Indirect consequences of hypolimnetic hypoxia on zooplankton growth in a large eutrophic lake

Diel vertical migration (DVM) of some zooplankters in eutrophic lakes is often compressed
during peak hypoxia. To better understand the indirect consequences of seasonal
hypolimnetic hypoxia, we integrated laboratory-based experimental and field-based observational
approaches to quantify how compressed DVM can affect growth of a cladoceran, Daphnia
mendotae, in central Lake Erie, North America. To evaluate hypoxia tolerance of D. mendotae, we
conducted a survivorship experiment with varying dissolved oxygen concentrations, which
demonstrated high sensitivity of D. mendotae to hypoxia (≤2 mg O2 l⁻¹), supporting the field observations
of their behavioral avoidance of the hypoxic hypolimnion. To investigate the effect of temporary
changes in habitat conditions associated with the compressed DVM, we quantified the
growth of D. mendotae, using a 3 (food quantity) × 2 (temperature) factorial design laboratory
experiment. Neither food quantity nor temperature affected short-term growth in body length of
D. mendotae. However, D. mendotae RNA content (an index of short-term condition) decreased
under starvation, indicating an immediate response of short-term feeding on condition. We further
evaluated the effect of hypoxia-induced upward shifts in vertical distribution by quantifying the
RNA content of D. mendotae from central Lake Erie before and during peak hypoxia. Despite high
temperature and food quantity in the upper water column, RNA content in field-collected D. mendotae
remained low during peak hypoxia. Furthermore,D. mendotae collected during peak
hypoxia consisted of only small-bodied (<~1.25 mm) individuals, suggesting that behavioral
avoidance of the hypoxic hypolimnion may also have indirect fitness costs.