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OCR for page 102
6
Algal and Invertebrate Biota in the;
Colorado River: Comparison of Pre- and
Post-Dam Conditions
DEAN W. BLINN, Northern Arizona University, Flagstaff, Arizona
GERALD A. COLE, Arizona State University, Tempe, Arizona
INTRODUCTION
This paper reports present knowledge of the algal and invertebrate com-
munities in the Colorado River between Glen Canyon Dam and Lake Mead.
Both preimpoundment and postimpoundment conditions are discussed, in-
cluding recent studies on the proposed changes of regulated flow on algal
and macroinvertebrates in the Colorado River. Case studies in other lotic
ecosystems with regulated flow that are germane to the algal and macro-
invertebrate communities in the Colorado River are also reviewed. Recom-
mendations for future studies and management operations are provided.
PREIMPOUNDMENT CONDITIONS
Algal Communities
There are few published reports on the physicochemical conditions of the
preimpounded Colorado River that relate to the growth and development of
algal communities. Woodbury et al. (1959) reported that the stretch of river
through Glen Canyon contained high sediment loads during the periodic
torrential flows of winter (>2,832 m3 so ), but was nearly clear during the
low flows (113 m3 so ~ of late summer. The average suspended sediment
load in the lower Colorado River before impoundment was 3.5 times higher
than after the construction of Glen Canyon Dam (Dolan et al., 1974; Stanford
and Ward, 1986; Stanford and Ward, this volume). Substratum available
102
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ALGAL AND INVERTEBRATE BIOTA...
103
for algal attachment included (1) scoured rock faces in areas of rapids and
cataracts and (2) fine sediment in backwaters and along the inner side of
river bends. (Woodbury et al., 1959) reported no data on the levels of algal
nutrients (nitrogen, phosphorus, and silica).
Flowers (1959) reported 53 taxa of rivenne algae, including 28 chlorophytes
and 20 diatom taxa, in selected tributaries of Glen Canyon. Spirogyra, Zygnema
and Cladophora were the dominant filamentous chlorophytes in these col-
lections. He did not include quantitative estimates of algae, nor did he
provide a list of algal species restricted to the mainstream.
Williams and Scott (1962) presented seasonal quantitative data on the dia-
tom taxa from a site near Page, Arizona, and Williams (1964) reported densi-
ties (400-1600 cells ml~i ~ of the dominant diatom taxa, Diatoma vulgare
Bory, Gomphonema olivaceum (Lyngb.) Kutz., Navicula Yiridula (Kutz.) Kutz.,
Synedra ulna (Nitz.) Ehr., and two species of Surirella at several sites along
the Colorado River below Cataract Canyon. Weber (1971) reported similar
results at a water pollution surveillance station near Page, Arizona.
Aquatic Invertebrates
An accurate appraisal of the changes that have occurred in the composi-
tion of the aquatic invertebrates in the Colorado River since Glen Canyon
Dam was closed is difficult because of the limited literature on preimpoundment
fauna and the introduction of exotic species from other parts of the conti-
nent. Prior to 1963, studies were concerned with species living in tribu~ries
or nearby springs; the main channel was neglected (Pilsbry and Ferriss,
191 1; Moore and Hungerford, 1922; Gregory and Moore, 1931; S earl 193la,b;
Woodbury et al., 1959~. Musser (1959) listed aquatic insects from the Colo-
rado River in Glen Canyon and from some of the tributaries. Although not
concerned with the reaches of the river below the dam, his data are the best
comparable material on insects that we have.
There are published accounts of events in other western canyons that
permit some generalizations. Of these, the published data from research in
Green River, Utah may be especially important. The Green River is the
largest tributary entering the Colorado River, and information collected be-
fore and after the closing of Flaming Gorge Dam is especially applicable to
the events that have occurred in the Colorado River below Glen Canyon
Dam (Pearson, 1967~.
/
POSTIMPOUNDMENT CONDITIONS
Algal Communities
Several algal surveys have been conducted in the Colorado River since
the closure of Glen Canyon Dam in 1963. Sommerfeld et al. (1976) and
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104
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
Crayton and Sommerfeld (1978, 1981) reported 127 species of sestonic
phytoplankton in the Colorado River and concluded that many of the plank-
tonic species were attached forms that had been dislodged due to widely
fluctuating river levels and high current velocity. Nearly 58% of the phyto-
plankton community was composed of diatoms, and the dominant species
were Diatoma vulgare, Rhoicosphenia curvata (Kutz.) Grun., and Cocconeis
pediculus Ehr. Many of the phytoplankters apparently originated from Lake
Powell. For example, of the 20 dominant algal species listed for Warm
Creek Bay, Lake Powell (Stewart and Blinn, 1976; Czarnecki and Blinn,
1977), eight species were found to be common to both river and lake.
Crayton and Sommerfeld (1978) reported that cell densities for diatom spe-
cies in the Colorado River after impoundment were over 1,600-fold lower
than cell densities prior to the impoundment of the river (Williams, 1964~.
Crayton and Sommerfeld (1979) also reported on the composition and abun-
dance of phytoplankton in the tributaries of the lower Colorado River.
Cladophora glomerata (L.) Kutz. is presently the dominant, attached
filamentous green alga in the Canyon system, especially between Glen Can-
yon Dam and the Paria River, and at the mouths of major tributaries (Usher
et al., 1986~. The alga is also common in sestonic stream drift in the Colo-
rado River during high flows (Haury, 1981; Leibfried and Blinn, 1986~.
Recently, Usher and Blinn (1990) estimated the average biomass of C. glomerata
for sites at and above Lee's Ferry to be 144 g m~2 (standard error [SE] +
4.1), compared with 17.2 g m~2 (SE + 5.5) at the mouths of selected tributar-
ies below Lee's Ferry. The relatively high biomasses of C. glomerata in the
upstream tailwater sites may result from the abundance of stable rock sur-
faces for attachment and the clear, nutrient enriched waters from the hypolimnial
releases of Glen Canyon Dam (Stanford and Ward, 1986~. The rather abrupt
decrease in standing crop of C. glomerata below Lee's Ferry most probably
resulted from the periodic inputs of suspended sediment (limited light pen-
etration) from major tributaries such as the Paria River and the Little Colo-
rado River (Cole and Kubly, 1976~.
Cladophora growth was frequently reduced or absent on the upstream
side of boulders at Nankoweap (river km 84) because of the intense sand-
blasting by suspended sediment (D. W. Blinn, personal observation). Fur-
thermore, the limited growth that did occur on the sand- impacted upstream
faces frequently formed condensed mosslike tufts rather than the highly
branched growth form on the more stable rock subs~ates.
Usher et al. (1986) reported a progressive increase in biomass of C.
glomerata with increased channel depth at sites above Lee's Ferry but an
overall decrease in biomass with depth at a site below Lee's Ferry (Nankoweap,
84.5 km). The decrease in Cladophora biomass with increased channel depth
at the lower site may have resulted from the rapid attenuation of light due to
periodic high sediment loads.
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ALGAL AND INVERTEBRATE BIOTA...
105
C. glomerata is an important biotic component in the overall structure of
the Colorado River ecosystem, especially in the tailwaters (25 km) below
Glen Canyon Dam, and therefore warrants further study. This filamentous
green alga serves as a substratum for invertebrates and refuge from fish
predators and fast-moving water. Leibfried and Blinn (1986) found a sig-
nificant positive relationship between the standing crops of C. glomerata
and Gammarus lacustris Sars at Lee's Ferry during the months of July and
October 1985. Other investigations also have reported a close association
between Cladophora and invertebrates in other lotic ecosystems (Blum,
1957; Whitton, 1970; Neel, 1963~.
The highly branched filaments of C. glomerata also provide enormous
surface areas for the attachment of epiphytic diatoms which are used as
food by aquatic invertebrates (Blinn et al., 1986) and perhaps indirectly or
directly by fish (Montgomery et al., 1986~. On the other hand, dense stands
of attached filamentous algae interfere with anglers (Whitton, 1970) and
may interfere with spawning grounds of fishes, especially salmonids (Skulburg,
1984~.
Czarnecki et al. (1976) reported 345 taxa of periphytic algae (attached
algae) in the seeps, the mouths of tributaries, and the Colorado River in the
Grand Canyon. Greatest abundance of periphyti~c algae was recorded dur-
ing the summer (June-July). Of the tax a reported, 65% were diatoms, 24%
were cyanobacteria, and 10% were chlorophytes. A red alga, Batrachospermum
sp., was reported at the confluence of Diamond Creek (river km 362~. Re-
cently another red alga, Audouinella, appeared in We mainstream (personal
observation, 1984~. Audouinella is frequently attached to the filaments of
the green alga, C. glomerata, and is most commonly found in the deeper
sections of the river channel above the confluence of the Paria River. The
appearance of Audouinella in the Colorado River is not unexpected, since
most freshwater rhodophytes are confined to rivers and streams and species
of Audouinella can tolerate flows exceeding Sm s~~ (Sheath, l984~.
Czarnecki and Blinn (1978) reported 235 diatom taxa from the Colorado
River and tributaries and from springs in the Glen and Grand Canyon sys-
tems. The dominant taxa were Diatoma vulgare, Synedra ulna, and Cocconeis
pediculus. The two former species were reported to be common within the
river system prior to the impoundment of Lake Powell (Flowers, 1959), but
C. pediculus appears to have increased in relative importance since the time
of impoundment. This may be a function of the frequent epiphytic associa-
tion of C. pediculus on C. glomerata (Stevenson and Stoermer, 1982; Blinn
et al., 1989a) and perhaps indirectly implies that the biomass of Cladophora
has increased since the closure of Glen Canyon Dam.
Usher et al. (1986) quantified the epiphytic diatoms associated with Cladophora
at two sites above Lee's Ferry and at the confluences of four major tributar-
ies below Lee's Ferry. Achnanthes affinis Grun., Cocconeis pediculus, Diatoma
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106
COLORADO RlV'ER ECOLOGY AND DAM MANAGEMENT
vulgare, and Rhoicosphenia curvata made up over 80% of the diatom com-
position at sites above Lee's Ferry, while these four taxa were significantly
less important at the downstream sites. Gomphonema olivaceum, Cymbella
Alibis KHtz., and Nitzschia dissipata (KHtz.) Grun. became progressively
more important at downstream sites. The explanation for this change in
species composition is not yet known, but the relatively high tolerance to
suspended sediment by the latter three diatom species may be a factor (Lowe,
1974; Bahls et al., 1984~. There was also a fourfold decrease in densities of
epiphytic diatom populations at sites below Lee's Ferry compared with sites
above Lee's Ferry, and there was a significant decrease in cell density with
increasing channel depth. Both patterns may relate to the relationships be-
tween suspended sediment, water depth, and light and nutrient attenuation.
In other studies of epilithic diatoms, Carothers and Minckley (1981) listed
total numbers of taxa for eight major tributaries to the Grand Canyon sys-
tem, and Usher et al. (1984) listed the periphytic diatom taxa in Bright
Angel, Garden and Pipe creeks in the canyon system. The periphytic diatom
communities were quite distinct from the diatom community associated with
Cladophora in the main stream.
AQUATIC INVERTEBRATE COMMUNITIES
Records of Intentional Stocking
Even if studies had been made on the aquatic invertebrates of the Colo-
rado River before the dam was closed, the intentional introduction of exotic
species after 1963 (Stone and Rathbun, 1968, 1969) would have prevented
accurate comparisons. Some events occurring between April and the end of
July 1967 underscore this. Ten thousand immature ephemeropterans secured
from a commercial source in Minnesota were planted in three sites between
the dam and Lee's Ferry. Shortly afterward, 10,000 snails (Physa and Stagnicola),
5000 leeches, and thousands of insects representing at least 10 families
from the San Juan River in New Mexico were planted. Finally 2000 cray-
fish collected from the Little Colorado River near Springerville were intro-
duced (Stone and Queenan, 1967~.
Zooplankton
From Haury's (1986, 1988) analyses, it appears that zooplankters of the
Colorado River below Glen Canyon Dam are derived from lentic popula-
tions in Lake Powell, i.e. similar to phytoplankton. Haury focused on the
planktonic crustaceans and proposed that occasional surface releases from
spillways would enhance the river populations and nocturnal releases would
have the greatest influence. Populations of Gammarus would not be in-
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ALGAL AND INVERTEBRATE BIOTA...
107
creased by such manipulations, but cladoceran and copepod numbers would
rise temporarily. There is good evidence that reproduction of zooplankton
occurs in the river because microcrustaceans below the dam increase to at
least the mouth of Diamond Creek, about 388 km below Glen Canyon Dam.
Haury did note, however, that the percentage of copepod plankters in "poor
condition" increased downstream.
The earlier report of Cole and Kubly (1976) included, in addition to the
nonplanktonic Gammarus, only 4 cladocerans, 8 ostracods, and 4 copepods
compared with 13, 8, and 13 listed by Haury about a decade later. Of those
on Haury's list, only 16 are true plankters; the others are benthic, although
they sometimes drift downstream and are sampled with the euplankters.
These drifters contributed a greater biomass in Haury's samples. Cole and
Kubly (1976) also listed rotifers, a collembolan, and water mites in their
river collections. The last two forms are categorized best as epineustonic
and, in the case of the mites, perhaps nektonic also.
One can speculate concerning the origins of some crustacean plankters in
the river. During the early 1950s Aglaodiaptomus clavipes Schacht and
Leptodiaptomus siciloides Lillj. were collected from Lake Mead (Wilson,
1955~. Since then, Skistodiaptomus reighardi Marsh has been found in the
lake (Paulson et al., 1980) and was included in Haury's list. A. clavipes is a
versatile western species, found in many Arizona habitats from turbid stock
tanks to large impoundments; it occurs with L. siciloides in the impound-
ments on the Salt River (Cole, 1961~. L. siciloides is widespread throughout
most of the continent, as are most of the cyclopoid crustaceans reported by
Cole and Kubly (1976) and Haury (1988~. Distribution records suggest that
Skistodiaptomus pallidus Herrick, the only calanoid reported by Cole and
Kubly, has moved westward from the Mississippi Valley. Skistodiaptomus
reighardi, Skistodiaptomus ashlandi Marsh, and Leptodiaptomus Forbes have
extended their ranges southward.
The richness of faunas of backwaters along the borders of the river chan-
nel and at the mouths of tributaries should be noted, and care should be
taken to preserve these refugia. Stanford and Ward (1986) noted that the
much higher productivity of backwaters probably make them very impor-
tant as a native fish habitat. A comparative study of the zooplankters in
backwaters and in the main stream of the Colorado River during 1987-1989
revealed mean densities per cubic meter almost four times greater in the
former (D. M. Kubly, personal communication). The relative increase in
cladocerans was especially noteworthy. Kubly found approximately 14 times
as many in quiet backwaters. By contrast, Haury (1981) found that the
numbers of Daphnia in the terminal pools at the mouth of Kanab Creek and
National Canyon were remarkably reduced compared with populations in
the adjacent mainstream. He noted, however, that the cladocerans were all
small, suggesting that fish had been selectively preying on the larger indi-
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108
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
viduals. The value of cladocerans as a food supply for larval fish is espe-
cially important, for they are more vulnerable to attack than are copepod
plankters.
Macroinvertebrates
The macroinvertebrates occurring in the Colorado River consist of but a
few species even though there have been numerous introductions of inverte-
brates as discussed previously. Over the years investigators have recorded
those species found in drift samples, fish stomachs, and material dredged
from the riverbed. The papers of Stevens (1976) and Leibfried and Blinn
(1986) and subsequent observations by Stevens (1990, personal communi-
cation) seem to apply to present conditions. The faunal list includes species
of planariid flatworm, perhaps three species of the Oligochaeta, gastropod
molluscs of which Physa predominates, a clam possibly belonging to the
Sphaeriidae, the amphipod crustacean Gammarus lacustris, and members of
six insect families. A baetid mayfly (Baetis sp.) has been overlooked by
many observers, but it has been reported in other river systems in the tailwaters
below dams (Pearson, 1967), and it appears to be increasing in the Colorado
River (Stevens 1990, personal communication). Corixid bugs, a hydrophilic
beetle, and at least 10 species of dipterans complete the list. Of the flies, the
larvae of simuliids (at least three species), about six species of chironomids,
and a ceratopogonid appear in collections.
The analyses of Polhemus and Polhemus (1976) of 14 species representing
nine families of aquatic heteropterans in the Grand Canyon are instructive.
Most of their collections were made in tributiaries entering about 363 km of
the river below Lee's Ferry. They considered the fauna typical of the south-
western United States and western Mexico. In that order of insects at least,
there is no evidence of species introduced from distant parts of the continent.
Many other authors have detailed the diversity of benthic macroinvertebrates
in the tributaries and their mouths in comparison with the river populations.
Hofknecht's (1981) list of 52 insect families represented in 30 tributaries
and spnngs, compared with only 5 in the main river, is typical. Carothers
and Minckley (1981) presented many comparisons of biomass and density
of aquatic invertebrates at the confluence of 15 tributaries and the river
versus similar data from 200 m upstream in each tributary. Their data un-
derscore once more the importance of these backwater pools; in all seasons
there were marked decreases in the upper station.
Usher et al. (1984) found 42 taxa of insects and one taxon each from
Gastropoda, Pelecypoda, Oligochaeta, and Arachnida in the habitats of Roaring
Springs, Bright Angel, Garden and Pipe creeks of the canyon system. It is
noteworthy that the caddisfly larvae (~Oligopheboides sp.) was recently col-
lected from Bright Angel Creek and Roaring Springs. These are the first
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ALGAL AND INVERTEBRATE BIOTA...
109
records for this genus in Arizona (M. W. Sanderson, personal communica-
tion). The known seven species of Oligophlebodes are all nearctic and are
confined to mountainous regions of the western United States (Wiggins,
1977), and therefore it is very surprising to find representatives of this
group in the Grand Canyon. In addition, Musgrave (1935) reported the
coleopteran Helichus triangularis from Garden Creek and is the only known
record of this species north of the Huachuca and Chiricahua Mountains of
southwestern Arizona.
Kondolf et al. (1989) predicted progressive armoring of the river bottom
with cobbles above Lee's Ferry, which may eventually cause destruction of
trout spawning areas because no gravel sources exist upstream. Whereas,
the relict gravel from preimpoundment years is being washed away, the
more stable substratum provides good simuliid habitat. Larvae of the simuliid
flies are an important food item for fish and fisheries may benefit from
increased blackfly populations. Larval chironomids, by contrast, are found
in silty areas or in Cladophora beds.
Simuliid and chironomid dipterans that metamorphose from aquatic lar-
vae to flying adults are involved in both the aquatic and terrestrial food
webs in the Grand Canyon (Stevens and Waring, 1988~. This is natural and
has little or nothing to do with flow regulation. More unusual is the phe-
nomenon of Gammarus stranded in pools along the stream banks falling
prey to lizards (personal observation).
The Amphipod Gammarus Lacustris
One of the most important items in the diet of the Colorado River exotic
and native fishes is the amphipod crustacean Gammarus lacustris. This
species is incorrectly termed freshwater shrimp in many publications, whereas
"scud" is more appropriate. Details are lacking about its history in the river,
although it began in December 1932 when 50,000 individuals were planted
in the "moss" of Bright Angel Creek (Anonymous, no date). Scuds do cur-
rently occur in Bright Angel Creek, and other tributaries entering the river
below Glen Canyon Dam. In 1965 more scuds were introduced at Lee's
Ferry and in the spring of 1968 many more were planted near the diversion
tunnels below the dam (Stone and Rathbun, 1969~. The source or sources of
these introduced amphipods was not reported. Titcomb (1927) pointed out
that millions of Gammarus limnaeus (an older name for G. Iacustris) from a
commercial source in Caledonia, New York (about 20 km south of Roches-
ter), had been distributed for stocking trout waters in various parts of the
country more than six decades ago. The natural distribution of this crusta-
cean probably has been blurred by human activities.
G. Iacustris is widespread in northern Asia and Europe. It occurs in most
of Alaska and Canada, perhaps being the only freshwater species of Gammarus
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COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
that reached North America via past Bering Sea land bridges. Local south-
en~ populations are found in California, Nevada, Arizona, and New Mexico.
It is a cold-stenothermous form, not thriving in warm waters with low pH
and low salinity.
Exotic and Unusual Species
In addition to the existence of northern and eastern crustaceans in the
Colorado River plankton populations, we can expect the arrival of certain
cosmopolitan species. Artificial waters seem to serve as stepping stones for
exotic species as they spread geographically. The appearance of the medusa
Craspedacusta sowerbyi Lankester in Lake Mead (Deacon and Haskell, 1963)
and in the small impoundment of Lake Patagonia in southern Arizona (Kynard
and Tash, 1974) is typical. It could be expected in Lake Powell and hence
the Colorado River below the dam.
The unusual tubificid worm Branchiura sowerbyi Bedd. has appeared in
canals in Tempe, Arizona and in Saguaro Lake, an impoundment on the Salt
River about 40 km northeast (Cole, 1966~. One could predict future inva-
sion of this cosmopolitan worm into Lake Powell. Furthermore, the Asiatic
clam Corbicula sp. has appeared in other Arizona impoundments, canals,
and streams. Some references to sphaeriid clams in the Colorado River
below Glen Canyon Dam may be referable to Corbicula from a different
pelecypod family.
Carothers and Minckley (1981) discussed the apparent establishment of
the exotic parasite Lernaea cyprinacea Linn. In some tributaries, they found
this parasitic copepod attached to native fishes, and its numbers may be
increasing. It thrives best in still waters of fish hatcheries, ponds, and lakes
but has been known for many years in the Arizona Salt, Verde, and Gila
River drainage systems, where it parasitizes at least three species of native
fishes (James, 1968~. It probably arrived in Arizona in the late 1800s or
early 1900s. Also, it has been reported from the Jordan River, Utah, where
it attacks Gila atraria (Bradford and Grundmann, 1968~. The slow flow of
the Colorado River may not inhibit its increase, and this worldwide species,
perhaps originally from Asia, may become a threat if a strain resistant to
cold water develops. It is presently increasing in the tributaries (Carothers
and Minckley, 1981~.
MODIFICATIONS IN BIOTA DUE TO REGULATED FLOWS
Colorado River Studies Between Glen Canyon and Lake Mead
Stranding of plant and animal communities during sudden low-flow peri-
ods in regulated rivers may be very important in structuring bioiic commu-
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ALGAL AND INVERTEBRATE BIOTA...
111
pities. Usher and Blinn (1990) experimentally tested the influence of short
term stranding (exposure to air) on the biomass and chlorophyll a of Cladophora
glomerata in a simulated stream environment. Their study indicated that
exposures of 12 daylight hours or longer resulted in significant reductions
in C. glomerata biomass, and exposures of 1 d or longer decreased chloro-
phyll a concentrations per gram dry weight of Cladophora biomass by over
50% from submerged control treatments. There were no significant differ-
ences in biomass between control treatments and treatments exposed to the
atmosphere for 12 hours of darkness, perhaps suggesting lower desiccation
rates during the night. C. glomerata treatments subjected to 12- and 24-hour
exposure-submergence cycles for a 2-week period showed over a 45% de-
crease in Cladophora biomass from control treatments.
Cladophora is frequently an important component of seston drift in the
Colorado River (Haury, 1981; Leibfried and Blinn, 1986~. Periodic, short-
term exposures (12 to 24 hours) of river substratum during low flows may,
increase drift of Cladophora in the Colorado River. It seems logical that if
holdfast systems of plants are dried during extended periods of stranding
they would become weaken and detach, and would enter the water column
as drift during re-wetting (Usher and Blinn 1990~. In addition, the increased
three-dimensional development of algal populations due to growth would
increase the natural shearing effect by current and contribute to detachment
(Vogel 1981~. However, Leibfried and Blinn (1986) found no significant
differences in Cladophora drift rates in the Colorado River at Lee's Ferry
between months with relative steady flows (May-September 1985; flows
fluctuated <8,000 cfs) and fluctuating flow periods (October-December 1985;
flows fluctuated >20,000 cfs). It is noteworthy that stranding periods were
<12 hrs during months of regulated flow.
Leibfried and Blinn (1986) did find a significant increase in stream drift of
Gammarus during the rising arm of discharges following low flow periods
(<5,000 cfs). Gammarus increased from 10.7 animals per hour for months
with relatively steady flows to 42.3 animals per hour for months with more
fluctuating flows. They observed that Gammarus left the stranded Cladaphora
filaments during low flows and moved into shallow pools of water. When
flows increased the amphipods were swept downstream into the water column.
Further studies on exposure and on the importance of regulated flow and
discharge rates for detachment and drift are essential to help clarify the impor-
tance of these disturbances on the biotic communities in the Colorado River.
Cladophora glomerata does display a few adaptations to reduce the im-
pact of widely fluctuating flow regimens caused by dams. Recently, Usher
(1987) reviewed the literature on the physicochemical requirements of
Cladophora, and Usher and Blinn (1990) discussed the importance of the
general plant body design of Cladophora to help reduce the effects of expo-
sure. Typically, the longer, outer filaments of Cladophora collapse on themselves
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COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
and trap water like a sponge as the water level recedes in regulated rivers.
This process tends to protect the innermost filaments from high light and
reduce desiccation rates. In addition, the dense assemblages of epiphytic
diatoms with their mucilaginous secretions may further help to slow desic-
cation rates in Cladophora during periods of short-term stranding in regu-
lated rivers. These adaptations may, in part, explain why Cladophora popu-
lations are commonly associated with marine intertidal communities and
perhaps preadapted to tolerate fluctuating flow regimes in rivers.
Diatoms are a common, if not the dominant, attached algal component in
many lotic ecosystems (Whitton, 1975; Lowe, 1979), including streams in
southwestern United States (Blinn et al., 1981; Duncan and Blinn, 1989a).
The Colorado River ecosystem is no exception (Czarnecki and Blinn, 1977,
1978~. These microalgae provide food for invertebrates and fish as well as
oxygen for general community metabolism.
The U.S. Fish and Wildlife Service has suggested a modification in the
water release program at Glen Canyon Dam in order to improve the habitat
for the humpback chub (Gila cypha). The new plan proposes to release
warmer subsurface water (>18°C) from the epilimnion of Lake Powell in-
stead of the present, cool water (10-12°C) released from the hypolimnion of
the reservoir (Maddux et al., 1987~. Recently, Blinn et al. (1989a) examined
the importance of elevated water temperature on the community structure of
periphytic diatoms in the tailwaters (25 km below Glen Canyon Dam) of the
Colorado River. They found that the relative frequencies of the numerically
important larger upright species such as Rhoicosphenia curvata and Diatoma
vulgare decreased when water temperature was elevated to 18°C or higher.
In contrast, the relative abundance of the smaller and closely adnate cells of
Cocconeis pediculus increased with elevated water temperatures. This com-
positional shift in epiphytic diatoms as a result of elevated water tempera-
ture may be potentially important to the aquatic food chain in the Colorado
River because recent studies have demonstrated that larger upright diatom
species are commonly consumed by macroinvertebrate grazers in preference
to smaller adnate diatom taxa (Sumner and McIntire, 1982; Steinman et al.,
1987; Colletti et al., 1987; Blinn et al., l989b). Furthermore, Blinn et al.
(1986) reported that the diet of Gammarus lacustris, an important inverte-
brate grazer in the tailwaters of Glen Canyon Dam, consisted primarily of
upright taxa (D. Yulgare and R. curvata) and rarely of the closely adnate
species, C. pediculus. Montgomery et al. (1986) and Leibfried (1988) have
also suggested the importance of epiphytic diatoms as a food resource for
rainbow trout in the Colorado River.
Peterson (1987) studied the importance of desiccation on the structure of
diatom communities in the tailwaters of Lake Mead in the Colorado River.
He reported that biomass and density were reduced by desiccation on shel-
tered substrata, as a function of regulated flow.
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ALGAL AND INVERTEBRATE BIOTA...
STUDIES ON OTHER REGULATED RIVERS
113
Over the past several decades, authors have reviewed the various ways
that manmade reservoirs frequently modify downstream environments (Neel,
1963; Ward, 1974, 1975, 1976; Baxter, 1977; Ward and Stanford, 1979;
Obeng, 1981; Lillehammer and Saltveit, 1984; Pelts, 1984; Craig and Kemper,
1987~. The general consensus is that regulated rivers subject biological
communities to the following downstream conditions: (1) reductions in sea-
sonal flow variability, (2) alterations in the timing of extreme flow events,
(3) unnatural pulses in flow during periods of peak power demands, (4)
clarification of water, (5) diet and seasonal constancy of water tempera-
tures, (6) a significant increase in armored substrates, (7) modifications in
nutrient regimes, and (8) the appearance of lentic plankton beneath reser-
voirs. Ward and Stanford (1987) also suggest that reservoirs reduce mean
annual runoff because of high evaporation rates in reservoirs, which in turn
can increase salinity. It is noteworthy that the position of a dam along the
river continuum (Vannote et al., 1980) is instrumental in the magnitude of
these downstream modifications (Ward and Stanford, 1983~.
Plant Communities
Typically, the conditions of regulated flow described above increase the
standing crops of both attached algae and aquatic macrophytes, as well as
aquatic mosses (Neel, 1963; Lowe, 1979~. Ward (1976) reported 3-20 times
more epilithic algae in regulated portions of the South Platte River below
Cheesman Reservoir, Colorado, than in unregulated channels. Likewise, workers
have reported substantial increases in algal biomasses below other reser-
voirs in Colorado (Zimmerman and Ward, 1984; Dufford et al., 1985) and
in Utah (McConnell and Sigler, 1959), Montana (Gore, 1977), Great Britain
(Pests and Greenwood, 1981), Norway (Skulburg, 1984), and Australia (King
and Tyler, 1982~. These observations concur with reports by Usher and
Blinn (1990) in regard to the high algal biomass in the tailwaters (28 km) of
the Colorado River below Glen Canyon Dam. It is noteworthy that Ross and
Rushforth (1980) did not find significant differences in diatom communities
above and below a newly formed reservoir on Huntington Creek, Utah. In
addition, Walker (1979) reported low algal and macrophytic development in
the Murray River, Australia. He suggested that the potential effects of nutri-
ent enrichment and regulated flows, which normally enhance plant develop-
ment, are greatly overridden by the prevailing high turbidity in the lower
Murray River.
Commonly, filamentous green algae, especially Cladophora glomerata,
and in some instances Microspora amoena (Kutz.) Rabh. (Skulburg, 1984)
and Ulothrix zonata (Weber and Mohr) Kutz. (Racer and Ward, 1988), and
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114
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
the chrysophyte Hydrurus foetidus (Vill.) Trev. (Ward, 1976; Skulburg,
1984) dominate the attached macroalgal communities below reservoirs. The
development of C. glomerata implies nutrient enrichment and high water
turbulence (Whitton, 1970, 1975; Usher, 1988), while U. zonata and H.
foetidus are considered Goldwater rheobionts (Blum, 1960~; all of these
environmental conditions occur in the tailwaters of hypolimnion-released
reservoirs. Regulated streams tend to favor extensive developments of dia-
tom species that are typically Goldwater stenotherms and rheobionts (Lowe,
1979; Blum, 1960~. Epiphytic diatoms from the genera Achnanthes, Cocconeis,
Cymbella, Diatoms, Diatomella, and Synedra are frequently associated with
regulated streams (Lowe, 1979; Pelts, 1984; Dufford et al., 1987~. It is
noteworthy that Diatoms vulgare is one of the more common diatom taxa
associated with regulated streams, including the tailwaters of Glen Canyon
Dam, and is frequently epiphytic on Cladophora and prefers cool, flowing
water with high nutrients (Lowe, 1979~. All of these conditions commonly
prevail in the tailwaters of reservoirs.
The environmental conditions most frequently cited for the enhancement
of algal growth below dams include (1) high clarity of water due to the
truncation effect of reservoirs on the downstream transport of sediment, (2)
postponement or absence of freezing conditions as a result of the diet and
seasonal constancy of water temperature, (3) increased stability and avail-
ability of armored substrates for attachment, (4) absence of sudden spates
and droughts, and (5) increased nutrient availability. Spence and Hynes
(1971) also suggest that the constant cool waters below reservoirs reduce
the number of algal grazers.
The importance of nutrient enrichment below reservoirs on algal growth
has been reviewed by several investigators (Lowe, 1979; Hannan, 1979;
Skulburg, 1984; Petts, 1984; Ward and Stanford, 1987~. The nutrients con-
sidered to be most influential on algal populations below reservoirs include
phosphorus, nitrogen, and silica, but the roles of other nutrients need to be
examined. For example, Patrick et al. (1969) suggested the potential impor-
tance of trace elements on the composition of lotic algal communities.
Marcus (1980) reported that ammonia-nitrogen was the only stream physico-
chemical parameter that correlated with increased periphytic chlorophyll a.
Other factors normally considered important in causing variations in algal
growth in regulated rivers (i.e., water temperature, flow rate, and streambed
characteristics) apparently had only a secondary influence on algal growth
because these conditions were similar at all sites. Hannan and Young (1974)
also reported an increase in ammonia-nitrogen below a reservoir on the
Guadalupe River in Texas, while Armitage (1984) reported that nitrogen
levels in coarse particulate seston were higher in regulated sections of streams.
The mobilization of nitrogen may result from mineralization and nitrogen
fixation in upstream reservoirs (Rada and Wright, 1979~. Armitage (1984)
concluded that increases in plant growth as a result of nutrient inputs from
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ALGAL AND INVERTEBRATE BIOTA...
115
reservoirs alter the character of substrates for zoobenthos by the increasing
surface area available for colonization and increasing retention of particu-
late material through the filtering action of plants. Hannan (1979) and
Walker (1979) discussed the potential for reduced dissolved oxygen con-
centrations in the hypolimnetic waters of reservoirs, while Ward and Stanford
(1987) suggested the importance of reduced compounds such as H2S of
tailwater biotic communities, especially below eutrophic reservoirs during
late summer.
Periodic spates are important in regulating the abundance and structure
of algal communities in lotic environments (Fisher et al., 1982~. Although
current velocity is an important variable because the rapid exchange of
water around cell surfaces removes wastes and replenishes nutrients (Whitford
and Schumacher, 1961, 1964), extreme discharges can dislodge attached
algal communities and severely reduce standing crop (Fisher et al., 1982;
Skulburg, 1984; Power and Stewart, 1987; Duncan and Blinn, 1989~. It is
likely that the Colorado River did not develop extensive algal populations
prior to the closure of Glen Canyon Dam because of periodic scouring by
turbid flood waters (Woodbury et al., 1959~. Glen Canyon Dam has buff-
ered these spates and allowed Cladophora glomerata to proliferate in the
tailwaters (28 km below Glen Canyon Dam). However, the present rela-
tively high discharges from Glen Canyon Dam may be responsible for the
frequent occurrence of Cladophora in the drift (Haury, 1981; Leibfried and
Blinn, 1986~. In fact, Mullan et al. (1976) suggested that the extensive
Cladophora populations that developed within 6 years of the closure of
Glen Canyon Dam had been reduced substantially since that time as a result
of the scouring action of daily discharge fluctuations of up to 140 m3 so.
Aquatic macrophytes, including mosses, also show substantial increases in
biomass below reservoirs (Lowe, 19793. Commonly, lotic environments show
only limited development for aquatic macrophytes as a result of the fre-
quency of turbid flood waters (Westlake, 1975; Fisher et al., 1982~. How-
ever, with the stabilization of flow regimes and increased stability of sub-
strates, a number of studies show that macrophyte standing crops increase
below reservoirs (Hall and Pople, 1968; Holmes and Whitton, 1977; Haslam,
1978~. Aquatic macrophytes are not important in the mainstream of the
Colorado River, perhaps because of its deep channel and the magnitude of
discharge below Glen Canyon Darn.
Invertebrate Communities
The most common effect of impounded rivers on the macroinvertebrate
community is reduced species diversity accompanied by high density and
biomass (Pearson, 1967; Pearson et al., 1968; Spence and Hynes, 1971;
Fisher and LaVoy, 1972; Ward, 1976; Ward and Stanford, 1979; Zimmerrnann
and Ward, 1984; Brusven, 1984~. These findings agree with those of Leibfried
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116
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
and Blinn (1986) for the macroinvertebrate community in the tailwaters of
Glen Canyon Dam in the Colorado River. Plecopterans (stonefly) and
ephemeropterans (mayflies) are generally reduced (Ward and Stanford, 1979;
Winget, 1984; Pett, 1984; Rader and Ward, 1988), while amphipod crusta-
ceans and simuliid larvae (dipterans) are enhanced in many impounded riv-
ers (Pfitzer, 1954; Hilsenhoff, 1971; Spence and Hynes, 1971; Ward, 1976;
Ward and Stanford, 1979~. The dam's effects are lessened downstream and
eventually lower reaches of the stream may recall creimnoundment condi-
tions (Voelz and Ward, 19891.
There are several factors responsible for the observed changes in com-
munity structure of macroinvertebrates below reservoirs. Ward (1976) sug-
gested that the dense growths of algae prevent the establishment of certain
forms of insects that use suckers or friction pads. The lack of smooth rock
surfaces below impoundments, which are algal free, may be especially re-
strictive to heptageniid mayflies (Ward, 1976~. Furthermore, Spence and
Hynes (1971) suggested that oxygen depletion due to the high respiratory
demands of dense algal stands caused the disappearance of three predatory
plecopterans below the Shand Dam in Ontario, Canada.
In situations where deep hypolimnial water is released from reservoirs,
macroinvertebrate communities have been adversely affected. This results
from the loss of particulate organic matter in upstream reservoirs (Armitage,
1984~. Larger zooplankters do not persist in downstream flows, and plank-
tonic organisms from hypolimnion releases are not a suitable food source
for filter-feeding benthic forms downstream (Ward, 1975~.
Modifications in thermal conditions also contribute to the compositional
changes in macroinvertebrates below reservoirs. The thermal constancy and
seasonal temperature patterns below dams may disrupt thermal signals es-
sential for the completion of life cycles for certain macroinvertebrates. For
example, the annual number of degree days may not be sufficient for the
completion of a life cycle (Ward, 1976; Hauer and Stanford, 1982~.
On the other hand, conditions below reservoirs may enhance the devel-
opment of some species. For example, Gammarus lacustris prefers cool to
cold water conditions in the summer (Bousfield, 1958~. Spence and Hynes
(1971) also suggested that increases in amphipod crustaceans resulted from
the increase in available microhabitats and food within the algal mats. En-
hanced standing crop of certain macroinvertebrates have also been attrib-
uted to increased plankton levels (Cushing, 1963~.
RECOMMENDED RESEARCH
Based on our present knowledge on the algal and invertebrate communi-
ties in the Colorado River, the following list of general recommendations is
provided.
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ALGAL AND INVERTEBRATE BIOTA...
117
1. Further studies should be done on the seasonal abundance and distri-
bution of Cladophora glomerata within the Colorado River ecosystem. We
recommend that, in cooperation with the Bureau of Reclamation, collections
be made under different flow regimes (i.e., typical range of regulated dis-
charge and steady flow) at several locations in the tailwaters of Glen Can-
yon Dam at various depths within the river channel to establish comparative
baseline information on the population dynamics of this important primary
producer under regulated flow in the Colorado River ecosystem.
2. Since Cladophora glomerata is potentially an important biotic com-
ponent in the aquatic food web of the tailwaters of Colorado River ecosys-
tem, we recommend that threshold flow rates between optimum growth and
primary production as well as loss of plants through natural shearing force
be determined for the C. glomerata population in the Colorado River. It is
likely that some optimum flow schedule could be established that would
provide a level of Cladophora growth and maintenance that would most
benefit the overall Colorado River ecosystem.
3. The importance of Cladophora glomerata and associated epiphytes to
the Colorado River ecosystem should be established. For example one could
(a) determine the importance of C. glomerata as a habitat and as a refuge
for invertebrates from fish predators in the Colorado River ecosystem and
(b) determine the importance of C. glomerata and associated epiphytic dia-
toms as food for invertebrates and selected fish species in the Colorado
River ecosystem.
4. Develop a clear understanding of the food web in the tailwaters of
Glen Canyon Dam compared to areas downstream in the Grand Canyon.
5. Undertake phonological studies of invertebrates in the Colorado River
ecosystem, especially if temperature patterns are likely to change because
of different release patterns from Glen Canyon Dam, i.e., epilimnial versus
hypolimnial releases.
6. There is a need to determine the influence of different flow regimes
on the detachment and drift rates of Cladophora glomerata and epiphytic
diatoms as well as for macroinvertebrates. With the cooperation of the
Bureau of Reclamation, drift rates for Cladophora and macroinvertebrates
could be measured for various levels of water release from Glen Canyon
Dam. The importance of Cladophora and invertebrate drift to the overall
food web, especially fish populations, in the Colorado River needs clarifi-
cation.
7. We should study and protect the biota of backwater refugia in major
tributaries that enter the Colorado River.
8. There is a need to develop models that would determine the effects
that various modifications in physicochemical parameters have on plant and
animal communities in regulated rivers.
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118
COLORADO RIVER ECOLOGY AND DAM MANAGEMENT
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Representative terms from entire chapter:
glen canyon
