Egg development and hatching success in alpine chironomids

Abstract

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.

Glacial Stream Ecology: Structural and Functional Assets

“…The averaged original dataset revealed that at the coldest treatment, Diamesinae had a lower day-degree demand (36.2 ± 7.2) than Orthocladiinae with 52.8 ± 11.8 ( Figure 3A,B). As highlighted in the original study [19], the averaged degree-day demand across chironomid species was also significantly higher with a higher breeding temperature. The lowest demand was found for Diamesinae in the coldest breeding temperature (2 • C) with on average 36.2 ± 7 degree-days, whereas the highest demand of the same subfamily was at 12 • C with 95.7 ± 18.8 degrees-day ( Figure 3B).…”

“…This study was built upon recent field surveys carried out on alpine streams and rivers within our long-term monitoring program. Besides presenting data on environmental conditions, as well as structural and functional characteristics (assessments in 2014), we re-analyzed some experimental raw data from already published sources (physiological data [17], isotope data [18], and experimental data [19], assessments in 2014, 2015, and 2017, respectively), where we think these new results specifically mirror functional assets in harsh environments, such as glacial rivers. We specifically aimed at defining cause-effect relationships (glacier cover vs. functions) by overlaying the invertebrate community data and environmental settings with the functional characteristics.…”

“…To assess the potential influence of degree glacier cover in the catchment on the egg development of alpine stream invertebrate species, we used experimental data from Schütz and Füreder [19] and compared the identified day-degree demands at distinct breeding temperatures between typical glacial specialists (Diamesinae) and other chironomid species. We then used this published data set (day-degree demand per breeding temperature level, Schütz and Füreder [15]) and combined it with the environmental characters (water temperature and catchment glacier cover) and invertebrate communities of sampled glacial and non-glacial streams in the Hohe Tauern national park.…”

“…The lowest demand was found for Diamesinae in the coldest breeding temperature (2 • C) with on average 36.2 ± 7 degree-days, whereas the highest demand of the same subfamily was at 12 • C with 95.7 ± 18.8 degrees-day ( Figure 3B). Linking the experimental dataset and the invertebrate community data in sampled reaches, we found that the taxa selected for the experiments in [19] were dominant, especially in streams with high glacier cover in the catchment ( Figure 3D). While the selected Diamesa taxa contributed considerably to the occurring invertebrate communities in cold and highly glaciated streams ( Figure 3D), the contribution of Orthocladiinae taxa selected for the experiment was only marginal in natural non-glacial study streams ( Figure 3C).…”

“…While the selected Diamesa taxa contributed considerably to the occurring invertebrate communities in cold and highly glaciated streams ( Figure 3D), the contribution of Orthocladiinae taxa selected for the experiment was only marginal in natural non-glacial study streams ( Figure 3C). The temperatures selected for breeding experiments in [19] represent significantly (p < 0.001) a negative relationship between catchment glacier cover (in %) and water temperature of the study streams and reflect the natural gradient of summer water temperatures in the same reaches (compare experimental and natural water temperature range, e.g., Figure 3B vs. Figure 3D). negative relationship between catchment glacier cover (in %) and water temperature of the study streams and reflect the natural gradient of summer water temperatures in the same reaches (compare experimental and natural water temperature range, e.g., Figure 3B vs. Figure 3D).…”