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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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110 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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112 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 (>18C) from the epilimnion of Lake Powell in- stead of the present, cool water (10-12C) 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 18C 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 REFERENCES Anonymous. Fish plants, Grand Canyon National Park, 1920-1956. Unpublished manuscript. Armitage, P. D. 1984. Environmental changes induced by stream regulation and their effect on lotic macroi'nvertebrate communities, p. 139-166. In: A. Lillehammer and S. J. Saltveit (ads.), Regulated Rivers. Univ. of Oslo Press, Oslo, Norway. 540 p. Bahls, L. L., E. E. Weber, and J. O. Jarvie. 1984. Ecology and distribution of major diatom ecotypes in the Southem Fort Union coal region in Montana. Geological Survey Profes- sional Paper 1289. U.S. Government Printing Office, Washington, D.C. 151 p. Baxter, R. M. 1977. Environmental effects of dams and impoundments. Annul Rev. Ecol. Syst. 8:255-283. Blinn, D. W., M. Hurley, and L. Brokaw. 1981. The effect of saline seeps and restricted light upon the seasonal dynamics of phytoplankton communities within a southwestern (USA) desert canyon stream. Arch. Hydrobiol. 92:287-305. Blinn, D. W., C. Pinney, R. Truitt, and A. Pickart. 1986. Examination of the influence of elevated temperature on epiphytic diatom species in the tailwaters of Glen Canyon Dam and the importance of these epiphytic diatoms in the diet of Gammarus lacustris. Prelimi- nary report submitted to the Bureau of Reclamation. 8 p. Blinn, D. W., R. Truitt, and A. Pickart. 1989a. Response of epiphytic diatom communities from the tailwaters of Glen Canyon Dam, Arizona, to elevated water temperature. Regu- lated Rivers 4:91-96. Blinn, D. W., R. Truitt, and A. Pickan. 1989b. Feeding ecology and radular morphology of the freshwater limpet Ferrissia fragil~s. J. N. Am. Benthol. Soc. 8:237-242. Blum, J. L. 1957. An ecological study of the algae of the Saline River, Michigan. Hydrobiologia 9:361 -408. Blum, J. L. 1960. Algal populations in flowing waters. Spec. Publ. Pymatuning Lab. Field Biol. 2:11-21. Bousfield, E. L. 1958. Fresh-water amphipod crustaceans of glaciated North America. Can. FId. Nat. 72:55-113. Bradford, C. S., and A. W. Grundmann. 1968. Lernaea cyprinacea Linn., a parasitic copepod infecting fish in Utah waters. Utah Acad. Sci. Arts Lett. 45:128-129. Brusven, M. A. 1984. The distribution and abundance of benthic insects subjected to reservoir- release flows in the Clearwater River, Idaho, USA, p. 167-180. In: A. Lillehammer and S. J. Saliveit (eds.), Regulated Rivers. University of Oslo Press, Oslo Norway. 540 p. Carothers, S. W., and C. O. Minckley. 1981. A survey of the fishes, aquatic invertebrates, and aquatic plants of the Colorado River and selected tributaries from Lee's Ferry to Separation Rapids. Final Report to U.S.D.I. Water and Power Res. Serv. Lower Colorado Region, Boulder City, Nev. 401 p. Cole, G. A. 1961. Some calanoid copepods from Arizona with notes on congeneric occurrences of Diaptomus species. Limnol. Oceanogr. 6:432-442. Cole, G. A. 1966. Branchiura sowerbyi Beddard (Annelida: Oligochaeta) in Arizona. J. Ariz. Acad. Sci. 4:43. Cole, G. A., and D. M. Kubly. 1976. Limnologic studies on the Colorado River from Lee's Ferry to Diamond Creek. Colorado River Research Series Contribution No. 37. Grand Canyon National Park. 88 p. Colletti, P., D. W. Blinn, A. Pickart, and V. T. Wagner. 1987. Influence of different densities of the mayfly grazer Heptagenia criddlei on loiic diatom communities. J. N. Am. Benthol. Soc. 6:270-280. Craig, J. F., and J. B. Kemper (eds.). 1987. Regulated Streams: Advances in Ecology. Plenum Press,New York. 431 p. Crayton, W. M., and M. R. Sommerfeld. 1978. Phytoplankton of the lower Colorado River, Grand Canyon region. J. Ariz.-Nev. Acad. Sci. 13:19-24.

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ALGAL AND INVERTEBRATE BIOTA... 119 Crayton, W. M., and M. R. Sommerfeld. 1979. Composition and abundance of phytoplankton in the tributaries of the Lower Colorado River, Grand Canyon Region. Hydrobiologia 66:81-93. Crayton, W. M., and M. R. Sommerfeld. 1981. Impacts of a desert impoundment on the phytoplankton community of the Lower Colorado River, p. 1608-1617. In: H. G. Stefan (ed.), Surface Water Impoundments, vol. I. American Society of Civil Engineers, New York. Cushing, C. E. 1963. Filter-feeding insect distribution and planktonic foods in the Montreal River. Trans. Am. Fish. Soc. 92:216-219. Czarnecki, D. B., and D. W. Blinn. 1977. Diatoms of the Lower Lake Powell and Vicinity. Bibl. Phycol., Band 28, J. Cramer. 119 p. Czarnecki, D. B., and D. W. Blinn. 1978. Diatoms of the Colorado River in Grand Canyon National Park and vicinity. Bibl. Phycol., Band 38, J. Cramer. 181 p. Czamecki, D. B., D. W. Blinn, and T. Tompkins. 1976. A periphytic microflora analysis of the Colorado River and major tributaries in Grand Canyon National Park and vicinity. Colo- rado River Research Program Publication No. 6. 106 p. Deacon, J. E., and W. L. Haskell. 1963. Occurrence of the freshwater jellyfish, Craspedacusta sowerbyi, in Lake Dead, Nevada. Am. Midl. Nat. 70:504. Dolan, R. A., A. Howard, and A. Gallenson. 1974. Man's impact on the Colorado River in the Grand Canyon. Am. Sci. 62:392-401. Dufford, R. G., H. L. Zimmermann, L. D. Cline, and J. V. Ward. 1987. Responses of epilithic algae to regulation of Rocky Mountain streams, p. 383-390. In: J. F. Craig and J. B. Kemper (eds.), Regulated Streams: Advances in Ecology. Plenum Press, New York. 431 p. Duncan, S., and D. W. Blinn. 1989. Importance of physical variables on the seasonal dynamics of epilithic algae in a highly shaded canyon stream. J. Phycol. 25:455-461. Fisher, S. G., L. J. Gray, N. B. Grimm, and D. E. Busch. 1982. Temporal succession in a desert stream ecosystem following flash flooding. Ecol. Monogr. 52:93-110. Fisher, S. G., and A. LaVoy. 1972. Differences in littoral fauna due to fluctuating water levels below a hydroelectric dam. J. Fish. Res. Board Can. 29:1472-1476. Flowers, S. 1959. Algae collected in Glen Canyon. Appendix D, pp. 203-205. In: C. E. Dibble (ed.), Ecological Studies of the Flora and Faunain Glen Canyon. University of Utah An- thropology Papers, Salt Lake City. 226 p. Gore, J. A. 1977. Reservoir manipulations and benthic macroinvenebrates in a prairie river. Hydrobiologia 55:113-123. Gregory, H. E., and R. C. Moore. 1931. The Kaiparowitz region. Geological Survey Profes- sional Paper 164. U.S. Government Printing Office, Washington, D.C. 161 p. Hall, J. R., and W. Pople. 1968. Recent vegetational changes in the lower Volta River. Ghana J. Sci. 8:24-29. Hannan, H. H. 1979. Chemical modifications in reservoir-regulated streams, p. 75-94. In: J. V. Ward, and J. A. Stanford (eds.), The Ecology of Regulated Streams. Plenum Press, New York. 398 p. Hannan, H. H., and W. J. Young. 1974. The influence of a deep-storage reservoir on the physicochemical limnology of a central Texas River. Hydrobiologia 44:177-207. Haslam, S. M. 1978. River Plants. Cambridge University Press, New York. 396 p. Hauer, F. R., and J. A. Stanford. 1982. Ecological responses of hydropsychid caddisflies to stream regulation. Can. J. Fish. Aquat. Sci. 39:1235-1242. Haury, L. R. 1981. Cladophora drift and plankton crustaceans in the Colorado River: Lee's Ferry to Diamond Creek. Unpublished report to Museum of Northem Arizona, Flagstaff, Ariz. 25 p. Haury, L. R. 1986. Zooplankton of the Colorado River from Glen Canyon Dam to Diamond Creek. Glen Canyon Environmental Studies Technical Report. Bureau of Reclamation, Salt Lake City, Utah. GCES Rep. No. B-10. 411 p.

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120 COLORADO RIVER ECOLOGY AND DAM MANAGEMENT Haury, L. R. 1988. Zooplankton of the Colorado River, Glen Canyon Dam to Diamond Creek. p. 205-215. In: Executive Summaries of Technical Reports. Glen Canyon Environmental Studies. Hilsenhoff, W. L. 1971. Changes in downstream insect and amphipod fauna caused by an impoundment with a hypolirnnion drain. Ann. Entomol. Soc. Am. 64:743-746. Hofknecht, G. W. 1981. Seasonal community dynamics of aquatic invertebrates in the Colo- rado River and its tributaries within Grand Canyon, Arizona. M.S. Thesis. Northern An- zona University, Flagstaff. 105 p. Holmes, N. R. H., and B. A. Whitton. 1981. Phytobenthos of the River Tees and its tributaries. Freshwat. Biol. 11:139-163. James, A. E. 1968. Lernaea (Copepod) infection of three native fishes from the Salt River Basin, Arizona. M.S. Thesis. Arizona State University, Tempe. 37 p. King, R. D., and P. A. Tyler. 1982. Downstream effects of the Gordon River Power Develop- ment, South-West Tasmania. Aust. J. Mar. Freshwat. Res. 33:431-442. Kondolf, G. M., S. S. Cook, H. R. Maddox, and W. R. Persons. 1989. Spawning gravels of rainbow trout in Glen and Grand Canyons, Arizona. J. Ariz-Nev Acad. Sci. 23:19-28. Kynard, B. E., and J. C. Tash. 1974. Freshwater jellyfish (Craspedacusta sowerbyi) in Lake Patagonia, southern Arizona. J. Ariz. Acad. Sci. 9:76-77. Leibfried, W. C. 1988. The utilization of Cladophora glomerata and epiphytic diatoms as a food resource by rainbow trout in the Colorado River below Glen Canyon Dam, Arizona. M.S. Thesis. Northern Arizona University, Flagstaff. 41 p. Leibfried, W. C., and D. W. Blinn. 1986. The effects of steady versus fluctuating flows on aquatic macroinvenebrates in the Colorado River below Glen Canyon Dam, Arizona. NTIS No. PB88206362/AS. 55 pp. Lillehammer, A., and S. J. Saltveit (eds.). 1984. Regulated Rivers. University of Oslo Press, Oslo, Norway. 540 p. Lowe, R. 1974. Environmental requirements and pollution tolerance of freshwater diatoms. EPA-670/4-74-005. U.S.E.P.A., Cincinnati, Ohio. 333 p. Lowe, R. 1979. Phytobenthic ecology and regulated streams, p. 25-34, In: J. V. Ward and J. A. Stanford (eds.), The Ecology of Regulated Streams. Plenum Press, New York. 398 p. Maddox, H. R., D. M. Kubly, J. S. deVos, Jr., W. R. Persons, R. Staedicke, and R. L. Wnght. 1987. Effects of varied flow regimes on aquatic resources of Glen and Grand Canyons. Arizona Game & Fish Department Final Rep. 291 p. Marcus, M. D. 1980. Penphytic community response to chronic nutnent enrichment by a reservoir discharge. Ecology 6:387-399. McConnell, W. J., and W. F. Sigler. 1959. Chlorophyll and productivity in a mountain ever. Limnol. Oceanogr. 4:335-351. Montgomery, W. L., W. C. Leibfried, K. Gooby, and P. Pollak. 1986. Feeding by rainbow trout on Cladophora glomerata at Lee's Ferry, Colorado River, AZ. The roles of Cladophora and epiphytic diatoms in trout nutrition. Preliminary report to the Bureau of Reclamation, Nonhem Arizona University, Flagstaff. Moore, R. C., and H. B. Hungerford. 1922. Water insects from a ponion of the southem Utah desert. Kansas Univ. Sci. Bull. 14:409-422. Mullan, J. W., V. J. Starostka, J. L. Stone, R. W. Wiley, and W. J. Wiltzius. 1976. Factors affecting Upper Colorado River Reservoir tailwater trout fisheries, p. 405-423. In: J. F. Orsborn, and C. E. Allman (eds.), Instream Flow Needs, vol. II. American Fisheries Soci- ety, Bethesda, Md. 657 p. Musgrave, P. N. 1935. A synopsis of the genus Helichus Erichson in the United States and Canada, with description of a new species (Coleoptera: Dryopidae). Proc. Entomol. Soc. Washington 37:137-145. Musser, G. G. 1959. Annotated check list of aquatic insects of Glen Canyon. Appendix E, p.

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