A recent outbreak of mountain pine beetle in Colorado provided an opportunity to study the effects of widespread tree mortality on stream ecosystems. Timber harvest and storms often cause large (~400%) increases in nitrate concentrations in stream water, but beetle kill resulted in virtually no increase in stream-water nitrate. Beetle-killed forests have strong compensatory mechanisms that protect against nutrient loss, and knowledge of these mechanisms provides guidance for forest management (Rhoades et al. 2013). A severe storm that caused widespread flooding in Colorado in 2013 provided another opportunity to study disturbance and stream ecosystems. With support from the NSF Rapid Research Program, the Center is investigating the effects of this storm on food webs and energy flow in Colorado mountain streams.
Stable isotope analyses have shown that the traditional emphasis on vascular plant material as a food resource for consumers in running waters may be erroneous. Algae, while present at far lower abundances than vascular plant material, are the main source of organic matter entering consumer food chains in the Orinoco River floodplain (Lewis et al. 2000, Lewis et al. 2001) and in headwater streams (McCutchan and Lewis 2002). Recent work by T. Detmer has shown that introductions of fish to high-elevation lakes alter the size distribution and biomass of invertebrate communities but not invertebrate production; the lack of a response of invertebrate production to fish introductions probably is due to the high production-to-biomass ratios for small invertebrates.
Phosphorus (P) has been viewed as the master limiting nutrient in lakes and streams, but a collaboration between the Center and limnologists at Utah State University and the University of North Carolina showed that nitrogen (N) and P are equally likely to limit algal growth (Lewis and Wurtsbaugh 2008, Lewis et al. 2011). Although control of P may be the most efficient means of controlling algal growth in lakes and streams, further control can be achieved by control of both P and bioavailable N. In 2011, the Center produced the first comprehensive estimate of the global primary production of lakes (Lewis 2011; under background nutrient conditions, net primary production for lakes averaged ~160 gC m-2 y-1, but eutrophication has increased this value by about 60%.