DenitriWcation, the anaerobic reduction of nitrogen oxides to nitrogenous gases, is an extremely challenging process to measure and model. Much of this challenge arises from the fact that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of the denitriWcation activity that occurs in both terrestrial and aquatic ecosystems. In this paper, we describe the prospects for incorporating hotspot and hot moment phenomena into denitriWcation models in terrestrial soils, the interface between terrestrial and aquatic ecosystems, and in aquatic ecosystems. Our analysis suggests that while our data needs are strongest for hot moments, the greatest modeling challenges are for hotspots. Given the increasing availability of high temporal frequency climate data, models are promising tools for evaluating the importance of hot moments such as freeze-thaw cycles and drying/rewetting events. Spatial hotspots are less tractable due to our inability to get high resolution spatial approximations of denitriWcation drivers such as carbon substrate. Investigators need to consider the types of hotspots and hot moments that might be occurring at small, medium, 50 Biogeochemistry (2009) 93:49-77 123 and large spatial scales in the particular ecosystem type they are working in before starting a study or developing a new model. New experimental design and heterogeneity quantiWcation tools can then be applied from the outset and will result in better quantiWcation and more robust and widely applicable denitriWcation models.

Vidon, Philippe, Craig Allan, Douglas Burns, Tim P. Duval, Noel Gurwick, Shreeram Inamdar, Richard Lowrance, Judy Okay, Durelle Scott, and Steve Sebestyen, 2010. Hot Spots and Hot Moments in Riparian Zones: Potential for Improved Water Quality Management. Journal of the American Water Resources Association (JAWRA) 46(2):278‐298. DOI: 10.1111/j.1752‐1688.2010.00420.x Abstract:  Biogeochemical and hydrological processes in riparian zones regulate contaminant movement to receiving waters and often mitigate the impact of upland sources of contaminants on water quality. These heterogeneous processes have recently been conceptualized as “hot spots and moments” of retention, degradation, or production. Nevertheless, studies investigating the importance of hot phenomena (spots and moments) in riparian zones have thus far largely focused on nitrogen (N) despite compelling evidence that a variety of elements, chemicals, and particulate contaminant cycles are subject to the influence of both biogeochemical and transport hot spots and moments. In addition to N, this review summarizes current knowledge for phosphorus, organic matter, pesticides, and mercury across riparian zones, identifies variables controlling the occurrence and magnitude of hot phenomena in riparian zones for these contaminants, and discusses the implications for riparian zone management of recognizing the importance of hot phenomena in annual solute budgets at the watershed scale. Examples are presented to show that biogeochemical process‐driven hot spots and moments occur along the stream/riparian zone/upland interface for a wide variety of constituents. A basic understanding of the possible co‐occurrence of hot spots and moments for a variety of contaminants in riparian systems will increase our understanding of the influence of riparian zones on water quality and guide management strategies to enhance nutrient or pollutant removal at the landscape scale.

Dosskey, Michael G., Philippe Vidon, Noel P. Gurwick, Craig J. Allan, Tim P. Duval, and Richard Lowrance, 2010. The Role of Riparian Vegetation in Protecting and Improving Chemical Water Quality in Streams. Journal of the American Water Resources Association (JAWRA) 46(2):261‐277. DOI: 10.1111/j.1752‐1688.2010.00419.x Abstract:  We review the research literature and summarize the major processes by which riparian vegetation influences chemical water quality in streams, as well as how these processes vary among vegetation types, and discuss how these processes respond to removal and restoration of riparian vegetation and thereby determine the timing and level of response in stream water quality. Our emphasis is on the role that riparian vegetation plays in protecting streams from nonpoint source pollutants and in improving the quality of degraded stream water. Riparian vegetation influences stream water chemistry through diverse processes including direct chemical uptake and indirect influences such as by supply of organic matter to soils and channels, modification of water movement, and stabilization of soil. Some processes are more strongly expressed under certain site conditions, such as denitrification where groundwater is shallow, and by certain kinds of vegetation, such as channel stabilization by large wood and nutrient uptake by faster‐growing species. Whether stream chemistry can be managed effectively through deliberate selection and management of vegetation type, however, remains uncertain because few studies have been conducted on broad suites of processes that may include compensating or reinforcing interactions. Scant research has focused directly on the response of stream water chemistry to the loss of riparian vegetation or its restoration. Our analysis suggests that the level and time frame of a response to restoration depends strongly on the degree and time frame of vegetation loss. Legacy effects of past vegetation can continue to influence water quality for many years or decades and control the potential level and timing of water quality improvement after vegetation is restored. Through the collective action of many processes, vegetation exerts substantial influence over the well‐documented effect that riparian zones have on stream water quality. However, the degree to which stream water quality can be managed through the management of riparian vegetation remains to be clarified. An understanding of the underlying processes is important for effectively using vegetation condition as an indicator of water quality protection and for accurately gauging prospects for water quality improvement through restoration of permanent vegetation.

[1] We examined how landscape hydrogeologic characteristics influence groundwater nitrate removal by eight stream riparian sites on glacial till and outwash landscapes in southern Ontario, Canada. During high water table periods in 2000 -N input concentrations from adjacent cropland to the riparian sites ranged from 0.15 to 44.7 mg L À1 . Seven of the eight sites had a mean nitrate removal efficiency of >90%. This removal occurred within the first 15 m of the riparian zone at three sites with loamy sand and sandy loam soils overlying a shallow confining layer at 1-2 m. However, at four of five sites with more conductive sand and cobble sediments the width required for 90% nitrate removal varied from >25 m to a maximum of 176 m at a site with a confining layer at 6 m. Sites linked to an extensive thick (>6 m) upland aquifer with a slope gradient of >15% at the riparian perimeter had high nitrate inputs throughout the year and were large nitrate sinks. Sites with gentle topography (<4-5%) and <2 m of permeable sediments were minor nitrate sinks because of small nitrate inputs that were limited to the late autumn-spring period. A conceptual model linking landscape hydrogeologic characteristics to riparian zone nitrate removal capacity is developed to understand and predict the effectiveness of riparian buffers at the landscape scale.

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Dissolved organic carbon (DOC) dynamics in streams is important, yet few studies focus on DOC dynamics in Midwestern streams during storms. In this study, stream DOC dynamics during storms in two Midwestern watersheds with contrasting land uses, the change in character of stream DOC during storms, and the usability of DOC as a hydrologic tracer in artiWcially drained landscapes of the Midwest are investigated. Major cation/DOC concentrations, and DOC speciWc UV absorbance (SUVA) and Xuorescence index (FI) were monitored at 2-4 h intervals during three spring storms. Although DOC is less aromatic in the mixed land use watershed than in the agricultural watershed, land use has little impact on stream DOC concentration during storms. For both watersheds, DOC concentration follows discharge, and SUVA and FI values indicate an increase in stream DOC aromaticity and lignin content during storms. The comparison of DOC/major cation Xushing dynamics indicates that DOC is mainly exported via overland Xow/macropore Xow. In both watersheds, the increase in DOC concentration in the streams during storms corresponds to a shift in the source of DOC from DOC originating from mineral soil layers of the soil proWle at baseXow, to DOC originating from surWcial soil layers richer in aromatic substances and lignin during storms. Results also suggest that DOC, SUVA and FI could be used as hydrologic tracers in artiWcially drained landscapes of the Midwest. These results underscore the importance of sampling streams for DOC during high Xow periods in order to understand the fate of DOC in streams.

River networks modify material transfer from land to ocean. Understanding the factors regulating this function for different gaseous, dissolved, and particulate constituents is critical to quantify the local and global effects of climate and land use change. We propose the River Network Saturation (RNS) concept as a generalization of how river network regulation of material fluxes declines with increasing flows due to imbalances between supply and demand at network scales. River networks have a tendency to become saturated (supply) demand) under higher flow conditions because supplies increase faster than sink processes. However, the flow thresholds under which saturation occurs depends on a variety of factors, including the inherent process rate for a given constituent and the abundance of lentic waters such as lakes, ponds, reservoirs, and fluvial wetlands within the river network. As supply increases, saturation at network scales is initially limited by previously unmet demand in downstream aquatic ecosystems. The RNS concept describes a general tendency of river network function that can be

Nonpoint source pollution from agriculture and urbanization is increasing globally at the same time climate extremes have increased in frequency and intensity. We review >200 studies of hydrologic and gaseous fluxes and show how the interaction between land use and climate variability alters magnitude and frequency of carbon, nutrient, and greenhouse gas pulses in watersheds. Agricultural and urban watersheds respond similarly to climate variability due to headwater alteration and loss of ecosystem services to buffer runoff and temperature changes. Organic carbon concentrations/exports increase and organic carbon quality changes with runoff. Nitrogen and phosphorus exports increase during floods (sometimes by an order of magnitude) and decrease during droughts. Relationships between annual runoff and nitrogen and phosphorus exports differ across land use. CH 4 and N 2 O pulses in riparian zones/floodplains predominantly increase with: flooding, warming, low oxygen, nutrient enrichment, and organic carbon. CH 4 , N 2 O, and CO 2 pulses in streams/rivers increase due to similar factors but effects of floods are less known compared to base flow/droughts. Emerging questions include: (1) What factors influence lag times of contaminant pulses in response to extreme events? (2) What drives resistance/resilience to hydrologic and gaseous pulses? We conclude with eight recommendations for managing watershed pulses in response to interactive effects of land use and climate change.