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Title: Chemical Indicators of Anthropogenic Nitrogen Loading in Four Pacific Estuaries 
Author: Fry, Brian; Gace, Arian; McClelland, James W.
Date: 2003-01
Publisher: University of Hawai'i Press
Citation: Fry B, Gace A, McClelland JW. 2003. Chemical indicators of anthropogenic nitrogen loading in four Pacific estuaries. Pac Sci 57(1): 77-101.
Abstract: Watershed inputs of anthropogenic nitrogen (N) are altering the
trophic status of estuaries worldwide. In this study we compared two chemical
approaches for assessing watershed N inputs to estuaries: (1) use of conventional
nutrient concentration measurements, and (2) use of nitrogen isotope (d15N)
measurements in estuarine sediments and biota. Of special interest was testing
whether d15N assays were generally robust tracers of watershed N across different
estuarine systems. Four Pacific estuaries were chosen for study at widely
spaced intervals on the u.s. West Coast: Padilla Bay (northern Washington
State), South Slough (southern Oregon), Elkhorn Slough (central California),
and Tijuana River (southern California). These estuaries are part of the
National Estuarine Research Reserve (NERR) system. They are relatively small
and shallow, are well flushed by tides, and can receive substantial natural N-loading
from seasonally upwelled offshore waters. Results showed that none of
the estuaries was truly pristine, with high watershed DIN (dissolved inorganic
nitrogen) concentrations >500 mM especially in Elkhorn and Tijuana estuaries
that respectively received high agricultural and sewage inputs. Nitrogen isotope
assays failed to detect N-loading under conditions of very high ammonium
inputs from sewage, but were otherwise useful indicators of estuarine N status
in all four estuaries. Overall, using a combination of nutrient and isotope measurements
was the best strategy for detecting watershed N-loading in these
estuaries. The combination approach could be used to generate maps of low,
medium, and high inputs to each of the four study estuaries. The N isotope
measurements appear to be useful especially for tracing historical development
of N-based eutrophication and for showing entry of pollutant N into local food
ISSN: 0030-8870

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