Discussion

This experiment provides evidence that Zostera muelleri seeds from thermally affected areas have higher germination rates than those from ambient locations. Counterintuitively, this suggests that higher temperatures in situ may facilitate some form of transgenerational plasticity leading to increased seed germination in temperate environments under warming conditions. The differences in germination between thermally affected and control locations suggests that temperate Z . muelleri sourced from thermally affected areas are exhibiting some form of transgenerational plasticity which has reduced the inhibiting impact that higher temperatures have on germination. While most Zostera sp. has shown low germination rates at combinations of high temperatures and high salinities, there are exceptions. Tropical Z .muelleri appears to germinate more readily and not be inhibited as severely by high temperatures and high salinity (34 ppt), with much higher germination rates of 20 – 36 % depending on temperature (Tolet al. , 2021). Likewise, Z ostera marina, a different eelgrass species, saw increased germination at higher salinities with higher temperatures (Xu et al., 2016). Other transgenerational adaptions to increased temperatures have been observed in Z. marina which highlighted that exposure to a heat wave by a parent plant stimulated an increase in above-ground biomass and the favouring of offspring shoot production rather than parent shoot maintenance (DuBois et al., 2020). Similarly, population specific resilience has been identified in another seagrass species (Halophila ovalis ) for both abiotic (salinity fluctuations; Webster et al , 2021) and biotic factors (grazing; O’dea et al , 2022). Increasing germination tolerance under higher temperatures may be a way for Z . muelleri to maintain population resilience when faced with a warming climate. Seeds having a higher likelihood to germinate would allow seagrass species to germinate more frequently and in the absence of a storm surge (which would bring favourable temperatures + salinities) which is important for species recovery after a stress event (Smith et al., 2016). An increase in germination at higher temperatures and salinities could lead to greater population resilience by increasing the window of ‘suitable conditions’ for germination, which typically are shown to be cooler temperatures and lower salinities (Conacher et al., 1994; Stafford-Bell et al., 2016);conditions usually limited during winter months where storms are more frequent and temperatures cooler. This study also reinforced that Z . muelleri is likely dependent on freshwater pulses and concomitant cooler waters which come with storm surges to significantly increase germination rates. This is similar to other species where lower temperatures and salinities saw much higher germination rates (Stafford-Bell et al., 2016; Cumming et al , 2017). While seeds in this study experienced increased germination and faster mean time to germination at lower salinities. In a different species, Zostera nigricaulis, the effect of a freshwater pulse to begin the experiment eliminated the influence of salinity on seed germination (Cumming et al , 2017). This would suggest that in our study, which also had a freshwater pulse to begin the experiment, salinity should not have influenced germination. Possibly, the much larger difference in salinities between the 8 ppt treatment and the 34 ppt treatment of the present study explains why salinity still had an effect on our seeds, as Cummingset al (2017) examined a much tighter salinity range of 25 ppt – 35 ppt. Overall, the experiment saw very low germination rates however, this is not entirely uncommon for temperate Z . muelleri subject to salinity treatments of >32ppt. It is also important to consider that large amounts of seeds are being produced, which means low germination still results in a high number of seedlings. This is demonstrated by a closely related species, Zostera Marina , wherein situ experiments have seen similar germination rates of 4.7 – 13.8 % (n = 50,000; Orth et al. , 2003). Typically, higher germination rates are seen in low salinity treatments (< 20 ppt) and treatments of 30+ ppt rarely exceed 0-10 % germination in temperate Z . muelleri (See Conacher et al., 1994; Stafford-Bell et al., 2016). Due to these low numbers, it is likely that some other germination cue such as dissolved oxygen content (Brenchly and Probert, 1998) or the effect of sediment microbes is required (Tarquinio et al., 2019). While more suitable conditions may be uncommon in estuaries, they are not entirely unlikely in a near-shore environment where Z . muelleri resides which would receive high amounts of freshwater from runoff (York et al., 2013; Collier and Waycott, 2014). Despite low germination numbers, the outcomes from this paper highlight significant ecological implications and may mean that temperateZ . muelleri is approaching an optimum period of increased germination rates as temperatures trend upwards, given we recorded higher germination rates in seeds sourced from adult plants living in higher temperatures. The evidence presented here suggests that under certain scenarios, Z . muelleri will experience a net increase in germination as temperatures increase. However, this would depend on other reproductive metrics such as reproductive shoot densities, spathe counts and seed viabilities remaining static under increasing temperatures. It would also be advantageous to understand if this interaction is isolated to estuaries with thermal plumes or if systems which experience similar ambient temperatures to the plumes also show a similar germination response.