Hydropeaks occur when water is released from dams for power generation, causing water levels to rise abruptly, and to fall abruptly when the turbines are switched off. Hydropeaks impact aquatic fauna in a variety of ways, but one hypothesised effect is that sudden changes in water level may cause mass mortality of aquatic insect eggs – curbing recruitment of benthic larvae and, potentially, limiting population sizes.
Three studies have examined this effect for aquatic insects that lay eggs obligately on Emergent Rocks (rocks protruding from the water). All follow a simple model: (1) raised water levels submerge existing Emergent Rocks, but create “Ephemeral Emergent Rocks” (EERs) in areas that are temporarily inundated; (2) aquatic insects lay eggs on EERs; (3) abrupt recession exposes eggs on EERs to air, desiccation and mortality. Current studies assume parts 1 & 2, demonstrate desiccation of eggs exposed to air in laboratory tests (part 3 only) and claim severe implications for populations. As a counterpoint, we test the full model (1-3) in situ and challenge the assumption that aquatic insects lay eggs in environments harmful to their young.
For the caddisfly Ulmerochorema rubiconum (Hydrobiosidae), we conducted daily surveys of egg-laying in the upper Snowy River (NSW) below Guthega Power Station. Among days with (n = 5) and without (n = 9) hydropeaking flows, we compare: (1) patterns of habitat availability (Emergent Rocks); (2) where eggs were laid (among rocks; within rocks); (3) egg mortality (%) via desiccation.
Field data do not support a hypothesis that U. rubiconum lay eggs during hydropeaks. No eggs were laid on EERs and sudden changes in water level (0.5 m in ~3 min) had minimal effect on egg survival. Instead, small variations in baseflow caused egg mortality (desiccation) by exposing eggs that were laid close to the waterline during baseflows.