Population asynchrony within and between trophic levels have contrasting effects on consumer community stability in a subtropical lake.

Clarifying the effects of biodiversity on ecosystem stability in the context of global environmental change is crucial for maintaining ecosystem functions and services. Asynchronous changes between trophic levels over time (i.e. trophic community asynchrony) are expected to increase trophic mismatch and alter trophic interactions, which may consequently alter ecosystem stability. However, previous studies have often highlighted the stabilising mechanism of population asynchrony within a single trophic level, while rarely examining the mechanism of trophic community asynchrony between consumers and their food resources. In this study, we analysed the effects of population asynchrony within and between trophic levels on community stability under the disturbances of climate warming, fishery decline and de-eutrophication, based on an 18-year monthly monitoring dataset of 137 phytoplankton and 91 zooplankton in a subtropical lake. Our results showed that species diversity promoted community stability mainly by increasing population asynchrony both for phytoplankton and zooplankton. Trophic community asynchrony had a significant negative effect on zooplankton community stability rather than that of phytoplankton, which supports the match-mismatch hypothesis that trophic mismatch has negative effects on consumers. Furthermore, the results of the structural equation models showed that warming and top-down effects may simultaneously alter community stability through population dynamics processes within and between trophic levels, whereas nutrients act on community stability mainly through the processes within trophic levels. Moreover, we found that rising water temperature decreased trophic community asynchrony, which may challenge the prevailing idea that climate warming increases the trophic mismatch between primary producers and consumers. Overall, our study provides the first evidence that population and trophic community asynchrony have contrasting effects on consumer community stability, which offers a valuable insight for addressing global environmental change.

Eutrophication decreases ecological resilience by reducing species diversity and altering functional traits of submerged macrophytes.

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 (HP ) 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 HP 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.

Cumulative effects of fish disturbance and vegetative propagules reduce the growth advantage of Vallisneria natans.

Benthivorous fish disturbance and density-dependent competition from adjacent macrophytes are two important biotic factors that significantly impact the growth of submerged macrophyte pioneer species, which is crucial for the success of eutrophication lake restoration. We conducted an outdoor mesocosm experiment to explore the individual and combined effects of these two factors on water quality and the growth of Vallisneria natans. The experiment involved two levels of fish (Misgurnus anguillicaudatus) disturbance crossed with two levels of Hydrilla verticillata vegetative propagule (shoot) intensity. The results showed that fish disturbance significantly increased the water column total nitrogen (TN), ammonia nitrogen (N-NH4), total phosphorus (TP), and phosphate­phosphorus (P-PO4). V. natans exhibited restricted plant height elongation and decreased soluble carbohydrate (SC) and starch concentration in fish treatments. Fish disturbance inhibited the growth advantage of V. natans by increasing the extinction coefficient of the water column. There was no statistical significance in total biomass between the two macrophytes in increased vegetative propagule and fish treatments. H. verticillata exhibited a higher relative growth rate (RGR) and summed dominance ratio (SDR3) than V. natans in four treatments and the treatment with three shoots of H. verticillata and one M anguillicaudatus, respectively. Fish disturbance and vegetative propagules showed cumulative effects that negatively affected the RGR_V.H (V. natans relative to H. verticillata). Our findings indicated that benthivorous fish disturbance and vegetative propagules could individually and cumulatively reduce the growth advantage of the pioneer species, V. natans. Our study sheds light on the accumulated effects of multiple disturbances that simultaneously occur in lakes, which holds theoretical and practical importance for lake restoration efforts.

Characteristics of plant trait network and its influencing factors in impounded lakes and channel rivers of South-to-North Water Transfer Project, China.

Trait-based approaches have been widely used to evaluate the effects of variable environments on submerged macrophytes communities. However, little research focused on the response of submerged macrophytes to variable environmental factors in impounded lakes and channel rivers of water transfer project, especially from a whole plant trait network (PTN) perspective. Here, we conducted a field survey designed to clarify the characteristic of PTN topology among impounded lakes and channel rivers of the East Route of South-to-North Water Transfer Project (ERSNWTP) and to unravel the effects of determining factors on the PTN topology structure. Overall, our results showed that leaf-related traits and organ mass allocation traits were the hub traits of PTNs in impounded lakes and channel rivers of the ERSNWTP, which traits with high variability were more likely to be the hub traits. Moreover, PTNs showed different structures among impounded lakes and channel rivers, and PTNs topologies were related to the mean functional variation coefficients of lakes and channel rivers. Specially, higher mean functional variation coefficients represented tight PTN, and lower mean functional variation coefficients indicated loose PTN. The PTN structure was significantly affected by water total phosphorus and dissolved oxygen. Edge density increased, while average path length decreased with increasing total phosphorus. Edge density and average clustering coefficient showed significant decreases with increasing dissolved oxygen, while average path length and modularity exhibited significant increases with increasing dissolved oxygen. This study explores the changing patterns and determinants of trait networks along environmental gradients to improve our understanding of ecological rules regulating trait correlations.

Linking the network topology of plant traits with community structure, functioning, and adaptive strategies of submerged macrophytes.

Plant trait network analysis can calculate the topology of trait correlations and clarify the complex relationships among traits, providing new insights into ecological topics, including trait dimensions and phenotypic integration. However, few studies have focused on the relationships between network topology and community structure, functioning, and adaptive strategies, especially in natural submerged macrophyte communities. In this study, we collected 15 macrophyte community-level traits from 12 shallow lakes in the Yangtze River Basin in the process of eutrophication and analyzed the changes in trait network structure (i.e., total phosphorus, TP) by using a moving window method. Our results showed that water TP significantly changed the topology of trait networks. Specifically, under low or high nutrient levels, the network structure was more dispersed, with lower connectance and higher modularity than that found at moderate nutrient levels. We also found that network connectance was positively correlated with community biomass and homeostasis, while network modularity was negatively correlated with community biomass and homeostasis. In addition, modules and hub traits also changed with the intensity of eutrophication, which can reflect the trait integration and adaptation strategies of plants in a stressful environment. At low or high nutrient levels, more modules were differentiated, and those modules with higher strength were related to community nutrition. Our results clarified the dynamics of community structure and functioning from a new perspective of plant trait networks, which is key to predicting the response of ecosystems to environmental changes.

Phosphorus enrichment affects trait network topologies and the growth of submerged macrophytes.

Significant differences in the morphological and physiological characteristics of submerged macrophytes have been studied following nutrient addition, but little research has investigated the changes in plant trait network topology structures and trait interactions at the whole-plant perspective along nutrient gradients. Plant trait interactions and coordination strongly determine ecosystem structure and functioning. Thirty plant traits were collected from a three-month experiment to construct plant trait networks to clarify the variations in trait connections and network organization arising from five total phosphorus (TP) addition concentrations in water, including a control (CK), 0.1 (TP1), 0.2 (TP2), 0.4 (TP3), and 0.8 (TP4) mg L-1. Nonmetric multidimensional scaling analysis showed a clear difference in the distribution of plant trait space among the different TP treatments. Distinct network structures showed that water TP-deficiency and TP-repletion changed the plant trait network into loose assemblages of more modules, which was related to low plant carbohydrate levels. Most plant functions involving biomass accumulation and carbohydrate synthesis were reduced under high TP conditions compared to moderate TP enrichment. Moreover, the percentage of significant relationships between plant functions and corresponding network modules was lower in the CK and TP4 treatments. These results suggested that low plant carbohydrates in high TP environments induced by high water chlorophyll a and tissue phosphorus could not support rapid resource transport among organs and thus inefficiently performed plant functions. Plant carbohydrates were a vital variable that impacted the network edge density, trait interactions, and plant growth. In summary, we demonstrated that high water TP enrichment reduces plant trait network connectedness and plant functional potentials, which may be correlated with reducing tissue carbohydrates. This study explores the correlations between plant trait network topology and functions to improve our understanding of physiological and ecological rules regulating trait interactions among organs and plant growth under eutrophic conditions.

Stoichiometric and physiological mechanisms that link hub traits of submerged macrophytes with ecosystem structure and functioning.

Eutrophication strongly influences plant stoichiometric characteristics and physiological status by altering nutrient and light availability in the water column. However, the mechanisms linking plant functional traits with ecosystem structure and functioning to clarify the decline of submerged macrophytes have not been fully elucidated to date. Therefore, based on a field investigation of 26 macrophytic shallow lakes on the Yangtze Plain, we first constructed a plant trait network at the whole-plant level to determine the hub traits of submerged macrophytes that play central regulatory roles in plant phenotype. Our results suggested that organ (leaf, stem, and root) phosphorus (P), starch, and total nonstructural carbohydrate (TNC) contents were hub traits. Organ starch and TNC were consistent with those in the experiment-based network obtained from a three-month manipulation experiment. Next, the mechanisms underlying the relationships between the hub traits and vital aspects of ecological performance were carefully investigated using field investigation data. Specifically, stoichiometric homeostasis of P (HP), starch, and TNC were positively associated with dominance and biomass at the species level, and community biomass at the community level. Additionally, structural equation modeling clarified not only a hypothesized pathway from eutrophication to water clarity and community TNC, but also combined effects of community TNC and HP on community biomass. That is, ecosystems dominated by more homeostatic communities tended to have more carbon (C)-rich compounds in relatively oligotrophic conditions, which promoted the primary production of macrophytes. Eutrophication was determined to affect community structure by inhibiting the predominance of more homeostatic species and the production of carbohydrates. Finally, reduced community biomass and increased nutrient contents and nutrient:C ratios in plants induced by eutrophication implied a decrease in the C sink in biomass and may potentially lead to an enhancement of litter decomposition rates and nutrient cycling rates. By adjusting plant responses to eutrophication, stoichiometric and physiological mechanisms linking plant traits with ecosystem structure have important implications for understanding ecosystem processes, and these results may contribute to practical management to achieve the restoration of submerged macrophytes and ecosystem services.

Rainfall is a significant environmental factor of microplastic pollution in inland waters.

Worldwide, increasing attention is being paid to the issue of microplastic pollution in marine ecosystems, yet there is a relative lack of knowledge concerning the pollution of microplastic in inland water systems, although these microplastics are major sources of ocean pollution. In aquatic environments, previous efforts have mainly been devoted to exploring the impact of human-related activities on microplastic pollution, but little is known about non-anthropogenic effects on microplastic distribution. In this paper, the relationship between rainfall and the microplastic concentration of surface water was studied for the first time. The investigation was conducted in Lake Donghu (the largest urban lake in China) from July 23 to September 1, 2019. The abundance of microplastics in the lake water ranged from 7.4 to 29.6 items/L. The dominant size of the collected microplastics was less than 2 mm, with a dominant fiber shape and a dominant transparent color. Our results demonstrated that microplastic concentration was significantly related to rainfall. Therefore, high frequency sampling and rainfall data are needed to accurately evaluate microplastic pollution as well as its consequences.

Carbon, Nitrogen, and Phosphorus Allocation Strategy Among Organs in Submerged Macrophytes Is Altered by Eutrophication.

The allocation of limiting elements among plant organs is an important aspect of the adaptation of plants to their ambient environment. Although eutrophication can extremely alter light and nutrient availability, little is known about nutrient partitioning among organs of submerged macrophytes in response to eutrophication. Here, we analyzed the stoichiometric scaling of carbon (C), nitrogen (N), and phosphorus (P) concentrations among organs (leaf, stem, and root) of 327 individuals of seven common submerged macrophytes (three growth forms), sampled from 26 Yangtze plain lakes whose nutrient levels differed. Scaling exponents of stem nutrients to leaf (or root) nutrients varied among the growth forms. With increasing water total N (WTN) concentration, the scaling exponents of stem C to leaf (or root) C increased from <1 to >1, however, those of stem P to root P showed the opposite trend. These results indicated that, as plant nutrient content increased, plants growing in low WTN concentration accumulated leaf C (or stem P) at a faster rate, whereas those in high WTN concentration showed a faster increase in their stem C (or root P). Additionally, the scaling exponents of stem N to leaf (or root) N and stem P to leaf P were consistently large than 1, but decreased with a greater WTN concentration. This suggested that plants invested more N and P into stem than leaf tissues, with a higher investment of N in stem than root tissues, but eutrophication would decrease the allocation of N and P to stem. Such shifts in plant nutrient allocation strategies from low to high WTN concentration may be attributed to changed light and nutrient availability. In summary, eutrophication would alter nutrient allocation strategies of submerged macrophytes, which may influence their community structures by enhancing the competitive ability of some species in the process of eutrophication.

Spatial and interspecies differences in concentrations of eight trace elements in wild freshwater fishes at different trophic levels from middle and eastern China.

There have been numerous studies on concentrations of trace elements in aquatic ecosystems, but few have been conducted at a large spatial scale. This study collected 410 samples of five wild freshwater fishes at different trophic levels from middle and eastern China. Concentrations of eight trace elements, chromium (Cr), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), lead (Pb) and cadmium (Cd) and stable isotope ratios (δ13C and δ15N) were determined in dorsal muscle of fishes. Spatially, concentrations of trace elements were least in fishes from the Hai River Basin, while those in fishes from the Taihu Lake Basin were greatest. The carnivorous topmouth culter and omnivorous common carp and crucian carp accumulated greater amounts of trace elements than did the planktivorous silver carp and bighead carp. Trophic biomagnification was for Cu, Fe and Zn, but not for Cr, Ni, As, Pb and Cd. Concentrations of As in 15 muscle samples (3.7%) from Taihu Lake Basin exceeded the guidelines (1.0 mg/kg, wet mass) provided by FAO/WHO (2014), while the total target hazard quotient (TTHQ) values were <1.0, indicating no obvious non-carcinogenic risks to humans that consume those fishes. However, people who consume larger amounts of fish products, or people who are vulnerable, such as pregnant women, children and people with poor health, might be at greater risk. Also, exposure to trace metals through other routes cannot be ignored. Accumulations of trace elements in Chinese freshwater fishes were affected by both geographical conditions and human activities.

Effects of high ammonium enrichment in water column on the clonal growth of submerged macrophyte Vallisneria natans.

As we know, the survival of young ramets and stolons is essential for the clonal growth of many aquatic plants. However, few NH4+ enrichment experiments on clonal growth of submerged macrophytes have been conducted to provide possible evidences for their declines in eutrophic lakes. Here, the growth and physiological responses of V. natans to the enrichment of NH4+-N were examined under six inorganic nitrogen (IN, i.e., the sum of nitrate nitrogen (NO3--N) and ammonium nitrogen (NH4+-N)) concentrations (control, 2.5, 4.5, 6.5, 8.5, and 10.5 mg L-1). When NH4+-N concentration increased over 0.5 mg L-1, free amino acid (FAA) contents in leaves and stolons increased while soluble carbohydrate (SC) and starch contents decreased, and major growth indices (total biomass of plants, number of ramets, and stolon dry weight (DW)) also showed a degressive tendency. Remarkably, the stolon DW significantly declined with increasing FAA, but significantly positively related to SC and starch. These results indicated that clonal growth of V. natans was inhibited by high NH4+-N concentration, and imbalance of C-N metabolism of stolons partly explained the decline of submerged clonal macrophytes. In this study, the leaves of new and small (NS) ramets contained significantly more FAA and less SC than that of mature and mother (MM) plants, indicating that the C-N metabolism of young ramets was easier to be disrupted, consequently inhibiting the clonal growth of aquatic plants. Furthermore, under the condition of high NH4+-N concentration, FAA may be a useful indicator of both macrophyte growth and physiological stress of plants.