and why change occurs. For example, the cumulative effects of changes in climate on the spatial-temporal attributes of water quality and ecosystem response may be sudden and dramatic (Lipp et al., 2001). A wet period may be marked by greater than average frequency and intensity of sediment entrainment and transport, leading to higher nutrient, pesticide and pathogen loadings into a receiving water body. A dry or quiescent period would be marked by the subsequent biogeochemical transformation of these loads in the water-soil columns. A dynamic sampling strategy designed to capture specific events and changes and not designed to follow a strict periodicity, would be able to contribute to understanding the relationships of variable and multiple stressors and their effects.
As NAWQA moves forward with a more dynamic approach to its program, the distinction between sampling parameters for traditional water quality monitoring and sampling for dynamic water quality changes becomes more important. NAWQA has utilized a periodic approach in assessments of pesticides in hydrologic systems and found remarkable added value (Box 2). NAWQA leaders should continue to recognize that aquatic systems constantly fluctuate, rather than assume they operate uniformly such that sampling can be done only in a uniform way. As such, the NAWQA monitoring and modeling design should reflect a dynamic sampling strategy overlain on top of a periodic sampling design (Box 1 and 2). The dynamic part of the sampling design would be question based, supporting Goals 2, 3, and 4, whereas the traditional design maintains documenting long term trends in water quality (Goal 1). This pairing provides an opportunity for innovation through an adaptive monitoring system that follows some of the key questions in the Science Plan.
The Importance of Sampling for Dynamic Water Quality Monitoring
Beginning in the mid-1990s, NAWQA collected samples and probed the presence of the insecticide diazinon in an urban stream. Diazinon samples were collected yearly, rather than the 4 year rotational sampling design commonly employed by NAWQA, during Cycle 2. NAWQA continued sampling as diazinon was phased out both in indoor and outdoor residential use in the early 2000s. NAWQA developed a reliable time-series model for assessing long term changes in diazinon concentrations as residential use declined. The model showed a rapid water quality response to eliminating outdoor uses in 2002 and a continued decline in diazinon concentration through 2004. Furthermore, NAWQA examined the results as if the 4 year rotational sampling design was employed, i.e., if the model was based on sampling every 4th year. The resulting trend indicated an increase in diazinon through 2004, rather than the decrease in concentration that actually occurred as a result of phasing out use of the insecticide.
SOURCE: October 26th, 2010, personal communication, Robert J. Gilliom.
Aquatic ecosystems both impact and are impacted by water quality (NRC, 1995; NRC, 1992). By focusing on how water quality impacts ecosystems, the Science Plan addresses only half of the picture. Consequently, aquatic ecosystems only appear subjected to degraded water quality. The Science Plan should recognize that biogeochemical processes in aquatic ecosystems also condition the water quality in those ecosystems or explain that the biogeochemical processes that are characteristic of aquatic ecosystems in good condition help restore and maintain water quality, i.e., there are feedback loops in the system. In addition, the Science Plan presents human and ecosystem needs for water as though they are two separate issues. In fact, meeting ecosystem needs for water ensures the maintenance of