| dc.description.abstract | Impending natural resources development and concern about the effects of climate change have
spurred increased efforts to study and monitor aquatic habitats in the Mackenzie River system. As
part of Environment Canada’s attempt to survey the system in advance of the construction of the
Mackenzie Gas Pipeline, benthic macroinvertebrates were sampled at 50 streams spanning the
geographical range of the Mackenzie system in the Northwest Territories, Canada, to assess spatial
patterns in diversity and assemblage structure and the environmental factors driving them. Replicated,
quantitative D-net samples were collected during the late summer of 2005 through 2008, mostly at
crossings of the proposed pipeline route.
373 macroinvertebrate taxa were recorded, mainly aquatic insects, which were identified to
the genus or species levels; other groups were identified to higher taxonomic levels. Ephemeroptera
and Plecoptera diversity declined along a latitudinal gradient, while Trichoptera diversity declined in
the middle of the latitudinal range and rose towards the far north. Chironomidae (Diptera) increased
in diversity and abundance towards the far north, becoming dominant in the northern sub-arctic forest
and lowland tundra of the Mackenzie Delta. Diversity, measured as the average generic richness per
stream, correlated with a composite environmental variable representing stream size, but not much
else; spatial trends in local generic richness were only apparent in the far north of the study area.
Regional diversity was assessed using rarefaction curves and showed a clear decrease from south to
north across the study area for most taxa; the major exception was the chironomid subfamilies
Orthocladiinae and Chironomini, the former being diverse throughout the study area and the latter
increasing in diversity on the tundra. Odonata, Hemiptera and Coleoptera were well-represented in
the south of the study area, but decreased sharply in diversity and abundance in the north; another
common order, Megaloptera, was entirely absent from the study area, as were crayfish.
Community composition varied along a latitudinal gradient, with some species restricted to
northern latitudes and many more species restricted to the southern areas. Composition varied by
region, as did the environmental factors that control it. Streams in the north of the system are
connected to hundreds of small lakes and tend to freeze in the winter, which increases habitat
stability; assemblages in this region were characterized by relatively large chironomids that are
usually associated with lentic habitats and by a lack of taxa that are intolerant to freezing. Substrate
was the main factor explaining differences in assemblage composition in this region. Just to the south,
alluvial streams are more common and permafrost is continuous with very shallow active layers,
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which likely results in intense discharge peaks and ice scour in the spring and flashy summer
hydrographs. Invertebrates in this region were mainly short-lived, small sized orthoclads, baetids and
chloroperlids; the annual disturbance regime seems likely to be an important factor shaping
community composition in this region. Many streams in this region received input from saline
springs, resulting in perennial flow, and these streams harboured several taxa that were absent or rare
in other streams at similar latitudes, including several stoneflies (e.g. Pteronarcys, Sweltsa); the
presence of flow during the winter was found to be a major factor affecting community composition
in this region, which surrounded the town of Norman Wells, NT. Nutrient dynamics appeared to be
important in structuring benthic assemblages in the southern portion of the study region, with highnutrient
streams supporting a diverse fauna which included many taxa that were absent in the north,
while communities in low-nutrient streams were more similar to the northern alluvial stream fauna.
There was no spatial distinction between low- and high-nutrient streams in the southern region, and
the difference may be due to the local conditions of permafrost, which is patchy and discontinuous in
the region.
Evidence that winter ice and permafrost conditions are important drivers of benthic
invertebrate diversity and community composition in the Mackenzie system, along with the latitudinal
gradients which are consistent with a temperature/climate gradient, raises the possibility that benthic
assemblages may be useful as indicators of effects of global climate change on freshwater habitats in
the Canadian north. More immediately, construction of the Mackenzie Gas Pipeline may affect stream
habitat due to sedimentation, and plans for the operation of the pipeline have raised concerns about
potential effects on permafrost conditions. Implications for development of a biomonitoring program
utilizing benthic invertebrates and their potential as indicators of climate change are discussed. | en |
