ABSTRACT Hydropeaking, associated with flexible hydropower, produces frequent and dramatic water‐level fluctuations causing repeated drying of shallow areas near river banks and gravel bars. These marginal habitats are crucial for the reproduction of many aquatic and semi‐aquatic species including insects, whose complex life cycles involve transitions between aquatic and terrestrial environments. The sensitivity of stream insects to sub‐daily variation in flow velocity and water levels induced by hydropeaking has mainly been assessed for larvae, whereas the sensitivity of adults and eggs remains largely unknown. For species that attach their egg masses to substrates in shallow waters, fluctuating water levels could expose eggs to air, causing desiccation and mortality. Such hydropeaking‐driven constraints may determine population‐level reproductive success and potentially lead to population bottlenecks. The aim of this study was to determine the sensitivity of stream insect eggs to dry phases. An experiment was performed with Baetis spp. mayfly egg masses in flumes. Egg masses from a wild population were exposed to one of 10 treatments: a dry phase lasting between 1 and 18 h or a control treatment with no dry phase. The treatment was followed by an incubation period in controlled conditions allowing for development and hatching of viable eggs. The hatching proportion was calculated for each individual egg mass based on automated counting of the total number of eggs per mass and manual counting of the number of either hatched or unhatched eggs per mass. A mean of 88%–98% of eggs hatched after 0–5 h dry phases. The proportion declined to 45% after 6 h then to near zero after 9 h dry phases. Dense hyphae developed on egg masses exposed to dry phases lasting 6–18 h. Our study provides some of the first estimates of stream insect egg mass responses to dry phases under optimal and reproducible controlled conditions. Hatching failure at dry‐phase durations ≥ 6 h was likely due to desiccation‐induced egg mortality, which then allowed for the growth of aquatic oomycete hyphae, stream‐dwelling decomposers of organic matter. We highlight the need to consider impacts of flow regulation—and specifically, artificial dry phases associated with hydropeaking operations—on all life stages of aquatic organisms to inform sustainable management strategies that support biodiversity within regulated rivers.
Gautronneau et al. (Sat,) studied this question.