Thermal springs are characterized by year-round high temperatures and a total-dissolved-solids concentration that is generally higher than that of surface waters. Insects appear to encounter few constraints from the water chemistry of most thermal springs, but considerable constraint from the high water temperature. Indeed, because no insect lives above 50 °C and very few above 40 °C, few thermal springs offer favorable conditions for insects in the actual boil itself. Thermal spring insects live in the stream at some distance from the source, and they may be defined as living in habitats having temperature regimens that are influenced by geothermy in the sense that they are warmer than they otherwise would be. An annual mean water temperature that is 5 °C above the annual mean air temperature of the region can be used to define the downstream limit of geothermal influence.Thermal springs around the world have similar insect faunas; only four orders (Diptera, Coleoptera, Hemiptera, Odonata) are commonly represented, and each of these only by a handful of genera. Furthermore, the fauna of any one thermal spring is characterized by very few species, and the higher the temperature the lower the species richness. Both temperature and water chemistry may exclude certain species, and even whole orders, from thermal springs, these factors acting either directly, alone or in concert, or indirectly through competitive interactions. Even moderately warmed systems can significantly affect insect growth rates, and seasonal regulation of adult emergence through diapause is a common strategy of temperate-zone thermal spring insects.Thermal springs present many advantages to the ecologist, such as long-term habitat constancy, temperature stability, and taxonomic simplicity. They provide field laboratories for the study of temperature-related phenomena as well as the opportunity to explore a range of questions in biogeography and evolutionary biology. The challenge is to form the questions and select the systems critically.

Caddisfly collections from 25 springs across Canada reveal some general trends and some regional and habitat-related differences in spring faunas. In general the number of caddisfly species present in springs increases with habitat diversity. Limnocrenes and rheocrenes with low current and small-sized substrate particles support few caddisfly species, but may have large populations of individual species. Species categorized as grazers, shredders, and predators are common in springs, but filter-feeders are rare.Eastern and western springs have many genera but few species in common. About 35% of the species recorded in this study are from the family Limnephilidae, but the most frequently encountered genus was Parapsyche (Hydropsychidae), usually P. apicalis (Banks) in the east and P. elsis Milne in the west (British Columbia and Alberta). Other common genera in both east and west were Neophylax, Lepidostoma, and Rhyacophila. Common genera collected only in the west were Anagapetus, Homophylax, Psychoglypha, and Neothremma, and Frenesia and Pseudostenophylax were taken only in the east. Three analytical techniques — ordination by detrended correspondence analysis, constrained ordination by canonical correspondence analysis, and classification by two-way indicator species analysis — all confirmed an east/west geographical difference in caddisfly communities and pointed to elevation, extent of groundwater source, and summer temperature as environmental factors influencing, but not totally responsible for, east/west species distributions. Past and present barriers to migration both appear to be important. Riparian vegetation, current, substrate particle size, microhabitat diversity, and pH all have strong influences upon the composition of spring communities in both the east and the west. Springs in which caddisflies were primarily associated with detrital processing were dominated by Frenesia and Lepidostoma in the east but by Homophylax in the west. Scrapers and predators were abundant only in springs with relatively high microhabitat diversity, current speed, and PH.

The habitat preferences of each of the 663 species of aquatic Coleoptera known from Canada and Alaska were categorized as lentic, lotic, spring-inhabiting, other, or unknown. Most species were assigned to a single habitat type although some occur in more than one habitat. The distribution of species among these habitat types is as follows: lentic, 61%; lotic, 23%; springs, 8%; other, <1%, unknown, 8%. The 63 spring-inhabiting species are distributed among the families Dytiscidae (38 species), Hydrophilidae (nine), Hydraenidae (eight), Chrysomelidae (subfamily Donaciinae) (six), Haliplidae (one), and Dryopidae (one). The diversity of these families in springs only approximately parallels their diversity in the total fauna. Several relatively diverse families (Gyrinidae, Scirtidae, and Curculionidae) are absent from springs as are some predominantly lotic families (Amphizoidae, Elmidae, and Psephenidae). About half the spring species are western (occurring in Manitoba and west), about a quarter are eastern, and a quarter are transcontinental. Most of these spring species are known from the conterminous United States and it is suggested that spring habitats within Canada are being colonized slowly from southern refugia.The habitat affinities of spring-inhabiting Dytiscidae are examined in detail. Of the 260 species occurring in Canada, 38 species occur in springs and represent about 60% of all beetle species in Canadian springs. The spring fauna of dytiscids comprises four elements: nine (24%) inhabit springs only, 11 (29%) are lotic species that also occur in springs, 12 (31%) are species that use a broad range of habitats, and six (16%) are species known otherwise only from lentic habitats. The dytiscid fauna of springs is a heterogeneous assemblage derived from many separate phylogenetic elements.

This paper reviews and summarizes information on the systematics, distribution, life history, and ecology of water mites in spring habitats in Canada, primarily on the basis of new data. The fauna comprises over 115 species, representing 57 genera and 25 families, in three ecological groups adapted for living in helocrenes, rheocrenes, and limnocrenes, respectively, though many species are able to exploit more than one type of spring habitat. The evolution of adaptations in water mites for living in spring habitats is discussed within the context of new hypotheses on the origins and zoogeography of spring-inhabiting taxa. A synopsis of available data suggests that knowledge of the species composition and structure of water mite communities can be used to characterize springs, and to assess and monitor the impact of environmental changes on these habitats. Inadequacies in the current database on Canadian species are identified, specifically uncertain species identities, incomplete zoogeographic data, inconsistent definition of spring habitats, and inadequate collecting techniques. Future studies are proposed to encourage research designed to overcome these inadequacies, and improve understanding of the biological roles of water mites inhabiting springs.

From the literature 70 of the 212 chironomid genera known in the immature stage from the Holarctic Region are considered in this paper to be inhabitants of springs or seeps, or both. Three more are known only from adults collected at springs and seeps so that 73 out of 235 genera recognized from the Holarctic appear to be associated with these habitats. Literature records of distribution suggest that 65 of the 73 occur in northeastern North America. Collections from natural and manmade springs and seeps in Labrador yielded 45 genera; 40 of these genera belong to the set of 65 genera noted above. The other five are likely spill-overs into springs and seeps from adjacent habitats. Communities from manmade springs and seeps originating from the earth dykes and dams at Churchill Falls were compared with natural springs and seeps there and in Goose Bay, Labrador. Chironomid communities were similar at the generic level in both man-made and natural spring–seep habitats.

Ostracodes are a diverse group of marine and continental crustaceans that have radiated into virtually all oxygenated aquatic environments that persist for more than about a month. Continental ostracodes live in both surface water and groundwater.Ostracodes living in springs and seeps have typically been the subject of systematic rather than ecologic studies. These taxa may or may not occur in other surface-water bodies. Similarly, lacustrine taxa may or may not be found in springs. Spring taxa occurring in other surface waters are often found in ponds, marshes, streams, or on the edges of lakes where groundwater discharge is important. Groundwater discharge, unlike lake water, shows limited and predictable variability in chemistry and temperature during the year. That level of variability relative to lake water may define particular ostracode environmental gradients. The gradients would range from stable, high-volume discharge springs occupied principally by spring species to high variability lakes occupied largely by lacustrine species.Ostracode occurrences may also be described by parameters such as temperature, solute (dissolved ion) composition, solute concentration (salinity, conductivity, ionic strength), and calcite saturation indices. A plot of these parameters associated with the presence of a taxon illustrates its physiologic response to the environment, a field. Three general fields bounded by chemical parameters are delineated by existing data. Those fields are as follows: (1) a restricted range and (2) a full range of fresh water, and (3) both fresh and saline water. Fields bounded by temperature and chemistry are also recognized. The fields also offer a way of describing ostracode occurrences in terms of hydrogeology and climate.If ostracode occurrences are limited by major chemical and physical properties of the aquatic environment, then their habitat may be defined by certain physical–chemical principles. The same physical–chemical principles must apply to the past. The ecology of extinct taxa may, therefore, be defined in the same environmental terms as those for extant taxa.

Springs include a great variety of habitats, because many possible geological and ecological conditions intersect in any given spring.Available information on the arthropod fauna shows that springs contain a limited number of species of diverse origins, including groundwater, stream, and water-film inhabitants. There is a substantial number of spring-specialist species, many of them in distinctive genera, reflecting many independent invasions of spring habitats by various groups and subgroups of aquatic arthropods. Most of this diversity is present in cold water springs, though smaller numbers of distinctive elements occur in hot or in saline springs. The specialists of coldwater springs tend to show adaptations such as cold stenothermy and limited dispersal, but different species possess different suites of adaptations to the habitat, reflecting their evolutionary history and biology.Faunal differences among springs result from geographical differences (many species, though not as many genera, differ between eastern and western Canada), but within a given region reflect the variety of habitats and microhabitats that exists. Such variety means that except in very broad terms it is not possible to establish workable “definitions” for the range of spring types. Rather, we recommend that biologists adopt a few key descriptors, based on source geometry, water supply, temperature, chemistry, and persistence, to provide useful information about the sites in which they collect. The term “spring” should be used conservatively, to apply only to the area immediately around the point of groundwater issue, because conditions change rapidly farther away from this point.Some needs for the inventory (and protection) of springs and for more extensive sampling are summarized. Further taxonomic studies are required in several characteristic groups. Ecological work on the specialized species confined to springs is likely to be especially instructive.