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.