Cumulative effects matter: multi‐brood responses of Daphnia to hypoxia

Abstract

Summary Periods of hypoxia lasting up to weeks are now anticipated in fresh waters, owing to anthropogenic influences. However, the cumulative effects of hypoxia on Daphnia, over multiple broods, have received virtually no attention, and to establish and evaluate such responses there is a need to make measurements over a wide range of oxygen concentrations, potentially revealing non‐linear patterns. We predict that the effect of hypoxia on growth, survival and fecundity (i.e., production of new individuals) of Daphnia will increase over multiple broods, and with increasing oxygen these responses will approach asymptotic maxima, following a rectangular hyperbolic response. Daphnia similoides were exposed to 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 and 8.0 mg oxygen L−1. To determine effects on the first brood we examined: number of offspring; time to the first eggs; time to the first brood and size of the female at the first eggs and the first brood. To determine cumulative effects of oxygen over multiple broods (up to 8 broods over 21 days), we measured: total number of offspring produced by a female; survival time and total number of moults and broods. To investigate how the cumulative effects arose over the multiple broods, we examined the number of offspring per brood in each brood over eight broods. To assess treatment effects and indicate responses, functions were fit to data using the most parsimonious function that reflected trends in the data. Measurements associated with a single brood responded linearly, or not at all, with changing oxygen concentration, whereas measurements made over the 21 days followed a rectangular hyperbolic response, increasing to an asymptote as oxygen increased. For the first brood, as oxygen concentration was raised from 1 to 8 mg L−1 the number of offspring produced and the time required to produce the brood were not affected; the time required to produce eggs decreased ˜0.3‐fold; and the size of individuals at the time when the eggs and the brood were produced increased ˜0.1‐fold. Over the 21 days, between 1 and 8 mg L−1 the total number of offspring increased ˜3.4‐fold; individual survival and the number of moults increased ˜2‐fold, and the number of broods increased ˜1‐fold. For single‐brood responses, there was no effect of decreasing oxygen levels on the number of offspring in the first brood, but there were negative effects on the second‐to‐fourth broods; the number of offspring in the remaining broods was not significantly related to oxygen levels, as there were fewer data at low levels due to poor survival. We conclude that assessments of Daphnia demographics should not rely on estimates of the effect of oxygen concentration on single broods. Instead, studies should consider cumulative changes over multiple broods. Following our approach, researchers may now explore the impacts of hypoxia on congeners and other zooplankton, and investigate the mechanisms associated with multi‐brood responses.

Effects of Hypoxia and Hypomagnetic Field on Morphometric and Life-History Traits in Freshwater Cladoceran Daphnia magna

“…Thus, the moving of crustaceans to deep layers of the hypolimnion with low oxygen levels in natural habitats led to the depression of reproduction in terms of a high percentage of males in the population and the low egg ratio [27]. A decrease in the reproductive potential of daphnids and an increase in the period between broods with a reduction of dissolved oxygen concentration in experiments were registered earlier [28,29]. Another well-described effect of a decrease in dissolved oxygen level is body size reduction [30].…”

Effects of seasonal hypoxia on macroinvertebrate communities in a small reservoir

“…Although reduced fish predation associated with hypoxic conditions can benefit zooplankton populations, low dissolved oxygen concentrations (<1 mg/L) have been shown to negatively impact the growth (Nebeker et al, 1992), reproduction (Hanazato et al, 1989; Lyu et al, 2016), survival (Nebeker et al, 1992) and abundance (Roman et al, 1993) of zooplankton. The mid‐summer oxygen concentrations in the hypolimnion of Lake Alvin from 2009 to 2011 were typically <0.5 mg/L, a level toxic to Daphnia spp.…”

Inducible defensive traits of Daphnia pulex against predators are mainly determined by the offspring's environment rather than their mother's experience

“…In addition to the transmission of phenotypes between two generations, the transmission of phenotypes in multiple broods within a generation also shows differences (Donelson et al., 2018). Studies have found that the performance of Daphnia offspring varies according to the order of brood (Lyu et al., 2016), and a study of multi‐brood offspring of rotifers also showed a similar result (Yin et al., 2015). When mothers are continuously exposed to predator kairomone for a long time, it may be reasonably speculated that, due to cumulative effects, the offspring can better inherit the mothers' phenotypes and perform stronger defences.…”