Introduction
Climate change threatens marine ecosystems globally and stands to
disrupt thermal optimums to which species have adapted (IPCC, 2019). For
seagrasses, a rise in temperature may see species disappear as
temperature extremes in the shallow water rise (York et al., 2014). It is likely that projected increasing temperatures and
salinities in estuaries (Scanes et al., 2020) will significantly
inhibit temperate seagrass species’ ability to successfully germinate.
Seagrasses are a prominent habitat in estuaries on the east coast of
Australia which support fish as critical nursery habitat (Heck et al.,
2003; Nagelkerken, 2009; Olson et al., 2019). Further, they
significantly contribute to sediment stabilization (Christianen et
al., 2013), carbon sequestration (Macreadie et al., 2014) and
consequently have significant value economically (Costanza et
al., 2014; Röhr et al., 2016). Seagrass habitats are threatened
worldwide from a variety of factors such as urbanization, pollution and
climate change which has resulted in a decline of 25% of all known
seagrass meadows between 2000-2010 (Turschwell et al., 2021).
However, seagrasses are highly resilient to significant short-term
changes in temperature, light and salinity associated with near-shore
ecosystems (York et al., 2013). A key component forming this
resiliency in seagrass ecosystems has been attributed to efficient
regenerative strategies employed through asexual or sexual reproduction
(Smith et al., 2016). Australia hosts a rich variety of
seagrasses on the east coast, including Zostera muelleri ,Heterozostera nigricaulis , Posidonia australis ,Halophila ovalis and Halophila decipens (Thomson, 1959,
Macreadie et al. 2018). Zostera muelleri has the most wide spread
geographical distribution, forming large monospecific beds in estuaries
along the east Australian coastline (York et al., 2013).
Reproduction in Z. muelleri can be achieved through asexual or
sexual methods with a combination of both providing optimal reproduction
(Kendrick et al ., 2012; Macreadie et al., 2014;
Stafford-Bell et al., 2015). Asexual clonal growth through
rhizome elongation allows efficient expansion within a meadow which aids
in population resilience and can result in more successful colonization
of bare substrates but limits genetic diversity (Kendrick et
al., 2012; Macreadie et al., 2014; Stafford-Bell et al., 2015). Sexual reproduction allows for more genetic diversity and
detached reproductive shoots can travel long distances (up to 108km;
Harwell & Orth, 2002) before their negatively buoyant seeds dehisce
from the shoot and germinate (Harwell & Orth, 2002; McMahon et
al., 2014).
There are few studies which have investigated Z . muelleri germination rates despite it being a prolific species on the east coast
of Australia and in New Z ealand (Mills et al., 2009; Leeet al., 2016; Stafford-Bell et al. ¸ 2016). Successful
germination in Z . muelleri seeds mostly encompass low
temperatures (< 16°C ) and low salinities (0-16 ppt) (Conacheret al., 1994; Brenchley et al., 1998; Stafford-Bellet al., 2016). However, germination is likely affected by lesser
known cues as Z . muelleri and similar species can be
affected by factors such as anoxia, sediment microbes and burial depth
(Conacher et al., 1994; Stafford-Bell et al., 2016;
Cumming et al., 2017; Tarquinio et al., 2019). Treatments
with high salinities (32 ppt), 24 hr light cycles and temperatures above
16°C see little to no germination in temperate Zostera sp.
despite these being conditions that would be commonly seen in east
Australian estuaries (Stafford-Bell et al., 2016).
Optimal Z . muelleri germination conditions in the
laboratory do not reflect conditions experienced in Australian
estuaries, which are highly variable in morphologies and subsequently
the environmental parameters within them. Consequently, estuaries are
not subject to static salinity and temperature regimes, but rather a
wide range of both parameters due to tidal movements, rain events and
evaporation (Scanes et al., 2020). On the east coast of Australia
estuarine salinity rests between 25-35ppt, however, it is possible for
systems to experience salinities ranging between 0-35ppt during large
rain events particularly near river entrances, albeit for a short
duration (Eyre and Ferguson, 2006; Stafford-Bell et al., 2016).
Based on successful laboratory germination, periods of low salinities
are critical for successful Z. muelleri germination
(Stafford-Bell et al ., 2016). However, with rainfall on the east
coast of Australia decreasing due to climate change (Head et al., 2013) the likelihood of low salinity events inducing germination is
decreasing. Considering the importance of sexual reproduction and large
flowering events in population resilience (Stafford-Bell et al., 2015; Smith et al., 2016) and the strong influence temperature
and salinity have on germination (Stafford-Bell et al., 2016;
Smith et al., 2016; Cumming et al., 2017; Tol et
al., 2021), acclimation or adaption to warmer temperatures predicted to
occur due to climate change will be critical for the survival ofZ. muelleri in Australia (York et al., 2013; Collieret al., 2014).
This study used two thermal plumes generated by coal-fired power
stations as a proxy for a future climate change scenario. Thermal plumes
have been studied globally (Robinson, 1987; Steinbeck et al., 2005; Garthwin et al., 2014) and can provide an avenue to explorein situ effects from predicted warming scenarios. The study aims
to investigate if seeds sourced from plants which grow in elevated
temperatures during their development respond differently to temperature
treatments than those growing in ambient locations in the
thermally-affected and a control estuaries.