BES Long Term Stream and Watershed Study Update – Overview
The watershed approach is perhaps the most powerful and useful approach in the field of Ecosystem Ecology (Likens 1992). Development of the ability to quantify inputs and outputs of water, energy, nutrients and carbon to hydrologically defined drainage basins has provided a basis for evaluating whole ecosystem function and response to disturbance and environmental change. The watershed approach also has been fundamental to the development of theory about patterns of ecosystem development in time and space and about the regulation and maintenance of ecosystem function via mechanisms of resistance and resilience (Odum 1969, Holling 1973, Bormann and Likens 1979). Watershed-based approaches have been applied to fundamental studies of forest, grassland, wetland, desert, agricultural and other ecosystem types.
The watershed approach has been a central component of the Baltimore Ecosystem Study (BES) since its inception. We hypothesized that this approach would allow us to establish a basis for comparing the relatively novel urban, suburban and exurban ecosystems that we were studying with the more well studied less human-dominated systems in the LTER network. Would water and nutrient retention by biogeochemical processes in these novel systems be similar to freshly disturbed, highly leaky forest ecosystems or to more mature, aggrading forests dominated by young, actively growing vegetation? The watershed approach also provided an opportunity to develop theories to guide analysis of the role of humans as components of ecosystems. Would human fluxes of water and nutrients be independent of and/or overwhelm natural fluxes? Are there coherent feedback relationships between environmental changes and human actions that function to maintain a quasi "steady-state" in urban watersheds? Finally, how does the built environment and human social processes co-evolve with abiotic and biotic components as components of developing ecosystems to function as adaptive processes that underlie sustainability?
|Establishment of long-term watershed studies in Baltimore|
Continuous data on stream stage and stream discharge are collected by the USGS. USGS protocols for data collection and processing are well developed and widely used, allowing for comparison of the data collected at our sites with watersheds across the world. Data from most of the gages utilized by BES are available in "real time" (e.g., http://waterdata.usgs.gov/usa/nwis/uv?01589352).
|The long-term "signal" in Baltimore watersheds|
The long-term nitrate signals from the forested and agricultural watersheds are as expected. Forests are known to have conservative (highly retentive) nitrogen cycles, leading to low concentrations of nitrate in streams draining forested watersheds. Indeed, the nitrate concentrations in the BES forested reference watershed are quite similar to those observed at the forested Hubbard Brook, Coweeta, Harvard Forest and Andrews LTER sites. We note that, similar to some of the Coweeta watersheds, peak nitrate concentrations and loads in our forested reference watershed are in the summer, in contrast to the snowmelt-dominated Hubbard Brook site. The nitrate signal from the agricultural watershed is also not a surprise. Agricultural watersheds often have high nitrogen inputs from fertilizer and resulting high nitrate concentrations in streams. The concentrations of nitrate observed in the BES agricultural stream are similar to those observed in other agricultural streams that have been studied throughout the eastern U.S. In this small catchment, concentrations and loads are very sensitive to local agricultural practice, and the large rise in concentrations is probably related to manure application and management.
The long-term forested reference and agricultural watershed nitrate signals provide an established context, and are a platform for detailed, process-level research for the urban and suburban watersheds that are the focus of BES. Why do suburban watersheds have nitrate concentrations that are clearly higher than concentrations in forested watersheds and lower than agricultural watersheds? What mix of nitrogen sources (e.g., fertilizers, atmospheric deposition, human waste, animal waste) and sinks (e.g., plant uptake, wetland denitrification) and hydrologic processes produce these patterns? Can we develop models capable of depicting these patterns and predicting nitrate concentrations in the future that can then be verified by continued monitoring? Just as in other LTER sites, long-term watershed-scale monitoring is useful for propelling the progress of our analysis of urban watershed ecosystems.
We observed marked variation in nitrogen retention both within and among watersheds that appears to be driven by precipitation (climate) and land use (urbanization) (Figure 4) (Kaushal et al. 2008, Bettez et al. 2013). While retention was consistently high in the forested reference watershed (> 95%), retention in urban and agricultural watersheds ranged from 33 - 95% between 1999 and 2010. There were marked decreases in retention with increased precipitation and urbanization. If hydroclimatic variability increases with climate change, our results suggest that this will lead to marked increases in the temporal range of nitrate export from these watersheds, with the potential for very high loading rates from urbanized areas during wet years.
Bettez, N. D., P. M. Groffman, J. M. Duncan, L. E. Band, J. O'Neil-Dunne, S. S. Kaushal, and K. T. Belt. 2013. Effects of urbanization and climate on nitrogen retention in coastal watersheds. Ecosystems Submitted.
Bormann, F. H. and G. E. Likens. 1979. Pattern and Process in a Forested Ecosystem. Springer-Verlag, New York.
Doheny, E. J. 1999. Index of hydrologic characteristics and data resources for the Gwynns Falls watershed, Baltimore County and Baltimore City, Maryland. USGS Report OFR 99-213.U.S. Geological Survey, Denver, CO. 24 pp.
Groffman, P. M., N. L. Law, K. T. Belt, L. E. Band, and G. T. Fisher. 2004. Nitrogen fluxes and retention in urban watershed ecosystems. Ecosystems 7:393-403.
Holling, C. S. 1973. Resilience and stability of ecological systems. Annual Review of Ecology and Systematics 4:1-23.
Kaushal, S. S., P. M. Groffman, L. E. Band, C. A. Shields, R. P. Morgan, M. A. Palmer, K. T. Belt, C. M. Swan, S. E. G. Findlay, and G. T. Fisher. 2008. Interaction between urbanization and climate variability amplifies watershed nitrate export in Maryland. Environmental Science & Technology 42:5872-5878.
Likens, G. E. 1992. The Ecosystem Approach: Its Use and Abuse. The Ecology Institute, Oldendork-Luhe, Germany.
Odum, E. P. 1969. Fundamentals of Ecology, 3rd Edition. Philadelphia, W.B. Saunders & Company.
|This research was supported by funding from the NSF Long-term Ecological Research (LTER) Program. This material is based upon work supported by the National Science Foundation under Grant No. 1027188. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.|