Abstract
Varying degrees of connectivity between side channels and the main river channel are vital for sustainable ecological processes and functions for both aquatic and terrestrial communities. Within the Mississippi River, restoring side channel functional diversity is a top priority for many natural resource agencies. Buffalo Chute, located at river kilometer 41, is one of the several side channels, island complexes that becomes isolated from the main channel during low river stages leading to thermal and chemical stratifications and anoxic conditions. The purpose of this study was to better understand the impacts of side-channel isolation in the middle Mississippi River (MMR) by measuring fish community and water quality characteristics within an isolated waterbody. Therefore, we selected Buffalo Chute a representative side channel of the MMR to document water quality and fish community characteristics. Water quality measurements were summarized for the 2-year study. Thermal and chemical stratifications coupled with high water temperatures and anoxic conditions were observed in both years during summer. Oxygen reduction potential was lower in summer than winter, which could be attributed to excessive growth of microorganisms and increased biological oxygen demand. Specific conductivity, total phosphorus, total nitrogen, and chlorophyll a were higher in summer compared to winter. Differences in water quality characteristics may have resulted in lower mean fish species richness, diversity, and evenness observed during the following winter. We surveyed 45 sites over 2 years using multiple gears to assess fish assemblage characteristics. The data collected in Buffalo Chute provide some insight into how isolated side channels function during low flow periods.
Introduction
The Mississippi River is a dynamic system with a fluctuating hydrologic regime. The river retains natural processes, but many of these processes have been altered due to anthropogenic activities that have degraded the floodplain and backwater habitats. The construction and maintenance of river channel training structures (e.g., dikes, levees, weirs, closing structures, and revetments) and locks and dams have straightened, narrowed, and deepened the main channel for navigational purposes (Southall and Hubert Citation1984; Beckett and Pennington Citation1986; Grubaugh and Anderson Citation1988; Sparks Citation1995). Training structures have fixed the river into its current configuration, increased sedimentation, and reduced river width, which has led to loss of side channel and backwater habitats (Simons et al. Citation1975; Theiling Citation1999).
The Mississippi River is artificially divided into the upper Mississippi River [(UMR), located between the headwaters of the Mississippi River near Lake Itasca, Minnesota, and the confluence of the Ohio River near Cairo, Illinois; approximately 1394 river kilometers (rkm)] and the lower Mississippi River (from the confluence of the Ohio River to the Gulf of Mexico near New Orleans, Louisiana; approximately rkm 1534). The UMR north of St. Louis, Missouri, is characterized by pools created behind the installation of a lock and dam system, while the UMR south of St. Louis lacks locks and dams, but the planform and natural processes have been strongly influenced by wing dikes, closing structures, and levees. This lower portion of the UMR still contains a variety of habitat types (e.g., main and side channels, channel borders with and without dikes, island or point bars, and intermittent wetlands) and is routinely defined as the middle Mississippi River (MMR; the stretch of the UMR between its confluence with the Missouri River near St. Louis, Missouri, and the Ohio River; approximately rkm 323).
The MMR still contains many of the aforementioned habitat types (including 23 side channels) in addition to remnant off-channel and floodplain habitats that have been cut off or isolated through continued modification and sedimentation (Sparks Citation1995). Off-channel and floodplain habitats have been severely fragmented in the MMR because of the construction and maintenance of levees and training structures and the closure of secondary channels by dikes. Off-channel habitats (e.g., side channels, sloughs, and backwaters) are areas having reduced current velocity compared to the main channel and providing necessary physical conditions required for the survival of many aquatic and terrestrial organisms (Sheaffer and Nickum Citation1986; Gore and Shields Citation1995; Sparks Citation1995; Zigler et al. Citation1999). Many riverine fish species [(e.g., the federally endangered pallid sturgeon (Scaphirhynchus albus) and the commercially important paddlefish (Polyodon spathula)] benefit from these slow current velocity habitats, using them for reproduction, foraging, overwintering, and refuges from intolerable conditions (Sheaffer and Nickum Citation1986; Barko and Herzog Citation2003; Zigler et al. Citation2003). The preservation or restoration of function of these habitats is crucial in sustaining the physical and biological composition of large river processes to a level of sustainable ecological function (Gore and Shields Citation1995). Understanding current biological status and physical functions of side channels in the MMR is imperative for modeling future restoration efforts on the Mississippi River.
Buffalo Chute is one of several island complexes that become isolated from the main channel during low river stages (approximately 3 m at Price Landing gage; rkm 45.4). This isolation results in a loss of aquatic habitat and ecological function through disconnection and sedimentation (Barko and Herzog Citation2003). The isolation between Buffalo Chute and the main channel typically occurs in intervals beginning in mid-June through early March of the subsequent year. With this disconnection, we assumed the isolated pools stratify and become anoxic during warmer months. Stratification during warmer months (mid-May through September) has been observed in lakes resulting in anoxic conditions and has been shown to be detrimental to fish communities (Gebhart and Summerfelt Citation1978; Gent et al. Citation1995). In addition, exceedingly warm temperatures may surpass the thermal thresholds of many fishes leading to declines in species number (Todd et al. Citation2008). Furthermore, aquatic communities are shaped by water quality characteristics since they are sensitive to changes in many environmental factors (Karr Citation1981; Linam and Kleinsasser Citation1996). Despite the apparent relevance, there is a lack of knowledge with respect to the impacts of side-channel disconnection on water quality and fish community characteristics, especially for the MMR. Therefore, we evaluated Buffalo Chute during a disconnected situation to better understand these characteristics.
Materials and methods
Buffalo Chute is located on the right descending bank of the Mississippi River at rkm 41 to 38 in the MMR. The side channel averages 97.5 m (±3.2 m) wide, ranging from 73 to 183 m. Bathymetry indicates the average bottom elevation is approximately +0.61 m Low Water Reference Plane (LWRP) and ranges from −7.62 to +6.10 m LWRP. Substrate is predominantly sands and silts. Revetment occurs along the right descending bank of the side channel from rkm 40.2 to the downstream mouth of the chute at rkm 38.6. A rock dike is located at the upstream end of the side channel at rkm 42.0 (Dike 26.1R), and two rock closing structures are located near the lower end (Figure 1). An internal closing structure (Dike 24.8R) is located at rkm 39.9, 0.48 km from the lower end of the side channel. Another, unnumbered closing structure parallels the main channel from the downstream tip of Buffalo Island to the right descending bank of the main channel. This unnumbered closing structure is at the extreme downstream extension of the side channel and has been buried under sand for many years.
Published online:
23 February 2012Figure 1. Map of Buffalo Chute, located at rkm 41, in the MMR, depicting winter and summer fish and water quality sampling sites from 2005 through 2006.
Figure 1. Map of Buffalo Chute, located at rkm 41, in the MMR, depicting winter and summer fish and water quality sampling sites from 2005 through 2006.
The fish community and water quality were sampled for 2 years (2005 and 2006) during summer (August) and winter (December). Sampling was initiated approximately 2 weeks after the side channel became isolated from the main channel and when the water temperature reached ≥20°C (defining the summer sampling period) or decreased to ≤10°C (defining the winter sampling period).
Water quality sampling used a stratified random sampling design. The stratification of sampling sites occurred spatially (upper, middle, and lower portions of the chute) to incorporate the entire chute and was associated with relatively deep scour holes in these locations. At each water quality sampling site, Hydrolab DS 5X Datasondes™ were deployed to continuously record specific water quality parameters. The parameters measured at the surface and bottom of each site were water temperature, oxygen reduction potential (ORP), dissolved oxygen, pH, and specific conductance. In addition, surface and bottom samples were taken using a Van Dorn vertical sampler following the methods of Lind (Citation1979) and Wetzel and Likens (Citation1991) for total Kjeldahl nitrogen, total phosphorus, and nitrate–nitrite nitrogen. Chlorophyll a was also measured using an extraction method 10200H (Eaton et al. Citation1995). In Buffalo Chute, summer water quality logging began on 1 August 2005 and again on 5 August 2006; it was discontinued on 22 August 2005 and 26 August 2006. A total of 18 water chemical measurements was taken each season. Winter water quality logging began on 3 December 2005 and again on 1 December 2006 and was discontinued on 24 December 2005 and 22 December 2006. A total of 18 water chemical measurements was taken each season. Water quality was summarized (means, standard error, and minimum and maximum values) by combining data from both years by season.
In both study years, the fish community was sampled using a multiple gear approach, which included seining, mini-Missouri trawling, and day electrofishing (see Gutreuter et al. (Citation1995) and Herzog et al. (Citation2009) for gear descriptions). Sample sites were randomly selected for each gear type independently using numbered 200 m grids demarcated by Universal Transverse Mercator coordinates superimposed over the sampling area in ArcGIS 9.0. The minimum effort by gear type was three sites (i.e., three seining sites, three mini-Missouri trawl, and 3-day electrofishing sites) resulting in a minimum of nine sites sampled each season each year.
Seining was conducted downstream parallel to the shoreline in water not exceeding 1.2 m deep. Seining occurred in separate hauls and when combined, totaled 5 min of effort. A single seine haul was approximately 30 s long, but varied based on the ability to effectively sample the site (e.g., debris and snags required shorter hauls). Trawling was used to capture small-bodied benthic fishes in areas where seining was not possible. The unit of effort for mini-Missouri trawling was two 3 min hauls parallel to the shoreline or to a length of 200 m. One haul was conducted in the near-shore side of the random site, while the second haul was conducted in the mid-channel side. When sample cells included both bank lines, the near-shore sample site was determined based on the ability to effectively sample the site (e.g., avoiding the presence of abundant snags and revetment). Standardized day electrofishing (MBS-1D Boat Electrofishing Unit (Pulsed DC) with pulse rate 60 and duty cycle 25) was conducted where water depth ranged from 0.5 to 3.0 m deep. Each shoreline and all structures within the 200 m2 random site were sampled thoroughly for 15 min.
Species richness was calculated as the number of fish detected per season. To maximize species richness estimates, an annotated list of species captured was maintained after each unit of effort for each gear deployed. If a new species was captured within the third sample site, then a fourth site was randomly selected and sampled. Additional sites were randomly sampled for each gear until no additional species were captured. Species diversity was calculated for each season using Shannon's diversity index, which was selected because it includes proportional representation of species in a community and provides relatively even weighting to both richness and evenness (Barbour et al. Citation1999).
Species evenness was calculated using Camargo's index of evenness (E′; Camargo 1993). This measure is unaffected by species richness and has a range of 0 (species are not equally abundant) to 1 (each species is equally abundant; Smith and Wilson Citation1996; Krebs Citation1999; Stewart et al. Citation2005).
Results
Temperature was highest in summer and was inversely related to dissolved oxygen which was highest in winter (). In both years, water temperature and dissolved oxygen exhibited stratification during summer. Anoxic conditions occurred during both summer surveys. ORP exhibited higher winter readings than summer. In addition, chlorophyll a and conductance were highest in summer. Total phosphorus and total nitrogen values were highest in summer, but nitrate–nitrites were higher in winter than summer.
Table 1. Mean, standard error, and range of water quality parameters measured in Buffalo Chute from August 2005 through December 2006.
Summer fish community surveys were conducted in mid-August in both years and consisted of totaling six seine, seven mini-Missouri trawl, and nine daytime electrofishing samples. During summer, 46 species were captured totaling 7565 individuals (). The most abundant fish species collected included emerald shiner (Notropis atherinoides), channel shiner (Notropis wickliffi), and western mosquitofish (Gambusia affinis), representing 27.4%, 16.0%, and 14.1% of the total catch, respectively. The most common families sampled included Cyprinidae, Poeciliidae, Centrarchidae, and Clupeidae representing 55.6%, 14.1%, 13.8%, and 12.0% of the total catch, respectively. Three Missouri-listed fish (Missouri Natural Heritage Program Citation2010) species of conservation concern were collected during the summer samples which included eight sicklefin chub (Macrhybopsis meeki), 54 silver chub (Macrhybopsis storeriana), and one pugnose minnow (Opsopoeodus emiliae). Species diversity (H′) was 3.4 and species evenness (E′) was 0.28.
Table 2. Total count and relative proportion of fish species captured by family per season using three gear types combined (day electrofishing, mini-Missouri trawling, and seining) for all sites surveyed in Buffalo Chute, a side channel of the MMR, from 2005 through 2006.
Winter surveys were conducted in mid-December for both years totaling six seine, nine mini-Missouri trawl, and eight daytime electrofishing samples. During winter, 32 species were captured totaling 3108 individuals (). The most abundant fish species collected were red shiner (Cyprinella lutrensis), channel shiner, and emerald shiner representing 24.6%, 22.0%, and 20.8% of the total catch, respectively. The sample was dominated by Cyprinidae (82.4% of total species) followed by Centrarchidae at 9.7%. Only one Missouri-listed fish species, silver chub (n = 13) was captured. Species diversity (H′) was 3.1 and species evenness (E′) was 0.22.
Discussion
Chemical and thermal stratifications occurred in Buffalo Chute during summer in both years of the study. Stratification has been documented in summer and fall months in previous studies (Happey Citation1970; Gebhart and Summerfelt Citation1978; Gent et al. Citation1995). These studies were conducted on lakes, reservoirs, or backwaters but are relevant to this work because the disconnection of side channels on the MMR often mimic lake-like conditions. Water quality conditions associated with stratification (i.e., high ammonia and carbon dioxide, and low dissolved oxygen) can have adverse effects on production of benthic invertebrates decreasing a potential food source for fishes (Gebhart and Summerfelt Citation1978). In addition, anoxic conditions, which are considered stressful to fish, were also observed in most of the bottom logging stations throughout the chute during summer. Dissolved oxygen concentrations <5 mg L−1 are considered stressful to fish, and levels less than 1 mg L−1 are lethal (Kelsch and Shields Citation1996). ORP was higher in winter than in summer, which could be attributed to increased biological oxygen demand due to increased primary productivity as indicated by the high chlorophyll a concentrations. Both total phosphorus and total nitrogen were higher in summer than in winter, which could be indicative of the side channel acting as a nutrient sink.
Buffalo Chute becomes isolated from mid-June through March of the subsequent year. To improve conditions within the chute, measures should be taken to decrease the periodicity of isolation from the main channel during typically low flows. Increasing connectivity of Buffalo Chute to the main channel would restore flow, reduce length of isolation, and should reduce chemical and thermal stratifications. This could eliminate anoxic conditions and provide quality off-channel habitat that can be used for foraging, spawning, and nursery areas for native aquatic organisms. Declines in fish relative abundance, species richness, species diversity, and species evenness from summer to winter could be related to a suite of physicochemical factors but are likely the result of winter isolation from the main channel. For example, threadfin shad (Dorosoma petenense) relative abundance declined from summer to winter (5.7–0.4%). This decline may be related to the sensitivity of threadfin shad to low water temperatures (summer ∼25°C, winter ∼6°C). Pflieger (Citation1997) reports massive fish kills may occur for this species when water temperature is below 7.2°C. Despite the decline in relative abundance of threadfin shad from summer to winter in this study, a fish kill was not observed during field sampling. Western mosquitofish also exhibited a seasonal decline in both years from summer to winter (14.1–2.5%). These declines may be explained by sensitivity to fluctuating cold temperature environments (Hubbs Citation1971). The higher relative abundance of western mosquitofish in spring (∼14%) could be related to the high fecundity of the species (Pflieger Citation1997).
Our fish surveys captured few predatory, sport, or commercially targeted fishes in Buffalo Chute. We postulate that poor water quality, in particular low dissolved oxygen, may explain low numbers of these fishes. A fish assemblage study conducted in the Wisconsin River by Coble (Citation1982) concluded that there was a higher percentage of sport fish and centrarchids collected where dissolved oxygen level was above 5 mg L−1. Dissolved oxygen recorded within Buffalo Chute was <5 mg L−1 from mid-summer to winter.
Distribution of fishes may be affected by the physicochemical composition of a waterbody. Linam and Kleinsasser (Citation1996) found in the Pecos River, Texas areas of higher conductivity were dominated by species tolerant to increased salinity levels. In Buffalo Chute, the conductivity was higher in summer, which could be impacting less tolerant fish species and may in part explain lower species richness between the seasons. They also stated that the red shiner is rarely found in clear, flowing streams and can become numerous when competition with other minnows is reduced and flow is decreased. In Buffalo Chute, flow was reduced from mid-June to March, and response of red shiner from summer to winter was similar to that observed in the Pecos River (Linam and Kleinsasser Citation1996). The increase in relative abundance of red shiner from summer (2.5%) to winter (24.6%) could be related to the decrease in flow just prior to the side channel becoming isolated and/or a result of a behavioral response (e.g., overwintering).
Buffalo Chute and other MMR side channels are hydrologically variable systems and are periodically inundated by high Mississippi River flows. Annual variation is important, but based on data collected in this study, seasonal variation may be an even more important factor directing side-channel function and shaping fish communities that use these channels. We found that water quality conditions in Buffalo Chute during isolation from the mainstem river were not conducive to supporting healthy native fish communities. We hypothesize that improving water quality conditions during isolation would result in an increase in fish usage (Gent et al. Citation1995) and species richness in off-channel areas. River restoration efforts have focused more on species-specific problems and little has been done to improve habitat and natural processes. Restoring connectivity (i.e., increasing the duration of connection between the main channel and side channel) is an effective measure to maintain off-channel habitat and restore a temperature regime tolerable to most large river fishes (Stanford et al. Citation1996). Future Mississippi River restoration must encompass both biological and biochemical processes to achieve success.
| Summer | Winter | |||||||
|---|---|---|---|---|---|---|---|---|
| Parameter | Mean | SE | Minimum | Maximum | Mean | SE | Minimum | Maximum |
| Temperature (°C) | 25.4 | 1.9 | 15.3 | 30.6 | 6.3 | 0.6 | 4.7 | 15.4 |
| pH | 7.6 | 0.2 | 6.55 | 8.28 | 8.2 | 0.1 | 7.58 | 8.76 |
| ORP (mV) | 101.1 | 124.7 | 756.1 | 480 | 314.3 | 7.5 | 198.6 | 365.4 |
| Specific conductance (µS cm−1) | 634.9 | 70.3 | 448.1 | 1132 | 572 | 16.7 | 324 | 682 |
| Dissolved oxygen (mg L−1) | 3.6 | 1.2 | 0.0 | 11.6 | 10.5 | 1.7 | 0.0 | 18.5 |
| Chlorophyll a (mg m−3) | 43.58 | 10.5 | 6.7 | 201 | 18.96 | 2.11 | 5.7 | 63.3 |
| Nitrates–nitrites (mg L−1) | 0.04 | 0.01 | 0.022 | 0.14 | 1.63 | 0.04 | 0.028 | 3.0 |
| Total phosphorus (mg L−1) | 0.58 | 0.11 | 0.08 | 1.4 | 0.25 | 0.21 | 0.09 | 0.38 |
| Total nitrogen (mg L−1) | 2.87 | 0.46 | 0.66 | 5.6 | 1.2 | 0.05 | 0.82 | 1.8 |
| Note: ORP, oxygen reduction potential. | ||||||||
| Summer | Winter | |||||
|---|---|---|---|---|---|---|
| Family name | Scientific name | Common name | Count | % | Count | % |
| Acipenseridae | Scaphirhynchus platorynchus | Shovelnose sturgeon | 1 | 0.01 | – | |
| Lepisosteidae | Lepisosteus platostomus | Shortnose gar | 9 | 0.12 | 6 | 0.19 |
| Hiodontidae | Hiodon alosoides | Goldeye | 3 | 0.04 | 4 | 0.13 |
| Clupeidae | Alosa chrysochloris | Skipjack herring | 9 | 0.12 | 1 | 0.03 |
| Dorosoma cepedianum | Gizzard shad | 468 | 6.19 | 66 | 2.12 | |
| Dorosoma petenense | Threadfin shad | 429 | 5.67 | 11 | 0.35 | |
| Cyprinidae | Ctenopharyngodon idella | Grass carp | 1 | 0.01 | – | |
| Cyprinella lutrensis | Red shiner | 185 | 2.45 | 765 | 24.61 | |
| Cyprinella venusta | Blacktail shiner | 17 | 0.22 | 28 | 0.90 | |
| Cyprinus carpio | Common carp | 17 | 0.22 | 37 | 1.19 | |
| Hypophthalmichthys molitrix | Silver carp | 4 | 0.05 | |||
| Macrhybopsis hyostoma | Shoal chub | 397 | 5.25 | 125 | 4.02 | |
| Macrhybopsis meeki | Sicklefin chub | 8 | 0.11 | – | ||
| Macrhybopsis storeriana | Silver chub | 54 | 0.71 | 13 | 0.42 | |
| Notemigonus crysoleucas | Golden shiner | 1 | 0.01 | |||
| Notropis atherinoides | Emerald shiner | 2076 | 27.44 | 647 | 20.82 | |
| Notropis blennius | River shiner | 48 | 0.63 | 145 | 4.67 | |
| Notropis shumardi | Silverband shiner | 3 | 0.04 | 41 | 1.32 | |
| Notropis wickliffi | Channel shiner | 1213 | 16.03 | 685 | 22.04 | |
| Opsopoeodus emiliae | Pugnose minnow | 1 | 0.01 | |||
| Pimephales notatus | Bluntnose minnow | 16 | 0.21 | 40 | 1.29 | |
| Pimephales vigilax | Bullhead minnow | 163 | 2.15 | 35 | 1.13 | |
| Catostomidae | Carpiodes carpio | River carpsucker | 231 | 3.05 | 31 | 1.00 |
| Ictiobus bubalus | Smallmouth buffalo | 3 | 0.04 | 7 | 0.23 | |
| Ictiobus cyprinellus | Bigmouth buffalo | 25 | 0.33 | 16 | 0.51 | |
| Ictiobus niger | Black buffalo | 7 | 0.09 | 8 | 0.26 | |
| Moxostoma macrolepidotum | Shorthead redhorse | 3 | 0.04 | |||
| Ictaluridae | Ictalurus furcatus | Blue catfish | 3 | 0.04 | 1 | 0.03 |
| Ictalurus punctatus | Channel catfish | 34 | 0.45 | 4 | 0.13 | |
| Pylodictis olivaris | Flathead catfish | 9 | 0.12 | 3 | 0.10 | |
| Atherinopsidae | Menidia beryllina | Inland silverside | 4 | 0.05 | ||
| Fundulidae | Fundulus notatus | Blackstripe topminnow | 2 | 0.03 | ||
| Poeciliidae | Gambusia affinis | Western mosquitofish | 1064 | 14.06 | 77 | 2.48 |
| Moronidae | Morone chrysops | White bass | 9 | 0.12 | ||
| Morone saxatilis | Striped bass | 1 | 0.01 | |||
| Centrarchidae | Lepomis cyanellus | Green sunfish | 22 | 0.29 | 15 | 0.48 |
| Lepomis gulosus | Warmouth | 2 | 0.03 | 1 | 0.03 | |
| Lepomis humilis | Orangespotted sunfish | 47 | 0.62 | 74 | 2.38 | |
| Lepomis macrochirus | Bluegill | 720 | 9.52 | 134 | 4.31 | |
| Lepomis megalotis | Longear sunfish | 34 | 0.45 | 65 | 2.09 | |
| Micropterus dolomieu | Smallmouth bass | 2 | 0.03 | |||
| Micropterus punctulatus | Spotted bass | 20 | 0.26 | 6 | 0.19 | |
| Micropterus salmoides | Largemouth bass | 5 | 0.07 | 2 | 0.06 | |
| Pomoxis annularis | White crappie | 2 | 0.03 | 3 | 0.10 | |
| Pomoxis nigromaculatus | Black crappie | 2 | 0.03 | |||
| Unidentified sunfish | 79 | 1.04 | ||||
| Sciaenidae | Aplodinotus grunniens | Freshwater drum | 112 | 1.48 | 12 | 0.39 |
| Total count | 7565 | 3108 | ||||
| Total species by season | 46 | 32 | ||||
| Shannon's diversity index (H′) | 3.4 | 3.1 | ||||
| Camargo's index of evenness (E′) | 0.28 | 0.22 | ||||
Acknowledgements
We thank the US Army Corps of Engineers for funding this project through the Navigation and Environmental Sustainability Program (NESP). We thank B. Johnson and K. Slattery of the St. Louis Corps for their assistance and corporation throughout the 2 years of this study. We also recognize D. Ostendorf, J. Ridings, A. Kelley, Z. Fratto, J. McMullen, A. Givens, L. Solomon, B. Swallows, C. Gump, A. Lamb, D. Huff, B. Landwer, and M. Elder for assisting with data collection.
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