The Effect of Controlled Floods on the Lower Aquatic Communities in the Grand Canyon by Sarah E

The Effect of Controlled Floods on the Lower Aquatic Communities in the Grand Canyon By Sarah E. Purdy ABSTRACT Glen Canyon Dam on the Colorado River has induced many changes in the physical and biological processes in the river. For the past decade, managers have tried to restore some of the important biological processes in the canyon by instituting an adaptive management strategy of controlled periods of high release from Glen Canyon Dam. These “controlled floods” are a large perturbation to the river system and have radical effects on the all of the organisms associated with it. This paper discusses the effect of the controlled floods on the aquatic food base, which includes algae, plants, zooplankton and macroinvertebrates. The Grand Canyon is also home to one of two remaining populations of the endangered Kanab Ambersnail. The controlled flooding has potentially detrimental consequences for their continued persistence as their habitat is within the high water zone. Additionally, the invasive New Zealand Mud Snail has recently colonized the tail waters of Glen Canyon Dam. This snail’s presence is altering the entire food web in the Grand Canyon and could further imperil the status of the native fish in the river, most of which are threatened or endangered. Introduction Regulated rivers exhibit profound changes in the physical processes that govern their structure and function. These changes alter the complex biological communities associated with rivers, especially by changes in the number and type of species present. The lowest levels of the food web are made up of primary producers, detritus and lower faunal organisms such as zooplankton and micro- and macro-invertebrates (Shannon et al. 2001). The primary producers in a system are organisms such as algae, cyanobacteria, diatoms, and macrophytes (large vascular plants) that transform solar energy into stored starch that is available to other organisms as food. Detritus consists of organic material derived from decomposing plants, animals and fish wastes. The animal portion of the food base consists of aquatic insects, zooplankton, aquatic worms, and mollusks that colonize and eat the primary producers. The aquatic food base is the fundamental resource that supports all of the vertebrate forms of wildlife within the system (Shannon et al. 2001). Nearly all of the wildlife in the Grand Canyon is either directly or S.E. Purdy 9/8/05 indirectly linked to the aquatic food base including both native and non-native fish, insectivorous birds and bats, reptiles, waterfowl and a multitude of animals that feed on fish such as the Bald Eagles, Western Grebes, Kingfishers (Carothers and Brown 1991, Stevens et al. 1997a, Shannon et al. 2001). In this paper I will discuss how the aquatic food base responds to controlled flooding and is changing over time, the impact of the controlled floods on the endangered Kanab ambersnail, and the recent invasion of the Grand Canyon by the New Zealand mudsnail. The Aquatic Food Base Glen Canyon Dam on the Colorado River was put into place in 1963. Prior to that time, the Colorado River was a warm, sediment rich system that had large variation in flow throughout the year, ranging from a trickle in the dry months to extremely high flows as snow melted in the upper basin during spring and early summer (Shannon et al 2001, Blinn et al. 1998, Schmidt et al. 1998). Violent summer monsoons caused flooding in tributary streams, and many tons of sediment entered the river as sudden rains scoured the sparsely vegetated soft sandstone rock that makes up the Colorado plateau. The high amount of sediment in the river caused it to become turbid. High turbidity prevents energy from the sun from penetrating the water; so fewer plants and other primary producers were historically present in the river. Glen Canyon Dam drastically changed nearly all of the fundamental processes that formed and maintained the river and its associated biota. Sediment became trapped behind Glen Canyon Dam so that the tail-waters below the dam run clear. Water released from the dam comes from the layer of cold, dense water near the bottom of the reservoir (hypolimnion) and maintains a much colder average temperature than the river had in the past. Changes in water temperature and clarity led to related changes in the aquatic food base. The Colorado River historically received most of its organic inputs from sources exterior to the river such as riparian trees, vegetation and debris washed into tributary streams. This is termed allochthonous material. When organic matter is produced within the stream itself, via algae and aquatic plants, it is termed autochthonous. There remains a large discrepancy in the scientific literature regarding the historic food base in the lower Colorado River. McDonald and Dobson (1960) reported a “sparse and depauperate” community of aquatic insects in the Glen Canyon area with 20 species reported. Persons et al. (1985) maintained this assumption and Page 2 of 20 S.E. Purdy 9/8/05 suggested that the post-dam species assemblages and densities were virtually unchanged from those in the pre-dam Colorado mainstem. Creel censuses and limnological studies performed by the Arizona Game and Fish Department from 1963 to 1972 indicated a highly disturbed system that possessed low densities of macroinvertebrates limited to Helisoma sp. (black snail) and chironomids (Stone 1964). This led Arizona Game and Fish Department to introduce an assortment of invertebrate species to the tailwater between 1966-1969 including the amphipod, Gammarus lacustris, chironomid species and snails. Most of the introductions failed however, the Gammarus and chironomids became the dominant food base taxa for trout in the tailwater. More recent studies question the contention that the pre-dam Colorado supported so few insect taxa. Stanford and Ward (1986) and Haden et al. (2003) argue that the upper Colorado River and the Green River in Canyonlands National Park provide the best example of historic conditions in the Grand Canyon because they retain many of the distinct properties associated with free flowing desert rivers. The Green and Colorado Rivers are warm in the summer, sediment laden, and dependent on allochthonous carbon sources for their primary productivity. The species assemblage found by Haden et al. (2003) in Canyonlands National Park reflects such characteristics by the feeding guilds that are present in the macroinvertebrate population. Multi- species feeding guilds (determined by how and what an organism eats), rather than the presence of individual species, have long been used to indicate stream function (Cummins 1974; Wallace and Webster 1996; Covich et al. 1999). The aquatic insect assemblage in Canyonlands National park is large (49 taxa from 13 orders), diverse and dominated by filterer and collector organisms such as Ephemeroptera (mayflies) and Trichoptera (caddis flies) that make their living by filtering bits of detritus and coarse particulate organic matter out of the water column (Haden et al. 2003). It is particularly significant that such a high diversity of Ephemeroptera, Plecoptera and Trichoptera were found in the upper watershed, as these species are often very sensitive to sedimentation and their presence is used as an indicator of good water quality (Cummins 1974). The fact that the hydrograph and sediment loads through Canyonlands National Park are virtually unchanged from historic data indicates that the pre-dam food base was likely far richer and more abundant than previously thought. Page 3 of 20 S.E. Purdy 9/8/05 Figure 1. Map of the Colorado River through Grand Canyon National Park. Boxes show river ecosystem energy shifting from autochthonous sources in the Clearwater segment to largely allochthonous sources in the turbid downstream reaches. Dashed lines without arrows in the boxes indicate weak trophic linkages. (Stevens et al. 1998). The 26-km reach from Lee’s Ferry to the mouth of its first major tributary, the Paria River has been transformed into a very different system by the presence of Glen Canyon Dam. This stretch of the river has very high aquatic plant productivity due to the newfound clarity of the water, its consistent temperature and the nutrient-rich inputs from Lake Powell reservoir. A cold, clear stream with a stable substrate is the ideal condition for the prolific growth of algae (Blinn et al. 1998). The system along this reach is dominated by the alga species, Cladophora glomerata (Shannon et al. 2001) but densities of aquatic macrophytes such as Chara contraria, Potamogeton pectinatus, and Elodea species have been increasing since 1993 (McKinney et al. 1996, 1997). This thick carpet of vegetation on the scoured river bottom is the basis of an entirely new food web at the base of Glen Canyon Dam. Cladophora hosts epiphytic diatoms that are the preferred food for many macroinvertebrates (Blinn and Cole 1991; Shannon et al. 1994). This clear water habitat has created a community of aquatic organisms that would ordinarily be found in nearctic regions and cannot tolerate sediment laden water as was historically present (See Table 1). Caddisfly and stonefly shredders presumably present in the Page 4 of 20 S.E. Purdy 9/8/05 pre-dam Colorado are absent in the tail waters of the Glen Canyon Dam, largely due to the low input of allochthonous carbon (Pomeroy et al. 2000). Grazers that feed on the plentiful algae, epiphytic diatoms and macrophytes dominate the new species assemblage. These plants are plentiful in the reach at Lee’s Ferry because of increased clarity in the water, consistent temperatures and lack of sediment scour (Stevens et al. 1997b, Shannon et al. 2001, Haden et al. 2003). While the aquatic food base is very different than it once was, most native fish are opportunistic omnivores and consume the new species when they are present (Minckley 1991).

Load more