Biomass production and nutrient removal efficiency of Suaeda salsa in eutrophic saline water using a floating mat treatment system

Coastal aquaculture plays an important role in the social-economic development of most coastal communities on the Eastern coastal tidal flats of China. With the accelerated pace of the development and utilization of tidal flat resources in China, the aquaculture industry has been showing a year by year increasing trend, which is more prominent in Jiangsu Province, Southeastern China, with its large tidal flat areas accounting for 28% of the total tidal flats in China (Zhao et al. 2015). However, the rapid development of coastal aquaculture has resulted in water quality degradation due to massive buildup of nutrients, such as N and P, in the aquaculture water. Such nutrients input to waters causes eutrophication and is responsible for degradation of aquatic ecosystems (Ansari & Khan 2008; Ansari & Khan 2009) and plants biodiversity (Ansari et al. 2011). As a consequence, it is urgent to take effective measures to control eutrophication and restore eutrophic water bodies in tidal flat areas.

Water eutrophication has become a worldwide environmental problem in recent years (Roy et al. 2013). To solve this serious problem, a series of new technologies have been developed for controlling or remediating eutrophic water over the past two decades. Among these technologies developed, phytoremediation is a promising and an environmentally friendly technique. Phytoremediation technique is a cost-effective and sustainable measure used for the remediation of eutrophic waters (Yeh et al. 2015; Pavlineri et al. 2017). It has been increasingly used for purification of contaminated soil and water systems because of its lower costs and fewer negative effects than physical or chemical engineering techniques (Mahujchariyawong & Ikeda 2001; Ali et al. 2013; Vymazal 2013). In phytoremediation, floating mat economic plant-based treatment systems (FMEPTSs) are an ecological technique used for phytoremediation of wastewater, and have promising potential for purifying wastewater by highly effective removal of nutrients and metals out of polluted water (Chang et al. 2013; Wang & Sample 2014; Dierssen et al. 2015). In FMEPTSs, plants with high economic value can be widely used to achieve a balance of ecological and economic benefits (Zhu et al. 2013; Wang & Sample 2014; Dierssen et al. 2015). However, nutrient removal efficiency and adversity resistance of floating-bed plants are the prerequisites for their application with FMEPTS in a polluted environment. Therefore, it is important and necessary to explore and evaluate a unique plant resource suitable for the phytoremediation of eutrophic saline water derived from different types of pollution sources and different geographical regions.

Suaeda salsa (S. salsa) is a native annual halophyte in salt-affected soils around coastal areas and has adapted to a high salt flooding region on the east coast of China (Alhdad et al. 2013). It is of significant economic importance as its fresh branches are highly valuable as a vegetable and its seeds can produce edible oil that is rich in unsaturated fatty acids (Sun & Zhou 2010). Due to its capability for removing salts and heavy metals from saline soil, it can be used in the restoration of heavily salinized land (Song et al. 2009). It has also been reported to have the ability to live in areas flooded with saline water, such as coastal salt marshes (Flowers & Colmer 2015). However, there is still little report available on using S. salsa to restore eutrophic saline water in tidal flat areas. To this end, S. salsa might be effective in treating nutrient pollution, as well as in restoration of aquaculture ecosystems. Nevertheless, before real action should be undertaken, a stimulated in situ phytoremediation system using FMEPTS should be carefully designed and run to test its capabilities in purifying eutrophically polluted saline waters.

For this purpose, halophyte S. salsa collected from a coastal tidal flat area was used to remediate eutrophic saline water based on an FMEPTS. The optimum salinity of hydroponic solution for S. salsa growth was screened based on the eutrophic water with three different levels of salinity. Then, an indoor experiment was conducted including three different eutrophic saline waters with uniform FMEPTS implemented to purify the eutrophication waters. The primary goal of this study was to investigate growth and biomass production of halophyte S. salsa in eutrophic saline water, and to evaluate the nutrient removal potential of S. salsa to purify wastewater. An economic and practical phytoremediation strategy especially adapted to remediate hypereutrophic water with high salt content, which is causing serious ecological and environmental problems in the area of coastal tidal flats in China, was assessed.

Seedlings of S. salsa with an average height of 15 cm were collected in May 2015 from coastal tidal flats in Nantong (32° 20′ N latitude, 120° 57′ E longitude), an eastern coastal city of Jiangsu Province in China. The plant seedlings were transferred to a well-ventilated glass house at the Institute of Botany, Jiangsu Province and the Chinese Academy of Sciences (32° 07′ N latitude, 118° 48′ E longitude), Nanjing, China. After being rinsed with deionized water, the plants were pre-cultured in an incubator (length 54 cm, width 39 cm and height 15 cm) filled with Hongland nutrient liquid and containing an extra ingredient of 50 mmol L⁻¹ sodium chloride (NaCl). The process of plant pretreatment lasted one week in the glass house at 28/15°C (day/night) with a 12-h photoperiod and ∼75% relative humidity.

Simulated eutrophic saline water was prepared based on the data of background values of total nitrogen (TN), total phosphorus (TP) and NaCl contents in aquaculture water from Nantong coastal tidal flats. The salinity of the aquaculture water ranged from 8‰ to 16‰, and the concentrations of TN and TP reached up to 2.4 mM and 0.05 mM, respectively, with a pH value of 8.0 ± 0.1. To obtain the salt concentration required for optimal growth of halophytes in the hydroponic culture, salt-treatment experiments were carried out before eutrophic treatment of S. salsa plants under hydroponic conditions.

In the salt-treatment experiments, three different levels of salinity were designed as low salinity (LS), moderate salinity (MS) and high salinity (HS), corresponding to final NaCl concentrations of 137, 274 and 410 mM, respectively, in nutrient solutions, which also contained nitrogen, phosphorous and other trace elements (see Table 1). The three different salinities in the study were based on the actual salt content of tidal flats in Nantong coastal region, in which the salinity usually ranged from 8% (137 mM) to 16% (274 mM). In addition, a salinity of 24% (410 mM) was used to investigate the effects of salinity fluctuation. To avoid osmotic shock, the salt for the MS and HS experiments was gradually put into the solutions by rising with 50 mM salinity per day until the required final concentration was reached. Considering the autopurification of eutrophic water, simulated eutrophic water containing no NaCl and covered by floating mats without S. salsa plants was used as a control which consisted of nitrogen, phosphorous and other trace elements as the three saline eutrophic water. 20 mL samples of salt-waters were collected after 0, 5, 10, 15, 20 and 25 days, and the plant was also sampled at the end of the experiment. Six replicates were conducted for each of the salinity treatments, and three repetitions for each sampling and survey analysis were performed.

For simulating eutrophication of salt water, different quantities of nitrogen (in the forms of NH₄NO₃ and KNO₃) and phosphorus (as KH₂PO₄) were dissolved in the screened salt water, to investigate the removal efficiency of S. salsa for the nitrogen and phosphorus in the water. Three water bodies with different eutrophic levels were prepared in this study, which corresponded to eutrophication (EP), moderate eutrophication (MEP) and hypereutrophication (HEP). The detailed composition of the eutrophic saline water is shown in Table 2. About 20 mL water for each sample was taken for laboratory analysis after 0, 5, 10, 15, 20 and 25 days of treatment. Similar to the salt-treatment experiments, each simulated eutrophic saline water experiment had six replicates, and three repetitions per water sample.

As shown in Figure 1, a floating mat treatment system was made of polyethylene foam and had a size of 55 × 43 × 3 cm³ (length × width × height). In the floating mats, S. salsa was inserted into three rows of holes on each foam, and supported by filling with sponges in the hole. The floating mats were placed on rectangular plastic containers of size 54 × 39 × 15 cm³ (Figure 1), filled with simulated eutrophic saline water as described above. After measuring the initial fresh weights of S. salsa, nine plants were transplanted evenly in the corresponding holes of each floating mat from the pre-culture incubator. All plants were continuously aerated to increase the dissolved oxygen in the water. Water losses due to evaporation and evapotranspiration were controlled by adding deionized water to the original level every alternate day.

An integrated system consisting of 20 mat subsystems was used, with each subsystem made of high-density polyethylene foam and of size 200 × 100 × 5 cm³ (length × width × height). There were 40 holes, each with a diameter of 1.0 cm, for each subsystem, and plant seedlings were arranged in four rows with six seedlings (or clusters) per row. Polyvinyl chloride (PVC) pipes (∅ 10 mm in diameter) set into each hole were used to prevent plant lodging. The subsystems in the floating island system were fixed and connected by self-locking nylon ties (5 × 300 mm²), and fixed thin PVC pipes (∅ 20 mm) were used to prevent spread of the mat subsystems. The bed structure of the floating island system was PVC pipes (∅ 100 mm). A model of the basic construction of the floating island system is shown in Figure 2.

Water samples were taken from each treatment every 5 days during the experiments and were stored at 4°C until analysis. All water samples were analyzed for TN and TP according to standard methods (APHA et al. 2005). Removal rates of TN and TP were calculated using the equation W_i = (C₀ − C_i)/C₀ × 100%, where W_i (%) represents the removal rate at i days; C₀ represents the initial concentration of the nitrogen or phosphorus of the simulated eutrophic water; C_i represents the concentration of nitrogen or phosphorus in the eutrophic water at i days; and i represents the time in days.

The initial fresh weight and height of the plants and their root were measured at the onset of the experiment. Plants were harvested at 25 days, and the final weight and height were also recorded. After plant collection, the organs were carefully separated and their fresh weights were measured. All samples (about 200 g) were dried at 105°C for 30 min and thereafter dried at 80°C for 48 h to attain a constant weight. They were then ground to pass through size 60 meshes for analysis of plant nitrogen and phosphorus. The nitrogen content of plants was determined using the Semimicro Kjeldahl method (Jensen 1991), and the phosphorus content was measured using the method of Christensen & Wigand (1998).

All compositional analyses were performed in triplicate, and all data were expressed as mean ± standard error. Differences between the treatments were determined by One-way analysis of variance (ANOVA) and Tukey's comparison test at the 0.05 significance level, and significance between treatments was considered as statistically significant if the p-value ≤0.05.

The effects of salt on the growth of S. salsa in the simulated eutrophic saline water are shown in Table 3. After 25 days of treatment, the average increases of plant height were 1.50 cm, 2.81 cm and 0.90 cm in LS, MS and HS eutrophic water, respectively; the elongations of roots in the three salinities were 15.12 cm, 19.46 cm and 8.75 cm, respectively; and the fresh weights and dry weights of the plants increased by 32.13 g and 2.59 g, 50.97 g and 4.11 g, and 10.91 g and 1.39 g in low, moderate and high salinities, respectively (Table 3). It is clear that MS can achieve an optimal growth of S. salsa, which is indicated by the increase in plant height or root elongation, as well as increased plant fresh and dry weights.

It has previously been found that different types of halophytes usually have different requirements for optimum salinity for growth (Flowers & Colmer 2015), and the growth of halophytes is ultimately reduced by too high salinity, although the halophytes have different salt tolerances (Bankaji et al. 2014). In the present study, the growth response of S. salsa in eutrophic saline water demonstrated that the growth of S. salsa requires a corresponding environment with a certain salinity, and different salinities have a considerable influence on biomass accumulation of S. salsa. Hydroponic conditions with 274 mM NaCl give optimal growth of S. salsa even in eutrophic water.

The TN removal profiles of the microcosm systems planted with S. salsa for the simulated eutrophic saline water are shown in Figure 3. The treatments of all three different salinity levels of eutrophic water showed higher TN removal efficiency compared with the control. After 25 days of salt treatment in eutrophic water, the TN concentrations of LS, MS and HS decreased from an initial value of 2.5 mM–0.65 mM, 0.99 mM and 1.40 mM, respectively, which were significantly lower than the 1.75 mM of the control (Figure 3(a)). The TN removal efficiency of MS calculated from Figure 3(a) reached 73.23%, which was significantly higher than that of HS (59.31%) and LS (41.94%), i.e. TN removal efficiency was as follows: LS < HS < MS. These results demonstrate that S. salsa can effectively remove the TN from eutrophic water, and the removal efficiency can be influenced by the salinity of water bodies, with moderate saline treatment having the highest removal efficiency.

In microcosm systems planted with S. salsa, salt treatments (LS, MS and HS) promoted TP removal rates from eutrophic water (Figure 3(b)). Similar to TN removal, the removal rates of TP reached 66.46%, 72.21% and 49.14% for LS, MS and HS treatments, respectively, thus showing HS < LS < MS. These results suggest that an appropriate salt-containing (274 mM NaCl) growing environment is of more benefit to TP removal by halophytes from eutrophic water.

Although removal performance could be enhanced by increasing the salinity, too high a salinity (i.e. 410 mM NaCl) could inhibit the removal efficiencies for TN and TP. The effect of salinity on purifying performance of eutrophic water was completely consistent with the growth response of S. salsa in the saline water. Therefore, the net increase of S. salsa biomass could reflect the removal efficiency of excessive nutrients from eutrophic saline water. This is similar to the restoration of eutrophic water by other aquatic plants (Zhao et al. 2014).

Salinity is one of the important environmental factors that affects the activity of S. salsa in its natural habitat (Cheng et al. 2014; Wang et al. 2015), while nutrients such as nitrogen and phosphorus are essential elements for the plant growth. After acquiring the optimal growth salinity of S. salsa, the effects of different degrees of eutrophication on growth of S. salsa were further investigated (see Table 4). After 25 days of growth in three different degrees of eutrophication of saline water, the fresh weight increase of S. salsa ranged from 19.10 to 50.97 g, and the dry weight increase ranged from 1.50 to 4.11 g in EP and HEP, respectively. The fresh or dry weights of S. salsa in HEP were significantly improved compared to those in EP and MEP. Although plant (shoot) height and root length of S. salsa increased during the whole growth period, the increased differences among the three different eutrophication waters were not statistically significant (p > 0.05). On the other hand, the root and branch numbers of the plant displayed significant differences among the three levels of EP, MEP and HEP in saline water (Table 4). These results indicate that plenteous nitrogen and phosphorus nutrition are conducive to biomass accumulation of S. salsa. In the present study, the plant root and branch numbers are the crucial factors in causing biomass differences under different eutrophication conditions.

Based on the optimal salinity for the growth of S. salsa, the removal effects of nitrogen and phosphorus were investigated under different degrees of eutrophication waters. The concentration of TN decreased sharply in the first 10 days for each degree of eutrophication. After 10 days of treatment, TN concentrations in EP, MEP and HEP decreased from 0.84, 1.60, and 2.44 to 0.06, 0.53, and 1.37 mmol·L⁻¹ respectively, with a corresponding removal rate of 92.9%, 66.9% and 43.9%. At the end of the experiment, the removal efficiencies for TN close to 100% in EP , reached 83.4% and 73.2% in MEP and HEP, respectively (Figure 4(a)). Hence, TN concentration changed significantly among the three different degrees of eutrophication of water, and the removal efficiency of TN exhibited the following trend: EP > MEP > HEP.

Similar to TN, TP concentration changed significantly among the three eutrophic saline waters. After 10 days treatment, the TP concentrations in EP, MEP and HEP decreased from 0.01, 0.03 and 0.05 to 0.002, 0.01 and 0.024 mmol·L⁻¹, respectively. The removal rates of TP in EP, MEP and HEP were 91.6%, 81.6% and 74.4%, respectively, also showing the trend EP > MEP > HEP (Figure 4(b)). These results indicate that S. salsa has good simultaneous nitrogen and phosphorus removal performance, and its removal efficiency changes significantly with eutrophic level or TN and TP contents in eutrophic saline water. The removal rate decreases with increasing TN and TP concentrations.

Based on the above results, we developed an integrated floating island system suitable for planting S. salsa as shown in Figure 2. In the following year, a practical application of the floating island system with S. salsa was applied to remediate eutrophic water of mariculture ponds located in Nantong coastal tidal flats. In May 2016, the plant seedlings with an average height of 15 cm were transplanted from local coastal tidal flats to the mariculture ponds using the floating island system. In total, more than 10,000 plants were transplanted to the five integrated floating island systems, which consisted of 100 (20 × 5) mat subsystems covering around 400 m² of pond water surface. As we expected, the plants are not only capable of completing their life cycle in the mariculture ponds (Figure 5), but also significantly improve aquaculture water quality (Figure 6).

In this study, we first developed an indoor system to cultivate S. salsa in eutrophic saline water and to subsequently clarify the resistance of the combined stress of salt and eutrophication under hydroponic conditions. The results suggest that S. salsa has good ability to tolerate the combined stress of salt and eutrophication under hydroponic conditions. S. salsa can grow normally in the salinity range of 8% (137 mM) to 24% (410 mM) under the TN and TP greater than 0.84 mM and 0.01 mM, respectively. Based on the simulation of salinity of coastal tidal flats in Nantong, China, the euhalophyte S. salsa has optimal growth in eutrophic saline water of MS, corresponding to 274 mM NaCl. A floating mat based treatment system planted with S. salsa had remarkable removal effects on nitrogen and phosphorus in eutrophic saline water, and the removal efficiency was closely related to the degree of eutrophication, with the following trend: EP > MEP > HEP. Furthermore, we developed an integrated floating island system suitable for planting S. salsa adapted to remediate hypereutrophic water with high salt content in situ (on coastal tidal flats in China), and the plant grew well and the improvement effect of water quality was remarkable. Thus, our study indicated that S. salsa could be applied to improve wastewater quality originating from large-scale mariculture, and the model of the floating island system could be used to mariculture S. salsa or seawater plants.

The authors thank the Public Science and Technology Research Funds Projects of Ocean (Grant No. 201505023). Thanks also go to Jiangsu Key Laboratory for Bioresources of Saline Soils (Grant No. YT2014-02) and Jiangsu Provincial Environmental Protection Research Project (Grant No. 2016044).