QUANTIFYING SURFACE WATER NITRATE CONCENTRATIONS THAT ELICIT DIATOM COMMUNITY CHANGES IN LAKES ACROSS THE ROCKY MOUNTAINS

QUANTIFYING SURFACE WATER NITRATE CONCENTRATIONS

THAT ELICIT DIATOM COMMUNITY CHANGES

IN LAKES ACROSS THE ROCKY MOUNTAINS

By: Heather Ann Arnett

Advisor: Dr. Jasmine Saros

A Lay Abstract of the Thesis Presented

Submitted in Partial Fulfillment of the Requirements for the

Degree of Master of Science

(in Ecology and Environmental Sciences)

May, 2010

Nitrogen (N) limited high elevation mountain lakes have been receiving enhanced atmospheric N deposition over the last century. This increase has caused dramatic changes in algal community structure. However, the surface water concentrations of nitrate that cause this shift remain unclear. Diatoms, a form of algae that contain silica, are used to attempt to quantify this change because they are sensitive to their environment and fossilize in lake sediments. We attempted to quantify these concentrations by developing a diatom calibration set using lake-bottom sediment samples from 46 lakes across the U.S. Rocky Mountain Range. Out of the sixteen measured environmental variables, statistical ordination analysis identified that nitrate, conductivity, total phosphorus, and temperature affect diatom distributions. A transfer function, using a combination of diatom abundances and their respective nitrate optima, was developed for nitrate and applied to a suite of fossil diatom profiles from lakes across the Rockies. Bootstrapping validation of the model suggested high potential (r²=0.6100) for reconstructing lakewater nitrate concentrations over time. However, comparisons of diatom-inferred (model) nitrate to historical values revealed very poor performance of the model, with diatom-inferred values almost always much higher than observed ones. The frequent domination of the diatom communities by three generalist species is likely the key problem with this model. This complication underscores the utility of the indicator species approach, using key ecological species, in understanding the effects of nitrate enrichment in oligotrophic lake ecosystems.

KEYWORDS: diatoms, nitrate, transfer function, calibration set, alpine lakes, Rocky Mountains