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.