Elsevier

Marine Pollution Bulletin

Volume 140, March 2019, Pages 462-471
Marine Pollution Bulletin

Effects of desalination brine and seawater with the same elevated salinity on growth, physiology and seedling development of the seagrass Posidonia australis

https://doi.org/10.1016/j.marpolbul.2019.02.001Get rights and content

Highlights

  • Brine had a greater effect on adult plants of the seagrass Posidonia australis than increased salinity only.

  • Brine had negative impacts on photosynthesis, water relations and leaf growth of adult plants within 2 weeks.

  • Seedlings survived over seven weeks in concentrated brine, and showed some recovery afterwards in natural seawater.

Abstract

Desalination has the potential to provide an important source of potable water to growing coastal populations but it also produces highly saline brines with chemical additives, posing a possible threat to benthic marine communities. The effects of brine (0%, 50%, 100%) were compared to seawater treatments with the same salinity (37, 46, 54 psu) for seagrass (Posidonia australis) in mesocosms over 2 weeks. There were significant differences between brine and salinity treatments for photosynthesis, water relations and growth. Germinating seedlings of P. australis were also tested in brine treatments (0%, 25%, 50%, 100%) over 7 weeks followed by 2.5 weeks recovery in seawater. Growth was severely inhibited only in 100% brine. These experiments demonstrated that brine increased the speed and symptoms of stress in adult plants compared to treatments with the same salinity, whereas seedlings tolerated far longer brine exposure, and so could potentially contribute to seagrass recovery through recruitment.

Introduction

Desalination plants are being increasingly developed worldwide to provide potable water but in coastal areas where brines are discharged into the sea, they are considered a potential threat to benthic marine communities (Gacia et al., 2007; Roberts et al., 2010; Portillo et al., 2014a, Portillo et al., 2014b). Marine desalination using reverse osmosis (RO) uses high pressure to force water molecules through a semi-permeable membrane that produces brines consisting of essentially concentrated seawater, but also containing a number of other pollutants, such as anti-scale additives, biocides, surface active agents or solid residues from back flushing of filters (Morton et al., 1996; Einav et al., 2002; Drami et al., 2011; Belkin et al., 2017), as well as pulses of sodium meta-bisulfate from shock cleaning of membranes (Portillo et al., 2014b). Much of the present focus on impacts of desalination outfalls has been on the effect of increased salinity in brines (Sánchez-Lizaso et al., 2008; Fernández-Torquemada and Sánchez-Lizaso, 2005, Fernández-Torquemada and Sánchez-Lizaso, 2011; Marín-Guirao et al., 2011; Sandoval-Gil et al., 2012a, Sandoval-Gil et al., 2012b; Cambridge et al., 2017).

At Geographe Bay in south–western Australia, hypersaline brines from a large (93 GL) reverse osmosis desalination plant are discharged into waters with benthic communities of seagrasses, reef pavement and sand habitats on a wave-exposed coast. Brines with salinities ranging from 44 to 54 psu are pumped through a diffuser array ~400 m offshore and ~12 m deep into waters with a high wave-energy climate (Dunn et al., 2014). Establishing causal relationships between brine effluent and impacts on key benthic marine species are a research and management priority (Roberts et al., 2010). Here we address the impact of brines on a major seagrass species found in temperate Australia, Posidonia australis under controlled experimental conditions.

Some studies have demonstrated the direct effect of brines with in situ monitoring of key species (e.g. Fernández-Torquemada et al., 2005; Gacia et al., 2007). Ruiz et al. (2009) conducted a complex underwater experiment, whereby brine from a pilot desalination plant was piped directly to undersea diffusers encircling a seagrass meadow, which was intensively monitored for changes over 3 months. These in situ studies have shown that some seagrass species are vulnerable to conditions created by brine plumes, such as the Mediterranean Posidonia oceanica (Fernández-Torquemada and Sánchez-Lizaso, 2005; Ruiz et al., 2009) and Cymodocea nodosa (Garrote-Moreno et al., 2014) whereas others are relatively tolerant to brine discharges, such as Thalassia testudinum, which showed very limited or no effects when exposed to increases of up to 4 units of salinity from RO discharge plumes (Tomasko et al., 2000). In the Canary Islands (subtropical eastern Atlantic) Cymodocea nodosa appeared to be unaffected by nearby brine (Pérez-Talavera and Quesada-Ruiz, 2001) but later studies by Portillo et al., 2014a, Portillo et al., 2014b showed a zone of impact corresponding to the trajectory of brine discharge.

In contrast to field studies, there are a number of detailed studies in mesocosms examining the effects of raised salinity under controlled conditions based on the assumption that adverse effects of exposure to brine are primarily the result of salt stress (Torchette, 2007; Koch et al., 2007). Salinity tolerance is highly variable in plant species (Munns, 2002). High salinity results in osmotic stress and ion toxicity, which affect plant water relations, ion concentrations in cytoplasm and the vacuole, and growth and photosynthesis. Physiological responses of P. oceanica to raised salinity have shown that after 6 weeks of hypersaline exposure, organic osmolytes had accumulated in leaf tissues, thereby avoiding dehydration, but at a metabolic cost (Sandoval-Gil et al., 2012a). Photosynthetic rates were lower but respiration rates increased, which was associated with reduced growth, and caused some shoots to die (Fernández-Torquemada and Sánchez-Lizaso, 2005; Marín-Guirao et al., 2011, Marín-Guirao et al., 2013; Sandoval-Gil et al., 2012a, Sandoval-Gil et al., 2012b; Garrote-Moreno et al., 2015). The Australian species, Posidonia australis has been shown to have a broad range of tolerance at salinities ranging from 27 to >60 psu (Tyerman et al., 1984), reflecting its occurrence in both estuarine and hypersaline habitats (e.g., Walker et al., 1988; Cambridge and Kendrick, 2009; Garrote-Moreno et al., 2016) but Marín-Guirao et al. (2017) have also investigated P. oceanica growing in a lagoon at higher than normal salinities on the south-eastern coast of Spain.

In this study we considered the combined effects of salinity and other toxic compounds in actual brines on seagrass to consider the question “is high salinity the main stressor in brine discharge, or do the other substances in brine impose effects that are additive to salt stress?" Several new desalination developments have been sited around the temperate southern half of Australia, so we used P. australis as the test species because of its distribution over some 6000 km, from near Shark Bay on the west coast to north of Sydney on the east coast. We also examined the responses of the earliest life-history stage of P. australis, exposing developing seedlings to brines for a longer time to examine their tolerance to extreme conditions and potential for recovery in seawater. In two mesocosm experiments, we tested the hypotheses that the effects of brine (0%, 50%, 100%) differed from treatments with seawater with the same salinity (37, 46, 54 psu) for photosynthesis, water relations and growth in the seagrass Posidonia australis.

In a separate experiment, developing seedlings of P. australis were tested for differences in biomass, leaf and root growth in brine treatments (0%, 25%, 50%, 100%) over 7 weeks followed by 2.5 weeks recovery in seawater.

Section snippets

Methods

Two experiments each with treatments lasting two weeks were carried out on adult plants of the seagrass P. australis (Table 1). The first experiment in Oct.–Nov. 2013 tested responses (photosynthesis, water relations, leaf growth) to treatments consisting of two raised salinities, 46 and 54 psu with four tanks per treatment. Because of delays in accessing desalination brine, experiments could not be run simultaneously, but were instead carried out in the same season, approx. 1 year apart, using

Photosynthesis

There were significant differences between the controls for brine and hypersalinity treatments for ETRmax (P < 0.0001), alpha (P < 0.005) and Ek (P < 0.001).

Maximum electron transport rates (ETRmax) showed significant reduction only at the highest concentration of salinity (54 psu), and 100% brine (P > 0.05) (Fig. 1a, b). There were no significant treatment effects for alpha in raised salinity but there was a significant reduction in 100% brine compared to controls (Fig. 1c, d). Maximum quantum

Discussion

Establishing causal relationships between brine discharge and impacts on key species may be difficult in environments where other activities confound or mask the effects of brine plumes, or areas are inaccessible for monitoring surveys on a regular basis. In these experiments carried out under controlled conditions in mesocosms, we separated salinity responses from the effects of whole brine that include both raised salinity and chemical additives. After two weeks of increased brine

Management implications

Brine varies in composition and discharge rate, depending on operational processes in a desalinisation plant such as chlorination, pH adjustment, coagulation, flocculation, dechlorination, antiscaling and membrane cleaning (Falkenberg and Styan, 2015). The concentration of seawater during reverse osmosis results in highly saline brines and a number of other additives, such as anti-scale additives, biocides, surface active agents and solid residues from back flushing of filters, as well as

Conclusions

Brine had a greater effect on adult plants of the seagrass Posidonia australis than increased salinity only. Negative impacts to adult plants were evident within two weeks, particularly in undiluted brine but seedlings proved to be more resilient to longer exposure to brine. Photosynthesis, water relations and leaf growth were affected, dependent on concentration, and there were increased concentrations of sugars and some amino acids in leaves that indicated the rapid onset of physiological

Acknowledgements

We thank Renae Hovey, Rob Creasy, Elizabeth Halladin, John Quealy and Ian Dapsen for technical assistance during experiments, and Juanma Ruiz for early discussions. Patrick Hayes carried out additional data analysis and statistical testing and Ylva Olsen produced revisions to figures. This work was undertaken with funding provided by the National Centre of Excellence in Desalination Australia (NCEDA), Project 08699.

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