Marcarelli,A.M. and Wurtsbaugh,W.A.(2009): Nitrogen fixation varies spatially and seasonally in linked stream-lake ecosystems. Biogeochemistry, 94, 95-110.

『窒素固定は繋がった河川−湖生態系において空間的かつ季節的に変動する』


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


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