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Dissolved Organic Matter in Running Waters
A separate study has demonstrated a strong relationship between the concentration of dissolved organic carbon (DOC) and rates of denitrification in the South Platte River (McCutchan and Lewis 2008). The cause of this relationship is not fully understood, but the link between DOC and denitrification has implications for management of water quality in the South Platte River and for a general understanding of the controls on denitrification in running waters.
Nutrient Fluxes and Nutrient Limitation in Lakes
Phosphorus (P) often has been viewed as the master nutrient in lakes because N-fixing cyanobacteria can overcome N limitation. Even so, many studies show that phytoplankton production can be limited by N deficiency. For N-limited lakes, the reduction in P concentration necessary to achieve a reduction in phytoplankton biomass can be estimated from nutrient concentrations, nutrient enrichment experiments, and information on the stoichiometry of phytoplankton, as illustrated recently for a Colorado reservoir (Lewis et al. 2008).
More recently, Claire McGrath has studied interactions between non-native brook trout and native cutthroat trout in Colorado mountain streams. Efforts to restore populations of greenback cutthroats in Colorado have been hampered by the presence of non-native salmonids. Dr. McGrath's work investigated predation and competitions as mechanisms for displacement of cutthroat trout by book trout (McGrath and Lewis 2007). Neither of these mechanisms were found to significantly affect adult cutthroats, but population data suggested that displacement of cutthroats by brook trout occurs through an effect on mortality of eggs or juvenile fish.
Community Metabolism and Modeling
Application of the open-channel method in Puerto Rico (Ortiz-Zayas et al. 2005) revealed high rates of ecosystem respiration in a tropical river compared to most temperate streams; although rates of photosynthesis were high in middle and lower reaches, intense herbivory kept algal biomass low throughout the Rio Mameyes. Another study (Cronin et al. 2007) has shown that changes in water management over the past 70 years have led to a decrease in photosynthetic rates in the South Platte River.
Additional research has extended the open-channel method to the process of denitrification (McCutchan et al. 2003). Recent studies have focused on spatial and temporal patterns of denitrification (Pribyl et al. 2005, McCutchan and Lewis 2008), as well as nitrogen ebullition (bubbling) associated with biogeochemical "hot spots" in the South Platte River (Higgins et al. 2008).
The Center has developed a generalized model (AMMTOX) that predicts ammonia concentrations in streams and also reach-specific models that include both nitrogen transformations and oxygen mass balance. These models are used by USEPA and by the State of Colorado for setting permit limits for wastewater discharge to streams and lakes.
Global Nitrogen Budget