Dataset and scripts from: Dietary sterol availability modulates heat tolerance of Daphnia

The increasing frequency and intensity of summer heat waves is pushing
freshwater zooplankton towards their upper thermal tolerance limits. At
the same time, higher temperatures and prolonged water column
stratification can favor the dominance of cyanobacteria in phytoplankton.
Even when not toxic or grazing resistant, these prokaryotes lack
phytosterols as essential precursors for cholesterol, the main sterol in
animal tissues. Cholesterol plays a crucial role in the physiological
adaptation of ectotherms to high temperature. Therefore, the shift to
cyanobacteria-dominated systems may increase the vulnerability of
zooplankton to heatwaves by intensifying cholesterol
limitation. Here, we used death time curves that take into
consideration the intensity and duration of a thermal challenge and a
dynamic model to study the effects of cholesterol limitation on the heat
tolerance of the keystone species Daphnia magna and to simulate the
cumulative mortality that could occur in a fluctuating environment over
several days of heatwave.  We show that increasing cholesterol
limitation does not affect the slope between time-to-immobilization and
temperature, but does decrease the maximal temperature that Daphnia can
withstand by up to 0.74°C. This seemingly small difference is sufficient
to halve the time individuals can survive heat stress.  Our
simulations predicted that, when facing heatwaves over several days, the
differences in survival caused by cholesterol limitation build up rapidly.
Considering the anticipated intensity and duration of future (2070-2099)
heatwaves, cholesterol limitation could increase mortality by up to 45%
and 72% under low- and medium-greenhouse-gas-emission scenarios,
respectively. These results suggest that the increasing risk of
cholesterol limitation due to more frequent cyanobacterial blooms could
compromise the resistance of zooplankton populations to future heatwaves.
More generally, this study shows the importance of considering the
nutritional context in any attempt to predict ectotherm mortality with
increasing temperatures in the field.