Michael J. McBride and Edward J. Peters, Department of Forestry, Fisheries, & Wildlife, University of Nebraska, Lincoln.
INTRODUCTION
Historical inferences of natural vegetation patterns show that the central Platte River consisted of wide channels bordered by wetlands, grasslands, and scattered woodland vegetation (Johnson 1994). Alteration of the natural hydrograph, suppression of natural prairie fires, and elimination of the bison (Bos bison) have worked collectively to alter the structure and function of this ecosystem (Williams 1978, Sidle et al. 1989). These changes have allowed for cottonwood (Populus sp.) and willow (Salix sp.) establishment throughout riparian areas of the central Platte River (Johnson 1994), and have significantly altered habitat for many avian species, including the endangered whooping crane (Grus americana), least tern (Sterna antillarum), and piping plover (Charadrius melodus) (Currier 1985). While forested riparian areas currently dominate the central Platte River, nonforested areas still exist and management for endangered species includes creation of open areas along the River. How these changes have influenced the benthic macroinvertebrate community is unknown, since baseline data for comparison is limited. However, significant changes may be expected since streamside plant communities are major determinants of the abundance and quality of nutritional resources for benthic macroinvertebrates (Cummins et al. 1989).
While considerable attention has been given to stream community organization in the past three decades (see Minshall 1988 for review), little information is available on the stream communities in the Great Plains region of the United states (Wiley et al. 1990). In addition, there are no published investigations about the relationships of benthic macroinvertebrates in braided prairie streams. The central Platte River in Nebraska is a fifth order stream. However, it is unknown how the braided channels with differing riparian vegetation types influence the benthic community. Previous collections of benthic macroinvertebrates along the central Platte River (Bazata 1991, Chadwick & Associates 1990, Chapman 1972) have not accounted for differences in riparian vegetation types.
If the benthic community is affected by riparian vegetation, differences in the functional organization of aquatic invertebrates would be expected (Cummins 1989, Gregory et al. 1991). If woody vegetation affects forested areas more than open areas it should affect benthic community structure by altering abundances of functional feeding groups. Groups specialized to process coarse particulate organic matter, known as shredders (Merritt and Cummins 1978) should be more abundant in forested areas than in open areas. If woody vegetation affects both forested and nonforested riparian areas to the same degree, then the composition of functional feeding groups would be the same in the two habitat types. The objective of this paper is to evaluate the benthic macroinvertebrate communities associated with open and forested riparian vegetation types along the central Platte River.
METHODS
Study Sites
Four pairs of sampling stations, each with a forested and open site were selected (Figure 1). The Elm Creek open and forested sites are located approximately 4.6-km east of U.S. Highway 183 at river mile (RM) 230 and 229, respectively on the south channel of the central Platte River. These are two of the narrowest study sites (Table 1). The Cottonwood Ranch open and forested sites occur on the north channel and are located approximately 4.8-km west of U.S. Highway 183 at RM 235 and 234, respectively. These sites are intermediate in channel width compared to the other sites. The Alda forested and open sites occur along the north channel, but are separated by a distance of 44.8-km. The Alda forested site is located approximately 3.2-km east of State Highway 10 at RM 207, and is intermediate in width. The Alda site open is located approximately 3-km south of Alda at RM 182 and is the widest of all the eight study sites. The Wildrose Ranch open and forested sites occur along the middle channel at RM 180 and 179, respectively. The Wildrose open site is one the widest sites (77-m), while Wildrose forested is one of the narrowest sites (10-m).
Field Sampling Protocols
Each study site was 100-m long, and consisted of six transects spaced 20-m apart (Figure 2). Benthic macroinvertebrates were sampled along four randomly chosen transects using a modified Hess sampler. A total of eight Hess samples were collected at each site (four bank samples and four mid-channel samples). Two samples were collected on the north bank and two on the south. Mid-channel samples were collected at a point along the transects 0.4x channel width from the respective bank sample. Detritus, organic matter, and invertebrates were placed in a glass quart jar, labeled, and preserved in 10% formalin. Physical habitat variables including depth, cover type, and substrate type were recorded along each of the six transects at 0.5-m,
1.0-m, or 2.0-m intervals, depending upon channel width, to standardize the number of observations at each site. Depth was measured using a standard wading rod. Instream and cover was recorded as being present or absent. If present, instream cover observations were classified as wood or herbaceous plant material. Substrate data were classified as silt, sand, gravel, and organic matter by inspection. The sampling unit was defined as that point where the wading rod touched the substrate. Both dominant and sub-dominant substrate types were recorded.
Laboratory Analysis
Benthic macroinvertebrate samples were placed in Petri dishes, sorted using a dissecting microscope, and placed in vials containing 70% ethanol. All taxa, except those belonging to the family Chironomidae (Diptera), were identified to the lowest practical taxonomic category using a dissecting microscope. Chironomid larvae were mounted on glass slides using CMC-10 and identified to the lowest practical taxonomic level using a compound microscope. Common taxonomic keys used for identification included Weiderholm (1983) for identifying Chironomidae larvae. Taxa were assigned to functional feeding groups using Merritt & Cummins (1978). After the organisms were removed, organic matter was separated from the inorganic debris and strained through a #60 sieve. This material was then dried for 24 hours at 200o C, weighed, combusted at 500o C, and reweighed to determine particulate organic matter (POM) concentrations (APHA 1992). Data were compiled on a seasonal basis in order to determine differences between sites.
Statistical Analysis
To test for significant differences in occurrences of substrate types and instream cover between forested and open areas, percentages of each were compared seasonally using the General Linear Model Procedure (SAS 1989). Percentages from each category were normalized by taking the square root before tests were performed. Concentrations of POM were compared between forested and open sites, between bank and mid-channel sampling positions, and among stations. This analysis was performed separately for the spring, summer, and fall 1993 seasons using the General Linear Model Procedure (SAS 1989).
Total densities of benthic macroinvertebrates per sample within each functional feeding group were compared using the Categorical Data Modeling Procedure (SAS 1989). Sources of variation included treatment (forested vs. nonforested), station, sampling position (bank vs. mid-channel), and a stationXtreatment interaction term.
Taxonomic richness among sites was compared using the Shannon Weiner diversity index and its associated evenness index (Washington 1984). A cluster analysis was performed using the Cluster Procedure (SAS 1989) which compared sites using the Percentage Similarity Index (Washington 1984).
RESULTS
Physical Habitat
No significant differences (p>0.10) in occurrences of substrate types were found between forested and open sites during any of the four seasons. The occurrence of instream cover was significantly higher (p=0.076) in forested areas during spring 1993. Particulate organic matter (POM) varied both within and among sites. Samples from bank habitats had significantly higher POM than samples from mid-channel areas during spring, summer, and fall 1993 (p=0.002, 0.003, and 0.022, respectively). Samples from forested areas had significantly higher POM (p=0.017) than those taken from open areas in spring 1993 (Table 2). No significant differences in POM concentrations were found among stations.
Benthic Invertebrates
Overall diversity values were highest at the Elm Creek forested site in fall 1992 and 1993, and lowest at the Elm Creek site open in spring 1993 (Table 3). Taxonomic richness was highest at the Elm Creek forested site in fall 1993 with 37 taxa present. In each of the four seasons, taxonomic richness was highest at the Elm Creek forested and open sites. Taxonomic evenness was highest in summer and fall 1993. In summer 1993, evenness values were highest at the Alda open and Alda forested sites. In fall 1993, samples taken from the Cottonwood Ranch open, Alda forested, and Wildrose Ranch forested sites had the highest evenness scores. The lowest evenness scores were found at the Elm Creek open site in spring 1993.
Functional Feeding Group Analysis
Collector-filterers and collector-gatherers were the most common functional feeding groups at all sampling locations over the four seasons (Figure's 3 and 4). Numbers of each functional feeding group per sample varied significantly between treatments (forested/open), between positions (bank and mid-channel), and among stations (Table 4). Numbers of collector-filterers varied significantly among stations during all four sampling seasons (p=0.064, 0.0001, 0.08, 0.09) and were most abundant at the Elm Creek station (Figure 3). The Elm Creek open site produced the highest numbers of collector-filterers per sample during fall 1992, spring 1993 and fall 1993).
Collector-gatherers varied significantly among stations during spring (p=0.0001), summer (p=0.0001), and fall (p=0.0001) 1993 , by position during spring (p=0.001), summer (p=0.0.021), and fall (p=0.007) 1993, and by treatment during spring (p=0.006) and summer (p=0.029) 1993 (Table 4). They were most abundant at the Elm Creek station during all four sampling seasons (figure 4).
Significantly higher numbers of shredders were collected in bank samples during spring (p=0.0007), summer (p=0.0537), and fall (p=0.0178) 1993 (table 4; figure 5). In spring and fall 1993 numbers of shredders varied significantly among stations (p=0.0155 and 0.0041, respectively) and were highest at the Elm Creek station.
Predator numbers varied significantly in relation to sampling position and station (Table 4, Figure 6). In spring and fall 1993, total numbers of predators were significantly higher along bank areas (p=0.0480 and 0.0300, respectively). Position was also found to be a significant factor in summer 1993 (p=0.0139), but total numbers of predators were not consistently higher in bank samples than in mid-channel samples. The Wildrose Ranch open, Alda forested, Elm Creek open, and Cottonwood Ranch forested sites had higher numbers of predators along bank areas, while the Wildrose Ranch forested, Elm Creek forested, and Cottonwood Ranch open sites produced higher numbers at mid-channel locations.
Densities of scrapers varied relative to station and sampling position (Table 4, Figure 7). The Elm Creek station exhibited significantly higher numbers of scrapers in the fall 1992 (p=0.0001), and summer 1993 (p=0.0044) samples. Scraper densities were significantly higher along bank areas in the spring (0.0311), summer (p=0.0332), and fall (p=0.0931) 1993 samples.
Taxonomic Similarity of Study Sites
The taxonomic composition of the benthic macroinvertebrate community was found to vary between the eight study sites, as revealed by the cluster analysis (figure 8). While certain sites do appear to be relatively similar, taxonomic peculiarities were found at each of the sites.
The cluster analysis showed the Cottonwood Ranch open and Cottonwood ranch forested to be the most similar of the eight study sites. Dipteran larvae were the most abundant taxa at both of these sites (Figure 9). Simulid and Ceratopogonid larvae were the most abundant Dipterans at the Cottonwood Ranch forested site. Chironomid larvae were the most abundant Dipteran at Cottonwood Ranch open site. Ephemeroptera was the second most abundant order, with Caenis sp. comprising most of the mayflies found at both sites.
Taxonomically, the Wildrose Ranch open and forested sites were found to be most similar to the Cottonwood Ranch sites (Figure 8). Ephemeroptera larvae were nearly equal in abundance at both Wildrose Ranch sites with most of these being caenis sp.. Samples taken from the Wildrose Ranch forested site had a greater proportion of chironomid larvae than those taken from the Wildrose Ranch open site (Figure 10). Orthocladinae were the most abundant midge larvae at the Wildrose Ranch open site. Chironomini larvae and Paracladopelma sp. were dominant at Wildrose Ranch forested. Paracladopelma sp. was only abundant at the Wildrose Ranch forested site.
The Alda open and Elm Creek forested sites were taxonomically more similar to each other than the other six sites (Figure 8). Samples from the Elm Creek forested site contained high total numbers and a high proportion of Chironomids dominated by Tanytarsini (figure 11). Physa sp. and Fossaria sp. Gastropods, along with Orthocladin and polypedilum sp. midges were the most abundant taxa at the Alda open site (Figure 12). The Alda forested and Elm Creek open sites were the least similar to each other and the rest of the study sites (Figure 8). Tubificid worms, Chironomini and Chironomus sp. midge larvae were the most abundant taxa at the Alda forested site (Figure 12). Samples taken from the Elm Creek open site were dominated by Dipteran midge larvae, which accounted for seventy seven percent of the organisms collected (Figure 11).
DISCUSSION
In this study the patchiness of the benthic invertebrate community in the central Platte River was evident spatially and temporally over several scales. This patchiness was measured along several environmental axes which included among sampling stations, among study sites, between forested and open sites, and between sampling positions at a site.
If the abundance of functional feeding groups reflects the quality and quantity of energy present in lotic ecosystems (Peterson and Cummins 1974, Cummins et al. 1980, Vannotte et al. 1980, Cummins 1989), then it is apparent that differences exist in the distribution of organic matter in the central Platte River. Significantly higher concentrations of particulate organic matter from forested areas compared with samples taken from open areas indicates that differences may exist in the relative contribution of allochthonous material (such as leaves) from riparian vegetation. Significantly greater occurrences of instream cover at spring 1993 forested sites related to significantly higher concentrations of POM in the same season. Retention of leaves by organic debris has been documented by many other studies (Bilby 1981, Bilby and Likens 1980, Speaker et al. 1984, Smock et al. 1989, Petersen et al. 1989, Trotter 1990, Bilby and Ward 1991, Jones and Smock 1991, Ehrman and Lamberti 1992, Hill et al. 1992).
Previous investigations of how riparian vegetation effects community structure of benthic macroinvertebrates in lotic ecosystems have yielded differing results. Dudgeon (1988) investigated the functional community structure in four Hong Kong streams with differing riparian vegetation. Shredders were most abundant at forested sites and were associated with high levels of detritus. Scrapers were most abundant in unshaded streams. In the central Platte River, evidence from the analysis of functional feeding groups suggests that the distribution of riparian vegetation does not significantly influence the distribution of benthic invertebrates. Variation in total numbers between open and forested sites occurred only within the Collector-gatherers during the spring and summer 1993 seasons.
Community structure was also found to vary between channels, and lateral sampling position. Differences in communities between bank and mid-channel areas are an integral component of Ward's (1988) theory of the multidimensional nature of stream ecosystems. Abiotic factors such as substrate type, instream cover, and POM concentrations all differ between bank and mid-channel areas. Bank areas are characterized by relatively high amounts of organic matter, silt, and instream cover, while mid-channel habitats are characterized by a shifting sand substrate with less instream cover and lower amounts of organic matter and silt (Table 5). The benthic invertebrate community reflects this dichotomy through differences in relative abundances and densities. Unstable sandy substrates provide little habitat for benthic organisms and have low rates of organic matter retention. Rhodes and Hubert (1991) found that bank areas formed 8.5% of the lateral habitat, but contained 44% of benthic invertebrates in July and 30% in August. Analysis of densities within each functional feeding group supports the idea that patches are based upon lateral position and separate braided channels. Based on densities, the most common source of variation occurred between stations. Differences in total numbers between lateral sampling positions was found to be the second most common factor. Sites located on narrower channels, such as Elm Creek open and forested, tended to have higher occurrences of silt, organic matter, and instream cover compared to wider channels such as Alda open and Wildrose Ranch open. Significant variation between paired sites indicates differences in densities between channels in the central Platte River. Greater occurrences of instream cover and organic matter may indicate that the smaller channels differed less in relation to sampling position when compared to wider channels, and provided a more stable environment for colonization. While significant differences in particulate organic matter were not detected among stations, higher numbers of both collector-filterers and collector-gatherers at the Elm Creek open and forested sites indicate differences in the transport and retention of allochthonous material. Community composition also varies taxonomically between the separate braided channels. This indicates that the taxonomic template may be based upon geographic locality, with physical factors such as substrate composition ultimately influencing community structure (Townsend and Hildrew 1994).
Results of this study agree, in part, with models of community organization in lotic ecosystems (Vannotte et al. 1980; Minshall et al. 1985). The high proportion of collector-filterers and collector-gatherers at all eight sites agrees with predictions with these models in that the majority of detritus consists of fine particulate organic matter in higher order rivers. Macfarlane (1983) investigated community structure in a Minnesota plains stream, and found high proportions of both collector groups. While large river systems, such as the Platte River, may be thought of as being homogeneous in terms of their biotic community and functional organization (Vannotte et al. 1980), the results of this study indicate that such systems are not homogeneous, but rather a mosaic of patches. This may be especially true for braided rivers, like the central Platte, where differences in community structure may reflect differences in the physical processes acting on individual channels. It is apparent that differences in community structure exist between and along channels of the central Platte River. Further research is needed to elucidate the factors and mechanisms responsible for patterns identified in this study.
ACKNOWLEDGMENTS
Funding for this project was provided by the United States Fish and Wildlife Service. Nebraska Public Power District and The Platte River Whooping Crane Critical Maintenance Trust provided access to land from which I was able to select my study sites. I would like to thank Dr. Edward J. Peters for providing assistance and support throughout this project. Dr. Richard S. Holland contributed time and effort which was essential to the completion of this project. Finally, I would like to thank Ken Bazata who assisted in the identification of Chironomid larvae.
LITERATURE CITED
APHA. 1992. Standard methods for the examine of water and wastewater. 18th edition. American Public Health Association, Washington, D.C.
Bazata, K.B. 1991. Nebraska stream classification study. Nebraska Department of Environmental Quality. Lincoln, NE.
Bilby, R. E., and G.E. Likens. 1980. Importance of organic debris dams in the structure and function of stream ecosystems. Ecology 61:1107-1113.
Bilby, R.E. 1981. Role of organic debris dams in regulating the export of dissolved and particulate matter from a forested watershed. Ecology 62(5):1234-1243.
Bilby, R.E. and J.W. Ward. 1991. Changes in characteristics and function of woody debris with increasing size of streams in western Washington. Trans. Amer. Fish. Soc. 118:368-378.
Chadwick and Associates. 1990. Invertebrate survey of the upper Platte River system. Report prepared for the Nebraska Public Power and Irrigation District. Chadwick and Associates, Littleton, Colorado.
Chapman, J. 1972. Effects of a diversion dam on the benthos and macroinvertebrate drift of the Platte River. Unpublished. M.A. thesis. Kearney State College, Kearney, Nebraska.
Cummins, K.W., G.L. Spengler, G.M. Ward, R.M. Speaker, R.W. Ovink, and D.C. Mahan. 1980. Processing of confined and naturally entrained leaf litter in a woodland stream ecosystem. Limnol. Oceanogr. 25:952-957.
Cummins, K.W., M.A. Wilzbach, D.M. Gates, J.B. Perry, and W.B. Taliaferro. 1989. Shredders and riparian vegetation. BioScience 39(1):24-30.
Currier, P.J., G.R. Lingle, and J. G. VanDerwalder. 1985. Migratory bird habitat on the Platte and North Platte rivers in Nebraska. Platte River Whooping Crane Critical Habitat Maintenance Trust, Grand Island, NE.
Dudgeon, D. 1988. The influence of riparian vegetation on the functional organization of four Hong Kong stream communities. Hydrobiologia. 179:183-194.
Ehrman, T.P., and G.A. Lamberti. 1992. Hydraulic and particulate matter retention in a 3rd-order Indiana stream. J. N. Amer. Benthol. Soc.11:341-349.
Gregory, S.V., F.J. Swanson, W.A. McKee, and K.W. Cummins. 1991. An ecosystem perspective of riparian zones. Bioscience 41(8):540-551.
Hill, B.H., T.J. Gardner, and O.F. Ekisola. 1992. Benthic organic matter dynamics in Texas prairie streams. Hydrobiologia 242:1-5.
Jones, J.B., and L.A. Smock. 1991. Transport and retention of particulate organic matter in two low gradient headwater stream. J. N. Am. Benthol. Soc. 10:115-126.
Johnson, W.C. 1994. Woodland expansion in the Platte River Nebraska: patterns and causes. Ecol. Monog. 64(1):45-84.
Merritt, R.W., and K.W. cummins. 1978. an introduction to the aquatic insects of North America. Kendall-Hunt, Dubuque, Iowa, USA.
Minshall, G.W. 1988. Stream ecosystem theory: a global perspective. J. N. Am. Benthol. Soc. 7(4):263-288.
Minshall, G.W., K.W. Cummins, R.C. Petersen, C.E. Cushing, D.A.Bruns, J.R. Sedell, and R.L. Vannote. 1985. Developments in stream ecosystem theory. Can. J. Fish. Aquat. Sci. 42:1045-1055.
Petersen, R.C., K.W. Cummins, and G.M. Ward. 1989. Microbial and animal processing of detritus in a woodland stream. Ecol. Monogr. 59(1):21-39.
Rhodes, H.A. and Hubert, W.A. 1991. Submerged undercut banks as macroinvertebrate habitat in a subalpine meadow stream. Hydrobiol. 213:149-153.
SAS Institute, Inc. 1989. SAS/STAT user's guide. Release 6.04 edition. SAS Institute, Inc., Cary, North Carolina.
Smock, L.A., G.M. Metzler, and J.E. Gladden. 1989. Role of debris dams in the structure and functioning of low-gradient headwater stream. Ecology 70:764-775.
Sidle, J.G., E.D. Miller, and Paul J. Currier. 1989. Changing habitats in the Platte River Valley of Nebraska. Prairie Nat. 21(2):91-104.
Speaker, R., K. Moore, and S. Gregory. 1984. Analysis of the process of retention of organic matter in stream ecosystems. Verhandlungen der dInternationalen Vereinigung fur Theoretische und Angewandte Limnologie 22:1835-1841.
Townsend, C.R. and A.G. Hildrew. 1994. Species traits in relation to a habitat templet for river systems. Freshwat. Biol. 31:265-275.
Trotter, E.H. 1990. Woody debris, forest-stream succession, and catchment geomorphology. J. N. Am. Benthol. Soc. 9(2):141-156.
Vannote, R.L., G.W. Minshall, K.W. Cummins, J.R. Sedell, and C.E. Cushing. 1980. The river continuum concept. Can. J. Fish. Aquat. Sci. 37:130-137.
Ward, J.V. 1988. The four dimensional nature of lotic ecosystems. J. N. Am. Benthol. Soc., 8(1):2-8.
Washington, H.G. 1984. Diversity, biotic, and similarity indices: a review with special relevance to aquatic ecosystems. Water Res. Vol 19, No. 6, pp. 653-694.
Wiederholm, T. (ed.). 1983. Chironomidae of the Holarctic region. Keys and diagnoses. Part 1. Larvae. Ent. Scand. Suppl. No. 19.
Williams, G.P. 1978. The case of the shrinking channels- the North Platte & Platte Rivers in NE.
Wiley, M.J., L.L. Osborne, and R.W. Larimore. 1990. Longitudinal structure of an agricultural prairie river system and its relationship to current stream ecosystem theory. Can. J. Fish. Aquat. Sci. 47:373-384.
Return to 1995 Platte River Basin Ecosystem Symposium 
Last updated by Darren A. Jack on 9/8/97