[L&O Featured Article]Vol 46, Issue 8 (Dec 2001)

[lo-feature-admin@aslo.org]

The featured article in the December 2001 issue of L&O is:

Tobias, Craig R., Stephen A. Macko, Iris C. Anderson, Elizabeth A. 
Canuel, and Judson W. Harvey. 2001. Tracking the fate of a high 
concentration groundwater nitrate plume through a fringing marsh: A 
combined groundwater tracer and in situ isotope enrichment study. 
Limnol. Oceanogr. 46(8): 1977-1989.

 It can be read online or downloaded by clicking here. 

          http://aslo.org/lo/toc/vol_46/issue_8/1977.pdf

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Introductory comments by Pat Mulholland (the Associate Editor for 
this paper)

Coastal eutrophication is an increasingly important issue, and 
increased nitrogen loading is thought to be the primary culprit.  
Although much interest has centered on the inputs of nitrogen via 
rainfall and rivers draining agricultural and urban areas, 
groundwater discharges are also a significant source of N to some 
estuaries.  This month's featured paper by Tobias et al. (2001) 
describes a thorough study of the pathways and rates of nitrate 
reduction in groundwater as it discharges through the sediments of 
a mesohaline coastal marsh in Southeastern Virginia.  The study 
used an experimental in situ addition of 15N-labelled nitrate.

Changes in concentration and isotope enrichment of NO3, N2O, N2, 
NH4 and PON during plume transport through the marsh were 
measured to determine dissimilatory nitrate reduction to ammonium 
(DNRA), dentrification, and immobilization and storage of N.  Up to 
90% of the groundwater nitrate load discharging to the marsh was 
reduced rapidly in the upper 10 cm of sediment.  Although 25% of 
the nitrate load was denitrified and exported to the atmosphere, 
DNRA was the primary fate of groundwater nitrate.  However, 
immobilization of the NH4 into sediments or plants was rapid enough 
to prevent large NH4 accumulations and export despite high rates of 
NH4 production via DNRA.  I believe that this is one of the first 
studies to identify such a large role for DNRA in the N cycle of 
coastal ecosystems.

The significance of this study lies as much in the method as in its 
results.  The authors used a relatively novel experimental approach 
that combined a conservative groundwater tracer (bromide) with an 
in situ 15N-NO3 addition in order to track the processing of high 
nitrate loads in groundwater.  The approach allows determination of 
pathways and rates under ambient conditions without the artifacts of 
enclosures, and distinguishes dilution from nitrate concentration 
reductions via biogeochemical processes. Although this approach 
has been used in streams and small estuaries (Peterson et al. 1997, 
Holmes et al. 2000), its application to the groundwater discharge 
zone of coastal marshes as described here is an important advance 
in the study of groundwater-surface water ecotones.  This paper 
should stimulate and serve as a model for future studies in this area.


References:

Holmes, R. M., B. J. Peterson, L. A. Deegan, J. E. Hughes, and B. 
Fry.  2000.  Nitrogen biogeochemistry in the oligohaline zone of a 
New England estuary.  Ecology 81: 416-432.

Peterson, B. J., M. Bahr, and G. W. Kling.  1997.  A tracer 
investigation of nitrogen cycling in a pristine tundra river.  Can. J. 
Fish. Aquat. Sci. 54: 2361-2367.

Tobias, C. R., S. A. Macko, I. C. Anderson, E. A. Canuel, and J. 
W. Harvey.  2001. Tracking the fate of a high concentration 
groundwater nitrate plume through a fringing marsh: A combined 
groundwater tracer and in situ isotope enrichment study. Limnol. 
Oceanogr. 46: 1977-1989.