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Current Research

Dissolved Organic Matter in Running Waters
Research on two high elevation watersheds in the Colorado Rockies has shown that dissolved organic nitrogen (DON) can be an important source of N for microbes when inorganic nitrogen is depleted (Kaushal and Lewis 2005). This information runs contrary to the common supposition that organic nitrogen is highly refractory and thus unusable in the food chain.

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
In 2007, James Anthony completed his doctoral dissertation work on sediment-water exchanges of nitrogen and phosphorus in lakes. One of Dr. Anthony's most important findings was a sharp increase in nutrient flux near the seasonal maximum epilimnetic temperature. This threshold for nutrient release indicates potential for large changes in nutrient flux associated with climate warming.

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).

Aquatic Foodwebs
Extensive study of aquatic foodwebs in Colorado by the use of stable isotopes has shown that the traditional emphasis on vascular plant material (primarily leaf litter) as a nutritional mainstay for river and stream foodwebs may be erroneous. Algae, while present at far lower abundances than vascular plant organic matter, proved to be the main source of organic matter entering the food chain (McCutchan and Lewis 2001, 2002).

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
Measuring system-level processes, like production and respiration, in natural streams presents significant methodological challenges. Research at the Center has yielded important advances in the open-channel method used to estimate these processes (McCutchan et al. 1998, 2002, McCutchan and Lewis 2006).

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
The non-anthropogenic baseline for delivery of nitrogen from watersheds to oceans is unknown. Because the nitrogen cycle is very strongly perturbed, it is difficult to observe nitrogen yields from watersheds under pristine conditions, especially at temperate latitudes. Over the last several years, the Center for Limnology worked out a method for estimating background conditions from a series of benchmark watersheds throughout the United States, and produced equations that relate yield and nitrogen partitioning to runoff, thus allowing extrapolation to temperate latitudes generally (Lewis et al. 1999, Lewis 2002).


Research Locations

Lake San Cristobal
Lake Fork Gunnison River

Rio Icacos
Rio Icacos in the Luquillo Rain Forest, Puerto Rico

North Fork of the Poudre
North Fork of the Cache la Poudre River near Rabbit Creek

South Fork of the Poudre
South Fork of the Cache la Poudre River

McPhee Reservoir
McPhee Reservoir