The Kawesas Watershed Assessment

V: Benthic Invertebrate Communities

Robert Wisseman

One of the most daunting challenges that ecologists and land managers face is the measurement of ecosystem condition, or integrity. The aquatic component of most ecosystems is highly sensitive to many processes that occur on nearby terrestrial landscapes, especially sedimentation and the supply of large wood, subjects addressed in previous chapters on geomorphology, vegetation, and stream habitat. Hence the condition of aquatic systems can serve as a useful indicator of the health of a larger landscape; this is particularly true when the unit of concern is a watershed.

Benthic invertebrates offer an excellent alternative to more costly measures of ecosystem integrity. Since the metabolism of many Pacific Coast rain forest aquatic systems depends on a productive and diverse benthic invertebrate fauna, these organisms provide a useful measure of ecosystem integrity, and can lend themselves to a cost-effective long-term monitoring system (Fore et al 1996, Nelson and Dewberry 1996). As part of the 1995 Kawesas watershed assessment, information on benthic invertebrate communities in the watershed was collected to provide the baseline of such a long-term monitoring system.

Primary objectives of the aquatic invertebrate portion of the study were to:

• Acquire baseline biomonitoring data on benthic invertebrate communities from various locations and habitat types found along the mainstem of the Kawesas River and in some of the lower tributaries. Baseline data, collected while the watershed is still in a pristine condition, will be invaluable for assessing impacts of any future development.
• Provide a preliminary faunistic survey of the aquatic invertebrates found in the watershed.

Secondary objectives included:

• Predict impacts that logging and reading activities in the Kawesas River floodplain and lower elevation forests would have on the integrity of the benthic invertebrate community.
• Acquire baseline information from the pristine Kawesas River system that can be compared with benthic invertebrate biomonitoring data from other coastal rivers in British Columbia and southeast Alaska that have been affected by development.
  

Methods

Collections of aquatic invertebrate communities of the Kawesas River watershed were made from August 2–5, 1995. Because of the limited time available, and particularly because of limited transportation, sites were concentrated along the lower river mainstem and tributaries, between the river mouth and the vicinity of Cole Creek. Jet boat access above Cole Creek was not possible. Access to another set of sites about 26 kilometres upstream of the mouth (in the vicinity of the confluence of the east and west forks of the Kawesas) was made possible through helicopter transport.

Biomonitoring samples were acquired from:

• Mainstem riverine sites and erosional habitats (riffles, glides, cascades). Samples were acquired, where possible, from the confluence of the east and west forks (about 26 kilometres upstream of mouth) downstream to tidally influenced reaches near the mouth.
• Large tributary streams near their base in the Kawesas valley; erosional habitats. These were all in the lower valley, from Cole Creek downstream to the mouth.
• Small tributary streams of various types; erosional habitats. These were also all in the lower valley, from Cole Creek downstream to the mouth.
• Dip-net and hand-picked collections were also made in a number of lentic (standing water) and depositional habitats in the river's flood plain. A traveling kick-net (500 micron mesh) method was employed to acquire qualitative samples from erosional habitats. This involves holding the kick-net downstream, while using feet to stir and roll surface substrates over a distance of 5–10 metres. The current carries dislodged invertebrates into the net.

Invertebrate samples were preserved with isopropyl alcohol. They were sorted in the lab with the aid of dissecting microscopes. Identifications and data analysis were performed by Robert W. Wisseman and Tracey Anderson of Aquatic Biology Associates, Inc. Most identifications of larval insect material was to the genus level, since taxonomy to species level for most western North American aquatic genera is not completely known.

Results and discussion

General faunal characteristics

About 30 different lotic (flowing water) and lentic (non-flowing water) sites were sampled for this study. Although most of the common lotic aquatic invertebrates found in the watershed were probably encountered in the August 1995 survey, the fauna list would expand considerably if more lentic and higher elevation sites were sampled. Of the 137 taxa encountered, 62, or nearly half, were dipterans (flies + midges). Caddis-flies were the next most diverse insect order (27 taxa), followed by mayflies (15), stoneflies (11), dragon and damselflies (9), true bugs (3), and beetles (3). Few non-insect invertebrate taxa were encountered.

The aquatic invertebrates of the Kawesas watershed are a considerably reduced subset of a common western North American montane fauna. The higher latitude of this river system, and the relatively recent and extensive glaciation of the watershed, combine to limit the potential fauna. Nearly all the taxa found in lotic habitats are common and widespread in western, montane North America. There is no distinctive maritime element to the fauna, as can be found farther south in the Pacific Coastal ranges. Only a few of the taxa have ranges restricted to maritime mountains. There is also no distinctive boreal component of the lotic fauna. A boreal component may be more evident once lentic habitats in the watershed are more thoroughly studied. With a few exceptions, the lotic taxa are most typical of Rocky Mountain streams.

Notable absences from the lotic fauna are: molluscs, crustacea (other than microcrustaceans); gomphid dragonflies; baetid mayflies other than Baetis bicaudatus; perlid and pteronarcid stoneflies; hydropsychid, glossosomatid, and hydroptilid caddisflies; riffle beetles (Elmidae); and megalopterans. All of these taxa are common in montane streams in more southerly areas of the Pacific Northwest. Extensive glaciation probably accounts for the absence of some groups (e.g. molluscs and crustacea). The high latitude with cold water temperatures year-round, plus extensive scouring and resorting of substrates during high flows, limits the colonization of the watershed by other groups.

Total taxa richness averaged 22 at the riverine sites, 29 in the large tributaries, and 31 in the small tributaries. Taxa richness was highest where habitats were more stable, that is, those that experienced less scour and resorting of substrates during high flows. More stable substrates and more complex habitat were encountered most often in smaller tributaries. Comparable taxa richness in more southerly, pristine Pacific Northwest montane streams, would be >60 taxa for streams, and >50 taxa for riverine sites.

EFT taxa richness (Ephemeroptera+Plecoptera+Trichoptera) averaged between 11 and 13 taxa for all lotic sites. This is relatively depauperate, when compared with >35 taxa found in more southerly and pristine Pacific Northwest montane streams. Cold-adapted and "intolerant" taxa (unable to tolerate elevated water temperatures, reduced dissolved oxygen, increased fine sediment, and "fouling" by filamentous algae) are common, particularly among the mayflies.

Most aquatic invertebrate taxa encountered in the Kawesas watershed have univoltine [1 generation per year) life cycles. Some taxa, principally the midges, may have a multivoltine (>1 generation per year) life cycle. The relative abundance of semivoltine (more than one year required to complete a life cycle) taxa is low in riverine and large tributary lotic habitats, where disturbance during high flows is greater. Semivoltine taxa are more common in small tributaries, where disturbance is less severe and/or frequent. However, small streams in avalanche tracks are subject to considerable disturbance, and did not have semivoltine taxa present.

Riverine habitats

Samples were acquired over about a 26 km riverine segment from the river's mouth upstream to the confluence of the east and west forks. This region can be characterized as the lower river, where gradient is low to moderate. From their mouth, both the east and west forks rapidly ascend the ridge side, becoming high gradient channels dominated by cascades and falls. These ridge side, riverine segments were not sampled, though the fauna present should be substantially similar to the large tributary communities found in Cole and Coho Creeks.

After ascending the ridge sides, both forks traverse higher elevation, U-shaped valleys. Gradient appeared moderate in this high valley segment from an aerial survey. Extensive muskeg could be seen adjacent to the river channel in these valleys. Coniferous trees are more scattered and smaller. Soils are thin, with much exposed bedrock at higher elevations. Both forks are partially fed by glacial melt-water. There was no opportunity during the August 1995 survey to sample these higher elevation riverine and stream habitats. Invertebrate communities in the lower river can be characterized as follows:

• Low total taxa richness (average 22 taxa).
• Low EPT richness (average 12 taxa)
• High dominance of the community by a single taxon (average 41%)
• High dominance by mayflies (average 62%)
• Dominance by scrapers and collector-gatherers feeding groups
• High proportion of intolerant mayflies, but other intolerant assemblages relatively rare
• Few semivoltine, or long-lived taxa present

There was no discernible longitudinal change in benthic invertebrate communities along the approximately 26 km of the lower river sampled. Communities from similar habitat types (e.g. riffles) were essentially the same from the mouth of the river to the sites 26 km upstream.

Greater taxa richness and especially EPT richness was found in riffles and cascades (compared with glides) undoubtedly because larger, more stable substrates (i.e. cobble/boulder) were found in the riffles/cascades. Glides most often had sand/gravel substrates that are more subject to scour and resorting during higher flows.

Dominant benthic invertebrates found at the riverine sites are listed in the KWA Technical Report. The dominant invertebrates are univoltine taxa that are relatively mobile; they can swim or drift to colonize optimum patches for shelter and feeding. Thus, they are able to persist in habitats where substrate stability is low.

Cobble and gravel substrates dominate riffles and glides in the lower river. Sand and fine gravel substrates are dominant in slower glides, pools, and slackwater areas along the banks. Much of the river bottom appears to receive substantial scour from bedload transport of sand/gravel during peak flows (during spring snow-melt or rain-on-snow events in the fall). A substantial portion of the river bottom also appears to be subject to annual resorting of surface deposits during peak flows. Little detritus was found entrained in the armor layer at I any site, nor were detrital accumulations common in slackwater areas. The river has sufficient hydraulic power to push most bole-wood out of the system or to leave it stranded on the banks. Overall habitat complexity and stability is low. The primary energy source for the benthic invertebrate community appears to be diatom production.

Large tributary streams

Coho and Cole Creeks were the principal large tributary streams sampled. Both of these streams are located in the lower Kawesas River valley. Headwater tributaries were not sampled. Invertebrate communities in the larger tributaries can be characterized as follows:

• Higher total and EPT taxa richness than found in riverine habitats.
• High dominance of the communities by a single taxon (average 35.3%).
• Very high dominance by mayflies (average 63.8%).
• Increased dominance by collector-gatherers compared with riverine habitats.
• High proportion of intolerant mayflies.
• Higher proportion of intolerant stoneflies, caddis-flies and dipterans, than found in riverine habitats.

Though there is some increase in the total number of taxa to be found in the large tributaries, and some reordering of the dominant taxa, the invertebrate communities present are substantially similar to the riverine habitats.

Annual scour and resorting of large stream substrates, though probably not as great as in the main river, is high. However, there is a larger proportion of boulder substrate in the large tributary streams that is relatively stable and adds to habitat complexity.

Small tributary streams

Invertebrate communities in small tributary streams were more diverse and significantly different than those found in the large tributaries and riverine reaches. There was also considerable variation in community composition between small tributaries. Much of this variation is probably related to substrate stability and frequency and magnitude of disturbances. Small streams in avalanche tracks that experience high and frequent disturbance had communities substantially different from small, forested tributaries where disturbance was low and infrequent. Small tributary invertebrate communities can be characterized as follows:

• Higher average taxa richness than riverine and large tributary sites
• Higher taxa richness and percent contribution of chironomid midges
• Much lower percent contribution of mayflies
• Higher caddisfly (Trichoptera) percent contribution
• Surprisingly low stonefly (Plecoptera) richness and percent contribution
• Dominance by collector-gatherers
• Higher proportion of semivoltine taxa

Dominant organisms present in the small streams were quite variable. When averaged across all sites, the 10 dominant taxa were spread among most of the insect orders, and included non-insects.

Lentic and pool habitats

A limited number of qualitative samples were taken from lentic and pool habitats. These included pools in muskeg areas of the Kawesas River floodplain, side-channel sloughs off the main river, beaver ponds on tributary streams, and pools in the delta marsh created by grizzly bears digging for roots.

The limited lentic and pool collections considerably expanded the total list of aquatic invertebrates collected from the Kawesas River watershed. Lentic habitats with the highest richness were ponds and muskeg pools in the river's floodplain. Slough and side-channel pools off the main river did not display much taxa richness. However, production of benthic invertebrates in off-channel sloughs and ponds may have particular importance for sustaining juvenile salmonids that use these areas as rearing habitats. Further baseline information on these habitats is needed.

Chironomid midges were richly represented in the lentic habitats. Taxa were found that represented a full range of trophic conditions, from ultra-eutrophic (nutrient rich) to oligotrophic (nutrient poor). Several caddisflies typical of boreal North American lentic habitats were found (e.g. Glyphopsyche irrorata and Banksiola).

One adult of the dragonfly Tanypteryx hageni was captured in the vicinity of Cole Creek. Larvae of this semi-aquatic dragonfly are assumed to be associated with muskeg areas in the floodplain. In Canada, this rare dragonfly has only been reported from southern B.C.. The Kawesas River collection represents a considerable range extension, and has resulted in a note submitted for publication elsewhere (Anderson and Wisseman in press).

Conclusions

On the whole, running waters in the Kawesas watershed support a typical, but truncated version of benthic invertebrate communities common to western montane streams of lower latitudes.

The Kawesas was easily sampled in less than a week's time, and can be resampled on an annual basis to provide a long-term baseline for gauging the health of the watershed. Trend monitoring can be assessed with a multimetric index of biological integrity (IBI), as well as multivariate approaches (Fore et al. 1996).

Comparative data from watersheds experiencing a range of human impacts in the general region of the Kawesas is undoubtedly scant or non-existent. This is the sort of information necessary for metric selection and scoring criteria calibration used in constructing a sensitive IBI for general application across the region.

An interim IBI, specific for trend monitoring in the Kawesas, can be constructed using baseline data gathered preferably over a several year period. Subsequent years' trend monitoring data can then be compared and scored against the baseline.

Potential metrics for an initial IBI specific for the Kawesas watershed are: (1.) Total number of taxa; (2.) Number of Ephemeroptera + Plecoptera + Trichoptera taxa (or the number of taxa in each of these insect orders separately); (3.) Percent single or percent 3 dominant taxa; (4.) Percent intolerant or cold-water taxa, either as an aggregate percentage, or separated out as assemblages (e.g. intolerant mayflies); (5.) Percent of certain insect orders (e.g. Ephemeroptera, Plecoptera, Trichoptera, Diptera); (6.) Number of taxa and percent contribution of "tolerant" invertebrate groups.

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