9410

• UCREFRP Catalog Number: 9410
• Author: Wydoski, R. S. and E. J. Wick.
• Title: Ecological Value of Floodplain Habitats to Razorback Suckers in the Upper Colorado River Basin.
• USFW Year: 1998.
• USFW - Doc Type: Denver.
• Copyright Material: NO

I

I

large riverine environments (Welcomme 1985). The most comprehensive
studies of 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 cope pods
(the most abundant taxa) were 421 organisms per liter in a floodplain
(Laguna la Orsinera). Welcomme (1985, 1989) summarized the range of
zooplankton densities (combined species) in floodplains ranged between 0.2
and 24,000 per liter. Various studies have reported zooplankton densities
that were 30 (Welcomme 1989) to 100 (Hamilton et al. 1990) times greater
in floodplain habitats than in the adjacent river channels. The
differences in zooplankton densities are due to seasonal pulses by
different species (Welcomme 1985).

I

I
I

I

Information on zooplankton densities in temperate rivers of North America
are limited. The mean number of zooplankton in backwaters of the Missouri
River between April and October was 6.7 organisms per liter (Kallemeyn and
Novotny 1977). Data for zooplankton in floodplains of the Missouri River
were not available because extensive channelization has eliminated
periodic inundation of the floodplain. Only one study included rotifers
among the zooplankton taxa studied in the Upper Colorado River Basin.
Grabowski and Hiebert (1989) sampled rotifers in 1988 using a 25 micron
plankton net and reported between 0 and 0.1 rotifers per liter in the main
channel of the middle Green River and between 0 and 14.9 rotifers per
liter in backwater habitats along this river reach. They did not sample
floodplain habitats during their study.

I

I

I

Although direct comparisons of zooplankton densities in the Upper Colorado
River Basin cannot be made because different sampling methods 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 floodplain habitats (Tables 1 and 2). The
following discussion focuses on cladocerans and copepods since these two
taxa constitute the most abundant prey items in the upper basin for which
data are available. The upper value in mean number of cladocerans and
copepods per liter in main channels of the Upper Basin was 1.3, 13.1 for
backwaters and 81.5 for floodplain habitats (Table 1). This summary of
Upper Basin zooplankton studies demonstrates that habitats with lower
water velocities (backwaters and floodplains) are more productive than
habitats with higher water velocities (main channels) and that floodplains
are the most productive habitats.

I

I

I

I

While the seasonal dynamics of zooplankton have been documented in the
literature (Hynes 1970; Welcomme 1985), few studies have been done on this
aspect of zooplankton dynamics in Upper Basin river/floodplain habitats.
The number of zooplankton (combined species) in a 147-ha wetland
depression (Old Charley Wash) along the middle Green River were highest
(43 [1995] to 54 [1996] organisms/liter) in June during the descending
limb of the spring runoff (Figure 17 of Modde 1997). During the summer of
1991, the mean number of zooplankton increased from - 200 per liter in
June to nearly 700 per liter in August in the same wetland depression
(Mabey and Shiozawa 1993; Table 2). However, the mean number of
cladocerans and copepods per liter was higher in the main channel of the
middle Green River during July, 1991 (1.3/1) than in August, 1991 (0.3/1)
and in a small backwater, 7.1/1 in July and 1.4 in August (Table 2) The
higher mean values in the backwaters in July were probably due either to
(1) escapement of zooplankton from floodplain habitats when they are
connected by high spring streamflows or (2) displacement into the river
from off-channel habitats as the streamflows subsided (Kallemeyn and
Novotny 1977; Welcomme, 1985, 1989).

I

I

I

I

I
I

11