Increased salinity reduces seed germination and impacts upon seedling development in Nymphaea L. (Nymphaeaceae) from northern Australia’s freshwater wetlands

Highlights

• We assessed regeneration from seeds of Australian Nymphaea under increased salinity.

• Significant reductions in seed germination were observed at ≥100 mM NaCl.

• Early seedling growth was also significantly affected by salinities ≥100 mM.

• Increased salinity in freshwater wetlands will reduce habitat for Nymphaea.

Abstract

Coastal, freshwater wetlands in northern Australia are at risk of increased saltwater intrusion associated with sea-level rise, which threatens the persistence of numerous freshwater plant species by increasing salinity. Waterlilies (Nymphaea) are widespread in northern Australia, and their loss from these wetlands will be detrimental, both ecologically and culturally. This study aimed to define the regeneration tolerance of Nymphaea to increased salinity through quantifying the effects of salinity on seed germination and early seedling growth in four Nymphaea species. Seed germination and seedling growth were assessed under a gradient of salinity concentrations. Seeds that did not germinate after salinity exposure were assessed for their ability to recover in fresh water. For all species, there was a significant reduction in germination when seeds were exposed to salinities of ≥100 mM NaCl. Total seedling biomass was less sensitive to increasing salinity than germination, however declined significantly across all species at salinities ≥100 mM NaCl. Ungerminated seeds from all salinity treatments displayed some degree of recovery when transferred to fresh water. For the majority of species, however, seed germination of these transferred seeds never reached the percentages observed in the non-saline controls. For most species of Nymphaea, any salinization event reaching ≥100 mM NaCl will significantly reduce recruitment from seeds, irrespective of whether saline water is flushed from the system. The predicted future increases of saltwater intrusion into coastal freshwater wetlands in Northern Australia associated with global sea-level rise will likely result in significant habitat loss for many Nymphaea species.

Introduction

Northern Australia’s wet-dry tropics encompass a vast area of coastal wetlands, rivers and floodplains that support numerous freshwater aquatic communities that are at risk from climate change (Hennessy et al., 2007; Steffan et al., 2013, 2014). Low-lying coastal wetlands are particularly susceptible to saltwater inundation associated with rising sea levels and the increasing intensity of storm surges accompanying tropical cyclones and monsoonal lows (Eliot et al., 1999; BMT WBM, 2010; Bayliss et al., 2018). Saltwater intrusion in the East Alligator Rivers region of Kakadu National Park has already contributed to a nine-fold expansion of saline mud flats and a reduction of freshwater Melaleuca fringed wetlands of more than 60 % (Winn et al., 2006; Steffan et al., 2014). The Mary River floodplain in the Northern Territory has experienced similar levels of saltwater intrusion resulting in 17,000 ha of freshwater floodplain transitioning to mangrove swampland (Knighton et al., 1991; Mulrennan and Woodroffe, 1998). Since 1993, global sea levels have risen by up to 3.2 mm per year (Church and White, 2011) and current projections for Australia predict an average sea-level rise by up to 82 cm by 2100 (CSIRO and Bureau of Meteorology, 2015). Given that many of northern Australia’s coastal freshwater wetlands are less than two metres above sea level, detrimental consequences of rising sea levels and greater frequency of storm tides on these unique systems could be widespread (Steffan et al., 2014). Therefore, our ability to predict the impact of increasing salinity on vulnerable freshwater wetland species is essential for the ongoing conservation and management of these species (Bayliss et al., 2018; Pettit et al., 2016).

Eighteen species of the basal angiosperm genus Nymphaea (Nymphaeaceae, commonly known as water lilies), occupy the freshwater wetlands of the wet-dry tropics in northern Australia. Many Nymphaea species inhabit low-lying coastal floodplains and wetlands (Cowie et al., 2000; Jacobs and Hellquist, 2011) and are likely to be affected by sea-level rise this century (Pettit et al., 2016; Bayliss et al., 2018). Nymphaea are important keystone species within freshwater wetlands as they provide in-stream sediment stabilisation, act to reduce water turbidity, and provide habitat and food for aquatic animals and migratory waterbirds (Finlayson and Woodroffe, 1996; Sainty and Jacobs, 2003; Finlayson et al., 2006). Nymphaea are also culturally significant to Indigenous Australians as they traditionally serve as an important food source through the dry season (Issacs, 1987; Brockwell et al., 1995; Karadada et al., 2011) and are a significant indicator of wetland health (Ens et al., 2016). The loss of these plants would be highly detrimental, both ecologically and culturally. Within the Magela Creek floodplain in the Northern Territory, Nymphaea occur in vegetation types dominated by Eleocharis, Hymenachne, Nelumbo, Oryza and Pseudoraphis (Cowie et al., 2000; Whiteside and Bartolo, 2014). The majority of these vegetation types occur in freshwater areas of the floodplains (Cowie et al., 2000). While there is some information available regarding the salinity tolerance of a number of freshwater macrophytes common in northern Australia (see Pettit et al. (2016) for a summary of known, published tolerances), very little information exists regarding the salinity tolerances of Nymphaea species. However, the widespread species N. violacea is known to occur in freshwater only, and N. macrosperma is thought to tolerate marginally brackish conditions (Cowie et al., 2000).

Many tropical Nymphaea species can reproduce vegetatively via detachable tubers or rhizomes in addition to reproducing via the production of seeds (Wiersema, 1988). For many Australian freshwater macrophytes, the sub-lethal effects of increased salinity on adult plants are apparent at salinities of 1000 mg L⁻¹ NaCl (<2 % that of seawater), and can include reduced growth rates, plant size and vigour (Hart et al., 1991; James and Hart, 1993). However, the more vulnerable life stages associated with plant regeneration and recruitment, such as seeds, seed germination, and early seedling growth, may exhibit even lower tolerance thresholds and show sensitivity to very small fluctuations in salinity (James et al., 2003; Nielsen et al., 2003a, b). While the impact of increased salinity may be observed in adult plants in situ (e.g. via the observation of local stands of mature individuals) in real time during salinization events, the impacts on regeneration via seeds and seedlings may be less apparent.

The potential impacts of oceanic saltwater intrusion on freshwater macrophytes is also influenced by the spatial and temporal variability associated with salinization event. The effects of saltwater intrusion may be lessened by low tides, or high precipitation in wetland catchments which may act to flush the system with fresh water, overall reducing water salinity (Flynn et al., 1995). As such, freshwater wetlands may only be exposed to a short period of salinization before returning to fresh water. Therefore, it is important to understand not only the ability of ungerminated seeds to survive a period of exposure to increased salinity, but also their ability to recover and germinate in fresh water, in order to assess whether they are able to survive and regenerate after short-term exposure to increased salinity.

The aim of our study was to determine the salinity thresholds limiting regeneration from seed in four species of Australian Nymphaea. Specifically, we assessed: (1) the salinity tolerance thresholds at the seed germination and seedling growth life stages, and; (2) whether ungerminated seeds can recover from prior exposure to salinity and germinate once transferred to fresh water. We tested these objectives using seeds from three widespread species commonly found in coastal freshwater wetlands throughout northern Australia, and one geographically restricted species found in inland waterways, to assess whether coastal species are more, or less, resilient to increased salinity.