Species-specific functional trait responses of canopy-forming and rosette-forming macrophytes to nitrogen loading: Implications for water-sediment interactions.

Globally intensified lake eutrophication, attributed to excessive anthropogenic nitrogen loading, emerges as a significant driver of submerged vegetation degradation. Consequently, the impact of nitrogen on the decline of submerged macrophytes has received increasing attention. However, a functional trait-based approach to exploring the response of submerged macrophytes to nitrogen loading and its environmental feedback mechanism was unclear. Our study utilized two different growth forms of submerged macrophytes (canopy-forming Myriophyllum spicatum, and rosette-forming Vallisneria natans) to established "submerged macrophytes-water-sediment" microcosms. We assessed the influence of nitrogen loading, across four targeted total nitrogen concentrations (original control, 2, 5, 10 mg/L), on plant traits, water parameters, sediment properties, enzyme activities, and microbial characteristics. Our findings revealed that high nitrogen (10 mg/L) adversely impacted the relative growth rate of fresh biomass and total chlorophyll content in canopy-forming M. spicatum, while the chlorophyll a/b and free amino acid content increased. On the contrary, the growth and photosynthetic traits of resource-conservative V. natans were not affected by nitrogen loading. Functional traits (growth, photosynthetic, and stoichiometric) of M. spicatum but not V. natans exhibited significant correlations with environmental variables. Nitrogen loading significantly increased the concentration of nitrogen components in overlying water and pore water. The presence of submerged macrophytes significantly reduced the ammonia nitrogen and total nitrogen both in overlying water and pore water, and decreased total organic carbon in pore water. Nitrogen loading significantly inhibited sediment extracellular enzyme activities, but the planting of submerged macrophytes mitigated their negative effects. Furthermore, rhizosphere bacterial interactions were less compact compared to bare control, while eukaryotic communities exhibited increased complexity and connectivity. Path modeling indicated that submerged macrophytes mitigated the direct effects of nitrogen loading on overlying water and amplified the indirect effects on pore water, while also attenuating the direct negative effects of pore water on extracellular enzymes. The findings indicated that the restoration of submerged vegetation can mitigate eutrophication resulting from increased nitrogen loading through species-specific changes in functional traits and direct or indirect feedback mechanisms in the water-sediment system.

Adaptive responses and transgenerational plasticity of a submerged plant to benthivorous fish disturbance.

Submerged macrophytes play a key role in the restoration of shallow eutrophic lakes. However, in some subtropical lakes, benthivorous fishes dominate the fish assemblages and influence the growth of submerged plants. A comprehensive understanding of the direct and indirect effects of benthivorous fishes on submerged plants is important. We conducted mesocosm experiments to examine the effects of three densities of benthivorous fish, Misgurnus anguillicaudatus, on the water properties, the growth, asexual reproduction, and the germination of turions of Potamogeton crispus L. Our results showed that fish disturbance increased TN, TP, PO4-P, NH4-N, and NO3-N of the water, raising the extinction coefficient K, Chl a, and the periphyton biomass. Benthivorous fish disturbance reduced the total biomass, root length, relative growth rate (RGR), and branching number while increasing the plant height of P. crispus. The P stoichiometric homeostasis coefficient (H P) (except turions) and H N was lower in plant tissues due to fish disturbance. Benthivorous fish disturbances promoted turions formation (e.g., increased turions total numbers and biomass) of P. crispus. Moreover, P. crispus exhibited transgenerational plasticity for benthivorous fish affecting turion emergence. The maximum final germination rate occurred only when fish density in the mother plant grow experiment matched that in the turion germination experiment. Turions generated by P. crispus disturbed by low-density fish exhibited increased germination rates. Our findings suggest that controlling benthivorous fish reduces its indirect and direct effects on submerged vegetation, facilitating the successful restoration of these plants.

In situ remediation mechanism of internal nitrogen and phosphorus regeneration and release in shallow eutrophic lakes by combining multiple remediation techniques.

Large anthropogenic inputs of N and P alter the nutrient cycle and exacerbate global eutrophication problems in aquatic ecosystems. This study in Lake Datong, China, investigates the remediation mechanism of multiple remediation technique combinations (dredging, adsorbent amendment, and planting aquatic vegetation) on sediment N and P loads based on two high-resolution sampling techniques (HR-Peeper and DGT) and P sequential extraction procedures. The results showed that high temperature and low dissolved oxygen considerably enhanced pore water dissolved reactive P (DRP) and NH4+ concentrations attributable to abundant Fe-P and organic matter content in the sediment. Fe reduction is critical for regulating pore water DRP release and promoting N removal. Overall, for Lake Datong, combining multiple remediation techniques is more effective in controlling sediment P loads (pore water DRP, P fluxes, forms of P, and labile P), from a long-term perspective, than a single remediation. Lanthanum-modified bentonite (LMB) inactivation treatment can transfer mobile P in the surface sediment into more refractory forms over time, thereby reducing the risk of sediment labile P release. However, it is difficult to effectively remediate internal P loads owing to inappropriate dredging depths and low biomass of aquatic vegetation. Future lake restoration practices should optimize the selection of different remediation technique combinations based on internal N and P pollution characteristics, while reducing external wastewater input. These results are important for understanding the remediation mechanisms of internal N and P and provide suggestions for sediment management of shallow eutrophic lakes.

The spatiotemporal characteristics of water quality and phytoplankton community in a shallow eutrophic lake: Implications for submerged vegetation restoration.

One of the most serious consequences of eutrophication in shallow lakes is deterioration of water quality, proliferation of phytoplankton and disappearance of submerged macrophytes. After removing herbivorous and plankti-benthivorous fish, submerged macrophyte restoration was utilized at the entire lake (82.7 km2) to combat eutrophication and improve water quality in the shallow subtropical aquaculture of Lake Datong. We conducted two years of monitoring, from March 2018 to February 2020. During the first year of restoration, 80% of the area of Lake Datong (approximately 60 km2) was successfully recovered by submerged vegetation, and the water quality was improved. For example, the phosphorous (P) content (including total P (TP), dissolved reactive P (DRP) and total dissolved P (TDP)) and turbidity decreased, and the Secchi depth (SD) increased. However, the submerged vegetation disappeared from autumn 2019 in the intermittent recovery area (MN), while the continuous recovery area (DX) continued to recover with an abundance of submerged vegetation. During the second year, the water quality continued to improve significantly in the DX area, with high biomass and coverage of submerged vegetation. In the MN area, although turbidity and ammonia nitrogen (NH4+-N) increased significantly and SD decreased significantly, the P content (TP, TDP, and DRP) still continued to decrease. The restoration of submerged macrophytes could significantly decrease the density of phytoplankton. Over time, there was a regime shift in Lake Datong. The structural equation model (SEM) results illustrated that the water level and submerged plant coverage were the primary drivers that triggered changes in the state of the lake ecosystem. Our results highlight the potential of restoring submerged vegetation to control water eutrophication at the whole-lake scale. However, the water level in spring was the primary driver that triggered changes in the state of the lake ecosystem. Water level management should be emphasized during the early stages of recovery of submerged plants.