Freshwater Mussel and Water Quality: a Review of the Effects of Hydrologic and Instream Habitat Alterations
Proceedings of the First Freshwater Mollusk Conservation Society Symposium, 1999, pages 261-274 © 2000 Ohio Biological Survey Freshwater mussels and water quality: A review of the effects of hydrologic and instream habitat alterations G. Thomas Watters1 Ohio Biological Survey and Aquatic Ecology Laboratory; 1315 Kinnear Road, The Ohio State University, Columbus, OH 43212 ABSTRACT: Hydraulic impacts represent a suite of habitat alterations that, although having different causes, often have similar methods of affecting the mussel fauna.
For instance, logging and channelization are very different disturbances, but both generate sediments. These ? hydraulic impacts? thus overlap each other to one degree or another. I have attempted to break them down into categories based on the type of disturbance, but what applies to 1 impact often may apply to others. By far, there is more published information on the effects of impoundments than on all other hydrologic impacts combined, and this review is dominated by that subject. Other subjects are not covered in any detail because they are too infrequent or ancillary to North American mussel conservation.
For example, log runs in Finland are known to damage mussel populations (Valovirta 1990), but this is probably not a widespread problem. Keywords: freshwater mussels, water quality, impoundments, hydrology, habitat Impoundments Perhaps mankind? s earliest attempt to manipulate freeflowing water was the dam. Dams could be used to divert water to mills and turbines, where its seemingly limitless power ground grain, cut lumber, and later generated electricity, generally freeing humanity to toil elsewhere. Dams could be used to divert water to irrigate ground that would not otherwise support crops.
They could, in theory, alleviate flooding if the amount of water passing through the dam could be regulated. Dams could make a shallow river deep, allowing watercraft to operate. Impounded rivers could act as reservoirs for holding water to support the populace. Pristine natural areas could be turned into a recreational goldmine through impoundment. Real, perceived, or pork barrel, there were many reasons to dam rivers. It must be stressed that an artificial impoundment is not analogous to a naturally occurring pool within a river. Impoundments typically become deeper toward their downstream end, until they abut the dam.
In contrast, natural pools are deepest toward their middle, then becoming shallow, forming runs and riffles. This results in a very different water flow pattern through the pool/impoundment, and subsequently downstream. These hydrologic differences result in faunal differences. For example, van der Schalie (1938) found 15 mussel species in Lake Cooper, a man-made 3 impoundment on the Mississippi River. In adjacent Lake Pepin, a naturally-formed pool, 30 species were encountered. The general impact of impoundments on existing aquatic habitats was reviewed by many authors.
Yeager? s (1993, 1994) reviews are particularly thorough. Ellis (1942) gave an early review of the biotic and abiotic effects of impounding a river. He noted such deleterious consequences as silt accumulation, loss of shallow water habitat, stagnation, accumulation of pollutants, and nutrient-poor water. He concluded that ? the initial period of high productivity may be very short in some reservoirs, and longer in others but the decline will inevitably come unless man makes some adjustments.? Baxter (1977) gave an excellent review drawing from examples worldwide.
The author characterized impoundments as a distinct type of ecosystem, characterized by complicated flow patterns that may involve hypolimnion discharge, long periods of flooding, and heavy sediment loads. Downstream areas may be affected as well, particularly by flow regime. Baxter sagely noted that ? since not all the hydraulic head of the world? s rivers has yet been utilized, it seems likely that more remain to be built.? Neel (1963) gave a somewhat lopsided review of reservoirs, where they were largely viewed as desirable features. However, he ? romantically? oted that fish migration was blocked, with ? frustrated piscine migrants, monotonously working their way, time after time, toward the overpowering jets.? Although the benthic fauna was ? adversely affected,? he speculated (in error) that ? no certainty exists that missing species 261 For correspondence contact G. T. Watters (Email: watters. [email protected] edu) have become extinct or cannot return.? Despite Neel? s (and others) claims, the evidence that impoundment is detrimental to aquatic life, and most mussels in particular, is overwhelming and indisputable. Even below mpoundments benthic diversity in general is reduced (Yeager 1993, 1994). We know that perhaps several dozen mussel species, and numerous more freshwater snails, were driven to extinction wholly or in large part by the construction of dams (Layzer et al. 1993, Lydeard and Mayden 1995, Stansbery 1973). Almost without exception, rivers that have been impounded, have lost or changed their mussel faunas. Examples A. Tennessee River: Isom (1969) reported that the Tennessee River? s mussel diversity had decreased from 100 species to 44, largely because of changes associated with dams.
Although caused in part by over-harvesting, the author attributed most of the decline to changes in habitat associated with the dam. Water flow in the impoundment had decreased to the point where silt could accumulate on the river bed, smothering mussels. Coincidentally, silt-tolerant species were expanding their ranges in the river. In the Fort Loudoun Reservoir on the Tennessee River, Isom (1971) found a drastic decline in mussel diversity post-impoundment. A survey in 1970 of the reservoir found four mussel species. Prior to its impoundment, Ortmann (1918) reported 64 species from the same general area.
In addition, the 1970 survey found mussels primarily on flooded preimpoundment land (overbank), not in the original river channel habitat. This probably was due to critically depleted oxygen levels in the channel. Elsewhere, during construction of the Nickajack Dam, the Tennessee River was dewatered at the construction site (Isom 1972). Seventeen species were found. Ortmann (1925) had found 25, for a cumulative total of 33 species reported for this reach. Missing in the latter survey were many endangered and rare species. Whether this was due to the presence of the Hales Bar Dam, 6. miles upstream, or other causes was not known. It may be argued that mussel faunal composition changes over time whether dams are built or not. However, Parmalee et al. (1982) documented a fauna that remained essentially unchanged for several millennia until impounded by a dam. The Chickamauga Reservoir of the Tennessee River supported 46 species for perhaps 2,000 years prior to impoundment. After impoundment, 28 species were 262 extirpated, and several are now extinct. Five species, mainly soft-substrate tolerant taxa, have invaded the reservoir.
Four original species that survived the impoundment also have increased in abundance. Mussel diversity has declined from 64 species to 30 in the upper Chickamauga Reservoir of the Tennessee River (Ahlstedt and McDonough 1994). Although relict populations of four federally endangered mussels still occurred there, based on pre-impoundment shell middens, two of these were once the most abundant species in the river reach. In the Tennessee River portion of Wheeler Reservoir, 19 of the historical 38 species were either absent or present only as relicts (non-reproducing individuals) (Ahlstedt and McDonough, in press).
Thick-shelled species may have been severely affected during a 6-y drought in the 1980? s. Overall abundance had declined from an estimated 39 million mussels in 1960, to 14 million in 1991. B. Cumberland River: At least 37 of the 60 preimpoundment species of the Caney Fork River of the Cumberland River have been extirpated (Layzer et al. 1993). Two are extinct. The authors attributed this, in large part, to the presence of the Center Hill Dam. No living mussels were found for 12 km below the dam, which they attributed to the discharge of cold hypolimnetic water, periodic scouring, and dewatering.
Near-anoxic conditions occasionally occurred in the metalimnion and hypolimnion of the impoundment. This acted as a barrier to fish movement across the impoundment, isolating mussel and fish populations in tributaries. Sedimentation of the impoundment also was taking place. Schmidt (1986), in a separate survey, found 36 mussel species in the Caney Fork River prior to impoundment. By this estimate, the construction of the Center Hill hydroelectric dam resulted in the loss of 78% of those species.
The author concluded that the extirpated species could not adapt to the widely fluctuating daily flows, cold water discharges, and loss of nutrients. In ? Lake? Barkley on the Cumberland River, 64% of the pre-impoundment mussel fauna was lost as a consequence of impoundment (Blalock and Sickel 1996). Prior to impoundment, 25 mussel species were reported from this reach. This study found only nine species 29 years after completion of the reservoir. However, 15 of the original species still occurred below the dam. Species had either invaded the impoundment, or once insignificant ones had became ominant, including anodontines and species of Quadrula. The authors attributed the overall decline, in part, to the accumulation of sediments, anoxic conditions in the impoundment, and loss of fish hosts. In the lowest reach of the Cumberland River, only 25 of the 45 pre-impoundment mussel species remained (Sickel and Chandler 1996). Several new species had invaded, probably from the impounded Ohio River. C. Little Tennessee River: Only 6 of the original 50 mussel species at Tellico Lake of the Little Tennessee River remained after impoundment (Parmalee and Hughes 1993).
Although diversity had declined before completion of the dam, additional species were lost when they were impounded. By 1972, this diversity had dropped to 18 species. Twelve years after impoundment and the formation of Tellico Lake, only 6 of the original species still lived there. However, another 8 species were encountered for the first time. These species were apparently able to colonize the extensive sand/mud/silt substrate of the new embayments and overbanks. D. Kaskaskia River: Forty mussel species were recorded from the Kaskaskia River prior to impoundment.
Approximately 8 years after impoundment, only 24 species were found (Suloway et al. 1981). Some sites no longer supported any mussels, and overall density had declined. Once clean sandgravel substrates were overlain with silt and debris caused by bank erosion, substrate instability, and runoff. Changes in host abundance also may have occurred. E. Tombigbee River: The once highly diverse mussel fauna of the Tombigbee River was nearly obliterated by the construction of the Tenn-Tom Waterway and associated dams (Williams et al. 1992).
Comparisons with unimpounded and pre-impounded areas revealed that perhaps two-thirds of the fauna were eliminated in impounded areas by loss of necessary habitat due to decreased water flow, increased depth, and sedimentation. These and other case studies reveal clear-cut patterns and trends in the change associated with converting a free-flowing riverine habitat into an impoundment. Clearly, impoundments interfere with the basic ecological processes of free-flowing systems (Sparks 1995). Most indigenous mussel species are extirpated from the impounded region.
These may be replaced by soft-substrate adapted species, such as anodontines and heelsplitters. The reason for this lies in the nature of the habitat modifications caused by impoundment. 263 Mussel habitat in impoundments A free-flowing river has great habitat heterogeneity: riffles, runs, pools, shoals, water-willow stands, and meanders, often with considerable tree canopy. After impoundment, 3 – 4 habitats remain, usually with no tree canopy. These are either new habitats or highly modified existing ones (Bates 1962, Blalock and Sickel 1996). These remaining habitats are discussed below.
Original channel The first is the original channel, which remains intact but under deeper water. Studies demonstrate that mussels, in general, are most abundant in shallow water, with relatively few species able to tolerate impoundment depths (Haukioja and Hakala 1974, Lewandowski and Stanczykowska 1975). Haag and Thorpe (1991) examined the relationship between depth and substrate type on benthic invertebrates at a site approximately 1. 5 km downstream of Kentucky Lock and Dam on the Tennessee River. Although mussel abundance was not correlated with substrate type, abundance decreased with depth.
As water velocity decreases, water loses its ability to carry sediment. The old river channel effectively becomes a sediment ? trap? , eventually smothering mussels that cannot adapt to soft substrates (Isom 1969). This is particularly true near the upstream side of the dam, where the substrate may be composed of mud mixed with debris and rubbish (Clark and Gillette 1911). Ellis (1936) summarized the effects of silt and sediment on the aquatic habitat in general, and on mussels in particular. Silt resulted in the loss of light penetration, causing diminished algal abundance, an important food of mussels.
Thermal changes also occurred, specifically the creation of an annual lag in the cooling and warming of the hypolimnion. Silt caused organic material to be retained on the bottom, leading to oxygen depletion. And finally, silt smothered the benthic fauna. Ellis (1936) demonstrated this using over 2,000 mussels of 18 species in an artificial stream to which silt was added. A silt accumulation of 0. 6? 2. 5 cm depth resulted in mortality approaching 90%. Even before extirpation by smothering occurs, recruitment may be diminished or stopped, and growth rates reduced.
Bates (1962), for example, found no evidence of mussel recruitment in the channel of Kentucky Lake, although adult mussels were present. Semenova et al. (1992) compared growth rates of Margaritifera margaritifera (Linnaeus, 1758) between rivers having different habitats, and found that rivers having high levels of suspended solids inhibited mussel growth. Bauer et al. (1980) determined that M. margaritifera juveniles required substrate with low organic content, conditions not met in submerged channels. Buddensiek et al. 1993) showed that juvenile mussels, which live completely buried for several years, required a substrate where water could be freely exchanged between the overlying water column and the interstitial water. These interstices are clogged by silt in impoundment conditions. The hypolimnion of the channel also may become excessively cold, near-anoxic, and nutrient-poor. Cold water ( 25 m), and any new juvenile mussels that may been deposited there earlier in the year, now exposed. Dams and impoundments as barriers to mussels and hosts It was recognized early that dams acted as barriers to fish movement (Coker 1914).
Coker (1914) believed impoundments had both good and bad qualities for mussels, and envisioned the tremendously increased surface area of flooded dry land as potential mussel and fish habitat. But he realized that dams would form effective barriers to fish host dispersal. Hubbs and Pigg (1976) showed that the decline in fish diversity in Oklahoma was due, at least in part, to these barriers. Migrating fishes, in particular, would be affected (Coker 1914, Branson 1974, Unkenholz 1986, Alexander 1987). Some fishes are now rare, due in large part to these migration obstacles (Stuebner 1993).
In some cases, such as the Hells Canyon Complex in Idaho, migration has become impossible for some species of fishes (Collier et al. 1996). For freshwater mussels, this meant that hosts may not be accessible to their glochidial parasites. Otherwise healthy mussel populations would simply grow old and die without recruiting. Such a loss of hosts has been implicated in the decline of mussels in several areas (Burkhead et al. 1992, Jones 1991, Suloway et al. 1981). Watters (1996) showed that dams as low as 1 m in height restricted the distribution of some mussel species.
Because the dams caused minimal habitat disturbance, this effect was probably due to interference with host movements and migrations. Even if not absolutely trapped by dams, the route a fish must take to surmount a dam are often circuitous and, at best, unlikely. Clark and Gillete (1911) noted that as the result of Sullivan? s Dam on the Little River, a fish trying to move upstream would have to backtrack downstream to the Big Arkansas River, enter Chisholm Creek and move upstream until it encountered a cut-off ditch that communicated back to 265 the Little River above the dam. Even then, this ditch ended in a floodgate hat was only open during normal flow. Impoundments themselves, beyond the physical presence of the dam, may act as barriers to hosts as well. Turbid, anoxic water in the main impoundment may isolate mussels and fishes in tributaries (Layzer et al. 1993). Mussel habitat in tailwaters The habitat in the tailwaters is influenced by the dam (Ligon et al. 1995). Depending on the use of the dam, water levels may fluctuate on a regular interval (for hydroelectric purposes) or at random (for flood control). In some areas, water levels may be become shallow enough that thermal buffering is lost, allowing extreme temperatures to occur.
Blinn et al. (1995) reported that substrate subjected to 2-12 h exposures to air required more than 4 mo. to regain a biomass similar to unexposed habitat. Federally endangered mussel species were reported by Neck and Howells (1994) as casualties of scheduled dewatering processes. Riggs and Webb (1956) reported that several thousand mussels died in the tailwaters of Lake Texoma, an impoundment of the Red River formed by Denison Dam, when water levels dropped, allowing the water to become excessively warm (>26 °C). This area was exposed for at least 20 days before being again inundated. Exposure to cold air may be equally lethal.
Nagel (1987) believed mussels were more sensitive to cold water during frosts than warm water during temporary droughts, and Blinn et al. (1995) showed that a single overnight exposure to subzero temperatures resulted in at least a 90% loss of invertebrate mass. Valovirta (1990) reported that mussels were killed when water froze to the river bottom. Harman (1974) found few mussels within the first 1734 km below impoundments in the Delaware River system due to cold water discharges. Low temperatures associated with an impoundment hypolimnion were thought to result in slow growth and inhibited reproduction.
This was dramatically shown by Heinricher and Layzer (1999), who transplanted non-reproducing mussels out of a cold water discharge to warmer water where the mussels began to reproduce. Associated with fluctuating water levels are dramatic changes in water velocity. Neck and Howells (1994) noted that high-volume water discharges and abrupt stoppages resulted in a river bed composed of large rocks and shifting sand, habitat inhospitable to most mussels. In the River Schwalm in Germany, Nagel (1992) compared growth rates of three mussel species (Unio pictorum, Pseudanodonta complanata, Anodonta iscinalis) along the length of the river. Little change in growth rate was associated with either cool- or warm-water discharges. More important were differences in water flow velocity. A similar result was found for the declining mussel population in the Licking River of Kentucky (McMurray et al. , 1999). The authors believed that spikes of cold hypolimnionic discharge from the Cave Run Lake dam was more important to the mussel fauna than were changes in long-term discharges. Scour immediately below dams also may preclude colonization by mussels (Miller and Payne 1992).
These authors characterized a large mussel bed below a dam by density, recruitment rates, and demography. Compared with earlier surveys, there was little indication that this bed had changed as the result of the presence of the dam, except the area closest to the dam. That area experienced greater scour and erosion than downstream and had less mussel diversity. However, in other rivers this unstable zone may be extensive. In Texas, the Possum Kingdom Reservoir on the Brazos River exhibited unstable substrate for 150 km below the dam (Yeager 1993).
These areas of erosion may move downstream at a rate of up to tens of km yr-1 (Fedorov 1969) until water velocity falls below the threshold necessary to transport sediments (Yeager 1993). Beyond the area of scour sedimentation below dams may be extreme. The Rio Grande at El Paso, below the Elephant Butte Dam, accumulated nearly 4 m of sediment in 26 y (Reinhardt 1937). This sedimentation reduced fish diversity and abundance by altering habitat necessary for spawning and overwintering (Petts 1984, Holland and Huston 1985, Nelson et al. 1987).
Changes in fish faunal composition as the result of impoundments have been demonstrated for the Clinch River in Tennessee (Fitz 1968), the Barren River in Kentucky (Swink and Jacobs 1983), and the Guadeloupe River in Texas (Edwards 1978). Clearly, silt deposited in the tailwaters may smother mussels. But Chutter (1969) showed that even sublethal amounts of silt and sand rendered the tailwaters inhospitable. In case studies on aquatic invertebrates in South Africa, he found that increases in the amount of sand and silt below dams caused instability of the river substrate.
Therefore, it was not necessary to smother the substrate before deleterious changes in habitat occurred. Neck and Howells (1994) proposed this as one cause (of several) for the decline of the imperiled Potamilus amphichaenus (Frierson, 1898). Dikes and levees The overall biotic and abiotic impacts of dikes and levees were summarized by Pennington and Shields 266 (1993). In otherwise habitat-homogeneous impounded rivers, these structures may offer a small measure of habitat diversity for some macroinvertebrates (Beckett et al. 983, Burress et al. 1982). However, dikes and levees often trap fine-grained sediments. These areas would be colonized by soft-substrate adapted mussels. The rip-rap often used with these structures would not be suitable for colonization by mussels (Libois and Hallet-Libois 1987). Upland reservoirs and changes in the water table There is little information on the effects of upland reservoirs on mussel communities. Hauck and Edson (1976) discussed some consequences on fishes of upland storage ponds used for hydroelectric generation.
Some of these effects influence mussels, directly or indirectly. Pumping cycles produce fluctuating water levels that adversely affect mussels. Hosts also may be entrained incidentally by water intakes. Land use practices Logging, mining, construction, farming, livestock, and a host of other land uses often adversely affect mussel populations, generally by releasing runoff of sediments, salt, and other pollutants into the stream, as well as an increased volume of water (Allan and Flecker 1993, Osborne and Kovacic 1993, Patric and Aubertin 1977).
Houp (1993) documented that an otherwise pristine Wild River segment of the Red River in eastern Kentucky, was experiencing a decline in mussels due to sedimentation from mining, streamrelocation, logging, and farming, although these occurred miles upstream of the effected area. The result was a changing mussel community composition favoring more tolerant and less habitat-specific species. A similar condition was reported for the Buttahatchee River, Mississippi (Jones 1991), where mussel surveys in 1990 demonstrated pronounced declines in several areas since 1977.
These declines were attributed to impoundment associated with the Tenn-Tom Waterway, increased turbidity from an abandoned kaolin strip mine, runoff from logging areas, and sand and gravel mines. The substrate was unstable at several sites, perhaps due to gravel mining. Bank erosion had resulted from the removal of the riparian corridor through logging or agriculture. Mussel populations in the Powell River, Virginia based on early surveys of the 1900? s, were degraded by land mismanagement (Wolcott and Neves 1994). Runoff from coal mines, abandoned mine lands, and wastewater treatment plant effluents all had contributed to the decline.
Although mussels still lived at most sites, virtually no recruitment was found. Woodward (1990) noted several changes in land use in Scotland that harmed existing mussel populations: strip mines and peat extraction contributed to sediment and acid water runoff; fish farms discharged waste; and improvements in the road system allowing easier access to streams not only to pearl fishers, but to tourists, skiers, divers, and hikers, which were ? counter-productive due to their increasing presence through ready access resulting in the destruction of the natural environment which drew them to the area in the first place.?
Proximity of streams to roads may be detrimental by increasing the amount of salt, heavy metals, petroleum products, and other pollutants washed into the system (Van Hassel et al. 1980, Winger et al. 1985). With runoff, changes in host composition may occur as well (Boschung and O? Neil 1981, Berkman and Rabeni 1987, Houp 1993, Rutherford et al. 1992). Finally, sediments from channelization and other sources of runoff erode mussel shells rendering them susceptible to shelldissolving acids (Harmon 1974). Where recognized, erosional problems often require bank stabilization practices.
Libois and Hallet-Libois (1987) examined four mussel species in the River Meuse in Belgium during a maintenance dewatering. The banks of the river varied from several types of natural substrate to man-made reinforcements: riprap, gabions, concrete, and open stone pitching. Mussel density was highest in the natural habitats of mud, sand, and fine gravel, and lowest by several orders of magnitude on man-made reinforcements. They concluded that mussel density was significantly lower in the ? stabilized? areas than in unaltered habitat. Land mismanagement need not be confined to physical destruction of the riparian corridor.
In an age when everyone wants stream-front property with a green lawn to the water? s edge, obvious habitat alterations become apparent. Morris and Corkum (1996) studied differences in mussel community composition between a forested riparian corridor and one composed of grass. They found greater temperature fluctuations and higher ammonia and nitrogen concentrations at the grassy site. Although the two types of sites had the same mussel diversity, the grassy site was dominated by an anodontine species. The authors suggested that species had a higher tolerance to the luctuating temperature and different chemical composition found there. Watercraft Watercraft, particularly large vessels such as tugs pushing barges, cause changes in habitat, and may affect mussels in and adjacent to navigation channels. Water turbulence increases as vessels pass by, creating short-lived surges. These surges stir-up 267 bottom sediments, and increase the amount of suspended solids for undetermined amounts of time. Muddy plumes behind tugs are not uncommon, indicating that bottom sediments have been forcibly removed from the channel and suspended. Apocryphal reports exist of mussels being lifted from the bottom into tug propellers. ) Wake impinging on the shoreline may escalate erosion and introduce additional suspended solids into the water. It is believed that these factors interfere with mussel respiration and/or feeding, and may result in diminished health in mussels in these areas. Aldridge et al. (1987) studied the effect of propellerinduced turbulence in controlled laboratory conditions by exposing mussels to varying levels of suspended solids and periods of turbulence and measuring physiological rates.
They concluded that frequent turbulence (once every 0. 5 h) and high concentrations of suspended solids (600-750 mg L-1) changed the physiological energetics of the mussels by significantly decreasing food clearance rates and oxygen uptake, and nitrogen elimination. However, Miller and Payne (1995), in an in situ study, investigated the possible effects of navigation traffic on mussel beds in the Ohio River. They concluded that although there were changes in water velocity associated with passage, the duration was too short and the velocity change too small to affect mussels.
Clearly, more work needs to be done on this important aspect of mussel conservation and management. Trampling There is very little literature addressing the issue of trampling on mussels. By this I mean the physical destruction to individual mussels caused by being crushed by people, animals, and machinery. As early as the 1700? s, Scottish and German pearl fishers realized that mussels downstream of fords often had more pearls than in other places (Kunz and Stevenson 1908). This was due, no doubt, to the increased sediments (that became the pearl nuclei) generated by horses and wagons crossing the fords.
In areas where livestock have access to a waterway, crushed mussels, broken shells, and deformed living individuals are not uncommon. Similarly, fords across streams are often rendered devoid of mussels. In areas where waterways have a firm substrate, the practice of driving offroad vehicles in the river may be a chronic problem. However these usually represent fairly insignificant nuisances that act on small areas. More serious is the problem posed by canoeists and naturalists, who pose a peculiar paradox. Although these people are often supportive and appreciative of atural areas, the portage of canoes and people across riffles and runs must have a deleterious impact on mussels, particularly juveniles. Juveniles and adult mussels live buried in these riffles. Hosts (e. g. , darters, sculpins) must frequent these riffles to be parasitized. I have witnessed up to 40 canoes an hour being dragged across riffles supporting federally endangered species in Big Darby Creek, Ohio. This scenario, played repeatedly at every riffle, may have a substantial effect on mussel populations.
There is no doubt that the presence of hikers, divers, skiers, and tourists may be detrimental to a natural area (Woodward 1990). Denying use of this recreation, however, could have negative political and social repercussions. Channelization, dredging and snagging Channelization, dredging, and snagging, like impoundment, reduce habitat heterogeneity and aquatic diversity (Nelson 1993). Meanders are removed (Simpson et al. 1982) and the riffle-run-pool sequence is disrupted (Keller 1978, Wesche 1985) reducing available fish and mussel habitat.
Circulation patterns and substrate composition are altered (Loar et al. 1980). Macroinvertebrate assemblages and trapped organic matter that form integral parts of the trophic web are eliminated (Cummins et al. 1973, Ebert 1993). Potential mussel hosts may be lost as fish faunal composition changes. Examples of fish faunal changes associated with channelization and dredging were documented in Mississippi (Arner et al. 1976), Ohio (Trautman and Gartman 1974), Illinois (Smith 1968, 1971), and Pennsylvania (Lee 1973). The process of dredging and channelization may be catastrophic.
Most obviously, mussels caught in the dredge path are destroyed. But the effects of channelization are more far-ranging than just the immediate dredge area. Silt and other suspended solids generated by channelization may travel downstream and smother, or otherwise adversely affect mussels. Instream dredging for minerals has similar effects. Marking (1979) buried 3 mussel species to depths of 25 cm to determine how much smothering they could survive. Fifty percent of Lampsilis cardium (Rafinesque, 1820) and Lampsilis radiata (Gmelin, 1791) individuals were able to extricate themselves from up to 17. cm in depth. Fifty percent of Fusconaia flava (Rafinesque, 1820) individuals were able to extricate themselves from up to 10 cm in depth. But Ellis (1942), working with over 2,000 mussels of 18 species, reported that 90% mortality occurred with 25 mm of siltation. Granted that dredge spoil and silt may be two different things, but the disparity of these results beg for additional 268 tests. Besides smothering, sediments resulting from channelization may resuspend contaminants (Engler 1979), increase concentrations of inorganic plant nutrients, lower photosynthesis (Loar et al. 980), and increase BOD (Ebert 1993). Sediments generated by channelization may eventually erode shells, rendering them more susceptible to shell-dissolving acids (Harman 1974). Full-scale dredging is not necessary to disrupt mussel populations. Valovirta (1990) noted that simply removing large rocks from the channels to facilitate water flow caused the substrate to become unstable, resulting in mussel mortality. In southern Europe, M. margaritifera was found in only 25% of its original range, and many populations were not reproducing (Bauer 1986).
Bauer attributed this decline largely to eutrophication through pollution, but several cases were given of channelization adversely affecting mussel populations. Although there is not a large body of literature on the impact of channelization to mussels, channelization has been implicated in mussel declines (Grace and Buchanan 1981, Hartfield 1993, Schuster et al. 1989, Valovirta 1990, Yokley and Gooch 1976). As the forces of nature attempt to reclaim these altered habitats, it becomes necessary to perform maintenance to preserve the existing modifications.
Channels dredged for navigation or flood control quickly begin to refill, requiring a periodic re-dredging to ensure a minimum depth. Impoundments are dredged to maintain storage capacity. Dredge spoil and associated contaminants disposed of in upland areas may inevitably re-enter the river through surface runoff, biological uptake and cycling, and leaching into groundwater (Gambrell et al. 1978). The problems associated with channelization thus become chronic, resulting in long-term ecological changes. Channelization may result in an increase in softsubstrate adapted mussels.
Increasing the depth of channels may lower water velocity, causing sedimentation to take place (Hubbard et al. 1993), allowing these mussels to colonize the new habitat. On one occasion this increased the population of an endangered mussel species. The federally endangered Potamilus capax (Green, 1832) had colonized portions of the severely channelized St. Francis River flood-control channels in Arkansas (Ahlstedt and Jenkinson 1987). The modification had resulted in an apparently optimum habitat for that soft-substrate tolerant species.
Many of these mussels were relocated when re-channelization became necessary. The Hocking River in Ohio, channelized for flood control, was colonized by several otherwise uncommon species, including a then state endangered species (Watters 1988). These were all soft-substrate adapted species. It must be emphasized that these are unusual cases and that despite their rarity, these new mussel faunas flourished at the expense of the original faunas. Furthermore, most dredged areas are not quickly colonized by mussels of any kind. For example, Grace and Buchanan (1981) found no mussels in an area dredged 15 y earlier.
Snagging is a common practice to alleviate perceived ? flooding? (i. e. , preventing a river from inundating its natural floodplain). Fallen trees and debris are removed by dragging them from the stream bed. This action inevitably reduces the available habitat by creating a more homogeneous environment (Marzolf 1978). Habitat heterogeneity is important to fish diversity, and therefore mussel diversity. Snagging increases bank erosion (Hubbard et al. 1993) and creates unstable substrates as the stream recovers (Cobb and Kaufman 1993).
Channelization and snagging may actually increase flood heights (Belt 1975). This is due, in part, to a reduction of stream length and increased gradient (Hubbard et al. 1993). (One stretch of the Rio Grande, for instance, was shortened from 155 miles to 88 miles (Mueller 1975)). These amplified flood events create additional runoff, and additional remediation. Headcutting Headcuts are regions of disturbance moving upstream, in a zipper-like fashion, as the result of the upper boundary of the modification collapsing. Headcuts may move miles upstream, destroying habitat and mussels as they pass.
Headcutting is a form of channel modification that has operated undetected on many rivers and streams (Hartfield 1993). Instream modifications such as dredging obviously affect the mussels in the immediate area, as well as those downstream. But sometimes, mussels above the modification become extirpated as well. Canals To our ancestors, traveling on rivers was much easier than traveling by land, as long as the river went where you wanted it to go. It quickly became apparent that divine provenance had not placed rivers where they should be, an oversight humanity attempted to rectify.
By digging canals between rivers and lakes, watercraft could move great distances without laborious portage. No thought was given to the possibility that connecting two different river systems possessing different faunas might not be a desirable goal. 269 Nevertheless, once connected, we know that fishes and mussels populated these canals and certainly must have moved between systems. What effect this had on mussel and host distributions cannot be ascertained; the event is too distant in time with sketchy baseline data for comparison.
While it might be assumed that these canals were sediment laden, supporting only anodontines and other soft-substrate tolerant species, this was not always the case. Higgins (1858) described the Columbus [Ohio] feeder canal of the Ohio Canal: ? Many species have traversed the whole length of the canal, and many species there thrive and become abundant which are quite rare in the adjacent rivers.? Records at the Carnegie Museum of Natural History, Pittsburgh, also document now endangered species living in these canals. Most of these canals are now abandoned and no longer maintain connections between rivers.
However, in Texas, connections recently have been made between the Red and Trinity Rivers. Plans also are being considered to move water from the Neches or Sabine River, across the Trinity, Brazos, and Colorado River systems, to the Lavaca-Navidad, to be removed downstream for transfer to Corpus Christi (Howells in lit. 1997). Summary Hydraulic impacts on freshwater mussel habitats are often catastrophic, both immediately and over time. Most such impacts involve complicated interrelated actions; rarely is there a single causative agent for mussel declines.
For example, although impoundments may lead to the immediate smothering of some mussels, many effects may take years to become apparent: changes in seasonal temperatures within the impoundment, isolation from necessary hosts, changes in component fauna, etc. The data clearly support the fact that impoundments, dredging, snagging, channelization, and other improvements once taken for granted may have longterm detrimental effects on freshwater mussels. In many cases these changes appear to be irreversible. Dozens of freshwater mussels and snails have become extinct within the past 200 years as the result of these practices.
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