Hydrologie and biotic influences on nitrate removal in a subtropical spring-fed river
Abstract
We use a long-term chemical and hydrologic record in combination with longitudinal
sampling and highfrequency nitrate (NO3-) measurements from in situ sensors to describe
temporal and spatial patterns of nitrogen (N) inputs and removal in the spring-fed
Ichetucknee River (Columbia County, Florida) and to determine the hydrological, geomorphic,
and biological factors that influence those dynamics. Over a 20-yr period of record,
NO 3-N removal averaged 118 kg N d-1 (0.77 g N m-2 d-1 ) over the upper 5 km of the
Ichetucknee River. Three independent estimates of gross autotrophic N assimilation
(from gross primary production, diel NO3- variation, and standing biomass) agreed
closely but accounted for less than 20% of observed N removal. Longitudinal surveys
indicate negligible or negative dissolved organic nitrogen and ammonium (NH4+) production,
suggesting that denitrification is the predominant mechanism of N removal in this
river. A positive relationship between discharge and the magnitude of NO3-N removal
shows that interactions with the surrounding floodplain exert considerable influence
at high flows, and longitudinal NO3- patterns indicate that N removal may be influenced
by channel morphology. These results suggest a greater role for dissimilatory processes
and hydrologic connectivity with hyporheic and floodplain sediments than has been
previously recognized in highly productive spring-fed rivers of north Florida. While
hydrologic variation is the primary determinant of variation in NO 3- removal within
the Ichetucknee River, comparison across systems indicates that biotic characteristics
can cause significant deviation from predictions based on purely physical models of
relationships between river size and N removal. © 2010 by the American Society of
Limnology and Oceanography, Inc.
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Show full item recordScholars@Duke
James Brendan Heffernan
Associate Professor of Ecosystem Ecology and Ecohydrology
I am interested in major changes in ecosystem structure, particularly in streams,
rivers and wetlands. My work focuses on feedbacks among ecological, physical, and
biogeochemical processes, and uses a wide range of tools and approaches. I am particularly
interested in projects that address both basic ecological theory and pressing environmental
problems. Increasingly, we are applying tools and theories developed for local ecosystems
to better understand ecological patterns and mechanisms at regi

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