Driving forces and recovery potential of the macrophyte decline in East Taihu Lake

Floating-leaved and submerged macrophytes suppress filamentous cyanobacteria blooms and 2-MIB episodes in eutrophic shallow lakes

Although macrophytes can control filamentous cyanobacteria that produce 2-methylisoborneol (2-MIB) in lakes, there is a lack of evidence regarding the inhibitory effects of different macrophyte growth forms on 2-MIB producers, as well as the underlying mechanisms. To address this knowledge gap, this study compared the impact of floating-leaved and submerged macrophytes on Pseudanabaena growth and 2-MIB release, combing a field investigation and culture experiments. The field survey showed that both the Pseudanabaena cell density and 2-MIB concentrations were significantly lower in areas dominated by floating-leaved or submerged macrophytes than in phytoplankton-dominated areas. In the culture experiments, floating-leaved macrophytes exhibited overall stronger inhibitory effects on Pseudanabaena than submerged macrophytes. Allelopathic effects emerged as a more critical mechanism than nutrient competition and light limitation in controlling Pseudanabaena growth and 2-MIB release by regulating the photosynthetic activity, gene abundance, and cell density. Allelopathic experiments further confirmed that the dissolved organic matter released from Nymphoides peltate, Trapa bispinosa and Myriophyllum spicatum contained higher concentrations of allelochemicals than that released from Vallisneria natans and Ceratophyllum demersum, driving the photosynthetic inhibition pathway. These findings demonstrate that biological control has great promise as an effective method for odorant management in eutrophic shallow lakes.

Host selection and nutrient status jointly drive algal and bacterial interactions in epiphytic biofilms of submerged macrophytes: Structural and functional insights

Epiphytic biofilms play a crucial role in aquatic biogeochemical cycles but are simultaneously influenced by host selection and eutrophication. However, the compositional structure and interaction mechanisms of these factors on algal and bacterial communities remain poorly understood. In this study, we employed Confocal Laser Scanning Microscopy (CLSM), Scanning Electron Microscopy (SEM), and high-throughput sequencing to investigate the physicochemical properties, algal and bacterial diversity, and community structure of epiphytic biofilms on two submerged macrophytes - Vallisneria natans (VN) and Hydrilla verticillata (HV) - across three urban shallow lakes with varying trophic levels in the Yangtze River Basin. Our results revealed distinct algal and bacterial communities influenced by both host plants and lake nutrient conditions, with unique core species identified in VN, HV, and the surrounding water. Host-environment effects index (HEEI = 1.79) indicated that bacterial communities were predominantly shaped by host selection, exhibiting lower diversity in HV (1.66 ± 0.92) and VN (2.31 ± 1.12) biofilms compared to surrounding waters (3.80 ± 0.47). In contrast, algal communities were primarily regulated by environmental factors (HEEI = 0.43), with higher diversity in less eutrophic lakes. Algal-bacterial co-occurrence network analysis revealed greater network complexity in VN biofilms than that in HV, with predominantly synergistic interactions facilitating carbon and nitrogen cycling. Eutrophication increased biofilm thickness, nutrient content, and extracellular polymeric substance (EPS) production but reduced microbial diversity and altered community interaction patterns. This study advances our understanding of epiphytic biofilms and offers insights into optimizing host-microbe interactions for improving lake restoration strategies.

Decomposition of exotic versus native aquatic plant litter in a lake littoral zone: Stoichiometry and life form analyses

The decomposition rates and stoichiometric characteristics of many aquatic plants remain unclear, and our understanding of material flow and nutrient cycles within freshwater ecosystems is limited. In this study, an in-situ experiment involving 23 aquatic plants (16 native and 7 exotic species) was carried out via the litter bag method for 63 days, during which time the mass loss and nutrient content (carbon (C), nitrogen (N), and phosphorus (P)) of plants were measured. Floating-leaved plants exhibited the highest decomposition rate (0.038 ± 0.002 day-1), followed by submerged plants and free-floating plants (0.029 ± 0.002 day-1), and emergent plants had the lowest decomposition rate (0.019 ± 0.001 day-1). Mass loss by aquatic plants correlated with stoichiometric characteristics; the decomposition rate increased with an increasing P content and with a decreasing C content, C:N ratio, and C:P ratio. Notably, the decomposition rate of submerged exotic plants (0.044 ± 0.002 day-1) significantly exceeded that of native plants (0.026 ± 0.004 day-1), while the decomposition rate of emergent exotic plants was 55 ± 4 % higher than that of native plants. The decomposition rates of floating-leaved and free-floating plants did not significantly differ between the native and exotic species. During decomposition, emergent plants displayed an increase in C content and a decrease in N content, contrary to patterns observed in other life forms. The P content decreased for submerged (128 ± 7 %), emergent (90 ± 5 %), floating-leaved (104 ± 6 %), and free-floating plants (32 ± 6 %). Exotic plants released more C and P but accumulated more N than did native plants. In conclusion, the decomposition of aquatic plants is closely linked to litter quality and influences nutrient cycling in freshwater ecosystems. Given these findings, the invasion of the littoral zone by submerged and emergent exotic plants deserves further attention.

The Secchi disk depth to water depth ratio affects morphological traits of submerged macrophytes: Development patterns and ecological implications

Water clarity, represented by Secchi disk depth (SD), and water depth (WD) alter bottom light availability, and SD/WD is critical for morphological trait development of submerged macrophytes in freshwater ecosystems. However, the underlying mechanism and trait development patterns of submerged macrophytes to a decreasing SD/WD gradient remains largely unknown. Here, we performed a 42-day mesocosm experiment with the erect type submerged macrophyte, Hydrilla verticillata, along a decreasing SD/WD gradient to study the relationship of morphological trait development with light availability, to determine the critical SD/WD at which changes in the development of morphological traits occur, and to gain insights into the potential mechanism involved. The results indicate that most of the morphological traits, including biomass, relative growth rate, number of clonal propagules, and the root/shoot ratio decreased with a decrease in the SD/WD ratio. Conversely, plant height and shoot increment rate increased with a decrease in the SD/WD ratio. Principal component analysis indicated that the SD/WD ratio is critical in determining the growth, stability, and reproduction of H. verticillata, and that only SD/WD ratios ≥ 0.45 and ≥0.55 ensured growth ability and stability, respectively. Possible development patterns of functional traits in relation to SD/WD reduction were investigated, and patterns of key traits of H. verticillata were distinct from those of Vallisneria natans, indicating different strategies for the adaptation to conditions of decreasing light availability. These results highlight the role of adaptive changes in morphology, resource allocation and life strategies for the maintenance of growth, stability and resilience of submerged macrophytes in low light conditions. Our present study provides a basis from which we could enhance our understanding of the critical transition mechanisms involved in morphological trait development in response to bottom light availability.