Aquatic
Macroinvertebrate Surveys in the Clark Fork River,
1986 to 1988.
Daniel
L. McGuire
McGuire Consulting
Box 764
Espanola, NM 87532
Abstract-:
The Montana DHES conducts aquatic macroinvertebrate surveys as part of its
environmental surveillance of the Clark Fork River. Since 1986, annual
collections have consisted of four modified Hess samples from each of 25
stations between the headwaters of Silver Bow Creek and Thompson Falls
Reservoir. The purpose of these studies are to monitor the integrity of
macroinvertebrate assemblages in the river, to detect changes in water quality
and to provide information useful for water-quality management decisions.
Macroinvertebrates provided assessments
of metals and organic pollution as well as cumulative environmental stress in
the Clark Fork drainage. Metals pollution remained the principle agent of
perdition in Silver Bow Creek while degradation from both metals and organic
pollution were widespread in the upper reaches of the Clark Fork River. The
biological impacts of heavy metal contamination were significantly reduced below
the Warm Springs Ponds compared with conditions in Silver Bow Creek.
Nevertheless, metals clearly limited the number of macroinvertebrate species in
the Clark Fork River downstream at least as far as Bonita. Impacts attributable
to metals were relatively uniform in the upper river from Warm Springs Creek to
the confluence of the Little Blackfoot River. Further downstream, proximity to
"clean water" tributaries appeared to be an important factor
influencing the extent of metals impacts. Biologically significant metals
pollution was not detected downstream from the Clark Fork River' s confluence
with Rock Creek. Metals-related stress appeared more severe in 1987 than in
other years. Among all river reaches, the healthiest benthic fauna was found
from Milltown Dam to the confluence of the Bitterroot River. Downstream from the
Bitterroot, nutrient enrichment impacted the aquatic community.
The 1988 drought appeared to accentuate biological responses
to organic and nutrient enrichment. Symptoms of organic pollution were more
distinct and widespread than in previous years. Macroinvertebrate densities
during August 1988 were, on average, double those in 1986 and 87. Impacts
attributed to organic enrichment were widespread with the greatest effects in
the upper river below the Warm Springs Ponds, at Deer Lodge, Bonita and Turah
and, in the lower river, from Harper's B ridge downstream to Alberton. The
response of the fauna and the ability of Silver Bow Creek to assimilate organics
was limited by the prevailing toxic environment.
The Clark Fork River drains most of western Montana to the Columbia drainage.
The upper Clark Fork River and Silver Bow Creek, a tributary, have been plagued
by heavy metal contamination from mining for over 100 years. More recently,
concerns have extended to the trophic status of the river, which has shifted due
to increased nutrient loading and organic pollution. The Montana Department of
Health and Environmental Sciences conducts annual aquatic macroinvertebrate
surveys (6., 11, 13, 14) as part of its environmental surveillance. These
surveys provide a general assessment of pollution impacts and environmental
conditions throughout much of the drainage by evaluating the integrity of
indigenous macroinvertebrate assemblages.
Field
and Laboratory Procedures
Benthic macroinvertebrates were collected by Montana Water Quality Bureau (WQB)
staff during mid August of 1986,1987 and 1988. Each year, a 'modified Hess
sampler (0.1 m2 diameter, 0.45 mm mesh netting) was used to obtain four samples
at each of 25 stations between the headwaters of Silver Bow Creek (SBC) and
Thompson Falls Reservoir (fig.1). Samples were preserved in ethanol containing
rose bengal dye and, when needed, formalin. In the laboratory,
macroinvertebrates were sorted from the debris, identified to the lowest
practical taxonomic level, and counted. The Montana WQB maintains a reference
collection of macroinvertebrates from this study.
Six measures of community structure and function were calculated as the primary
method of impact assessment: taxa richness, total density, Shannon diversity
(25), EPT (Ephemeroptera, Plecoptera and
To provide an overview of heavy metal and organic pollution,
the study area was divided into seven stream reaches (table
1) based on
Community Similarity Coefficients and the geographic proximity of stations (13).
For comparison, the lower Blackfoot River was included as a control reach. The
Blackfoot River is a tributary to the Clark Fork River and, at the seven
stations incorporated into this analysis, was considered free of significant
environmental degradation (10). Blackfoot River data from August, 1988 (10) were
used as an estimate of the Clark Fork River's potential to support aquatic life.
Metals, including copper, cadmium, iron, lead, zinc and arsenic, contaminate
Silver Bow Creek and much of the Clark Fork River, Elevated metal concentrations
are found throughout the upper basin in water (8). sediments (2, 16). benthic
insects (1) and fish (!8), The distribution. exposure and concentration patterns
of most metals appeared similar. with copper considered the metal of greatest
biological significance (7, l7), The mean copper concentration and the frequency
of copper concentrations exceeding chronic and acute aquatic life criteria were
used to summarize metal pollution in each stream reach during this study (table
2).
Plant nutrients comprise another major category of pollutants
in the Clark Fork River drainage. By stimulating algal growth, nitrogen and
phosphorus can have a profound influence on the abundance and composition of
macroinvertebrate assemblages. The mean concentrations of Total Soluble
Inorganic Nitrogen (TSIN) and Orthophosphate (OP) at one station in each stream
reach from July 1986 to August 1988 are presented in table
2. Other papers in
these proceedings consider nutrients (2) and algal growth (2:4) in more detail.
Ammonia concentrations toxic to most aquatic life occur frequently in Silver Bow
Creek (8).
In general, the Clark Fork River supported an abundant benthic fauna that,
during August. was dominated by hydropsychid caddis flies. Caddis flies
accounted for approximately 55% of the macroinvertebrates collected while
dipterans ac- counted for approximately 30%. Dipterans were relatively more
abundant in Silver Bow Creek (fig. 2). Overall, mayflies comprised slightly less
than 10% of the fauna while stoneflies and beetles each contributed less than
five percent. The amphipod, Hyalella azteca, was a major faunal component in the
Warm Springs Ponds during 1986 and 1988.
With a study area consisting of 25 stations scattered over
471 km (294 miles) of river, it was not surprising to find 165 taxa among the
more than 300,000 macroinvertebrates collected. Dipterans accounted for 59 taxa,
including 43 midges. Caddis flies, mayflies, beetles and stoneflies were also
well represented with 30, 23, 15 and 12 taxa, respectively. One to three taxa in
the insect orders Hemiptera, Lepidoptera, Megaloptera and Odonata were also
collected. Non- insects, although minor components of the fauna, accounted for
an additional 20 taxa.
Chronic
Pollution
The integrity of the aquatic communities in Silver Bow Creek (SBC) and most
reaches of the Clark Fork River (CFR) were impaired when compared with those in
the Blackfoot River (table 1). Impacts were most severe in Silver Bow Creek.
Where macroinvertebrate density, taxa richness, EPT richness and Shannon
diversity were significantly lower and the percent contribution of the dominant
taxon was significantly higher than for all other stream reaches. Varying
degrees of environmental stress were indicated by one or more indices in the
Warm Springs Ponds outflow (WSP) and all reaches of the Clark Fork River .
SILVER
BOW CREEK (SBC AND WSP)
Prior to 1975, aquatic macroinvertebrates were absent from Silver Bow Creek.
Chadwick et al. (5) documented the establishment of a metals-tolerant benthic
fauna between 1975 and 1983. Macroinvertebrate colonization was concurrent with
the effective secondary treatment of the effluent from the Anaconda Minerals
Company metal concentrator and with cessation of sludge discharge by the Butte
sewage treatment plant ill). Chironomids and empidids, the earliest colonizers,
were present throughout Silver Bow Creek by 1981. A few additional species,
including hydropsychids, were established in the lower reach by 1983. The
biological recovery in Silver Bow Creek stagnated during the 1980's. The
macroinvertebrate fauna has remained virtually unchanged since the early
eighties, and no new species have established populations in Silver Bow Creek in
recent years. Silver Bow Creek remains the most impaired reach of the Clark Fork
River drainage. The impoverished macroinvertebrate fauna was limited to a few
metals tolerant species including Cricotopus spp., Cardiocladius sp.,Pagastia
sp., Simulium vittatun and Hydropsyche slossonae. Mayflies and stoneflies were
conspicuously absent. Severe environmental stress and pervasive heavy metal
contamination were indicated by the low macroinvertebrate density, taxa
richness, EPT richness and Shannon diversity as well as the high relative
abundance of the numerically dominant taxon (fig.
3). Despite extremely high
nutrient concentrations (Table 2), Silver Bow Creek's macroinvertebrate fauna
exhibited few indications of organic or nutrient pollution. The stream's ability
to assimilate nutrients and the fauna's ability to respond to organics appeared
limited by the prevailing toxic environment.
The Warm Springs Ponds provide additional treatment of Silver
Bow Creek water prior to its entering the Clark Fork River (table
2). The
macroinvertebrate fauna in the Warm Springs Pond #2 outflow was typical of a
lake outflow assemblage. At this site, the mean macroinvertebrate density
increased nearly eight fold, and taxa richness was double that in Silver Bow
Creek. Both were good indications of reduced toxicity. While the severity
lessened, biological impairment by metals remained apparent. Taxa richness and,
particularly, EPT richness were significantly lower than in downstream reaches (table
1). Nutrients transported by Silver Bow Creek had a substantial impact on
the biota as indicated by the high macroinvertebrate density and the large
percentage of filterers in the Warm Springs Pond #2 outflow (fig.
3). During
August, the most abundant taxa were Cheumatopsyche sp., Hydropsyche vexa,
Simulium spp. and Hyalella azteca. Cumulative environmental stress was moderate
to severe at this site.
UPPER
CLARK FORK RIVER (CFR 1 AND 2)
The biological integrity of the upper Clark Fork River was much improved
compared to historic conditions in this reach and existing conditions in Silver
Bow Creek. Investigators in the 1950's (1, 22) and 1960's (21) reported sparse
insect populations and the virtual absence of mayflies, stoneflies and caddis
flies in the Clark Fork River upstream from the confluence of the Little
Blackfoot River. Impact from metals contamination diminished substantially
during the 1970' s (4) Nevertheless, degradation from both metals and organic
pollution remain evident in CFRl, and cumulative impact appeared moderate to
severe.
Downstream from the confluence of the Little Blackfoot River
(CFR2) was clearly a transition zone with regard to heavy metals and other
pollutants (table 2). Impacts were apparent at some stations but not others.
During 1986 and 1988, taxa richness and Shannon diversity were significantly
higher at stations 11 (CFR at Gold Creek Bridge) and 13 (CFR at Turah) than at
station 12 (CFR at Bonita). For the reach as a whole, cumulative impacts
attributable to metals were slight. Taxa richness and EPT richness were not
significantly different from downstream Clark Fork River reaches; although, both
indices were significantly lower than for the Blackfoot River (Table
1).
Nutrient loading clearly impacted this reach of the Clark
Fork River. Macroinvertebrate densities were, on average, higher than elsewhere
in the drainage (fig. 3), an indication that large amounts of nutrients were
assimilated in this reach. The benthic fauna was comprised of taxa tolerant to
organic and nutrient enrichment including H. occidentalis, Simulium spp.,
Polypedilum sp. and numerous species of Orthocladinae midges.
MIDDLE
CLARK FORK RIVER (CFR 3, 4 AND S)
Historically t pollution has had a less dramatic impact on the biota in the
middle reaches of the Clark Fork River than in the headwaters (1, 21, 22).
Organic wastes from the Missoula WWTP t the Stone Container Mill and
metals-laden sediment flushed from Milltown Dam have been the pollutants of
concern (23). With improved wastewater treatment at the mill (1975) and the
Missoula WWTP (1978), recent impacts from these sources have primarily been
limited to nutrient enrichment (Qt 2J)).
Significant heavy metal pollution, as measured by taxa and
EPT richness (Fig 3), was not indicated downstream from Milltown Dam during this
study. Among all reaches of the Clark Fork River, the highest biological
integrity was found from Milltown Dam downstream to the confluence of the
Bitterroot River (CFR3) and from Superior downstream to the confluence of the
Flathead River (CFR5). As measured by Shannon diversity and the contribution of
the dominant taxon, environmental stresses were significantly less in these two
reaches than elsewhere in the Clark Fork River. Values of these indices, along
with taxa and EPT richness, approached those for the reference reach on the
Blackfoot River. The higher relative abundance of filter feeders in the middle
Clark Fork River compared to the Blackfoot River indicated higher nutrient
levels in the former.
A slight to moderate impact of nutrient enrichment was
evident in reach CFR4, from the confluence of the Bitterroot River to Alberton.
Shannon diversity was significantly lower in this reach than in adjacent
reaches. Additional evidence of nutrient-related impacts included the higher
macroinvertebrate densities, percentage of filter feeders and contribution of
the dominant taxon compared with adjacent reaches. Hydropsyche occidentalis was
abundant throughout the middle Clark Fork River and was the most abundant
species in August samples from reach CFR4. Interestingly, it was replaced by H.
cockerelli as the most numerous species in reach CFR3 and by Cheumatopsyche sp.
As the dominate in reach CFR5.
Understanding temporal and spatial variation can increase our understanding of
the underlying conditions and mechanisms influencing aquatic communities. In
addition, knowledge of annual variation is a prerequisite to successfully
predicting and assessing long-term trends.
Drought conditions prevailed during 1987 and 1988 with near
record low stream flows during much of 1988. The drought affected many facets of
the environment and, consequently, had a major influence on aquatic communities
in the Clark Fork River drainage. For instance, the effects of organic/nutrient
enrichment were more apparent in 1988 than in other years. Among the 16 stations
on the Clark Fork’s mainstem, macroinvertebrate density averaged 27,000
organisms per square meter during 1988, nearly triple the density estimates for
1986 and 1987. The increase in macroinvertebrate density was not uniform among
locations, rather pronounced increases in macroinvertebrate density occurred at
some stations (Fig. 4) and stream reaches
(fig. 3).
Nutrients in municiple discharges were relatively more important during
periods of low streamflows (8) and, in reaches were these nutrients were
assimilated, macroinvertebrates were quite abundant. Based on total
macroinvertebrate density, nutrient and organic enrichment had its greatest
impact in the Clark Fork River at Deer Lodge, Bonita, Turah and Harper’s
Bridge (st. 09,12, 13 and 20, respectively). The relatively high
macroinvertebrate densities in Silver Bow Creek (st. 01), the Warm Springs Pond
#2 effluent (st. 04) and the Clark Fork River stations 07, 22 and 24 also
indicated widespread nutrient enrichment. A dramatic reduction in total
macroinvertebrate density at station 11 in 1988 was attributed to excessive
sediment deposition during the drought.
As measured by taxa richness, chronic
heavy metal impacts in Silver Bow Creek and the upper Clark Fork River appeared
to be reduced in 1988 when compared with 1986 and 1987 (fig.
4). During 1987 and
1988, low streamflows resulted in reduced metals concentrations and loading in
these stream reaches (8) and mean taxa richness was significantly higher for
both years than in 1986. However, for 1987, a statistically significant
interaction between year and location was noted, with taxa richness being
relatively low at upstream stations but relatively high at downstream sites.
This was attributed to runoff from a rainstorm on July 3, 1987, which resulted
in elevated metals concentrations and/or pH fluctuations sufficient to kill
several thousand salmonids in the Mill-Willow Bypass and upper Clark Fork River.
Impacts attributable to an episode of acute toxicity were evident during August
of 1987, in Silver Bow Creek, the Warm Springs Pond effluent, the Mill-Willow
Bypass and the upper Clark Fork River.
Surprisingly, the impact was most evident in the Warm Springs Pond #2 outflow.
The ponds are generally effective in reducing metals toxicity (table
2) cause
they are well buffered and have a relatively long retention time. Prevailing
conditions allowed a large population of Hyalella azteca, a relatively
intolerant species, to became established in the ponds. Subsequently, when the
buffering capacity of the ponds was overwhelmed, the H. azteca population was
severely diminished (fig. 2) and community structure was greatly altered
(fig. 3).
Impacts from metal pollution were also more severe in Silver
Bow Creek during 1987. Simulium vittatum and Hydropsyche slossonae appeared to
be the species least tolerant of heavy metals in Silver Bow Creek, and both were
severely reduced in abundance during 1987. In other years, these species
comprised most of macroinvertebrate fauna and nearly all of the filter feeders
in Silver Bow Creek. Their demise was registered by the relative abundance of
orders, filterers and total number of macroinvertebrates (fig.
3).
At most stations in the upper Clark Fork River, community
level indices exhibited little response to the episodic perturbation, although
taxa considered relatively intolerant of heavy metals suffered significantly
reduced densities and restricted distributions during 1987. Micropseclra sp.,
Arclopsyche grandis, and Hexaloma sp. were considered the best indicator species
of changes in the severity of metals pollution in the upper Clark Fork River
(H). Measures of the macroinvertebrate fauna as a whole showed only subtle
changes. This was probably due to chronic metals pollution, which had already
limited relatively intolerant species to a minor component of the fauna.
1.
Silver Bow Creek remained severely polluted by heavy metals. The
macroinvertebrate fauna was impoverished and restricted to a few tolerant
species.
2.
Biological impacts of heavy metal contamination were significantly
reduced below the Warm Springs Ponds, however, the capacity of the ponds to
reduce metals toxicity in the outflow appeared to be overwhelmed by a rainstorm
induced pulse of metals-laden runoff during July, 1987.
3.
Indications of organic/nutrient enrichment were minimal in Silver Bow Creek.
Apparently, the stream's ability to assimilate nutrients and the faunas ability
to respond to this form of pollution were limited by the prevailing toxic
environment.
4.
Nutrients transported by Silver Bow Creek contributed to the high
macroinvertebrate densities in the Warm Springs Ponds discharge. Nutrients
transported to the Warm Springs Ponds had a major impact on the downstream
fauna.
5.
The biological integrity of the upper Clark Fork River was much improved
compared to that of Silver Bow Creek. However, degradation from both metals and
nutrients were pervasive. Cumulative impacts were severe in CFR 1 and moderate
in CFR2.
6.
Impacts attributed to organic enrichment were most severe below Warm Springs
Creek, and at Deer Lodge, Bonita and Turah (stations 07, 09, 12 and 13,
respectively).
7.
Impacts from metals were relatively uniform from the Mill- Willow Creek
Bypass downstream to the confluence of the Little Blackfoot River (CFRI).
8.
Metals pollution appeared intermittent and less significant from the Little
Blackfoot River to Milltown Dam (CFR2). In this reach, station 12 (CFR at
Bonita) appeared to be more severely impacted than other stations.
9.
Biological impacts attributable to metals were not detected downstream from the
Clark Fork River's confluence with Rock Creek.
10.
Among all river reaches, the healthiest benthic fauna was found from Milltown
Dam to the confluence of the Bitterroot River (CFR3) and from Superior to the
confluence of the Flathead River (CFR5). Slight nutrient enrichment was
indic3led in these reaches.
11.
No impacts were detected at stations immediately below designated mixing
zones for me Missoula WWTP and the Stone Container kraft mill. Nutrients from
these sources contributed to downstream impacts.
12.
From the confluence of the Bitterroot River to Albel1on (CFR4), moderate
impairment due to nutrient enrichment was indicated. Within this reach, the
impact was most severe at Harper’s Bridge (station 20).
13.
During August 1988, construction at Milltown Dam did not appear to have a
deleterious impact on the downstream benthic fauna.
14.
The 1988 drought affected many facets of the environment and,
consequently, had a major influence on aquatic communities in the Clark Fork
River drainage. Reduced stream flows appeared to limited metals impact in Silver
Bow Creek and the upper Clark Fork River, but created some localized sediment
problems. Drought-related conditions accentuated biological responses to organic
and nutrient enrichment.
The author wishes to recognize the efforts of Gary Ingman. Mark Kerr and Eric
Weber, who conducted the field work and the Montana Water Quality Bureau, whose
continued support made this report possible.
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