Current Research

Organic Nitrogen
Research at the Center for Limnology has shown over the last two years that organic nitrogen has a surprisingly high availability to microbes, i.e., it can be a significant biotic nitrogen source even when inorganic nitrogen is depleted (Kaushal and Lewis). This new information runs contrary to the common supposition that organic nitrogen is highly refractory and thus unusable in the food chain. This finding has implications for analysis of the nitrogen cycle and interpretation of the consequences of anthropogenic nitrogen enrichment of watersheds.

Ammonia Concentrations
The Center completed during 2002 a model (AMMTOX) that predicts ammonia concentrations in streams. The model, which is based on extensive calibration work from Colorado, incorporates physical processes as well as biological processes such as nitrification, autotrophic assimilation, and ammonification. The model will be used by USEPA and by the State of Colorado for setting permit limits for ammonia in wastewater discharge to streams and lakes.

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. Organisms have specialized mechanisms for efficient collection of algal organic matter because this source is more easily metabolized than the organic matter derived from vascular plants (McCutchan and Lewis 2001, 2002).

Community Metabolism
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). Additional research has extended the open-channel method to the process of denitrification (McCutchan et al. 2003).

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 few 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

Rio Icacos in the Luquillo Rain Forest, Puerto Rico

North Fork of the Cache la Poudre River near Rabbit Creek

South Fork of the Cache la Poudre River

McPhee Reservoir