Nitrogen fixation varies spatially and seasonally in linked stream-lake ecosystems

Abstract

We performed surveys of nitrogen (N 2 )-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 N 2 fixation?, and (2) How does N 2 fixation compare to the hydrologic flux of nitrogen? A seasonal survey showed that N 2 fixation in a single lake and its outlet stream peaked in late summer, when hydrologic N fluxes were lowest. Benthic lake N 2 -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 N 2 fixation was below detection. Stream N 2 -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 N 2 -fixation rates peaked in the lake benthos at shallow depths and on rock substrates in outlet streams. Scaling N 2 -fixation rates to whole lake and stream areas revealed that N 2 fixation could exceed the nitrate, and sometimes the total dissolved nitrogen flux during baseflow in lakes and outlet streams. Despite low rates, total N 2 -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 N 2 -fixation rates and despite low surface areas. This study suggests that N 2 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 N 2 -fixation contributions to watersheds by providing a large area for within-lake N 2 fixation, and creating conditions favorable for N 2 fixation in outlet streams.

Differences in phytoplankton dynamics and community structure between a wet year and dry year in the Zhoucun Reservoir

“…Some studies have indicated that N-fixing cyanobacteria become dominant in summer with a nitrogen shortage (Mugidde et al 2003;Marcarelli and Wurtsbaugh 2009;Paterson et al 2011). However, N-fixing cyanobacteria did not dominate the phytoplankton community in several additional studies under analogous circumstances (Havens et al 2003;Scott and McCarthy 2010;Gikuma-Njuru et al 2013).…”

Assessing nonpoint‐source nitrogen loading and nitrogen fixation in lakes using δ15N and nutrient stoichiometry

“…N fixation, like all biogeochemical processes, is a spatially and temporally patchy process; thus small-scale experiments may misrepresent its extent. There is also growing appreciation for the potential N fixation in benthic habitats (Marcarelli and Wurtsbaugh 2009) that is difficult if not impossible to quantify in microcosms. Thus, an approach to measuring N fixation that integrates over space and time has become increasingly needed to understand how eutrophication changes the importance of N fixation in aquatic ecosystems.…”

The Influence of Landscape Position and Catchment Characteristics on Aquatic Biogeochemistry in High-Elevation Lake-Chains

“…Alternatively, and perhaps more plausibly, if the majority of DON results from within-lake biological uptake of nitrate, as nitrate concentrations decline with increasing lake landscape position less new autochthonous organic nitrogen would be produced. With little landscape variability in rates of nitrogen fixation or uptake in streams (Arp and Baker 2007;Marcarelli and Wurtsbaugh 2009), DON concentrations would cease to increase with increasing lake landscape position as nitrogen was primarily cycled between inorganic and organic fractions in association with biological processes.…”