Cooperative Institute for Research in Environmental Sciences

FY 2010 Accomplishments and Impacts

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The Center for Limnology supports research and graduate education related to biogeochemistry and metabolic functions of aquatic ecosystems. During 2009, the Center continued its work on the biogeochemical effects of pine bark beetles on montane watersheds in Colorado. Strong disturbance of vegetation by practices such as clearcutting or herbicide treatment is known to cause radical changes in the nitrogen cycle of watersheds, as reflected in the chemistry of streams within watersheds. For example, classical studies of clearcutting and herbicide treatment in experimental watersheds of New Hampshire showed a 50-fold increase in the export of nitrate from watersheds as a result of treatment. Because of the work in New Hampshire and elsewhere, biogeochemically oriented scientists in the Rocky Mountain region have anticipated massive loss of nutrients, and especially inorganic nitrogen, from watersheds that are severely infested with pine beetles. Because vegetation takes up nutrients, elimination of vegetation by any mechanism, including severe pine beetle infestation, is likely to cause release of nutrients. Nutrient release has been studied by Center staff, including CIRES Ph.D. student Leigh Cooper.

The results of biogeochemical studies across a range of infestation intensities for pine beetle in the Colorado Rockies have shown effects that range from negligible to very mild (Figure 1). Thus, the observations are unexpected and require some explanation. Staff scientist James McCutchan has proposed an explanation that is derived from the mechanism of pine beetle infestation. The infestation comes on incrementally over a period of years. The effect of the incremental nature of infestation can be modeled by the use of watershed biogeochemistry models that predict the loss of nitrate and other nutrients from disturbed watersheds (Figure 2). Modifying the model to acknowledge the staged nature of vegetation losses associated with pine beetle mortality shows that the model predicts what we are now observing (i.e., a suppression of peak losses associated with sudden disturbances such as clearcutting or herbicide treatment). These findings are being prepared for publication in collaboration with other research groups working on the biogeochemical effects of pine beetle infestation.

The Center also continues to work on comparisons of food web structure in lakes that contain fish and lakes that lack fish. The lakes under study are in Rocky Mountain National Park, where stocking is currently prohibited. This is one of the few places where multiple waterbodies of similar morphometry and size can be compared on the basis of presence or absence of fish. CIRES Ph.D. student Thomas Detmer is leading the research.

Initial results from Detmer’s studies show that lakes without fish have a much stronger representation of large invertebrates such as mayfly larvae. When fish are present, large invertebrates are strongly suppressed by fish predation (Figure 3). Detmer believes that the shift from strong populations of large invertebrates to more cryptic populations of small invertebrates is accompanied by a change in the relative dependence of lake food webs on external carbon subsidies coming from leaves and other organic matter generated outside the lake. He is investigating this hypothesis by use of stable isotope signatures, which may differ for different carbon sources.

In 2009, we continued the of study of Little Gaynor Lake, a natural waterbody in Boulder County that receives most of its water supply through slow flow of alluvial waters rather than surface flows. The lake is slightly saline and supports large populations of bluegreen algae (cyanobacteria). Populations reach such high densities that the lake appears to support a density of algae that is unmatched by any waterbody for which abundances have been reported in the literature. The Center receives some biofuels research funding to investigate the manner in which the stability of these algal populations can be maintained, as well as measurements of their lipid content. A surprise came in 2008 when one genus of algae disappeared and a second genus replaced it. The characteristics of the second population are much the same as the first: a monoculture of high-density algal cells. Causal analysis is in progress.

The Center for Limnology also continues it studies of metabolism of the South Platte River downstream of Denver. Large changes in chemistry of the river are expected as new treatment facilities are constructed for removal of nitrogen and phosphorus, in compliance with new nutrient standards adopted by the state of Colorado. The interplay between nitrogen and organic matter will be especially interesting. Denitrifying microbes, which use nitrate as an electron acceptor, require abundant labile organic matter to grow. They also require anoxic conditions, which they find now below the sediment surface. A change in the balance between labile organic matter and nitrate in the future could drastically affect the denitrifying capabilities of the river, which are beneficial in reducing high concentrations of nitrate that originate from effluent and from nonpoint sources including agriculture. Modeling and experiments are underway to determine the probable effects of change in wastewater quality.

Impacts

Our research to understand the nitrogen cycle in mountain streams produced some unexpected results. Biogeochemical studies across a range of infestation intensities for pine beetle in the Colorado Rockies have shown effects that range from negligible to very mild.

Figure 1: Effect of tree mortality on nitrate concentration in stream water. For 60 montane watersheds in Colorado, nitrate concentrations were similar in undisturbed watersheds and watersheds with the highest level of tree mortality; nitrate concentrations were elevated slightly in watersheds with intermediate levels of tree mortality.

Figure 2: Predicted changes in stream-water nitrate concentration following clearcutting and tree mortality associated with mountain pine beetle.