, hydrologic exchange rates with transient storage zones, particle settling rates, nutrient uptake or denitrification, DOC breakdown rates, and bacterial breakdown rates. Each of these will be determined in streams of different sizes, with different flow conditions, and different seasons. We will use standard techniques to develop rate vs. flow condition relationships in different stream types (e.g., Raymond and Saiers 2010), accounting for land use and stream size. For gas exchange, we will conduct trace gas emissions using SF6, argon, or propane and measure loss rates from the water (Raymond et al. 2012). For hydrologic exchange, we will use Chloride tracer additions and transient storage zone modeling (Briggs et al. 2010). For GPP and R we will use single station metabolism methods (Bott 2006). For nutrient uptake, we will use reactive nitrogen additions (Wollheim et al. 2014). In addition to these whole ecosystem experiments, we will do input-output measurements in longer residence time systems, by sampling variables of interest at all inputs and the outflow of defined water bodies, and estimating percent retention, from which we can derive process rates if we know the hydrologic dimensions.6.4. Improve an existing model and apply it to different river networks in New England to understand water quality impairments, management options, and management priorities. We will apply a spatially distributed river network model to different watersheds in New England that has already been developed on previous AES and externally funded awards. The model, called the Framework for Aquatic Modeling of the Earth System, FrAMES, (Wollheim et al. 2008), has already been applied to understand water quality in response to agriculture and urban land use, including parameters such as dissolved inorganic nitrogen, dissolved organic carbon, water temperature, and salt (Mineau et al. 2015, 2016, Wollheim et al. 2015, Zuidema et al. In Review, Samal et al. In Review, Stewart et al. 2013). These applications were in different river networks throughout New England. Additional applications are being developed, including dissolved oxygen, aquatic metabolism, and bacteria (Huang 2016, Sheehan et al. In Review). The model is time and space varying, which allows testing of many of the issues discussed above, especially the effect of land use distribution, land use change, climate variation, and climate change (e.g. Samal et al. In Review).However, uncertainties emerged, in particular regarding the land use/climate interaction as an influence on loading, and various instream processes. The measurements described above will help to improve parameterization of this model. Further, we will develop new and improved modules to test our understanding of carbon dynamics in linked terrestrial-aquatic watersheds. Several proposals are also pending to further enhance these models. One proposal to be submitted to NSF-CHN will specifically incorporate alternative agricultural management strategies (high tunnels, silvopasture) to evaluate impacts on water quality at a regional scale.

Progress 10/01/17 to 09/30/20

Outputs
Target Audience:The target audiences were UNH undergraduate and graduate students, stakeholders (residents, managers, local and state government, aquaculturists, educators) of the coastal resources of New Hampshire (NH) and New England. Stakeholder engagement included the Piscataqua Region Estuary Partnership (PREP), NH Natural Resources Conservation Service (NRCS), the Town of Durham, the Oyster River School District (Elementary School Coordinator Ellen Ervin), EPA, and the NH Department of Environmental Services (DES). Wollheim became co-chair in 2019 (with Dr. Bonnie Brown) of the PREP Technical Advisory Committee to help identify research priorities related to the Great Bay estuary and its watersheds. He is currently advising PREP in updating their Research and Monitoring Plan for Great Bay. An additional target audience includes primary and secondary students through events such as Ocean Discovery Day (UNH), Little Harbour School's STEAM Day (Portsmouth NH), class presentations (Durham NH) and Skype a Scientist (global network). Throughout the project, AES Graduate Assistant Eliza Balch has met with several partners and stakeholders to discuss updates and collaborations. These meetings have included members of the Town of Durham town officers, NH NRCS, NH DES, and the City of Portsmouth. Balch presented findings at several meetings including: Lamprey River Symposium (January 2019 and 2020, Durham NH), the UNH Graduate Research Conference (April 2019 and 2020, UNH Durham), New Hampshire Water and Watersheds Conference (March 2019 and 2020, Plymouth NH), and the national meeting of the American Geophysical Union (December 2019, San Francisco CA). Wollheim hosted a high school student to "shadow" him during a field measurement campaign (June 2019) so the student could learn about water quality research. Wollheim and Balch have worked directly with civil engineering consultants Vanasse Hangen Brustlin, Inc. (VHB) to provide findings and background research on the impacts of the Mill Pond reservoir in Durham NH, on downstream nitrogen fluxes and water quality for their dam removal feasibility study. Wollheim presented findings and participated in discussions at a number of meetings and workshops including: Piscataqua Region Estuarine Partnership Technical Advisory Committee (June 2018, Kittery ME), the New Hampshire Coastal Climate Summit organized by the Coastal Adaptation Working Group (June 2018, Greenland NH), the Coastal Climate Resiliency Workshop organized by the Northeast Climate Adaptation center (October 2018, Boston MA), the New Hampshire Water and Watersheds Conference (March 2018, Plymouth MA). Wollheim also participated as instructor in the Stream Resiliency Research Coordinating Network workshop on Spatial Analysis (Boulder CO, Nov 2018), gave two seminars at the University of Alabama (Nov 2018), led a workshop on in situ sensors and presented at the American Geophysical Union (Washington DC, December 2018, and was an invited speaker at the Gordon Research Conference on Catchment Science (Andover NH, June 2019). Presentation over the three-year project period also were made the annual meetings of the Society of Freshwater Science (2018,2019), American Geophysical Union (2017,2018), Ecological Society of America. Undergraduate researchers have presented their findings at the COLSA URC, and at a summer research symposium for summer undergraduate researchers on the UNH campus, and at the Society of Freshwater Science annual meeting. Twenty-three undergraduate and graduate students in Wollheim's Aquatic Ecosystems class (NR 751/851) went on field trips to visit measurement locations to understand water quality impacts of agriculture and urbanization. Fifty undergraduate students in Wollheim's Freshwater Resources class (NR 504) visited measurement locations to understand water quality impacts of agriculture and urbanization. Over the past year, presentations have been greatly reduced due to COVID-19. Changes/Problems:COVID-19 has cause some research activities in Spring 2020 to be delayed or cancelled due to on-campus lab restrictions imposed during the Coronavirus pandemic shutdowns. Because the project was already far along, all our goals were attained. Also, presentations at meetings and outreach activities were greatly reduced in this last year for the same reason. What opportunities for training and professional development has the project provided?Four graduate students were trained on this project, including Dan Bolster (M.S.), Eliza Balch (M.S.), Drew Robison (Ph.D), and Chris Whitney (Ph.D). Dan completed his field work in 2018, but defended during summer 2020 (he got a job that slowed completion). He will graduate December of 2020. Eliza is an M.S. student funded by an AES graduate student fellowship between 2018 and 2020, and will defend her M.S. this semester while being a Teaching Assistant. Drew is in his final year and will graduate in May 2021 (and has a post doc lined up with Dr. Tom Battin in Switzerland), while Chris will likely graduate in May 2022. Each of these students has been trained in scientific processes, including proposal writing, paper writing, research project planning, field methods, and networking. Bolster presented his results at the Society of Freshwater Science (SFS) meeting in Detroit MI (May, 2018), Balch presented her results at the annual meeting of the American Geophysical Union (AGU) in San Francisco (December, 2019) and at the New Hampshire Water and Watershed Conference in March 2019 and 2020, Robison presented his results at AGU meeting in Washington, D.C. (2018) and San Francisco (2019) and SFS (May 2019, virtually in 2020). Whitney also presented his research at SFS in May 2019 and 2020. Balch also worked directly with civil engineers Vanasse Hangen Brustlin, Inc. with her work in a coastal reservoir and watershed. Several undergraduates were trained in research within this project, including Bonnie Turek, Maggie Phillips, Cynthia Bova, Carter Snay, Hannah Miller, and Julie Bobyock. All students were trained in field and laboratory activities, including: stream gaging, water sample collection, total suspended solids (TSS) measurement, and analytical methods in dissolved gas (i.e. CO2?CH4, and N2O) and N2:Ar measurements. Turek was funded through an NSF Research Experience for Undergraduates fellowship during the summer of 2018. She conducted research on methane ebullition in forested and urban headwater streams. She presented her research at the UNH Undergraduate Research Conference (URC) as well as at the Society for Freshwater Science (SFS) annual meeting in Salt Lake City in May 2019. She received a $500 undergraduate travel award from SFS, which she enhanced with a UNH Natural Resources Farrington Fund award, in order to be able to travel to the meeting. Phillips was funded during the summer of 2018 and studied the role of spatial heterogeneity within two NH reservoirs, sampling for dissolved oxygen, nitrogen gas (an indicator of denitrification) and dissolved nutrients. In May 2019, Phillips wrote her senior Environmental Science thesis on this research and presented results at the UNH Undergraduate Research Conference, receiving honorable mention for her work. In the summer of 2019, Cynthia Bova was funded on a SURF during the summer of 2019 and Carter Snay was funded on a Weeks Fellowship. They worked on methane emissions from streams draining different land use. Both Bova and Snay presented their findings at the UNH Undergraduate Research Conference (Virtual posters, 2020). Hannah Miller studied sediment characteristics and mercury content in two NH reservoirs, and presented her research at the UNH Undergraduate Research Conference as well as with civil engineers Vanasse Hangen Brustlin, Inc. Julie Bobyock, working remotely during summer of 2020, analysed existing data sets to determine whether town water withdrawals are detectable in USGS gaging station data during summer, and will be presenting her poster at the virtual AGU meeting in December 2020. She continues to do measurements on ecosystem processes of storm water detention ponds during Fall 2020. How have the results been disseminated to communities of interest?Wollheim and members of his lab group have presented their research findings summarized above at various meetings. Current lab members who have presented results from the AES work, in addition to Wollheim, include Zuidema, Samal, Robison, Balch, Turek, Phillips, and Bolster. Major society meetings include the American Geophysical Union annual meeting, the Ecological Society of America annual meeting, and the Society for Freshwater Science annual meeting. Major local and regional meetings include the Lamprey River Symposium (January 2019 and 2020, Durham NH), Piscataqua Region Estuarine Partnership Technical Advisory Committee (meets regularly), the New Hampshire Coastal Climate Summit organized by the Coastal Adaptation Working Group (June 2018, Greenland NH), the Coastal Climate Resiliency Workshop organized by the Northeast Climate Adaptation center (October 2018, Boston MA), and the New Hampshire Water and Watersheds Conference (March 2019 and 2020, Plymouth NH). Members of the group have also met with several partners and stakeholders throughout the course of the project including the Town of Durham, NH Natural Resources Conservation Service, NH Department of Environmental Services, the City of Portsmouth, civil engineering consultants Vanasse Hangen Brustlin, Inc. several oyster aquaculture farmers in Great Bay, and several private property owners. An additional target audience includes primary and secondary students through events such as Ocean Discovery Day (UNH), Little Harbour School's STEAM Day (Portsmouth NH), class presentations (Durham NH), Global Learning and Observations to Benefit the Environment (GLOBE) Program (global network) and Skype a Scientist (global network). Undergraduate researchers have presented their findings at the UNH Undergraduate Research Conference, a summer research symposium for summer undergraduate researchers on the UNH campus, the Society of Freshwater Science annual meeting, and to Vanasse Hangen Brustlin, Inc. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? Impact Water quality is one of the most persistent and pervasive environmental issues of our time. Human systems and ecosystems alike depend on the health of surface waters and their processes. Some water quality issues, such as nutrient-overloading from agricultural and urban land, are improved by the water bodies themselves, through natural biological processes. However, the factors that control these processes are poorly understood, particularly when considering entire networks of surface waters. The threat of land use and climate variability on water quality drives us to understand the conditions at which surface water networks can thrive. Our findings will help policy makers and natural resource managers make better decisions, including optimizing activities, and better plan for the future. Examples include: 1) Management of nitrogen in storm flows should be prioritized because river networks are able to remove much of the inorganic nitrogen during lower flows. 2) Management of nitrogen inputs close to the mouth of a watershed should prioritized, because nitrogen entering the most distant headwaters can be removed by river networks. 3) Dam removals should consider the impact of the reservoir in downstream nitrogen fluxes, which may be site specific. Although reservoirs are generally considered to be nitrogen sinks, some reservoirs, including the Mill Pond reservoir in Durham, NH, may just be transformers from inorganic to organic nitrogen, and may in fact at times be sources of additional nitrogen via N fixation. Our findings from the Mill Pond reservoir in Durham were incorporated into the Dam Removal Feasibility Study conducted by the consulting firm VHB Inc. The report was submitted to the Town of Durham to support the town's consideration of what to do about the town-owned dam. We recommended from a nitrogen perspective, the removal of the dam will not lead to increased nitrogen fluxes to the Great Bay estuary, a nitrogen impaired system. 4) Increased frequency of intense precipitation events will likely lead to a disproportionate increase in N fluxes to coastal zones, so coastal managers should begin to consider climate change interactions with population growth and land use change into long term planning of estuarine health. 5) Given the high proportion of NH streams impaired due to high salt content, the amount of road salt applied to New Hampshire roads should be optimized and that managers will need to be patient to see environmental improvements. Overall Goals and Applications Goal 1. Develop and apply a general conceptual model for supply and demand at river network scales that integrates point and non-point sources (supply) from agriculture and urban areas, and aquatic sinks (demand). Goal 2. Continue measurements of different water quality metrics including conductivity, dissolved oxygen, nitrogen, phosphorus, sediments, dissolved organic carbon and greenhouse gases across seasons and flow conditions in headwater streams and along larger rivers. Goal 3. Develop models for each of these variables to understand their variation in time and space throughout river networks and the role of aquatic processes in defining these distributions. Goal 4. Quantify the relative importance of different factors that control supply and demand of different variables in river networks. Key findings and implications Key findings over the life of this project and their implications, and the goals they applied to, include: 1) River networks have a finite capacity to attenuate nutrient fluxes from agricultural or urban land because supplies overwhelm demand, but that ponded waters significantly increase capacity. These results suggest that natural resource managers should prioritize loading reductions in watersheds with fewer ponded waters first, and that function of the entire range of water bodies from small streams to large rivers should be protected. (Goal 1) 2) River networks are able to retain a large proportion of nutrient inputs during small storms and much less during large storms. This indicates that if extreme flow events increase, we will be able to rely less on retention by aquatic ecosystems, and that stormwater controls that reduce flood peaks will result in increasing aquatic nutrient retention. 3) A large proportion of rivers in New Hampshire are impaired by high salt concentrations, mostly during summer, due to road salt applications in winter. The annual snowfall amount, which drives road salt loading, was a driver of chloride impairment in headwater streams during summers, whereas summer runoff from forested watersheds determines chloride levels in larger rivers. Future declines in snowfall have a delayed impact on stream chloride due to large groundwater storage pools. These results suggest that the amount of road salt applied to New Hampshire roads should be optimized and that managers will need to be patient to see environmental improvements. 4) Model projections show that climate will have a greater influence on future aquatic ecosystem services than plausible changes in land cover, though the latter exacerbates impacts. Minimal changes in aquatic environmental indicators are predicted through 2050, after which the high emissions scenarios show rapidly intensifying impacts. These findings suggest that actions that mitigate climate change are important to implement now and that we should prepare for our natural ecosystems to begin to drastically change in the near future. 5) as watersheds increase in size (as water flows downslope), the cumulative function of the river network can increase at a faster rate than does the rate of input, indicating the importance of larger rivers as a key component determining the role of river systems in regulating water quality (Wollheim et al. In Review). An implication is that large rivers should be managed in order to maintain their functionality. 6) reservoirs may at times be nitrogen sources not just nitrogen sinks. When algal blooms occur, all inorganic N is assimilated or denitrified, allowing N fixing algae to become abundant, introducing more N to the system (Phillips URC 2019). This has implications as to whether removal of dams in New England is likely to lead to increases or decreases in nitrogen fluxes to the coast. 7) based on modeling results, river networks have a greater capacity to improve bacterial contamination (E. coli) than previously thought due to the filtration by stream bottom sediments, through which surface water continuously exchanges. Previous models of E. coli attenuation only considered die-off in the water column (Huang et al. In Review). Managers of landscape sources of pathogen bacterial should incorporate the role of this natural ecosystem service. 8) Streams draining human-dominated watersheds can maintain ecosystem functions similar to or higher than those in more natural watersheds. Although they experience greater hydrologic disturbance than streams in more natural watersheds (more extreme and frequent high flows), streams in urban watersheds can recover quickly to high levels of function (e.g. gross primary production and respiration) (Bolster 2020). The implication is that some stream functions are not degraded by land use change, and it is important to identify how these watersheds can be better managed to maintain such function. 9) Small coastal reservoirs that are infilling may not be nitrogen sinks despite having wetland characteristics that suggest they should be sinks. Rather, they may be transformers from in organic (nitrate) to organic (dissolved organic nitrogen) forms that can be transported further downstream. Over some time periods, these systems may also be new sources of nitrogen via N fixation (Balch 2020).

Publications

• Type: Journal Articles Status: Published Year Published: 2020 Citation: Li W., L. Qiuliang, H. Yen, W. M. Wollheim, L. Zhai, W. Hu, L. Zhang, W. Qiu, J. Luo, H. Wang, T. Ren, H. Liu. 2020. The overlooked role of diffuse household livestock production in nitrogen pollution at the watershed scale. Journal of Cleaner Production. 272. 122758 https://doi.org/10.1016/j.jclepro.2020.122758
• Type: Other Status: Published Year Published: 2020 Citation: Wollheim, W.M. 2020. RE: Scientists and Disinformation on Social Media. eLetter response to editorial by H. Holden Thorp. Science. 368 p 1405. https://science.sciencemag.org/content/368/6498/1405/tab-e-letters
• Type: Journal Articles Status: Published Year Published: 2018 Citation: See additional publications reported in previous progress reports for additional products of this research.
• Type: Theses/Dissertations Status: Accepted Year Published: 2020 Citation: Bolster, D. 2020. Quantifying the effects of land use and flow regime on metabolism of New England Streams. M.S. Thesis. University of New Hampshire. 101pp.
• Type: Theses/Dissertations Status: Under Review Year Published: 2020 Citation: Balch, E.C. 2020. Taking nitrogen by storm: spatial and temporal controls on nitrogen processing in a small stream. M.S. Thesis. University of New Hampshire.

Progress 10/01/18 to 09/30/19

Outputs
Target Audience:The target audiences were UNH undergraduate and graduate students, and stakeholders interested in the coastal resources of New Hampshire and New England. Stakeholder engagement continued to include the Piscataqua Region Estuary Partnership (PREP), the Natural Resources Conservation Service, the Town of Durham, and the New Hampshire DES. Wollheim recently became co-chair (with Dr. Bonnie Brown) of the PREP Technical Advisory Committee to help identify research priorities related to the Great Bay estuary and its watersheds. Wollheim hosted a high school student to "shadow" him during a field measurement campaign (June 2019) so the student could learn about water quality research. Members of the group have also met with several partners and stakeholders throughout the year including the Town of Durham, NH NRCS, NH DES, the City of Portsmouth, and oyster aquaculture farmers in Great Bay. An additional target audience includes primary and secondary students through events such as Ocean Discovery Day (UNH), Little Harbour School's STEAM Day (Portsmouth NH), class presentations (Durham NH) and Skype a Scientist (global network). Undergraduate researchers have presented their findings at the COLSA URC, and at a summer research symposium for summer undergraduate researchers on the UNH campus, and at the Society of Freshwater Science annual meeting. Wollheim also participated as instructor in the Stream Resiliency Research Coordinating Network workshop on Spatial Analysis (Boulder CO, Nov 2018), gave two seminars at the University of Alabama (Nov 2018), led a workshop on in situ sensors and presented at the American Geophysical Union (Washington DC, December 2018, and was an invited speaker at the Gordon Research Conference on Catchment Science (Andover NH, June 2019). Fifty undergraduate students in Wollheim's Freshwater Resources class (NR 504) visited measurement locations to understand the water quality impacts of agriculture and urbanization. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?Three graduate students were trained on this project this past year, including Eliza Balch (M.S.), Drew Robison, and Chris Whitney (Ph.D's). Eliza is an M.S. student funded by an AES graduate student fellowship. Each of these students has been trained in scientific processes, including proposal writing, paper writing, research project planning, field methods, and networking. Eliza is working on her Master's thesis, Drew is approaching his final year, while Chris likely still has two years. Drew presented his results at the national meeting of the American Geophysical Union in Washington, D.C. in December 2018 and the Society of Freshwater Science (SFS) in May 2019. Chris also presented his research at SFS in May 2019. A number of undergraduates were trained in research this past year, including Bonnie Turek, Maggie Phillips, Cynthia Bova, and Carter Snay. All four students were trained in several field and laboratory activities, including stream gaging, water sample collection, total suspended solids (TSS) measurement, and analytical methods in dissolved gas (i.e. CO2?CH4, and N2O) measurements. Turek was funded through an NSF Research Experience for Undergraduates fellowship during the summer of 2018. She conducted research on methane ebullition in forested and urban headwater streams. She presented her research at the UNH Undergraduate Research Conference (URC) as well as at the Society for Freshwater Science (SFS) annual meeting in Salt Lake City in May 2019. She received a $500 undergraduate travel award from SFS, which she enhanced with a UNH Natural Resources Farrington Fund award, in order to be able to travel to the meeting. Phillips was funded on a UNH Summer Undergraduate Research Fellowship (SURF) during the summer of 2018 and studied the role of spatial heterogeneity within two NH reservoirs, sampling for dissolved oxygen, nitrogen gas (an indicator of denitrification) and dissolved nutrients. In May 2019, Phillips wrote her senior Environmental Science thesis on this research and presented results at the UNH Undergraduate Research Conference, receiving honorable mention for her work. Cynthia Bova was funded on a SURF during the summer of 2019 and Carter Snay was funded on a Weeks Fellowship. They worked on methane emissions from streams draining different land use, both expecting to present at the 2020 URC. Turek continues to work in the Wollheim lab on an hourly basis. How have the results been disseminated to communities of interest?Wollheim and members of his lab group have presented a number of the findings summarized above at various meetings. Current lab members who have presented results from the AES work, in addition to Wollheim, include Robison, Balch, Turek, Phillips, and Bolster. Major society meetings include the American Geophysical Union annual meeting and the Society for Freshwater Science annual meeting. Major local and regional meetings include the Lamprey River Symposium (January 2019, Durham NH), Piscataqua Region Estuarine Partnership Technical Advisory Committee (meets regularly). Members of the group have also met with several partners and stakeholders throughout the year including the Town of Durham, NH NRCS, NH DES, the City of Portsmouth, and oyster aquaculture farmers in Great Bay. An additional target audience includes primary and secondary students through events such as Ocean Discovery Day (UNH), Little Harbour School's STEAM Day (Portsmouth NH), class presentations (Durham NH) and Skype a Scientist (global network). Undergraduate researchers have presented their findings at the COLSA URC, and at a summer research symposium for summer undergraduate researchers on the UNH campus, and at the Society of Freshwater Science annual meeting. Wollheim also participated as instructor in the Stream Resiliency Research Coordinating Network workshop on Spatial Analysis (Boulder CO, Nov 2018), gave two seminars at the University of Alabama (Nov 2018), led a workshop on in situ sensors and presented at the American Geophysical Union (Washington DC, December 2018, and was an invited speaker at the Gordon Research Conference on Catchment Science (Andover NH, June 2019). Fifty undergraduate students in Wollheim's Freshwater Resources class (NR 504) visited measurement locations to understand water quality impacts of agriculture and urbanization. What do you plan to do during the next reporting period to accomplish the goals?We are making good progress towards all our goals. Dan Bolster, MS student is planning to defend his thesis in the coming year. Eliza Balch, MS student has conducted her Master's thesis research on understanding the function of small coastal reservoirs in transforming nutrient pollution during a range of flow conditions. She will present at New Hampshire Water and Watershed Conference in March 2020, Society for Freshwater Science's annual meeting in May 2020, and defend her thesis research in summer 2020. In addition, she will continue to meet with stakeholders and collaborators as needed, and work with local partners to enhance engagement through public presentations and events. Wollheim is also hosting a research scholar from China, Dr. Yue Hu, who will be here through summer 2020. She is currently working with data sets and models relevant to the AES work Modeling work will continue to investigate the role and fate of non-point pollutants on aquatic ecosystems, with the specific focuses moving towards understanding the fate and transport of pathogens, sediments, and dissolved oxygen. A proposal was also re-submitted to USDA this past summer, in collaboration with Dr. Jessica Ernakovich, to understand how agricultural activity impacts stream ecosystem function and the associated microbial communities. If this project is funded, it will start a new collaboration to improve understanding of how land use, microbial communities, and ecosystem function interact.

Impacts
What was accomplished under these goals? IMPACT: The major issues being addressed by this research include how water quality is impacted by agriculture and urbanization, how climate variability interacts with land use to affect water quality, and to what extent surface waters help clean up non-point pollution inputs. This past year, my research team focused more heavily on quantifying the impact of reservoirs on nutrient retention using input/output measurements; the dynamics within reservoirs that lead to the observed net retention; the dynamics in stream channels that lead to greenhouse gas emissions; and modeling the ability of entire networks of surface waters to mitigate pollution fluxes to critical downstream water bodies, including impaired estuaries and lakes. Key findings and implications include: 1) as watersheds increase in size (as water flows downslope), the cumulative function of the river network can increase at a faster rate than does the rate of input, indicating the importance of larger rivers as a key component determining the role of river systems in regulating water quality (Wollheim et al. In prep). An implication is that large rivers should be managed in order to maintain their functionality. 2) reservoirs may at times be nitrogen sources, not just nitrogen sinks. When algal blooms occur, all inorganic N is assimilated or denitrified, allowing N fixing algae to become abundant, introducing more N to the system (Phillips URC 2019). This has implications as to whether the removal of dams in New England is likely to lead to increases or decreases in nitrogen fluxes to the coast. 3) based on modeling results, river networks have a greater capacity to improve bacterial contamination (E. coli) than previously thought due to the filtration by stream bottom sediments, through which surface water continuously exchanges. Previous models of E. coli attenuation only considered die-off in the water column (Huang et al. In Review). Managers of landscape sources of pathogen bacterial should incorporate the role of this natural ecosystem service. Goal 1. Develop and apply a general conceptual model for supply and demand at river network scales that integrates point and non-point sources (supply) from agriculture and urban areas, and aquatic sinks (demand). The conceptual model that was published in 2018 (The River Network Saturation (RNS) Concept; Wollheim et al. 2018) continues to be expanded on, focusing now on how processes accumulate with increasing drainage area of the watershed. This new perspective reveals that as watershed size increases, aquatic ecosystem function increases at a faster rate, especially under high flow conditions. This idea builds on the Metabolic Theory of Ecology, which showed that at the scale on an individual organism or an individual ecosystem, metabolism increases at slower rate than organism or ecosystem size. Our analysis finds that in contrast, river networks behave fundamentally differently than other systems, and this turns out be because non-point material inputs are primarily to smaller headwater streams, while reactive river surface area continues to increase as watershed size increases. Models were developed to demonstrate the factors controlling this scaling characteristic. This work is currently in the final stages of preparation for a publication and will be submitted to the Proceedings of the National Academy of Sciences. Goal 2. Continue measurements of different water quality metrics including conductivity, dissolved oxygen, nitrogen, phosphorus, sediments, dissolved organic carbon and greenhouse gases across seasons and flow conditions in headwater streams and along larger rivers. We had four measurement campaigns this past year regarding water quality, led by AES Graduate Fellow Eliza Balch. First, we monitored continuous nutrient fluxes at the mouth of a local watershed, the Oyster R., using in situ, high-frequency sensors. Monitoring at the mouth of the river was paired with continuous nitrate and conductivity sensors upstream of the reservoir for a two-week period in summer 2019 to evaluate the role of the reservoir in transforming nitrate. Second, we monitored inputs and outputs to two coastal reservoirs for over a year to understand how small reservoirs contribute to river network scale nitrogen demand across flow conditions. The two reservoirs are the Mill Pond on the Oyster R. and the Sawyer Mill reservoir on the Bellamy R. Third, we conducted an intensive synoptic study of the Mill Pond reservoir to evaluate nitrogen processing in different areas of the reservoir during storm events. This study will be used to evaluate biogeochemical response to storm events, and spatial variability of that response. The fourth sampling campaign was a series of Rhodamine Water Tracer dye additions in the Mill Pond to aid in the understanding of how hydrological conditions influence the biogeochemical role of reservoirs. The dye movement was monitored with continuous sensors over the course of several days to quantify hydrologic variables such as residence time and connectivity of different areas of the reservoir during normal conditions and storm conditions. Results of all four studies will be synthesized in Eliza Balch's thesis work. We focused on the mass balance of nitrogen in particular, given it is a major cause of impairment of Great Bay. We added regular sampling of dissolved nitrogen gas (N2) and nitrous oxide gas (N2O) to quantify N removal via denitrification and potential tradeoffs with greenhouse gas release. We submitted the results from this work to the EPA Low Cost Nutrient Sensor Challenge Prize, Phase 2 competition. We were one of three winners nationwide, demonstrating the high-quality nature of our research. Goal 3. Develop models for each of these variables to understand their variation in time and space throughout river networks and the role of aquatic processes in defining these distributions. This past year, we applied the Framework for Aquatic Modeling of the Earth System (FrAMES) to understand the fate of bacterial pathogens as they are transported downstream (Huang et al. In Review). Wollheim was recently awarded (as UNH Co-I) an NSF INFEWS project to model the nitrogen dynamics of the Mississippi R. basin under different management scenarios. This project is led by Dr. Tom Hertel at Purdue University. Our initial goal is to understand how wetland mitigation interacts with nitrogen removal by the river network to influence N exports from the mouth of the Mississippi R. watershed, where a hypoxic dead zone remains a major problem. This will be an exciting application of research to address an important agricultural water quality problem. Goal 4. Quantify the relative importance of different factors that control the supply and demand of different variables in river networks. Based on the model applications described above, we have been able to identify key factors that contribute to a smaller imbalance between network supply and demand, representing times and places when downstream fluxes of pollutants are effectively regulated by surface water. These conditions include low stream flow periods, large watersheds, river systems with significant interactions with subsurface benthic sediments (filtration), presence of ponded waters like beaver ponds, lakes, and reservoirs, and disproportionate distribution of pollutant sources in the headwaters of a watershed rather than closer to the basin mouth. Each of these factors should be considered when prioritizing management activities, such as where to emphasize cover crops, fertilizer reductions, septic system upgrades, and installing stormwater management. We are currently working on a proposal to EPA program "Reducing Nutrients to Better Manage Harmful Algal Blooms" to synthesize multiple point and non-point sources, management strategies, and natural ecosystem services of surface water nutrient removal in a watershed context to identify management priorities.

Publications

• Type: Other Status: Published Year Published: 2019 Citation: Wollheim W.M., G. Mulukutla, E. Balch. 2019. Quantifying the impact of dam removals on nitrate retention using low cost nitrate sensors. Report for Phase 2 of the EPA Low Cost Nutrient Sensor Challenge. 22pp.
• Type: Journal Articles Status: Published Year Published: 2019 Citation: Borsuk M., G. Mavrommati, N. Samal, S. Zuidema, W. Wollheim, S. Rogers, A. Thorn, D. Lutz, M. Mineau, C. Grimm, C. Wake, R. Howarth, K. Gardner. 2019. Deliberative multiattribute valuation of ecosystem services across a range of regional land-use, socioeconomic, and climate scenarios for the upper Merrimack River watershed, New Hampshire, USA. Ecology and Society 24(2):11. https://doi.org/10.5751/ES-10806-240211
• Type: Journal Articles Status: Published Year Published: 2019 Citation: Bernal S., A. Lupon, W.M. Wollheim, F. Sabater, S. Poblador, E. Marti. 2019. Supply, demand, and in-stream retention of dissolved organic carbon and nitrate during storms in Mediterranean forested headwater streams. Frontiers in Environmental Science. doi: 10.3389/fenvs.2019.00060
• Type: Journal Articles Status: Published Year Published: 2019 Citation: Song C, WK Dodds, J Ruegg, A Argerich, CL Baker, WB Bowden, MM Douglas, KJ Farrell, MB Flinn, EA Garcia, AM Helton, TK Harms, S Jia, JB Jones, LE Koenig, JS Kominoski, WH McDowell, D McMaster, SP Parker, AD Rosemond, CM Ruffing, KR Sheehan, MT Trentman, MR Whiles, WM Wollheim, F Ballantyne. 2018. Warming induces asymmetric convergence of stream metabolic balance. Nature Geoscience. https://doi.org/10.1038/s41561-018-0125-5

Progress 10/01/17 to 09/30/18

Outputs
Target Audience:The target audiences were UNH undergraduate and graduate students, and stakeholders interested in the coastal resources of New Hampshire and New England. Stakeholder engagement included the Piscataqua Region Estuary Partnership, the Natural Resources Conservation Service, the Town of Durham, and the New Hampshire DES. Anadditional target audience included high school teachers interested in developing water quality related lesson plans through the Museum Institute for Teaching Science. Wollheim presented findings and participated in discussions at a number of meetings and workshops including: Piscataqua Region Estuarine Partnership Technical Advisory Committee (June 2018, Kittery ME), the New Hampshire Coastal Climate Summit organized by the Coastal Adaptation Working Group (June 2018, Greenland NH), the Coastal Climate Resiliency Workshop organized by the Northeast Climate Adaptation center (October 2018, Boston MA), and the New Hampshire Water and Watersheds Conference (March 2018, Plymouth MA). Presentations in the past year were also at the annual meetings of the Society of Freshwater Science, American Geophysical Union, and Ecological Society of America. Twenty-three undergraduate and graduate students in Wollheim's Aquatic Ecosystems class (NR 751/851) went on field trips to visit measurement locations to understand water quality impacts of agriculture and urbanization. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?Three graduate students were trained on this project, including Dan Bolster, Eliza Balch and Drew Robison (Ph.D.). Eliza is a new student funded by an AES graduate student fellowship. Each of these students has been trained in scientific processes, including proposal writing, paper writing, research project planning, field methods, and networking. Eliza is just beginning her graduate work, Drew defended his dissertation proposal and advanced to candidacy, and Dan is finishing up his Masters Thesis. Drew presented his results at the national meeting of the American Geophysical Union in New Orleans in December 2017. Dan presented his results at the national meeting of the Society of Freshwater Science in Detroit, in May 2018. A number of undergraduates were trained in research this past year, including Jake Gehrug, Mason Caceres, and Bonnie Turek. Bonnie was funded through an NSF Research Experience for Undergraduates fellowship, while Jake was funded through a Hamel Center Undergraduate Research Fellowship. Jake studied the role of spatial heterogeneity within two NH reservoirs, sampling for dissolved oxygen, nitrogen gas (an indicator of denitrification) and dissolved nutrients. Mason Caceres and Bonnie Turek were trained in several field and laboratory activities, including: stream gaging, water sample collection, total suspended solids (TSS) measurement, and analytical methods in dissolved gas (i.e. CO2¸CH4, and N2O) measurements. Both continue to work in the Wollheim lab on an hourly basis. How have the results been disseminated to communities of interest?Wollheim and members of his lab group have presented a number of the findings summarized above at various meetings. Current lab members who have presented results from the AES work, in addition to Wollheim, include Zuidema, Samal, Robison, and Bolster. Major society meetings include the American Geophysical Union annual meeting, the Ecological Society of America annual meeting, and the Society for Freshwater Science annual meeting. Major local and regional meetings include the Lamprey River Symposium at UNH, Piscataqua Region Estuarine Partnership Technical Advisory Committee (June 2018, Kittery ME), the New Hampshire Coastal Climate Summit organized by the Coastal Adaptation Working Group (June 2018, Greenland NH), the Coastal Climate Resiliency Workshop organized by the Northeast Climate Adaptation center (October 2018, Boston MA), and the New Hampshire Water and Watersheds Conference (March 2018, Plymouth MA). Anadditional target audience included high school teachers interested in developing water quality related lesson plans through the Museum Institute for Teaching Science. Undergraduate researchers have presented their findings at the COLSA URC, and at a summer research symposium for summer undergraduate researchers on the UNH campus. What do you plan to do during the next reporting period to accomplish the goals?We are making good progress towards all our goals. Dan Bolster, MS student is planning to defend his thesis in the coming year. New student Eliza Balch will write her Masters proposal focused on understanding the function of small coastal reservoirs in transforming nutrient pollution from agriculture and urban catchments. She is planning to continue to measure inputs and outputs, as well as internal dynamics within reservoirs to better understand the controls of mass balance. Modeling work will continue to investigate the role and fate of non-point pollutants on aquatic ecosystems, with the specific focuses moving towards understanding the fate and transport of pathogens, sediments, and dissolved oxygen. A proposal was also submitted to USDA this past summer, in collaboration with Dr. Jessica Ernakovich, to understand how agricultural activity impacts stream ecosystem function and the associated microbial communities. If this project is funded, it will start a new collaboration to improve understanding of how land use, microbial communities, and ecosystem function interact.

Impacts
What was accomplished under these goals? IMPACT: The major issues being addressed by this research include how water quality is impacted by agriculture and urbanization, how climate variability interacts with land use to affect water quality, and to what extent do surface waters help clean up non-point pollution inputs. My research team focused specifically on quantifying the nutrient fluxes from watersheds of different land use and drainage area; the impact of reservoirs on nutrient retention using measurements of inputs and outputs; and the ability of entire networks of surface waters within watersheds to mitigate nutrient fluxes to critical downstream water bodies, including impaired estuaries and lakes, using dynamic models. Key findings and implications include: 1) river networks have a finite capacity to attenuate nutrient fluxes from agricultural or urban land because supplies overwhelm demand, but that ponded waters significantly increase capacity. These results suggest that natural resource managers should prioritize loading reductions in watersheds with fewer ponded waters first, and that function of the entire range of water bodies from small streams to large rivers should be protected. 2) river networks are able to retain a large proportion of nutrient inputs during small storms and much less during large storms. This indicates that if extreme flow events increase, we will be able to rely less on retention by aquatic ecosystems, and that stormwater controls that reduce flood peaks will result in increasing aquatic nutrient retention. 3) a large proportion of rivers in New Hampshire are impaired by high salt concentrations, mostly during summer, due to road salt applications in winter. The annual snowfall amount, which drives road salt loading, was a driver of chloride impairment in headwater streams during summers, whereas summer runoff from forested watersheds determines chloride levels in larger rivers. Future declines in snowfall have a delayed impact on stream chloride due to large groundwater storage pools. These results suggest that the amount of road salt applied to New Hampshire roads should be optimized and that managers will need to be patient to see environmental improvements. 4) model projections show that climate will have a greater influence on future aquatic ecosystem services than plausible changes in land cover, though the latter exacerbates impacts. Minimal changes in aquatic environmental indicators are predicted through 2050, after which the high emissions scenarios show rapidly intensifying impacts. These findings suggest that actions that mitigate climate change are important to implement now and that we should prepare for our natural ecosystems to begin to drastically change in the near future. Goal 1. Develop and apply a general conceptual model for supply and demand at river network scales that integrates point and non-point sources (supply) from agriculture and urban areas, and aquatic sinks (demand). The River Network Saturation (RNS) Concept was published this summer in the journal Biogeochemistry presenting the conceptual model (Wollheim et al. 2018). The RNS integrates both supply and demand at river network scales to understand how flow levels interact with biological activity to determine when river networks regulate downstream fluxes of pollutants. A number of simple models were run to demonstrate the RNS concept, showing how network-scale nutrient removal declines with increasing flows, and why over a range of low flows, nutrient removal remains high. The model demonstrates that small rivers contribute substantially to network-scale nutrient removal under low flow conditions, but that large rivers are much more important under high flow conditions. The RNS Concept is a new way of thinking about the function of entire river networks using a simple mathematical relationship that can be applied to any solute that is transported by water. Thus, it applies towards understanding the fate of multiple non-point pollution sources, including different nutrients, sediments, and pathogens to understand how river networks function as unit. Goal 2. Continue measurements of different water quality metrics including conductivity, dissolved oxygen, nitrogen, phosphorus, sediments, dissolved organic carbon and greenhouse gases across seasons and flow conditions in headwater streams and along larger rivers. We had three measurement campaigns this past year. First, we continued to monitor continuous nutrient fluxes at the mouth of a local watershed, the Oyster R., using in situ, high-frequency sensors. The Oyster R. has a significant amount of agricultural activity (including several UNH research farms) and urbanization. Data streams include water temperature, conductivity, nitrate, turbidity (i.e. sediments), and dissolved organic matter. Second, we continued monthly water sampling at several representative headwater sites of different land use to track seasonality and trends of a variety of nutrients. Third, we began monitoring inputs and outputs to two reservoirs to understand their contribution to river network scale nitrogen demand across flow conditions. The two reservoirs are the Mill Pond on the Oyster R. and the Sawyer Mill reservoir on the Bellamy R. We focused on the mass balance of nitrogen in particular, given it is a major cause of impairment of Great Bay. To all these measurements, we added regular sampling of dissolved nitrogen gas (N2) to quantify N removal via denitrification Goal 3. Develop models for each of these variables to understand their variation in time and space throughout river networks and the role of aquatic processes in defining these distributions. Our underlying model is called the Framework for Aquatic Modeling of the Earth System (FrAMES). This model provides the hydrological underpinnings to understand the movement of pollutants from land to water and through the river network. We have coupled new functions for water quality variables, including water temperature, chloride, nitrogen, and pathogens (Stewart et al. 2013, Samal et al. 2017, Zuidema et al. 2018, Wollheim et al. 2018), and have begun to develop models for sediments, dissolved oxygen. The water quality models predict the distribution of water quality conditions and impairments. For example, Zuidema et al. (2018) identified the percent of total river length exceeding various chloride thresholds in the Merrimack R. watershed. The models are spatially distributed, accounting for the role of the location of source areas within watersheds, important for prioritizing water quality mitigation activities. Goal 4. Quantify the relative importance of different factors that control supply and demand of different variables in river networks. We used the model described above (Wollheim et al. 2018) to demonstrate that hydrology is the primary factor influencing the balance of supply and demand in almost all cases where pollutants (e.g. nitrate) have low to medium reactivity (e.g. biotic demand). High flows caused by storms lower the ability of river networks to regulate downstream fluxes. Only in some cases where reactivity is very high (e.g. ammonium) can biological activity keep up with increasing supply caused by storm flows. Low biological activity leads to river network saturation at much lower flows than solutes that have higher reactivity. Ponded waters increase the flow levels at which saturation occurs. We ran scenarios that indicate that increasing concentrations associated with land use change including agriculture result in a shift to saturation at lower flows, and a greater proportion of non-point sources being exported to downstream waters. These are fundamental changes in knowledge of how entire connected systems of surface waters (i.e. river networks) function as a unit, allowing improved understanding for management of non-point water pollution.

Publications

• Type: Journal Articles Status: Published Year Published: 2017 Citation: Samal, N., W.M. Wollheim, S. Zuidema, R. Stewart, M. Mineau, T. Huang, C. Wake, K. Gardner, M. Borsuk, G. Mavrommati, D. Lutz, Z. Zhou, S. Glidden, M. Huber. 2017. Projections of Coupled Terrestrial and Aquatic Ecosystem Change Relevant to Ecosystem Service Valuation at Regional Scales. Ecology and Society 22: 18. https://doi.org/10.5751/ES-09662-220418
• Type: Journal Articles Status: Published Year Published: 2017 Citation: Wollheim, W.M., G.K. Mulukutla, C. Cook, R.O. Carey. 2017. Aquatic nitrate retention at river network scales across flow conditions determined using nested in situ sensors. Water Resources Research. 10.1002/2017WR020644
• Type: Journal Articles Status: Published Year Published: 2018 Citation: Zuidema, S. W.M. Wollheim, M.M. Mineau, M.B. Green, R.J. Stewart. 2018. Chloride impairment in a New England river network: regional assessment using a dynamic watershed transport model. Journal of Environmental Quality. doi:10.2134/jeq2017.11.0418
• Type: Journal Articles Status: Published Year Published: 2018 Citation: Li W, Zhai L, Lei Q, Wollheim WM, Liu J, Liu H, Hu W, Ren T, Wang H, Liu S. 2018. Influences of agricultural land use composition and distribution on nitrogen export from a subtropical watershed in China. Science of the Total Environment. 642:21-32. https://doi.org/10.1016/j.scitotenv.2018.06.048
• Type: Journal Articles Status: Published Year Published: 2018 Citation: Talbot CJ, EM Bennett, K Cassell, DM Hanes, EC Minor, H Paerl, PA Raymond, R Vargas, PG Vidon, WM Wollheim, MA Xenopoulos. 2018. The impact of flooding on aquatic ecosystem services. Biogeochemistry. https://doi.org/10.1007/s10533-018-0449-7
• Type: Journal Articles Status: Published Year Published: 2018 Citation: Hopkinson CS, JT Morris, S Fagherazzi, WM Wollheim, PA Raymond. 2018. Lateral marsh edge erosion as a source of sediments for vertical marsh accretion. Journal of Geophysical Research: Biogeosciences. 123. https://doi.org/10.1029/2017JG004358
• Type: Journal Articles Status: Published Year Published: 2018 Citation: Wollheim W.M., Bernal S., Burns D.A., Czuba J.A., Driscoll C.T., Hansen A.T., Hensley R.T., Hosen J.D., Inamdar I., Kaushal S.S., Koenig L.E., Lu Y.H., Marzadri A., Raymond P.,A Scott D., Stewart R.J., Vidon P.G., Wohl E. 2018. River network saturation concept: factors influencing the balance of biogeochemical supply and demand of river networks. Biogeochemistry: https://doi.org/10.1007/s10533-018-0488-0