THE AQUATIC INVERTEBRATES OF
THE UPPER CLARK FORK RIVER, 1972-1984

 

Steven P. Canton and James W. Chadwick
Chadwick & associates
5721 South Spotswood Street
Littleton, CO 80120

  

Abstract--Benthic invertebrate communities of the upper Clark Fork River were sampled yearly from 1972 to 1984. Invertebrates were collected in fall using a modified Hess sampler at seven stations on the upper Clark Fork River, Mill and Willow Creek and Silver Bow Creek. The upstream control station on Mill and Willow Creek (CFRO) exhibited a balanced, diverse invertebrate community throughout the study. However, during the early part of the study no invertebrates were collected at the other headwater station on Silver Bow Creek (SBC5). Following improved mine wastewater treatment upstream of SBC5 in 1972, insects were eventually collected in 1975 and benthic densities and number of taxa have generally increased through 1984. Nonetheless, over the last 4 years, the densities (3708/m2) and number of taxa (26) in SBC5 have been considerably lower than those at CFRO (12 356/m2 and 57, respectively), The stations below the outflow of the Warm Springs Ponds have exhibited a noticeable response to the outflow of plankton from the ponds with benthic densities exceedil1g 60000/m2. These communities are composed primarily of filter feeding insects, both hydropsychid caddis flies and black flies. At the downstream stations, densities and number of taxa approach those originally observed at CFRO. However, the species composition at these lower stations 18 more typical of a large, low-gradient river rather than the headwater stream community found upstream.

 

INTRODUCTION

 

The Clark Fork River is located in the Columbia River basin. From its source in western Montana, it flows north and west through western Montana, northern Idaho, and northeastern Washington. The benthic invertebrate communities of the headwaters of the Clark Fork River and its tributary Silver Bow Creek have been adversely affected by historic metal mining activity and municipal wastewater (l,l,3,l). In the fall of 1972, major improvements in mine wastewater treatment and handling in Butte resulted in improved discharge quality (9), and a corresponding marked improvement in the quality of waters within Silver Bow Creek and the upper Clark Fork River (4). Since 1972, benthic invertebrate communities of the upper Clark Fork River have been sampled yearly during the fall to monitor the biological recovery following initiation of this improved mine wastewater treatment.

 Station Description

   Eleven stream stations were originally established in 1972 (fig.1). The five stations located on Silver Bow Creek are treated more fully elsewhere (3). The results from Station 5 are included in this study as a comparison to the upper Clark Fork River stations. Station 5 on Silver Bow Creek is located 5 km upstream of the Warm Springs settling ponds, just east of Opportunity, Montana. These settling ponds are located near the mouth of Silver Bow Creek. The stream is 7 m wide at this station, with sand-gravel-cobble substrate and little noticeable algal development. Recently, however, there has been an apparent increase in the amount of suspended algae.

    Clark Fork River Station 0 is located in the diverted channel for Mill and Willow Creeks 6.4 km upstream of the confluence with the discharge from the Warm Springs Ponds. The stream at Station 0 is approximately 4 m Widoe with gravel-rubble substrate. Periphytic algal growth has been evident in recent years. This station was originally established as an upstream control for the Clark Fork River. Station 2a is located in the discharge channel for Warm Springs Pond 2. This channel is approximately 6-8 m wide with rubble-gravel substrate. Abundant growths of filamentous algae were evident during the recent sampling. Station 2 is located in the Clark Fork River 7.1 km& downstream of Station 0 and 1.4 km upstream of its confluence with Warm Springs Creek. Station 2 is 0.6 km downstream of the input of discharge for the settling ponds. The stream is 10 m wide at this station with gravel-cobble substrate. Occasionally, aquatic vegetation has been observed, consisting primarily of sporadic growths of the alga Ctadophora glomerata and the macrophyte Myriophyttum exatbeuaens. Station 4 is located 8.8 km downstream of Station 0 and just downstream of the confluence of Warm Springs Creek with the upper Clark Fork River. The stream is 5-6 m wide with sand-gravel-cobble substrate. Riffle habitat is least abundant in this meandering section. Although there has been little algae observed at this site, leaf litter is abundant during the fall sampling episodes. Station 6 is located in Deer Lodge, Montana, 44.9 km downstream of Station 0. The stream is approximately 17 m wide here, with gravel-rubble substrate. Growths of the filamentous algae Ctadophora were generally present. The macrophyte Ceratophyllum demersum was also present but not abundant. The farthest downstream location is Station 8, located 82.4 km downstream of Station 0 near the town of Garrison. Montana. The stream is 20 m wide at this point with cobble-gravel substrate and a moderate algal growth of Cladophora glomerata and Ulothrix sp.

    For most of the study area, the upper Clark Fork River flows through a broad alluvial valley with relatively low gradient (0.3%). Elevation ranges from 1490 m at CFRO to 1272 m at CFR8.  

METHODS

   Benthic invertebrates were collected from the seven stations with modified Mess samplers (2,13), which enclose 0.1 m2 and have a collecting net with a mean mesh size of-700 ~m. Collections were made in October from 1972 to 1984 by taking three samples at each station. Beginning in 1981, these samples were kept separate to allow the use of statistical tests when analyzing the data. Samples were preserved in the field with 95% ethanol and returned to the lab where the organisms were sorted from the debris, identified, and counted. Organisms were identified to the lowest practical taxonomic level using available keys, with approximately 40% of the taxa identified to the species level. Analysis of the invertebrate community included the calculation of the Shannon-Weaver diversity index. The EPA encourages the use of this index as a measure of the effects of stress on benthic communities (14).

   Invertebrates generally exhibit "clumped" distribution in the substrate rather than "random" distribution. Therefore, prior to use of statistical tests on the data collected since 1981, the data were transformed (log base 10) to normalize its distribution and allow the use of parametric tests (2). Tests included one-way and two-way analysis of variance, which test differences between stations and years. In addition, cluster analyses were run using two similarity indices--the Coefficient of Community which measures similarity in terms of the presence/absence of species and Percentage Similarity which measures similarity using relative abundance of like species (12).  

RESULTS AND DISCUSSION  

Species Composition

   During the study, a total of 89 taxa were collected from the upper Clark Fork River, with only 12 taxa common to all seven stations (table 1). May-Flies exhibited their greatest diversity at Station O where 11 species were collected during the study. In lower Silver Bow Creek and the station immediately below the Warm Springs Ponds, mayflies were represented by only a few baetids and an occasional heptageniid. Additional species were found downstream at Stations 6 and 8 as headwater forms such as Caudatella hystrix and Rhithrogena hageni were replaced by taxa more common in big rivers, such as Drunella grandis and Tricorythodes minutus. This pattern was also observed in Dhe stoneflies where the headwaters species Doroneuria theodora and Capniidae were replaced further downstream by the perlodid stoneflies Isogenoides elongatus. As with mayflies, stoneflies were rarely collected at SBCS or CFR2a (table 1).

   Caddisflies also exhibited their greatest diversity at CFRO (table I). The downstream shift in species composition was again evident as Anagapetus sp., Miarasema sp., and Rhyaaophila spp. were replaced at the downstream stations by Ceraalea sp., Protoptila sp., and Psyahomyia sp. Dipterans were generally well distributed throughout the study area, with only a few taxa restricted to either upstream (i.e., Limnophora sp.) or downstream (i.e., prosimulium sp.) reaches.

    Beetles, damselflies, and many non-insect invertebrate taxa exhibited a distribution in the study area apparently tied to the influence of the outflow of the Warm Springs Ponds (table 1). Certain lentic forms such as the beetle Haliplus sp., the damselfly Enallagma sp., the amphipod Hyalella azteaa, the snail Lymnaea sp., and leeches were found only downstream of the pond discharge.

Yearly Trends

    Benthic density and number of taxa for each station from 1972 to 1984 are summarized in figure 2. At CFRO,     density and number of taxa exhibited an apparently random fluctuation in values over the years. These variations were probably a result of yearly fluctuation in physical parameters such as snow- melt runoff. No organisms were collected at SBC5 from 1972 to 1974. Improved water quality following treatment of mine wastewater at the headwaters of Silver Bow Creek was manifested in 1975 when benthic invertebrates were first collected at SBC5. Since 1975, benthic densities and number of taxa have exhibited a gradual increase. Nonetheless, in recent years density (3708/m2) and number of taxa (26) has remained considerably lower than those in nearby CFRO (12 356/m2 and 57, respectively).

    Station 2a, in the discharge channel of the Warm Springs Ponds, was not sampled until 1975. These ponds were built to serve as a settling sink for the metals in Silver Bow Creek prior to its confluence with the Clark Fork River. These ponds appeared to have a strong influence on the communities downstream, as benthic densities below the ponds since 1975 have been considerably greater than those at SBC5 (fig. 2). Densities have remained quite high with gradually increasing number of taxa over the years. Nutrient loading from upstream has made the ponds very productive and the outflow is rich with suspended algae and other organisms. This rich food source is utilized by invertebrate filter feeders, whose abundance (>60 OOO/m2) accounted for the high densities observed at CFR2a throughout the study. Station 2 started out with moderate densities. These densities, along with number of taxa, increased through 1974 and have since remained relatively constant. The high numbers of organisms at this station were due to the abundance of filter feeders, reflecting continued influence of the pond outflow.  

 Downstream Trends

Downstream trends in benthic density and number of taxa during the 13-year study are summarized in figure 3. The general trend exhibited over the years is for higher density at Stations 2 and 6, with Stations 0, 4 and 8 having similar values. The higher density at CFR2 is due to the influence of the pond outflow, as noted above. The reasons for the higher densities are CFR6 are unknown, but the greater algal abundance observed here suggests some nutrient input upstream of this station.
    The observed downstream pattern was tested for the 1981-84 period, when separate samples allowed the use of statistical tests. The difference between stations observed in figure 3 was statistically significant (ANOVA. p-O.OOO2). In addition, there were significant differences between years (ANOVA. p: 0.0007), which was evident in figure 2. However, the interaction between stations and years was not significant (ANOVA. p=0.177), indicating that the differences between stations were significant regardless of the year in question.

    Number of taxa generally exhibited a pattern with highest values at CFRO decreasing sharply at CFR2a and CFR2, followed by increasing values downstream (fig. 3). Station SBCS exhibited much lower number of taxa throughout the study.  

Relative Abundance

    The relative abundance of the major invertebrate groups has exhibited noticeable differences between stations and between years (fig. 4). The up- stream station, CFRO, exhibited a balanced community typical of small Rocky Mountain streams. Silver Bow Creek was initially dominated by dipterans, but has exhibited a gradual increase in importance of Trichoptera through the study. This pattern indicates a recovery of the benthic community following the 1972 improvements in mine wastewater treatment upstream of this site. Silver Bow Creek has also been shown to have some nutrient loading (1). After mine wastewater treatment was implemented, the Warm Springs Ponds apparently began to respond to this nutrient input with increased plankton productivity. The subsequent increase in suspended algae is reflected in the increasing abundance of filter-feeding hydropsychid caddis flies and black flies immediately downstream of the pond discharge. Filter feeding insects are often very abundant below surface outlets of lakes (6,11). However, their densities generally decline further downstream as the pond material is either removed from the water column by the filter feeders (11) or settles out. In the present study filter-feeding caddis flies remains relatively important at the lower stations CFR6 and 8 (fig. 4), indicating that suspended organic matter in this large river is still an important food source, as would be expected in this size of stream (10). Hydroptilid or "micro" caddis flies also increase in abundance at the downstream stations. These caddis flies feed on filamentous algae (16), which can be abundant at these stations.