.|  Baltimore Ecosystem Study
Stream and Watershed Research Projects

Land use and climatic impacts on watershed biogeochemistry
  • Sujay Kaushal - University of Maryland
Timing of Nutrient Export
 
Watershed export of nitrogen (N) and phosphorus (P) from anthropogenic sources has contributed to eutrophication in freshwater and coastal ecosystems. Our work has investigates the interactive effects of land use and climate on sources, transport, and transformation of nitrogen and other contaminants (e.g. Kaushal et al. 2008, Kaushal et al. 2010, Duan and Kaushal 2013). Previous work has shown that climate variability can play a large role in influencing watershed N export (Kaushal et al. 2008). We are exploring impacts of watershed urbanization on the magnitude and export flow distribution of P along an urban-rural gradient in 8 watersheds monitored as part of the BES LTER site. Exports of soluble reactive phosphorus (SRP) and total P (TP) were lowest in small watersheds with forest and low-density residential land use. In contrast, SRP and TP exports increased with watershed impervious surface coverage and reached highest values in a small urban watershed. Load duration curve analysis showed that increasing urbanization in watersheds was associated with shifts in P export to high-flow conditions (>2 mm/day; Fig. 2). SRP concentrations during low-flow conditions at urban headwater sites were highest during summer and lowest during winter.
 

Fig.1 Cumulative flow duration and SRP and TP export as a function of runoff in BES LTER small subwatersheds. Runoff is normalized per unit area and is shown in units of mm d-1.

Effects of Rising Stream Temperature
 
We are also exploring how alterations in water temperatures due to urbanization and climate can influence stream water P concentrations and P export from urban watersheds. There can be rising water temperatures in streams due to climate and land-use change (Kaushal et al. 2010). This warming can accelerate biogeochemical fluxes from sediments to streams (Figure 3) (Duan and Kaushal 2013). We investigated impacts of increased streamwater temperatures on sediment fluxes of dissolved organic carbon (DOC), nitrate, soluble reactive phosphorus (SRP) and sulfate. Results indicate: (1) warming significantly increases sediment DOC fluxes to overlying water across land-use but decreases DOC quality via increases in the humic-like to protein-like fractions (Fig. 4) (2) warming consistently increases SRP fluxes from sediments to overlying water across land-use (Fig. 5) (3) warming increases sulfate fluxes from sediments to overlying water at rural/suburban sites but decreases sulfate fluxes at urban sites likely due to sulfate reduction (4) nitrate fluxes show an increasing trend with temperature at forest and urban sites but with larger variability than SRP. Sediment fluxes of nitrate, SRP and sulfate are strongly related to watershed urbanization and organic matter content.
 

Fig. 2 Plots of DOC flux, or changes in protein-like fluorescence, humic-like fluorescence and their (P/H) ratio versus incubation temperature. Best fit regressions were exponential for DOC flux, or changes in protein-like fluorescence, humic-like fluorescence (in solid) and linear for P/H ratio (in dash). Protein-like and humic-like fluorescence are in Raman Units (RU).

Tracking Nonpoint Nitrogen Sources During Storms
 
Nonpoint source N pollution is a leading contributor to U.S. water quality impairments. We combine watershed N mass balances and stable isotopes to investigate fate and transport of nonpoint N in forest, agricultural, and urbanized watersheds at the BES LTER site. Analysis of ?15N-NO3-, and ?18O-NO3- indicates that wastewater is an important nitrate source in urbanized streams during baseflow conditions (Fig. 5). Isotopic information suggest that N from belowground leaking sewers is less susceptible to denitrification. Interestingly, N source contributions also change with storm magnitude (atmospheric sources contributed 50% at peak storm N loads) (Figure 6). N transformations were also observed in a storm drain (no natural drainage network) potentially due to organic carbon inputs (please see Urban Watershed Continuum webpage on the BES site). Overall,we are finding that nonpoint sources such as atmospheric deposition, wastewater, and fertilizer show different susceptibility to watershed N export. There can be large changes in nitrate sources as a function of runoff, and anticipating source changes in response to climate variability and storms will be critical for managing nonpoint N pollution.
 

Fig. 3 (A) Dual δ15N-NO3 and δ18O-NO3 source plots for rural watersheds. (B) Dual δ15N-NO3 and δ18O-NO3 source plots for suburban and urban watersheds.


Fig. 4 δ15N-NO3 sampled along 6 locations of the Dead Run (DRKR) watershed during summer storms.

References
 
Duan, S.W. and S.S. Kaushal. 2013. Warming increases carbon-nutrient fluxes from sediments in streams across land use. Biogeosciences 10: 1-15.
 
Kaushal, S.S., P.M. Groffman, L.E. Band, C.A. Shields, R.P. Morgan, M.A. Palmer, K.T. Belt, G. T. Fisher, C.M. Swan, and S.E.G. Findlay. 2008. Interaction between urbanization and climate variability amplifies watershed nitrate export in Maryland. Environmental Science & Technology 42, 5872-5878, 2008. 10.1021/es800264f
 
Kaushal, S.S., M.L. Pace, P.M. Groffman, L.E. Band, K.T. Belt, P.M. Mayer, and C. Welty. 2010. Land use and climate variability amplify contaminant pulses. EOS 91(25), 221-222
 
Kaushal, S.S., G.E. Likens, N.A. Jaworski, M.L. Pace, A.M. Sides, K.T. Belt, D. Secor, D. Seekell, and R. Wingate. 2010. Rising stream and river temperatures in the United States. Frontiers in Ecology and the Environment 100323112848094 DOI: 10.1890/090037
 
Kaushal, S.S., P.M. Groffman, L.E. Band, E. Elliott, and C.A. Kendall. 2011. Tracking nonpoint nitrogen pollution in human-impacted watersheds. Environmental Science & Technology 10.1021/es200779e