Positive feedback is key to producing alternative stable states and largely determines ecological resilience in response to external perturbations. Understanding the positive feedback mechanisms in macrophyte-dominated lakes is crucial for resilience-based management and restoration. Based on the field investigation of submerged macrophyte communities in 35 lakes in China, we found that morphological complexity (MC) and morphological plasticity (MP) are correlated with the stoichiometric homeostasis of phosphorus (H_P) and are related to ecosystem structure, functioning, and stability. We also found that the positive feedback strength of lakes dominated by macrophytes is biomass- and diversity-dependent. Eutrophication can decrease the community biomass by decreasing community MC, MP, and H_P and the species diversity through low-light availability, ultimately decreasing the positive feedback strength and resilience of clear-water states. We argue that functional traits and species diversity should be considered to build more resilient ecosystems in future changing environment scenarios.

Submerged macrophyte communities from 35 subtropical lakes in China were investigated between 2014–2021. 534 sites were sampled among those lakes distributed in the middle and lower reaches of the Yangtze Plain (2014 and 2018; an average of 16 sampling sites for each lake), Sichuan Basin (2020; an average of 11 sampling sites for each lake), and Yunnan Plateau (2021; an average of 17 sampling sites for each lake) .

Water clarity was measured by a Secchi disc (Secchi depth, SD). Ten species of submerged macrophytes widely distributed in subtropical lakes were studied: Potamogeton maackianus, Potamogeton malaianus, Ceratophyllum demersum, Hydrilla verticillata, Vallisneria natans, Myrophyllum verticillatum, Potamogeton pectinatus, Potamogeton perfoliatus, Najas marina, and Potamogeton lucens.

We measured the surface area of plants above the sediment level by portable leaf area meter .  An electronic balance was used to weigh the wet biomass. Plant height was defined as the length (cm) of the aboveground part. The phosphorus in plants and sediments was first digested using concentrated sulfuric acid and hydrogen peroxide, and the amount was determined using the ammonium molybdate-ascorbic acid method (Sparks et al.,1996). Total phosphorus (TP) in the water sample was digested with potassium persulfate (Sigma Aldrich, Germany) in a sterilization pot at 120 ℃ for 30 min.

We caculated P stoichiometric homeostasis (Sterner & Elser, 2002) by P concentration in sediments and plants and quantized morphological complexity and plasticity by specific surface area. Community trait is the overall mean of functional traits across all species weighted by the relative biomass. 

The community biomass of a lake is defined as the mean total biomass of all submerged macrophyte species in a quadrat, and community stability is indicated by the mean biomass to standard deviation ratio. Species diversity here is indicated by the H-Shannon diversity, which is calculated as the exponential of Shannon entropy. We define positive feedback strength as the ability of plants to improve water clarity per unit biomass.

Finally, general linear model and piecewise SEM were used to explored the relationships of TP, MC, MP, H, community biomass, stability, species diversity and positive feedback strength.

Sparks, D.L., Page, A.L., Helmke, P.A., Loeppert, R.H., Soltanpour, P.N. Tabatabai, M.A. et al. (1996). Methods of Soil Analysis. Part 3-Chemical Methods. Soil Science Society of America Inc. http://refhub.elsevier.com/S0043-1354(21)00590-X/sbref0043

Sterner, R.W. & Elser, J.J. (2002). Ecological stoichiometry: the Biology of Elements from Molecules to the Biosphere. Princeton University Press, Princeton. http://refhub.elsevier.com/S0043-1354(21)00590-X/sbref0046