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Karst regions occupy 15–20% of the Earth’s continental area, and the quantity of atmospheric CO 2 trapped by surface karstification is approximately 0.61 Gt C/year. However, the stability of the karst carbon sink remains questioned. Here, we focused on two typical submerged macrophytes ( H. verticillata and M. spicatum ) collected from a karst spring-fed river to explore their influences on the stability of karst carbon sink. The pH drift technique reveals that both H. verticillata and M. spicatum collected from the karst spring-fed river can use HCO 3 − as a carbon source, providing direct evidences of the karst carbon sink sequestrated by submerged macrophytes. In the DIC enrichment water, the mean RGR fresh weights are 0.092 day −1 and 0.047 day −1 for H. verticillata and M. spicatum , respectively, which are 5.38 and 10.23 times compared with that in the control group. Taking the morphological traits to indicate their biomass allocation strategies, we find that for both species the accumulated biomass is allocated to apical elongation and root production, the difference to lateral tillering for H. verticillata as well. Overall, H. verticillata has a relatively higher ΔBiomass:ΔHCO 3 − ratio than M. spicatum , this is in agreement with the results of pH drift and morphological traits. Moving forward, we establish a model ( Δ B i o m a s s = 0.0985 × Δ p H + 0.0969 , P  < 0.001) based on the experimental data so that can be applied to quantitatively estimate the karst carbon sequestrated by submerged macrophytes in karst spring-fed rivers. Considering submerged macrophyte is the cornerstone in biomass pyramid, our work highlights that submerged macrophytes act as a carbon pump in karst aquatic ecosystems.

This study presents research on the possibility of using lake macrophytes to create diagnostic and classification tools for trophically and morphologically diverse lakes. The diagnostic role of macrophytes was determined on the basis of the spatial and functional interrelationships of the various macrophyte groups, which include emergent, submerged, and floating species. In addition, an effort was made to reveal the morphometric features of the lakes that significantly affect the development of the distinguished macrophyte groups. The research was carried out comprehensively in the lakes of the Łęczna-Włodawa Lake District, a unique natural area due to the richness of hydrogenic areas. The lakes in the area are undergoing constant and rapid changes (despite the introduction of various forms of protection), which is reflected in the decreasing number of lakes and their eutrophication. Moreover, stoneworts, which are indicators of clean water, are disappearing from the lakes. The presence of specific groups of macrophytes led to the classification of these lakes as ecologically stable, meaning they are resistant to change and sustainable over time. The depth and surface area of the lakes were identified as the key morphometric features that most significantly influence the growth of macrophytes in eutrophic lakes, which were the most abundant in the study area. By analysing the consistent characteristics of macrophytes and lakes, four distinct classes were identified: emergent macrophyte dominated lakes, submerged macrophyte dominated lakes, low macrophyte density lakes, high macrophyte density lakes. The proposed classification can serve as a diagnostic framework for lakes, helping to identify them and improve our understanding of the changes occurring within these ecosystems.

Both non-rooted submerged vegetation dominated by coontail (Ceratophyllum demersum) and non-rooted floating duckweed vegetation (Lemna gibba) can maintain their stable dominance in small ponds and channels. We examined the competitive interactions between them and how Ceratophyllum can sustain its stable state against floating plants in a range of nutrient concentrations. Coontail and duckweed were co-cultured in static and semi-static microcosm experiments, and their impact on the nutrients (N, P, Fe, Mn) in the water column was analysed. Coontail strongly reduced the growth of duckweed under a low nitrogen level (0.2–2 mg N L⁻¹). This reduction seems to be due to the low availability of nutrients in the water as derived from the lower nutrient concentrations in duckweed tissue or high pH in water. High nitrogen levels in semi-static media (5–10 mg N L⁻¹) resulted in an increasingly higher chance to overgrow C. demersum by L. gibba. Field observations revealed that C. demersum dominated over L. gibba in water bodies with total N below 3 mg L⁻¹, while L. gibba dominance over C. demersum occurred above 5 mg L⁻¹ total N. Ceratophyllum occurrence correlated negatively with total N in the water, while Lemna showed a positive correlation. Furthermore, the occurrence of L. gibba was negatively correlated with the frequency of C. demersum. All findings together support the theory that under a certain nutrient range, rootless submerged macrophytes have a strong potential to inhibit the dominance of floating plants in ponds, ditches and channels, and thus, they stabilize the submerged vegetation state.

Fluxes of carbon dioxide (CO2) and methane (CH4) in shallow lakes are strongly affected by dominant primary producers which mostly has been studied in temperate and boreal regions. We compared summer CO2 and CH4 fluxes (diffusion and ebullition) in littoral and pelagic zones of three subtropical shallow lakes with contrasting regimes: clear-vegetated, phytoplankton-turbid, and sediment-turbid, and assessed fluxes in different seasons in the clear-vegetated system. Significant differences among the lakes occurred only for CH4 fluxes. In the sediment-turbid lake we found undersaturated CH4 concentrations were below atmospheric equilibrium, implying CH4 uptake (< 0 mg m−2 day−1), likely due to low availability of organic matter. Differences between zones occurred in the clear-vegetated and phytoplankton-turbid lakes, with higher total CH4 emissions in the littoral than in the pelagic zones (mean: 4342 ± 895 and 983 ± 801 mg m−2 day−1, respectively). CO2 uptake (< < 0 mg m−2 day−1) occurred in the littoral of the phytoplankton-turbid lake (in summer), and in the pelagic of the clear-vegetated lake even in winter, likely associated with submerged macrophytes dominance. Our work highlights the key role of different primary producers regulating carbon fluxes in shallow lakes and points out that, also in the subtropics, submerged macrophyte dominance may decrease carbon emissions to the atmosphere.

Background and aims Harvesting can regulate the overgrowth of submerged macrophytes and affect interspecific competition. However, the effects of harvesting on the growth and competition of submerged macrophytes with different growth forms, remains unclear. Methods Simulation experiments were conducted to study the morphological and physiological indicators, competition intensity, and received light intensity of rosette-forming Vallisneria natans ( V.natans ) under different harvesting intensities of canopy-forming Hydrilla verticillata ( H.verticillata ). Results V. natans had greater plant height and larger leaf area at a medium-intensity (harvest 30% and 45% of the plant height) harvesting than at other intensities. Medium-intensity harvesting was the most conducive to the accumulation of chlorophyll, whereas high-intensity (harvest 60% and 75% of the plant height) harvesting had the lowest chlorophyll content and highest malondialdehyde content in V. natans . Interspecific competition was observed between H. verticillata and V. natans , and the medium harvesting intensity of H. verticillata conferred a competitive advantage to V. natans . Harvesting H. verticillata affect the growth of V. natans by increasing the underwater light intensity, with a greater harvest intensity corresponding to stronger light intensity received by V. natans . Light conditions were the most suitable for the growth of V. natans under medium-intensity harvesting. Conclusions Medium-intensity harvesting not only improved the recovery of H. verticillata but also increased the competitiveness of V. natans , thus promoting the growth of V. natans . Therefore, in practice, medium-intensity harvesting can be applied to the regulation of H. verticillata and V. natans .

Epiphytic bacteria constitute a vital component of aquatic ecosystems, pivotal in regulating elemental cycling. Despite their significance, the diversity and functions of epiphytic bacterial communities adhering to various submerged macrophytes remain largely unexplored. In this study, we employed a metagenomic approach to investigate the diversity and function of epiphytic bacterial communities associated with six submerged macrophytes: Ceratophyllum demersum , Hydrilla verticillata , Myriophyllum verticillatum , Potamogeton lucens , Stuckenia pectinata , and Najas marina . The results revealed that the predominant epiphytic bacterial species for each plant type included Pseudomonas spp., Microbacterium spp., and Stenotrophomonas rhizophila . Multiple comparisons and linear discriminant analysis effect size indicated a significant divergence in the community composition of epiphytic bacteria among the six submerged macrophytes, with 0.3–1% of species uniquely identified. Epiphytic bacterial richness associated with S. pectinata significantly differed from that of both C. demersum and H. verticillata , although no significant differences were observed in diversity and evenness. Functionally, notable variations were observed in the relative abundances of genes associated with carbon, nitrogen, and phosphorus cycling within epiphytic bacterial communities on the submerged macrophyte hosts. Among these communities, H. verticillata exhibited enrichment in genes related to the 3-hydroxypropionate bicycle and nitrogen assimilation, translocation, and denitrification. Conversely, M. verticillatum showcased enrichment in genes linked to the reductive citric acid cycle (Arnon-Buchanan cycle), reductive pentose phosphate cycle (Calvin cycle), polyphosphate degradation, and organic nitrogen metabolism. In summary, our findings offer valuable insights into the diversity and function of epiphytic bacteria on submerged macrophyte leaves, shedding light on their roles in lake ecosystems.

Harvesting is a direct and simple artificial method to regulate submerged macrophyte communities. However, there is still a gap in whether the succession of different seasonal submerged macrophytes can be achieved by harvesting. The morphological, physiological index, competition intensity of Hydrilla verticillata ( H. verticillata ) and underwater light under different harvesting intensities of Potamogeton crispus ( P. crispus ) was studied by a controlled experiment. Under moderate harvesting intensity (harvest 30% and 45% of plant height) of P. crispus , H. verticillata was the least inhibited, resulting in accumulating more Chl a and higher plant height. The underwater light created by moderate-intensity harvesting was favorable for H. verticillata to compete with P. crispus . Thus, moderate-intensity harvesting of P. crispus can promote the growth of H. verticillata . In practice, moderate harvesting intensity of P. crispus can be used to create conditions for the growth of summer species to achieve seasonal succession of submerged macrophytes and maintain the clear-water state of the lake.

ABSTRACT Large amounts of epiphytic bacteria live on the leaf surfaces of submerged macrophytes in freshwater lakes. Despite their important roles in affecting host plant's health and biogeochemical cycling, knowledge about epiphytic bacteria assembly is not sufficient. We studied epiphytic bacteria on two cohabiting plant species in Taihu Lake, China. In comparison with plant identity and geographic distance, the plant-growing season played a prominent role in driving alpha and beta diversity (compositional variations) of epiphytic bacterial communities. Phylogeny-based null model analysis revealed that the growing season also drove the relative importance of deterministic versus stochastic processes underlying bacterial community assembly. In May when both plants start growth, the deterministic processes were most prominent, while in months later than June, the stochastic processes’ effects increased substantially. In addition, we found a significant positive relationship between alpha diversity and compositional stochasticity, implying that stochastic processes may have great effects on the maintenance of diversity and functioning of epiphytic bacteria in aquatic ecosystems. In summary, the growing season overwhelmed plant identity and spatial site in shaping epiphytic bacterial communities in Taihu Lake, which may suggest new clues in understanding the dynamics of epiphytic communities and their roles in large shallow lacustrine ecosystems. Growing season overwhelmed site (A, site DX; B, site GF in Taihu lake) and plant identity in influencing the alpha/beta diversity and assembly processes of epiphytic bacteria on submerged plants in a lacustrine ecosystem.

The nutrient threshold of collapse and recovery of submerged macrophytes have been widely reported for shallow lakes. However, understanding the threshold variation for lakes with water depth (Z) gradients remains limited. In this study, based on a field investigation of 9 lakes with varying water depths and nutrient levels in the Yunnan Plateau, southwest of China, we integrated water depth to predict the nutrient threshold of collapse and recovery of submerged macrophytes in lakes. Our results showed that: 1) Canopy-forming submerged macrophytes, i.e. and , had a higher resistance to high nutrients and turbidity; 2) Submerged macrophyte species richness had a significantly negative response to water depth, while biomass did not; 3) A multiplication of turbidity (Turb) with water depth provided the best explanation for the collapse and recovery of submerged macrophytes for lakes with large depth gradients compared to the single variables; 4) The thresholds of Z /Z were 0.06 for the collapse of submerged macrophytes and 0.53 for the recovery of submerged macrophytes; the corresponding thresholds were 81.6 and 9.92 NTU m for Turb*Z, respectively. Our findings on the thresholds of macrophyte collapse and recovery are expected to provide quantitative guidance for lake restoration of diverse water depths.

Aquatic ecosystems provide vital services, and macrophytes play a critical role in their functioning. Conceptual models indicate that in shallow lakes, plants with different growth strategies are expected to inhabit contrasting habitats. For shallow peat lakes, characterized by incohesive sediments, roles of growth forms, life-history strategies and environmental factors in determining the occurrence of aquatic vegetation remain unknown. In a field survey, we sampled 64 points in a peat lake complex and related macrophyte occurrence to growth forms (floating-leaved rooted and submerged), life-history strategies for overwintering (turions, seeds, rhizomes) and environmental factors (water depth, fetch, and porewater nutrients). Our survey showed that macrophyte occurrence relates to water depth, wind-fetch, and nutrients, and depends on growth form and life-history strategies. Specifically, rooted floating-leaved macrophytes occur at lower wind-fetch/shallower waters. Submerged macrophytes occur from low to greater wind-fetch/water depth, depending on life-history strategies; macrophytes with rhizomes occur at greater wind-fetch/depth relative to species that overwinter with seeds or turions. We conclude that growth form and life-history strategies for overwintering predict macrophytes occurrence regarding environmental factors in peat lakes. Therefore, we propose an adapted model for macrophyte occurrence for such lakes. Altogether, these results may aid in species-selection to revegetate peat lakes depending on its environment.