Biogeochemistry of Nitrogen

Much of my current work with students and other collaborators focuses on environmental cycling of nitrogen in numerous aquatic environments of Colorado. One example is the South Platte River below Denver, which receives a large amount of treated wastewater from Denver and its suburbs. Our recent studies show that the South Platte River below Denver supports very high rates of denitrification, which converts nitrate to N₂, an inert gas that is released to the atmosphere. This process is environmentally favorable because excessive nitrate derived from wastewater causes nuisance growth of algae and aquatic vascular plants.

Analysis of Food Webs in Lakes

Recent PhD graduate Thomas Detmer led an ambitious project in Rocky Mountain National Park related to the influence of fish on food webs. The RMNP has lakes with fish and lakes that lack fish. In comparing these lakes, Detmer showed that fish very strongly suppress the presence of large invertebrates such as stoneflies, thus shifting the composition of aquatic invertebrates in the lakes, as expected. Contrary to expectation, however, he showed that loss of the large invertebrates was compensated by a great increase in the abundance of small invertebrates. Ecologically speaking, the small invertebrates took the place of the larger invertebrates in the food web, so that changes in food web functions were negligible. This is an example of ecological resilience.

Production of Aquatic Biomass

Aquatic environments create new biomass through photosynthesis, which is conducted by algae and aquatic vascular plants. Our research on both lakes and streams in Colorado shows that photosynthesis is controlled in some cases by P, a nutrient, and sometimes by N, also a nutrient. In the past, limnologists have thought that P is a master nutrient, and that N does not commonly control production through photosynthesis. Our experiments in Colorado, and similar work conducted elsewhere, support a new view of nitrogen as a commonly limiting nutrient for aquatic photosynthesis.

Mixotrophy and nitrogen fixation in cyanobacteria