High elevation ecosystem responses to atmospheric deposition of nitrogen in the Colorado Rocky Mountains, USA
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Book chapter↗
Geology topics
B. M. LaFrancois
Publications and source records attributed to B. M. LaFrancois.
NO3 uptake in shallow, oligotrophic, mountain lakes: The influence of elevated NO3 concentrations
Nutrient enrichment experiments were conducted in 1.2-m deep enclosures in 2 shallow, oligotrophic, mountain lakes. 15 N-NO 3 isotope tracer was used to compare the importance of phytoplankton and benthic compartments (epilithon, surface sediment [epipelon], and subsurface sediment) for NO 3 uptake under high and low NO 3 conditions. NO 3 uptake approached saturation in the high-N lake, but not in the low-N lake. The capacity of phytoplankton and benthic compartments to take up NO 3 differed among treatments and between lakes, and depended on water-column nutrient conditions and the history of NO 3 availability. Phytoplankton productivity responded strongly to addition of limiting nutrients, and NO 3 uptake was related to phytoplankton biomass and photosynthesis. However, more NO 3 usually was taken up by benthic compartments (57–92% combined) than by phytoplankton, even though the response of benthic algal biomass to nutrient additions was less pronounced than that of phytoplankton and benthic NO 3 uptake was unrelated to benthic algal biomass. In the low-N lake where NO 3 uptake was unsaturated, C content or % was related to NO 3 uptake in benthic substrates, suggesting that heterotrophic bacterial processes could be important in benthic NO 3 uptake. These results suggest that phytoplankton are most sensitive to nutrient additions, but benthic processes are important for NO 3 uptake in shallow, oligotrophic lakes.
Journal of the North American Benthological Societ↗
Lake-specific responses to elevated atmospheric nitrogen deposition in the Colorado Rocky Mountains, U.S.A
We explored variability among subalpine lakes sharing very similar climate and atmospheric conditions, but differing in watershed characteristics, hydrology, and food web structure. Special attention was given to nitrogen (N) dynamics because the study area receives some of the highest levels of atmospheric N deposition in the Rocky Mountains. We asked if the effect of regional N deposition would be manifested uniformly among neighboring lakes both in terms of ambient conditions and responses to greater nutrient inputs. Catchment vegetation appeared to be the main determinant of ambient nitrate (NO 3 ), phosphate (PO 4 ), and dissolved organic carbon (DOC) concentrations, although in-lake differences in recycling produced variable and contrasting NH 4 levels. Phytoplankton chlorophyll a temporarily responded to early season NO 3 peaks in the lakes with rocky watersheds, but chlorophyll means over the ice-free season were remarkably similar among lakes despite differences in both nutrient supply and zooplankton grazing. In most cases, phosphorus was limiting to phytoplankton growth, although the importance of N deficiencies was greater in lakes with forested watersheds and fringing wetlands.
Colorado↗
Environmental characteristics and benthic invertebrate assemblages in Colorado mountain lakes
Twenty-two high-elevation lakes (>3000 m) in Rocky Mountain National Park and Indian Peaks Wilderness Area, Colorado, were surveyed during summer 1998 to explore relationships among benthic invertebrates, water chemistry (particularly nitrate concentrations), and other environmental variables. Water samples were collected from the deepest portion of each lake and analyzed for ions and other water chemistry parameters. Benthic invertebrates were collected from the littoral zone using both a sweep net and Hess sampler. Physical and geographical measurements were derived from maps. Relationships among benthic invertebrate assemblages and environmental variables were examined using canonical correspondence analysis, and the importance of sampling methodology and taxonomie resolution on these relationships was evaluated. Choice of sampling methodology strongly influenced the outcome of statistical analyses, whereas taxonomie resolution did not. Presence/absence of benthic invertebrate taxa among the study lakes was best explained by elevation and presence of fish. Relative abundance and density of benthic invertebrate taxa were more strongly influenced by sampling date and water chemistry. Nitrate (NO₃⁻) concentration, potentially on the rise due to regional nitrogen deposition, was unrelated to benthic invertebrate distribution regardless of sampling method or taxonomie resolution.
Western North American Naturalist↗