Epiphytic algae mitigate the inhibitory effects of two aquatic invasive plants, Pontederia crassipes and Pistia stratiotes, on a submerged plant community

191 Epiphytic algae mitigate the inhibitory effects of two aquatic invasive plants, Pontederia crassipes and Pistia stratiotes, on a submerged plant community Yi-Luan Shen1,2, Jing-Fang Cai1,2, Jing-Jing Xue1,2, Xiao-Li Yang1,2, Da-Zhi Wang1,2, Hong-Li Li1,2 1 School of Ecology and Nature Conservation, Beijing Forestry University, Beijing 100083, China 2 Key Laboratory of State Forestry and Grassland Administration on Ecological Protection in the Yellow River Basin, Beijing Forestry University, Beijing 100083, China Corresponding author: Hong-Li Li ([email protected]) Copyright: © Yi-Luan Shen et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Exotic plant invasions and epiphytic algae potentially influence the growth of submerged plants. However, the effect of epiphytic algae on the resistance of submerged native plants to plant invasion is elusive. Therefore, we conducted this study to explore the effects of invasion scenarios and epiphytic algae on a submerged plant community (Hydrilla verticillata, Potamogeton wrightii, and Vallisneria natans) and water quality. We set up a control experiment with four invasion scenarios (no invasion, Pontederia crassipes invasion, Pistia stratiotes invasion, and co-invasion of P. crassipes and P. stratiotes) and two epiphytic algal states (with and without epiphytic algae). The presence of invasive plants resulted in significantly lower biomass (P < 0.01) of the submerged plant community without epiphytic algae than that with epiphytic algae, and the community biomass was lowest in the case of co-invasion. The presence of epiphytic algae resulted in native plants exhibiting competitive interactions only during co-invasion, whereas native plants exhibited competitive interactions at all times in the absence of epiphytic algae. Total nitrogen and phosphorus contents in water were lowest in the absence of invasion. Additionally, total phosphorus contents was highest in the presence of epiphytic algae and co-invasion. These results demonstrate that co-invasion strongly inhibits the growth of submerged plant communities more than single invasions, and epiphytic algae potentially offset this suppression. These findings will provide a scientific basis for submerged native plant restoration in wetland ecosystem. Key words: Co-invasion, native plants, plant invasion, plants restoration, water quality Introduction Submerged plants are primary producers, providing abundant food and habitat for aquatic biota and aiding in water purification (Jeppesen et al. 1997; Li et al. 2023; Liao et al. 2024). Therefore, submerged plants are critical in maintaining the biodiversity and stability of shallow lake ecosystems (Liu 2021). However, invasions contribute significantly to the decline and extinction of submerged plant communities (Phillips et al. 2016). Invasive plants reproduce rapidly and occupy crucial ecological niches, floating invasive plants can form a dense meadow on the water surface, competing with submerged plants for light, nutrients, and other resources (Fleming and Dibble 2014). Plant invasions are frequently accompanied by changes in the aquatic environment, further degrading the submerged plant community (Zhou et al. 2017; Yuan et al. 2023). Academic editor: Pedro Anastácio Received: 10 December 2024 Accepted: 21 April 2025 Published: 7 October 2025 Citation: Shen Y-L, Cai J-F, Xue J-J, Yang X-L, Wang D-Z, Li H-L (2025) Epiphytic algae mitigate the inhibitory effects of two aquatic invasive plants, Pontederia crassipes and Pistia stratiotes, on a submerged plant community. In: Anastácio P, Ribeiro F, Chainho P (Eds) Invasions in Aquatic Systems. NeoBiota 102: 191–207. https://doi.org/10.3897/ neobiota.102.144004 NeoBiota 102: 191–207 (2025) DOI: 10.3897/neobiota.102.144004 Advancing research on alien species and biological invasions A peer-reviewed open-access journal NeoBiota 192 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects Plant invasions are prevalent, and ecological niches of certain invasive plants overlap such that ecosystems are co-invaded by two or more plants (Wang et al. 2020; Wei et al. 2020). The frequent interaction between invasive plants impacts native plant communities in these co-invaded ecosystems (Zhong et al. 2022; Li et al. 2024). Negative interactions between invaders can moderate their impacts on native plant communities (Wang 2016). On the other hand, invasive species can mutually contribute to their establishment, further accelerating the invasion process and causing more severe impacts on native plants, a phenomenon known as invasional meltdown (Simberloff and Von Holle 1999; Wang 2016). A study showed that the co-invasion of two invasive plants Erigeron annuus (L.) Pers. and Solidago canadensis L. can impose synergistic impacts on soil bacterial richness, thereby facilitating their subsequent invasion (Wei et al. 2020). Not only will the co-invasion of plants cause more serious damage, but the co-invasion of plants and herbivores will also have similar effects; herbivory by the invasive snail Pomacea canaliculata may enhance the invasiveness of Myriophyllum aquaticum (Vell.) Verdc. (Yan et al. 2022). Compelling evidence indicates that single-species invasions are detrimental to native plant communities, and the invasional meltdown hypothesis continues to be validated at the same time (Kuehne et al. 2016; Yan et al. 2022). However, the interactions between co-invading plants in aquatic ecosystems and their effects under extreme invasion on native submerged plant communities remain elusive. Numerous biological and abiotic factors regulate the effects of plant invasion on submerged plant communities, including epiphytic algae, water nutrients, temperature, and water depth (Huang et al. 2023; Peng et al. 2023; Yuan et al. 2023). Among these regulatory factors, epiphytic algae, as a biotic factor, also potentially impact the invasion resistance of submerged plant communities (Song et al. 2017; Lv et al. 2023). Epiphytic algae form a biofilm on the surface of submerged plant stems and leaves, forming complex interactions, competitive or symbiotic, with submerged plants (Cattaneo et al. 1998; Zhang et al. 2023; Alena et al. 2025). On the one hand, excessive epiphytic algae inhibit the growth of submerged plants by competing for light and nutrients, ultimately causing the collapse of submerged plants and turning the water from clear to turbid (Asaeda et al. 2004; Min et al. 2017). On the other hand, epiphytic algae can promote plant growth by aiding in nitrogen fixation (Dai et al. 2023), complicating the effects of epiphytic algae on submerged plants. However, in aquatic ecosystems where plants co-exist, the effect of epiphytic algae on the resistance of communities of submerged plants to invasion remains unknown. As common invasive species in aquatic ecosystems, Pontederia crassipes and Pistia stratiotes L. cause evident damage and ecological pollution to submerged plant communities, and there are numerous studies on their invasion mechanisms (Zhou et al. 2017; Qian et al. 2022). However, there is relatively limited research on the effects of epiphytic algae on the resistance of submerged plant communities to the co-invasion of P. crassipes and P. stratiotes. Therefore, in this experiment, we developed a submerged plant community with four invasion scenarios (no invasion, P. crassipes invasion, P. stratiotes invasion, and co-invasion of P. crassipes and P. stratiotes) and two epiphytic algae (with and without epiphytic algae) treatments to test our proposed hypotheses: (1) Co-invasion suppresses the submerged plant community more than single invasions; (2) Epiphytic algae increase the resistance of submerged plant community to co-invasion. 193 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects Materials and methods Experimental design The experiment was conducted in the experimental base of Sanqing Park, Beijing Forestry University, Haidian District, Beijing, China (40°0'28.17"N, 116°20'17.64"E), from June 10 to July 28, 2023. The submerged and invasive plants used in the experiment were collected from an aquatic plant breeding base in Suqian City, Jiangsu Province, and were cultured in water for two weeks before the commencement of the experiment. According to previous research (Fletcher et al. 2023; Ma et al. 2023), we set the experimental period as 48 days, and used 25 L plastic buckets (38 cm high, 32.5 cm diameter at the upper mouth, 27.5 cm diameter at the lower mouth) as the experimental container, with the bottom filled with 15 cm of substrate (sand, soil ratio 2: 3, soil was collected from the fields of Sanqing Park). Submerged plants with favorable and comparable growth conditions were selected and planted uniformly in the experimental buckets, each of which included four individuals of each submerged plant species (Hydrilla verticillata (L. f.) Royle, Potamogeton wrightii Morong, and Vallisneria natans (Lour.) H. Hara. Initial length of H. verticillata and P. wrightii was roughly 15 cm, initial length of V. natans was roughly 10 cm). The experimental setup comprised groups of epiphytic algae and non-epiphytic algae. Submerged plants with the epiphytic algae group were planted directly in the experimental buckets, while submerged plants without epiphytic algae group were carefully brushed with a soft-bristled brush in clean water to remove epiphytic algae on the surface of the leaves before planting in the experimental buckets. In addition, four different invasion scenarios were established, namely, no invasion, P. crassipes invasion with four individuals, P. stratiotes invasion with four individuals, and co-invasion of P. crassipes and P. stratiotes, with two individuals of each species (Fig. 1). Invasive plants covered about 95% of each experimental bucket. There were 8 treatments, each with 5 replicates, and plants were watered at regular intervals to maintain the water depth at 20 cm. Sampling and parameter detection All submerged and invasive plants were harvested at the end of the experiment, and water samples were collected from each experimental bucket. The biomass of submerged and invasive plants was measured by drying them in an oven at 70 °C for Figure 1. Experimental design. 194 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects 48 hours until a constant weight was achieved. Water quality tests for collected water samples included total nitrogen content (TN), total phosphorus content (TP), transparency, pH, and dissolved oxygen (DO). The water TN content was evaluated using ultraviolet spectrophotometry with alkaline potassium persulfate digestion, whereas the water TP content was assessed by ammonium molybdate spectrophotometry. Transparency was evaluated using the Secchi disk method, and water pH was determined using a water quality detection pen (TDS-PH-EC). The DO content was measured using a portable dissolved oxygen (JPB-607A) analyzer. Of these, TN and TP measurements were done in the laboratory, and DO, pH and transparency were measured at 10:00 a.m. on the day plants were harvested. The above measurement methods all refer to our previous research methods (Shen et al. 2023). Statistical analysis The competition effects (CE) of invasive plants on submerged plants were determined from the biomass of each plant acquired by drying and weighing using the following formula (Wang et al. 2023): CE = ln(R0/Rs) (1) Where R0 denotes the total biomass of the submerged plant community in the absence of invasive plants, and Rs denotes the total biomass of the submerged plant community in the presence of invasive plants. A positive value for the calculated competition effect value implied that the invasive plants inhibited the growth of the submerged plant community, and the larger the value, the stronger the inhibition effect. A negative value for the competition effect implied that the invasive plants promoted the growth of the submerged plant community. The relative dominance index (RDI) of invasive plants in treatments with the presence of invasive plants was calculated as follows (Yuan et al. 2013): RDI = A/(A+B) (2) Where A denotes the biomass of invasive plants in the treatment in the presence of invasive plants, and B denotes the total biomass of the submerged plant community in the same treatment. The diversity index of the community (Shannon-Wiener diversity index) was calculated according to the following formula: H = −∑Pi ln (Pi) (i = 1, 2, 3 …, S) (3) Where S denotes the number of species in the community, and Pi denotes the biomass of species i divided by the sum of the biomass of all community species. The evenness index (Pielou evenness index) was calculated from the diversity index using the following formula: E = H/ln (S) (4) Where S denotes the number of species in the community, and H denotes the Shannon-Wiener diversity index. 195 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects Data analysis A two-way ANOVA was applied to determine the effects of invasion scenarios, epiphytic algae, and the two-way interaction on community indexes (including diversity index, evenness index, total community biomass, and competition effect), the biomass and relative dominance index of invasion plants, as well as the TN, TP, transparency, pH, and DO of the water. A one-way analysis of variance (ANOVA) was employed to evaluate significant differences between treatments. Before ANOVA was performed, all data were tested for normality and homogeneity of variances. Pearson correlation analysis was employed to evaluate associations between invasive plants, physical and chemical properties of water, and submerged plant communities. Structural equation modeling was used with the piecewiseSEM R package to assess the effects of invasive plants’ and water’s physical and chemical properties on submerged plant communities. Structural equation modeling involves treating each influencing factor as an independent observed variable. In this view, we first constructed a model that incorporated all possible paths and then used regressions to refine the model, yielding the final structural equation model. The standard regression coefficient, or path coefficient between indicators, was used to quantify the effects of invading plants and physical and chemical properties of water on the submerged plant community. Furthermore, the model fit was evaluated using AIC, P-Value, Fisher’s C, and Chi-Square. Submerged plant community metrics were calculated in Excel, while correlation analyses of invasion scenarios, epiphytic algae, and submerged plant community growth were performed in R 4.3.3. All plots were generated in SigmaPlot 14.0. Results Characteristics of the submerged plant community The invasion scenarios and epiphytic algae had significant effects on the biomass of H. verticillata and P. wrightii. The interaction between the two factors had a significant effect on H. verticillata, while the invasion scenarios, epiphytic algae, and their interaction had no significant effect on the biomass of V. natans (P < 0.05, Table 1). The invasion scenarios had significant effects on all community indicators, including total biomass, Shannon-Wiener diversity index, Pielou evenness index, and competition effect (P < 0.05, Table 1). But epiphytic algae only significantly affected total biomass and competition effect. The interaction between the two factors was only significant for total biomass (P < 0.05, Table 1). Table 1. ANOVA results on the effects of invasion scenarios (I) and epiphytic algae (E) on the indicators of submerged plant communities. Indicators Invasion scenarios (I) Epiphytic algae (E) I × E Biomass of Hydrilla verticillata 11.135*** 12.377** 4.378* Biomass of Potamogeton wrightii 3.296* 16.839*** 2.066 Biomass of Vallisneria natans 0.747 2.667 2.025 Total biomass 4.479** 24.798*** 5.051** Shannon-Wiener diversity index 14.341*** 0.952 0.154 Pielou evenness index 16.058*** 0.087 1.172 Competition effect 3.492* 41.014*** 0.706 †: Significant values of F are shown in bold; *** denotes P < 0.001, and ** denotes P < 0.01. 196 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects The trend of changes in the biomass of H. verticillata was consistent with the changes in the total biomass of community. Only when there was no invasion, the biomass of H. verticillata without epiphytic algae was higher than that with epiphytic algae (Fig. 2A). Except for co-invasion, the biomass of P. wrightii with epiphytic algae under other conditions was higher than that without epiphytic algae (Fig. 2B). For V. natans, only when P. crassipes invaded alone, the biomass with epiphytic algae was higher than that without epiphytic algae, while under other treatment conditions, Pontederia crassipes Pistia stratiotes Co-invasion Control Biomass of Hydrilla verticillata (g) 0.0 0.4 0.8 1.2 1.6 With epiphytic algaeWithout epiphytic alga e E:** I:*** E×I:* Biomass of Potamogeton wrightii (g) 0.0 0.1 0.2 0.3 0.4 0.5 E:*** I:* E×I:ns Invasion scenarios Biomass of Vallisneria natans (g) 0.00 0.05 0.10 0.15 0.20 E:ns I:ns E×I:ns A B C a a a ab ab bc cc bc d ab cd a ccbc ab b a b ab ab b ab Figure 2. Effects of invasion scenarios (I) and epiphytic algae (E) on biomass of three submerged plants (mean + SE, n = 5). Control denotes “no invasion”; Pontederia crassipes denotes “Pontederia crassipes invasion”; Pistia stratiotes denotes “Pistia stratiotes invasion”; co-invasion denote “co-invasion of Pontederia crassipes and Pistia stratiotes”; * denotes “P < 0.05”; ** denotes “P < 0.01”; *** denotes “P < 0.001.” Different letters indicate significant differences among treatments (P < 0.05). 197 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects there was no significant difference (Fig. 2C). Total community biomass was higher in the absence of invasion without epiphytic algae than with epiphytic algae, but the opposite effect was observed in the presence of invasive plants, with total community biomass being lowest in the co-invasion scenario (Fig. 3A). The diversity and evenness indices of plant communities were lowest in the absence of invasion (Fig. 3B, C). Pontederia crassipes Pistia stratiotes Co-invasion Control Total biomass (g) 0 1 2 3 4 5 6 With epiphytic algae Without epiphytic algae E:*** I:** E×I:** Shannon–Wiener diversity index 0.0 0.2 0.4 0.6 0.8 1.0 1.2 E:ns I:*** E×I:ns Invasion scenarios Pielou evenness index 0.0 0.2 0.4 0.6 0.8 1.0 1.2 E:ns I:*** E×I:ns A B C a a a a a bb b b b aa aa aa b b aa aa a a Figure 3. Effects of invasion scenarios (I) and epiphytic algae (E) on indicators of submerged plant communities (mean + SE, n = 5). Control denotes “no invasion”; Pontederia crassipes denotes “Pontederia crassipes invasion”; Pistia stratiotes denotes “Pistia stratiotes invasion”; co-invasion denote “co-invasion of Pontederia crassipes and Pistia stratiotes”; ** denotes “P < 0.01”; *** denotes “P < 0.001.” Different letters indicate significant differences among treatments (P < 0.05). 198 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects The presence of epiphytic algae caused a negative competitive effect between the submerged plant community and invasion plants under the invasion of P. crassipes alone and P. stratiotes alone, but positive under the co-invasion (Fig. 4A). On the other hand, the absence of epiphytic algae resulted in a positive competition effect in all invasion scenarios, with co-invasion exerting the highest effect (Fig. 4B). Characteristics of invasive plants The findings revealed that epiphytic algae exerted a significant effect on the relative dominance index of invasive plants, but neither invasion scenarios nor epiphytic algae had a significant effect on invasive plant biomass (Table 2). Invasive plants had a higher biomass in the absence of epiphytic algae than in the presence of epiphytic algae (Fig. 5A). The relative dominance index of invasive plants was significantly higher in the absence of epiphytic algae than in the presence of epiphytic algae, with the highest value for co-invasions (Fig. 5B). Table 2. ANOVA results on the effects of invasion scenarios (I) and epiphytic algae (E) on the indicators of the invasion plants. Indicators Invasion scenarios (I) Epiphytic algae (E) I × E Invasion plants biomass 0.623 2.030 0.706 Relative dominance index 2.342 23.601*** 0.442 †: Significant values of F are shown in bold; *** denotes P < 0.001. Invasion scenarios Competition effect -2 -1 0 1 2 3 P. crassipesP. stratiotes Co-invasion Pontederia crassipes Pistia stratiotes Co-invasion Pontederia crassipes Pistia stratiotes Co-invasi on E:*** I:* E×I:ns cc c bb a With epiphytic algaeWithout epiphytic algae Figure 4. Effects of invasion scenarios (I) and epiphytic algae (E) on competition effect of submerged plant communities (mean + SE, n = 5). Pontederia crassipes denotes “Pontederia crassipes invasion”; Pistia stratiotes denotes “Pistia stratiotes invasion”; co-invasion denote “co-invasion of Pontederia crassipes and Pistia stratiotes”; * denotes “P < 0.05”; *** denotes “P < 0.001.” Different letters indicate significant differences among treatments (P < 0.05). 199 NeoBiota 102: 191–207 (2025), DOI: 10.3897/neobiota.102.144004 Yi-Luan Shen et al.: Epiphytic algae mitigate the inhibitory effects Physical and chemical properties of water Invasive scenarios had significant effects on TN, DO, pH, and transparency, and epiphytic algae exerted significant effects on TP and DO. However, the interaction between the two factors had no significant effect on any of the water quality indicators (Table 3). TN and TP were lowest in the absence of invasion. In the presence of epiphytic algae, TN was highest under the invasion of P. stratiotes alone, while in the absence of epiphytic algae, TN was highest under the invasion of P. crassipes alone (Fig. 6A). In the presence of epiphytic algae, TP increased with the presence of the invasive plants and was the highest level under co-invasion. Moreover, TP was significantly higher in the presence of epiphytic algae than in the absence of epiphytic algae (Fig. 6B). On the other hand, pH and DO were highest in the absence of invasive plants and significantly decreased in the presence of invasive plants, and transparency was lowest in the presence of P. stratiotes alone. The presence or absence of epiphytic algae had no significant effect on pH and transparency (Fig. 6C–E). Relevance of invasive plants and water quality to submerged plant community The Pearson correlation analysis revealed that total community biomass was negatively correlated with invasive plant biomass (IB), whereas the community diversity index (SI) and evenness index (PI) were negatively correlated with DO and pH Pontederia crassipes Pistia stratiotes Co-invasion Invasion scenarios Pontederia crassipes Pistia stratiotes Co-invasion Biomass of invasion plants (g) 0 5 10 15 20 25 With epiphytic algaeWithout epiphytic algae A E:ns I:ns E×I:ns Relative dominance index 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 E:*** I:ns E×I:ns B a a a aaa dcd bc ab ab a Figure 5. Effects of invasion scenarios (I) and epiphytic algae (E) on biomass and relative dominance index of invasion plants (mean + SE, n = 5). Pontederia crassipes denotes “Pontederia crassipes invasion”; Pistia stratiotes denotes “Pistia stratiotes invasion”; co-invasion denotes “co-invasion of Pontederia crassipes and Pistia stratiotes”; *** denotes “P < 0.001.” Different letters indicate significant differences among treatments (P < 0.05). Table 3 ANOVA results on the effects of invasion scenarios (I) and epiphytic algae (E) on the physical and chemical water properties. 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