DISSOLVED ORGANIC CARBON AND TOTAL NITROGEN 
CONCENTRATIONS UNDER DIFFERENT SPATIAL AND TEMPORAL 
CONDITIONS IN THE UPPER MISSISSIPPI RIVER

Danielle Quist and Michael Delong
Large River Studies Center, Biology Department, Winona State University, Winona, MN 
55987

Dissolved organic carbon (DOC) has been observed to change temporally in large rivers. 
An important resource for microbial production, DOC concentrations has not been 
examined thoroughly in backwaters, where DOC concentrations may be greater due to 
the abundance of macrophytes.  The inorganic fraction of total nitrogen (TN) limits 
primary production, with nutrient availability appearing to be strongly tied to 
hydrological patterns in large rivers. This study analyzed  dissolved organic carbon and 
total nitrogen concentrations to help better understand  nutrient availability backwater 
and main channel habitats of Reaches 5 and 6 of the Upper Mississippi River , June - 
August 2005.  Samples were obtained by pumping water from a depth of 1 m at each site.  
Water samples were passed through a 53-?m sieve in the field, then through a 1-?m glass 
fiber filter in the laboratory, leaving just the dissolved nutrients.  DOC and TN were 
determined using a Shimadzu TOC-V analyzer.  DOC was highest in the main channel 
(10.4 mg/L) in June and steadily declined to 8.8 mg/L by August.  TN also decreased 
over this time period (2.7 mg/L to 1.1 mg/L). This pattern coincided with declining 
discharge from June through August.  DOC and TN exhibited the same pattern in 
backwaters, but the difference in TN was more pronounced, dropping from 2.9 mg/L in 
June to 0.9 mg/L in August.  The pronounced habitat-related difference in TN is likely a 
result of differences in the influx of nitrogen to backwaters relative to the main channel.


Keywords: dissolved organic carbon, total nitrogen, main channel, backwater, temporal