Interaction between Urbanization and Climate Variability Amplifies Watershed Nitrate Export in Maryland SUJAY S. KAUSHAL,* ,† PETER M. GROFFMAN, ‡ LAWRENCE E. BAND, § CATHERINE A. SHIELDS, § RAYMOND P. MORGAN, | MARGARET A. PALMER, † KENNETH T. BELT, ⊥ CHRISTOPHER M. SWAN, # STUART E. G. FINDLAY, ‡ AND GARY T. FISHER ∇ University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, 1 Williams Street, P.O. Box 38, Solomons, Maryland 20688, Cary Institute of Ecosystem Studies, Box AB Route 44A, Millbrook, New York 12545, Department of Geography, University of North Carolina, Chapel Hill, North Carolina 27599, University of Maryland Center for Environmental Science Appalachian Laboratory, Frostburg, Maryland 21532, Urban Forestry Ecological Research Unit, U.S. Forest Service/Northern Research Station, University of Maryland at Baltimore County, Baltimore, Maryland 21227, Department of Geography and Environmental Systems, University of Maryland at Baltimore County, Baltimore, Maryland 21227, and U.S. Geological Survey at Baltimore, Baltimore, Maryland 21237 Received February 12, 2008. Revised manuscript received May 30, 2008. Accepted June 02, 2008. We investigated regional effects of urbanization and land use change on nitrate concentrations in approximately 1,000 small streams in Maryland during record drought and wet years in 2001-2003. We also investigated changes in nitrate-N export during the same time period in 8 intensively monitored small watersheds across an urbanization gradient in Baltimore, Maryland. Nitrate-N concentrations in Maryland were greatest in agricultural streams, urban streams, and forest streams respectively. During the period of record drought and wet years, nitrate-N exports in Baltimore showed substantial variation in 6 suburban/urban streams (2.9-15.3 kg/ha/y), 1 agricultural stream (3.4-38.9 kg/ha/y), and 1 forest stream (0.03-0.2 kg/ ha/y). Interannual variability was similar for small Baltimore streams and nearby well-monitored tributaries and coincided with record hypoxia in Chesapeake Bay. Discharge-weighted mean annual nitrate concentrations showed a variable tendency to decrease/increase with changes in annual runoff, although total N export generally increased with annual runoff. N retention in small Baltimore watersheds during the 2002 drought was 85%, 99%, and 94% for suburban, forest, and agricultural watersheds, respectively, and declined to 35%, 91%, and 41% during the wet year of 2003. Our results suggest that urban land use change can increase the vulnerability of ecosystem nitrogen retention functions to climatic variability. Further work is necessary to characterize patterns of nitrate-N export and retention in small urbanizing watersheds under varying climatic conditions to improve future forecasting and watershed scale restoration efforts aimed at improving nitrate-N retention. Introduction Approximately 50% of the world’s human population cur- rently lives within 100 km of a coastline, and this percentage is projected to increase in the future (1). Human land use in coastal watersheds has been linked to increasing nitrogen loads and coastal eutrophication (2–4). The watershed of the Chesapeake Bay, the largest estuary in the United States, has experienced large inputs of nitrogen from agriculture and increasing urbanization (3). Coincident with land use change, this region and nearby watersheds have also recently experienced increased frequency of extreme droughts and wet years (e.g., 5–8). Here we investigate how urbanization influences regional patterns in nitrate concentrations in small streams in Maryland, and analyze how interactions between urbanization and climate variability can decrease retention and amplify nitrate export from small suburban and urban watersheds. In 2002, the Chesapeake Bay watershed, along with 48% of the contiguous United States, experienced severe drought conditions (9) with some of the lowest rainfall amounts recorded over a 100-year period (10, 11). Severe drought was followed by near record high flows to Chesapeake Bay in 2003 (5–7). Extreme changes in precipitation during 2002 and 2003 coincided with some of the most severe water quality problems reported in Chesapeake Bay including hypoxia, changes in species composition, formation of harmful algal blooms, mortality in fish, oysters, aquatic grasses, and health risks to humans (5–7). A growing body of research suggests that ecological risks associated with climate variability are expected to increase in the future (12, 13). During “dry” years, nitrogen is stored, and during “wet” years, N is flushed from watersheds contributing to eutrophication and hypoxia (e.g., 4, 14–16). Recent work in exurban, suburban, and urban watersheds (e.g.. 15, 17–19) has raised concerns that hydrologic alterations associated with urbanization can amplify climate-driven export of nitrate-N from small watersheds. These alterations include “burial” of headwaters (20) and increases in imper- vious surfaces that alter hydrologic “connectivity” between urban streams and riparian areas, which is important in nitrate-N retention and removal (21). There has been a 60% increase in urbanization in the Chesapeake Bay watershed between 1990 and 2000 due to conversion of both agricultural and forest lands into residential areas (22). Understanding the effects of this increase on nitrogen export is critical for forecasting changes in water quality in response to climate and for guiding restoration and stormwater management practices in small watersheds (21, 23). We investigated the relationship between land use change and nitrate concentrations in approximately 1,000 randomly * Corresponding author e-mail: kaushal@cbl.umces.edu. † University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory. ‡ Cary Institute of Ecosystem Studies. § University of North Carolina, Chapel Hill. | University of Maryland Center for Environmental Science Appalachian Laboratory. ⊥ Urban Forestry Ecological Research Unit, U.S. Forest Service/ Northern Research Station. # Department of Geography and Environmental Systems, Uni- versity of Maryland at Baltimore County. ∇ U.S. Geological Survey at Baltimore. Environ. Sci. Technol. 2008, 42, 5872–5878 5872 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 16, 2008 10.1021/es800264f CCC: $40.75  2008 American Chemical Society Published on Web 07/09/2008