『Abstract
We performed surveys of nitrogen (N2)-fixation
in three oligotrophic lake-stream systems in the Sawtooth Mountains
of central Idaho to address two questions: (1) Which habitat types
within linked lake-stream systems (lake pelagic, lake benthic,
and stream) exhibit the highest rates of N2
fixation?, and (2) How does N2 fixation compare
to the hydrologic flux of nitrogen? A seasonal survey showed that
N2 fixation in a single lake and its outlet
stream peaked in late summer, when hydrologic N fluxes were lowest.
Benthic lake N2-fixation rates by epiphytes
were highest at mid-lake depths, where their percent cover was
highest, while rates by epipelon were greatest at shallow lake
depths. Pelagic N2 fixation was below detection.
Stream N2-fixation rates were greatest on
rock substrates and in the lake outlet stream. These patterns
were supported by a baseflow survey (late July) in three lake-stream
ecosystems which confirmed that N2-fixation
rates peaked in the lake benthos at shallow depths and on rock
substrates in outlet streams. Scaling N2-fixation
rates to whole lake and stream areas revealed that N2
fixation could exceed the nitrate, and sometimes the total dissolved
nitrogen flux during baseflow in lakes and outlet streams. Despite
low rates, total N2-fixation contributions
(kg/day) from lakes were greater because they had far larger surface
areas than the stream environments. Fixed nitrogen contributions
from stream outlets were also relatively high because of high
N2-fixation rates and despite low surface
areas. This study suggests that N2 fixation
could be a seasonally important nitrogen source to nutrient deficient
subalpine lake-stream ecosystems. In addition, the frequency and
location of lakes could control N2-fixation
contributions to watersheds by providing a large area for within-lake
N2 fixation, and creating conditions favorable
for N2 fixation in outlet streams.
Keywords: Hydrologic N flux; Linked lake-stream ecosystems; Nitrogen
fixation; Oligotrophic; Subalpine watersheds; Cyanobacteria』
Introduction
Method
Study area
Sample collection
Acetylene reduction and chlorophyll a analysis
N2 fixation and hydrologic N flux calculations
Statistical methods
Results
Lakes
Seasonal survey
Baseflow survey
Streams
Seasonal survey
Baseflow survey
Comparing N2-fixation rates in lakes and
streams to N flux
Discussion
The magnitude and importance of N2 fixation
in the study lakes and streams
Factors potentially controlling N2-fixation
rates in lake-stream systems
Conclusions
Acknowledgments
References