7796

• UCREFRP Catalog Number: 7796
• Author: Burdick, B. D.
• Title: Conceptual Management Plan for Habitat Enhancement in Flooded Bottomlands, Escalante State Wildlife Area, Gunnison River Downstream of Delta, Colorado.
• USFW Year: 1994.
• USFW - Doc Type: \
• Copyright Material: NO

conditions for phytoplankton production. Sediments deposited in these areas
where the water velocity is decreased provide nutrients and sunlight penetrates
the clearer water that allows phytoplankton to flourish as primary producers and
to stimulate production of the food chain. Low velocity off-channel habitats
become warmer than the riverine environment in the upper basin (Kaeding and
Osmundson 1988; Osmundson and Kaeding 1989). The combination of nutrients,
sunlight penetration of the water column, and warmer water temperatures in low
velocity habitats provide the best conditions for phytoplankton production in the
upper basin.
Importance of Low Velocity Habitats to Zooplankton Production. The
importance of low velocity habitats to the production of zooplankton for fish in
riverine environments has not been documented very well. Most riverine studies
have concentrated on macro invertebrates occurring in the drift (Waters 1969).
Mabey and Shiozawa (1993) reported that the most comprehensive studies have been
made of the plankton communities in the Orinoco River, Venezuela. Mean densities
of cladocerans and copepods (the most abundant taxa) were 421 organisms per liter
in the Laguna la Orsinera. Welcomme (1989) summarized zooplankton densities in
floodplains in a range of 270 to 10,000 organisms per liter. Mabey and Shiozawa
(1993) documented zooplankton densities in the middle Green River as 0.3 to 1.3
organisms per liter, 1.5 to 7.1 in the Ouray backwater, 63.4 at Intersection Wash
(another backwater), and 206 to 690 in Old Charlie Wash (Woods Bottom) on the
Ouray National Wildlife Refuge, located downstream from Vernal, Utah. Grabowski
and Hiebert (1989) reported 0 to 20 planktonic crustaceans (cladocerans and
copepods) per liter in the middle Green River channel and 0.02 to 17 organisms
per liter in backwaters during 1987 and 1988. In an open water bottomland
habitat of the Moab Slough on the Colorado River near Moab, Utah, the density of
planktonic crustaceans (cladocerans and copepods) averaged about 36 organisms per
liter in the summer of 1993 (Cooper and Severn 1994a). Cooper and Severn reported
a mean of only 2 organisms per liter for backwater sites. Samples of planktonic
crustaceans (cladocerans and copepods) from the Escalante Ranch site on the
middle Green River, upstream from Jensen, Utah, contained 0 organisms per liter
for the main channel, a mean of 41 organisms per liter for backwaters, and a mean
of 71 organisms per liter for an open water wetland (Cooper and Severn 1994b).
The mean number of cladocerans and copepods from a backwater of the Gunnison
River at the Escalante SWA, about 5 miles downstream from Delta was 11 organisms
per liter of water and the mean number from an open water wetland was 25
organisms per liter (Cooper and Severn 1994c). Zooplankton (cladocerans and
copepods) samples from a bottomland (Old Charlie Wash) on the Ouray National
Wildlife Refuge contained a mean of 31 organisms per liter of water (Cooper and
Severn 1994d). Samples taken from the middle Green River and a backwater on the
refuge did not contain any cladocerans or copepods.
Major Limiting Factor Affecting Endangered Colorado River Fish Populations.
The major limiting factor affecting the populations of endangered Colorado River
fishes is the lack of successful recruitment (USFWS 1987, 1990a, 1990b, 1991).
In the Lower Colorado River Basin, studies have demonstrated that predation by
non-native fishes and the lack of food may be limiting the survival of larval and
juvenile endangered fishes (Marsh and Langhorst 1988; Papoulias and Minckley
' 1990, Papoulias and Minckley 1992). Similar conclusions have been reached for
the endangered Colorado River fishes in the upper basin (Miller et al. 1982;
Maddux et al. 1993).
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