1 INTRODUCTION

Organisms commonly encounter environments that negatively impact their fitness, and many environmental stressors recur on a seasonal basis. Representative examples are temperate eutrophic lakes and ponds that experience seasonal algal blooms dominated by toxic cyanobacteria (O'Neil et al., 2012; Paerl & Huisman, 2008). Zooplankton that feed on phytoplankton, such as the non-selective grazer Daphnia, come into direct contact with cyanobacteria during algal blooms, which typically occur in mid- to late summer (Hansson et al., 2007; Sivonen et al., 1990; Sommer et al., 1986).

Cyanobacteria are known to affect Daphnia negatively in several different ways, including through their low nutritional value, mechanical interference with the filtering apparatus and through the production of toxins (e.g., Ger et al., 2016; Porter & Mcdonough, 1984; Rohrlack et al., 2001; von Elert et al., 2003, 2012). One of the best-known groups of cyanotoxins are the microcystins, which consist of c. 100 variants (Carmichael, 1994; Dawson, 1998; de Figueiredo et al., 2004; Meriluoto et al., 2017). In Daphnia, both purified and cell-bound microcystins have been shown to be acutely toxic and decrease survival (e.g., DeMott et al., 1991; Rohrlack et al., 2001), growth rates (e.g., Lürling, 2003) and reproductive output (e.g., Gustafsson et al., 2005). Despite the documented negative effects of the toxins, several zooplankton, including Daphnia, show a remarkable ability to co-exist in environments dominated by bloom-forming algae, and some studies even suggest that Daphnia not only can cope with, but also effectively graze down and reduce toxic cyanobacteria, even at high toxin concentrations (e.g., Chislock et al., 2013; Sarnelle, 2007).

Daphnia populations that live in lakes with regular cyanobacteria blooms can evolve local adaptation to their toxins, including microcystin (Hairston et al., 2001; Sarnelle & Wilson, 2005). However, this does not fully explain seasonal changes in susceptibility to microcystin-producing cyanobacteria (Hansson, Gustafsson, et al., 2007). Instead, Daphnia populations may only develop tolerance to microcystin during periods of high concentration, and lose that tolerance during times of the year when exposure is low (Schaffner et al., 2019). Such adaptive seasonal change could involve selective elimination of unfit individuals, lineages or genotypes, or via some form of non-genetic inheritance. A recent study by Schaffner et al. (2019) showed that clones collected during an algal bloom had higher juvenile growth rates when fed phytoplankton typical to a bloom event compared to clones collected before the bloom, suggesting adaptive change within months. Other studies suggest that Daphnia can respond adaptively to microcystin-producing cyanobacteria via phenotypic plasticity, and that such responses can accumulate across at least two generations (Gustafsson & Hansson, 2004, Gustafsson et al., 2005, Ortiz-Rodriguez et al., 2012; but see Radersma et al., 2018).

Studies like these typically are designed to test for differences between genotypes, not individuals, which means that often only a limited number of clones are isolated and kept for several generations before being tested. However, genetic differences and plastic responses in Daphnia reared for long periods in the laboratory may not reflect well the absolute and relative performance of individuals before, during and after exposure to toxic cyanobacteria in a natural environment. In fact, there are very limited data on how Daphnia that come directly from natural populations actually respond to cyanotoxins, and it is unknown if these responses change across the season and following a bloom in a manner expected under adaptive change (but see Schwarzenberger et al., 2013).

The aim of this study was to fill this gap in knowledge. We sampled Daphnia from five lakes in spring, summer and autumn, and quantified the body-size distribution of the different populations. To test whether or not Daphnia from these lakes and across the season differed in their tolerance to microcystin, we quantified reproduction and survival in a laboratory experiment in which we exposed individuals to cyanobacteria that either produce or do not produce the cyanotoxin microcystin. By measuring the concentration of microcystin in the five lakes, we also were able to assess if recent exposure to microcystin were associated with changes in body size, reproduction and survival.

Based on previous laboratory experiments (e.g., Gustafsson et al., 2005; Hansson, Gustafsson, et al., 2007; Radersma et al., 2018), we hypothesised that exposure to toxic cyanobacteria in the lakes would generally reduce body size, viability and reproduction. Thus, viability and reproductive fitness should be highest in spring (i.e., before blooms) and decline as the concentration of microcystin in the lakes increases. We also predicted that, if populations successfully adapt to toxic cyanobacteria (through genetic change or transgenerational plasticity), it would change the relative fitness of individuals on a toxic versus non-toxic diet. Specifically, the negative effects of the toxin on reproduction and survival relative to the controls should be smaller following the bloom (even if both reproduction and survival may be worse for individuals collected after the bloom than those collected before the bloom).