7996

• UCREFRP Catalog Number: 7996
• Author: Wydoski, R. S. and E. D. Wick.
• Title: Ecological Value of Flooded Bottomland Habitats to Endangered Fishes in the Upper Colorado River Basin.
• USFW Year: 1996.
• USFW - Doc Type: \
• Copyright Material: NO

river channel obstructs the penetration of sunlight that is needed for
phytop 1 an kton product i on. However, backwaters and embayments along the ma in
river channels and flooded bottomlands, ponds, and lakes in off-channel areas
provide favorable conditions for phytoplankton production. Sediments are
deposited in low water velocity areas that provide nutrients and sunlight
penetrates the clearer water allowing phytoplankton to flourish as primary
producers and to stimulate production of the food chain. Low velocity off-
channel habitats also become warmer than the riverine environment in the upper
basin that also aids phytoplankton production (Osmundson and Kaeding 1989a;
Osmundson and Kaeding 1989b). The combination of nutrients, sunlight penetration
of the water column, and warmer water temperatures in low velocity habitats
provide the best conditions for phytoplankton and zooplankton production in the
upper basin. Low velocity habitats provide nursery areas that are essential to
the survival of early life stages of the endangered fishes. The decline of the
endangered fishes is associated with poor survival of the early life stages,
resulting in the lack of adequate recruitment to maintain self-sustaining
populations.

V. IMPORTANCE OF LOW VELOCITY HABITATS TO ZOOPLANKTON PRODUCTION

Low velocity habitats are important to the production of zooplankton for fish in
large riverine environments (Welcomme 1985). The most comprehensive studies
plankton communities in rivers and floodplains have been made in tropical rivers
of Venezuela (Saunders and Lewis 1988a, 1988b, 1989; Twombly and Lewis 1987,
1989). Mean densities of cladocerans and copepods (the most abundant taxa) were
421 organisms per liter in the Laguna la Orsinera. Various studies have reported
zooplankton densities that were 30 (Welcomme 1989) to 100 (Hamilton et al. 1990)
times greater in off-channel habitats than the adjacent ri ver channels. We 1 comme
(1985) summarized zooplankton densities in floodplains that ranged between 0.2
and 24,000 organisms per liter and Welcomme (1989) summarized zooplankton
densities in floodplains in a range of 270 to 10,000 organisms per liter. The
mean number of zooplankton in backwaters of the Missouri River between April and
October was 6.7 organisms per liter (Kallemeyn and Novotny 1987). Seasonal
pulses of total zooplankton numbers are due to the cyclic nature of different
zooplankton species (Welcomme 1985).

Information on zooplankton densities in temperate rivers of North America are
limited. The early life stages of endangered Colorado River fishes required
small food items for survival. For example, larval razorback suckers first feed
on items such as diatoms and rotifers (Papoulias and Minckley 1992). Few studies
have included rotifers among the zooplankton taxa studied in the Upper Colorado
River Basin. Grabowski and Hiebert (1989) used a 25 micron plankton net in 1988
that provides some insight into rotifer densities. They reported between 0 and
0.1 rotifers per liter in the middle Green River and between 0 and 14.9 rotifers
per liter in backwater habitats along this river reach.

Although direct comparisons of zooplankton densities in the Upper Colorado River
Basin should not be made because different sampling techniques were used by the
various investigators, the trends in zooplankton density by habitat are the same
with the lowest density in the main river channels, higher in backwaters, and
highest in flooded bottomlands. Larval razorback suckers quickly convert to
larger zooplankton such as cladocerans and copepods. The following discussion

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