The watershed approach, i.e. monitoring inputs and hydrologic outputs of nutrients, has long been fundamental in ecosystem ecology, and is an important component of many sites in the NSF LTER network. This approach is ideal for comparison of different ecosystems (e.g., our urban ecosystems with the less human-dominated ecosystems in the LTER network) and has been extensively used for evaluation of the effects of disturbance on ecosystem function. The watershed approach is useful for bridging the gap between basic and applied science, providing data at a scale relevant to management of the land for protection of receiving waters.

Water quality in urban ecosystems has been relatively well studied, with a strong focus on the effect of storm runoff of receiving water quality. However, the vast majority of these studies have been short-term and focused on storm events. There have been very few attempts to evaluate long-term nutrient fluxes and budgets in urban watersheds similar to the approach taken in long-term ecological research. Long-term flux and budget studies are necessary if we hope to be able to compare urban ecosystems with the less intensively managed ecosystems that dominate the LTER network. Such studies should also provide a useful and unique addition to the database on pollutant delivery to receiving waters in urban watersheds.

In mixed land use watersheds, there is great interest in characterizing the water quality "signal" from different land use classes, e.g. forest, agriculture, urban/residential. Signals from agricultural and forest land uses tend to be more well characterized than from urban uses. There is a great need to better quantify pollutant delivery from urban ecosystems to receiving waters and to understand the factors (e.g., density, physical setting, social factors) that influence this delivery.

In the Baltimore urban LTER, we are using the watershed approach to evaluate integrated ecosystem function. The LTER research is centered on the Gwynns Falls watershed, a 17,150 ha catchment that traverses a gradient from the urban core of Baltimore, through older urban residential (1900 - 1950) and suburban (1950- 1980) zones, rapidly suburbanizing areas and a rural/suburban fringe. Our long-term sampling network includes four longitudinal sampling sites along the Gwynns Falls as well as four small (40 - 100 ha) watersheds located within or near to the Gwynns Falls. The longitudinal sites provide data on water and nutrient fluxes in the different land use zones of the watershed (rural/suburban, rapidly suburbanizing, old suburban, urban core) and the small watersheds provide more focused data on specific land use areas (forest, agriculture, rural/suburban, urban).

Each of the gaging sites is continuously monitored for discharge and is sampled weekly for chemistry. Additional chemical sampling is carried out in a supplemental set of sites to provide a greater range of land use. Water quality analysis includes major cations, nutrients including inorganic and organic forms, total suspended solids, temperature, dissolved oxygen.

The effects of land use/land cover patterns, hydrologic flowpath structure and dynamics and the presence and distribution of BMPs are evaluated relative to the behavior of the set of sampled catchments. The set of catchments along the urban-rural gradient are compared within our network and with other catchments in the LTER network.

We are extending and implementing distributed watershed hydroecological models, RHESSys and PLM, which have been used extensively in other biomes, to work with the distributed patch structure of urban and urbanizing areas. Both models incorporate spatial data processing and hydroecological flux process components within the framework of a nested or spatially distributed watershed system. The models simulate the spatial patterns of water, carbon and nutrient flux, along with land/atmosphere interactions. The sampling framework is used to parameterize, calibrate and test this model. A combination of the model operation, measurement scheme and information gained from other components of BES will be used to determine hydrologic and nutrient balances, and net primary productivity of the set of small catchments and the full Gwynns Falls watershed. The influence of urban structure and human activity on these mass balances and flux processes will also be estimated as part of this component.

In addition to our long-term stream monitoring, there is BES research on stream biotic communities.