The Long Term Resource Monitoring Program, Open River Field Station (ORFS) near
Cape Girardeau, Missouri has six side channels in it's 50-mile study reach. Four of the
six side channels become isolated from the main channel at various river levels. Two
of the four side channels have closing structures at the inlets. The other two are silted
in at the inlets and contain closing structures.

Since 1991, ORFS staff have observed phytoplankton blooms in isolated side
channels, particularly during summer. Phytoplankton biomass increases during
summer when side channels become isolated for an extended period. We wanted to
document whether these blooms were reducing dissolved oxygen (DO) levels to a
stressful level for aquatic organisms.

On September 4, 1998, we deployed two data recorders in Santa Fe Chute below the
inlet closing structure (river mile 38.9L). A Hydrolab Datasonde 3 was placed one
meter above the bottom and a Hydrolab Minisonde was placed one meter below the
surface in 8.5 meters (28 feet) of water. The recorders were set to record temperature,
pH, specific conductivity, and DO every 30 minutes. The data recorders were retrieved
after almost four days of continuous monitoring. Results showed that surface DO had a
maximum reading of 9.2 mg/L, water temperature was 28.1øC at 5:30 p.m. on
September 5, 1998 and a minimum of 2.0 mg/L (27.1øC) at 8:00 a.m. on September 8,
1998 (Figure 1). The bottom DO had a maximum of 0.5 mg/L (24.9øC) at 3:30 p.m. on
September 7, 1998 and a minimum of 0.2 mg/L (25.1øC) at 6:00 p.m. on September 4,
1998. During this period, the river fell 0.7 meters (2.2 feet) on the Cape Girardeau
gage.

Low DO on the surface concerns us, because values below 5 mg/L are considered
stressful to aquatic organisms. On the first day, DO was between 5.0 - 4.0 mg/L for 12
hours (10:00 p.m. - 10:00 a.m.). Two days later, DO was between 4.0 - 3.6 mg/L for
approximately 14 hours (7:00 p.m. - 9:00 a.m.). On the last day, DO was between 6.5 -
2.4 mg/L (12:00 a.m. - 7:30 a.m.) and fell to 2.0 mg/L at 8:00 a.m., when the data
recorders were retrieved.

The data suggest that isolated side channels, whether naturally occurring or created by
human alterations, are subject to dangerously low dissolved oxygen levels. It is
speculated that isolated side channels act as catchments for nutrients, which promote
rapid phytoplankton production under optimum environmental conditions. During such
conditions, nighttime respiration of phytoplankton uses DO, sometimes causing whole
water bodies, like those in isolated side channels, to go near-anoxic. These conditions
are stressful for aquatic organisms and could cause significant kills. Further data
recording will be needed to determine the extent and duration of low DO in Middle
Mississippi River side channels. Seasonal trend data is also needed.

This article was submitted by Mike Petersen and David Herzog who are employed
by the Missouri Department of Conservation in the Fisheries Research,
Assessment, and Monitoring Section of the Long Term Resource Monitoring
Program. They are located at the Open River Field Station, 3815 East Jackson
Boulevard, Jackson, MO 63755; Phone: 573/243-2659, E-mail:
Michael_Petersen@usgs.gov or David_Herzog@usgs.gov.