In streams with high temporal and spatial variability, such as hot desert streams in the southwestern United States, characterizing nutrient dynamics can be difficult. A number of factors can influence nutrient dynamics in these systems. A researcher team examined those factors and grouped them into four categories. Ultimate determinants are factors such as parent geology, climate, and catchment configuration. For instance, parent geology can affect the availability of nutrients to the environment; the size of soil particles can impact the rate of hydrologic conductivity in the stream; and the riparian zone can influence nutrient storage and transformation. Intermediate determinants are factors such as biological community structure and function. For instance, when floods occur, there can be changes in the biotic community, and ultimately in nutrient processing that can be affected. As another example, during dry periods, a stream area contracts and affects algal community composition. Proximal determinants include biological impact on processes that impact nutrient processing in the environment (e.g., denitrification, nitrogen fixation). Hydrological variability also can greatly impact nutrients in desert streams. For instance, because of the lack of riparian vegetation in deserts, more runoff reaches streams during precipitation than in more vegetated areas. At other times, a stream might be completely dry, with the associated impacts on the chemical and biological structure of the stream. All of these tiers of factors are important considerations in modeling nutrients in streams and in the development of nutrient criteria. Based on a review of the available literature, researchers outlined steps for characterizing the nutrient regime of desert stream systems, including selecting index sites and periods. They also provided a list of important factors for making the characterization, including physical and structural elements (e.g., altitude, land use), temporal elements (e.g., season, photoperiod), chemistry (e.g., CO2, total dissolved solids), and biology (e.g., algal community composition, benthic chlorophyll a) (USEPA 2000b).
Reference:
USEPA. 2000b. Nutrient Criteria Technical Guidance Manual: Rivers and Streams. EPA-822-B-00-002. U.S. Environmental Protection Agency, Office of Water, Office of Science and Technology, Washington, DC. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPDF.cgi/20003CVP.PDF?Dockey=20003CVP.PDF.