Ecology and production of invertebrates in a Canadian coldwater spring-springbrook system

Valley Spring is a hardwater rheocrene consisting of a single point issue and a springbrook some 60 m long. The fauna consists of > 60 taxa, with insects and mites predominating. During 1985–86, benthic densities varied both along the springbrook and at single stations over time (range 58 to 756 animals 900 cm⁻²). A clear longitudinal zonation of many of the major taxa occurred with the Diptera tending to be more abundant downstream, the Plecoptera and Acari more abundant upstream, and the Copepoda and Bivalvia most abundant in mid- and downstream sections. The Gastropoda and Trichoptera occurred throughout the system although, in the case of the latter, there were distributional differences at the species level (e.g. Diplectrona modesta downstream, Parapsyche apicalis and Lepidostoma vernale upstream). Life histories varied between major species and were grouped into: (A) those species in which there was a wide range of larval sizes for most of the year, with no easily discernible temporal succession of cohorts, e.g. L. vernale, P. apicalis, Sialis sp., Dixa sp., Metriocnemus sp., Trissopelopia sp., Micropsectra sp. and Mucronothrus nasalis (the generation time of species in this group ranged from < 1 year to > 2 years); and (B) those species in which there was a clear pattern of larval growth, with obvious temporal succession of cohorts, e.g. D. modesta, Nemoura trispinosa and Hyalella azteca (all of these species were univoltine). Differences between the “adaptive strategies” of these two life history types are discussed. Total secondary production in Valley Spring in 1985–86 was estimated to be 913.2 g wet wt yr⁻¹. Production upstream was more than twice that downstream (11.21 and 4.01 g wet wt m⁻² yr⁻¹, respectively). The three most abundant species of caddisfly together contributed > 30% of the total production, the stonefly N. trispinosa contributed > 17%, while the chironomids accounted for >16% of production at downstream stations but only 3% upstream.