Unexpected patterns of chironomid larval size in an extreme environment: a highly glaciated, alpine stream

Abstract

In this article, we report on the development and growth of alpine chironomid species in a highly glaciated headwater, using biometrical analyses. Glacially influenced alpine streams are characterized by year-round harsh environmental conditions. Only a few, highly adapted benthic insects, mainly chironomid larvae (genus Diamesa) live in these extreme conditions. Although several studies have shown patterns in ecosystem structure and function in alpine streams, cause-effect relationships of abiotic components on aquatic insects' life strategies are still unknown. Sampling was performed at Schlatenbach, a river draining the Schlatenkees (Hohe Tauern NP, Austria), at three sites and on six occasions from August to October 2015. Semi-quantitatively sampled Diamesa cinerella (Meigen 1835) and Diamesa steinboecki (Goetghebuer 1933) larvae were biometrically analysed, and they showed differences in larval size and biovolume with higher values close to the glacier.Considering the decreasing water temperatures but increasing benthic organic matter towards the glacier, food availability seems to play a crucial role for larval size in highly glaciated alpine headwaters. This is the first study to show that harsh conditions in these environments (low temperatures, high turbidity and flow dynamics) may exclude many taxa, but favour other, highly adapted species, when their essential needs (food quality and quantity) are guaranteed.

Egg development and hatching success in alpine chironomids

“…Only if the egg development is finished before summer will the larvae have enough time to grow, develop and morph into pupae in autumn. Considering the oviposition dates observed by the present study in spring, the high egg development rates obtained by the rearing experiments and the fast larval growth published by Schütz and Füreder (), not only univoltinism but bivoltinism is feasible for the alpine chironomids examined here.…”

“…() indicating a second main emergence window of alpine chironomid species (e.g. D. cinerella ) in late autumn, we concluded univoltine life cycles in cold‐adapted chironomids under extreme environmental conditions (Schütz & Füreder, ). Fast egg development in alpine chironomids, resulting in the setting of larval stages to summer, clearly is the key to their success in colonising cold alpine streams.…”

“…In another study, we observed fast larval development of Diamesa species in an alpine glacial headwater during summer and autumn as long as the essential needs of cold‐stenotherm chironomid larvae were met (low water temperatures, sufficient nutrients; Schütz & Füreder, ). Given the presence of pupae in early October, along with Füreder et al.…”

“…They can therefore be considered cold‐stenotherm and especially cold adapted (Marziali & Rossaro, ; Moog, ; Niedrist & Füreder, ). In particular, species of the genus Diamesa , specifically Diamesa cinerella and Diamesa zernyi, are known to inhabit even the uppermost stream regions of both spring‐ and glacier‐fed rivers, tolerating the extreme living conditions of the alpine zone (Füreder, Wallinger, & Burger, ; Moog, ; Schütz & Füreder, ). Egg rearing experiments confirmed the special cold‐hardiness of Diamesa species, with significantly higher development rates and thus lower degree‐day demands, especially at low temperatures, than Orthocladiinae.…”

“…Based on the models proposed by Milner and Petts (1994) and Milner, Brittain, Castella, and Petts (2001), Rossaro, Lencioni, Boggero, and Marziali (2006), Marziali and Rossaro (2013), and Niedrist and Füreder (2016) defined species-specific ranges of temperature for their occurrence, underlining the significance of water temperature for the dispersal of chironomid larvae. Recently, Schütz and Füreder (2018) were the first to report on unexpected fast larval development and high growth rates in chironomid larvae in a glacier-fed headwater, indicating their robustness and high productivity in these harsh ecosystems. Based on their observations, they suggested univoltine life cycles at these very low temperatures, which they assumed to be a highly efficient development strategy in cold stenothermic chironomid larvae.…”