Egg development and hatching success in alpine chironomids

Climate change affects both alpine freshwater ecosystems and their benthic communities, dominated by Chironomidae (Diptera). Previous studies assigned each chironomid species a narrow thermal niche and a correspondingly high sensitivity towards global warming. Nevertheless, to date no study has examined egg development although the potential effect of changing temperatures on this initial phase of life might be crucial to existence and/or survival in harsh environments. Adaption towards low temperatures might result in the most efficient egg development (lowest degree‐day demand) at the species’ temperature optimum close to the freezing point, theoretically allowing for bivoltinism. Similar to the eggs of other benthic insects, chironomid eggs could be the most temperature resistant life stage, not limiting the species distribution and persistence with regard to climate change effects. Here, we show the effect of water temperature on the egg development in five Diamesa (Diptera, Chironomidae) and five Orthocladiinae (Diptera, Chironomidae) species from a spring‐fed and a glacier‐fed alpine stream in the Stubai Alps (Austria). Laboratory breeding experiments were carried out at four different temperatures (2, 4, 8, and 12°C). Hatching success of all tested species proved to be consistently high throughout all experimental treatments, suggesting a high plasticity against a wide range of rearing temperatures. With increasing temperatures, the development rates increased. The degree‐day demands, however, also increased, indicating a special physiological adaption of cold‐stenotherm chironomids towards low temperatures. The knowledge presented herein regarding the temperature dependency of chironomid egg development was used to predict the potential date of hatching for each tested species in their natural environment, taking into account thermal profiles of their habitat and theoretically allowing for bivoltinism in cold‐streams. As this is only a first contribution towards the modelling of alpine chironomid life cycle dynamics under climate change scenarios, further experiments of this kind are urgently needed to determine the threshold temperatures for successful egg development, to include other abiotic parameters (e.g. photoperiod), and to expand the number of species.