Water Depth Gradient Research Articles

A Self-Constraint Underwater Positioning Method without the Assistance of Measured Sound Velocity Profile

Aiming at the problem that lack of the measured sound velocity profile (SVP) leads to the unreliable underwater positioning solution, this paper proposed an efficient underwater positioning method by the self-constraint conditions of water depth and sound velocity gradient. To construct the depth constraint condition, the sound propagation distance error model is deduced by acoustic ray tracing, and the sound vertical propagation error model related to the incident angle and sound velocity error is given firstly. By fitting the vertical propagation error model, the reference depth is solved, and the vertical propagation distances between the transducer and the underwater control points of all observation epochs are gotten. Then with the solved vertical distance of each epoch and the sound velocity gradient from neighbor SVPs as the constraint conditions, the SVP is retrieved by the simulated annealing (SA) algorithm. With the retrieved SVP, the underwater positioning can be performed when the measured SVP is absent. The proposed method was verified by an experiment in the 3000 m depth water area of the South China Sea. The results achieved 2.07 m/s of standard deviation of the SVP inversion, centimeter-level horizontal positioning accuracy and 0.54 m of vertical positioning accuracy under the circumstance of lack of SVP measurement.

ECOLOGICAL IMPACTS OF ECOHYDROLOGICAL INTERVENTION ON THE WETLANDS ALONG THE SOUTHEASTERN SHORE OF LAKE TANA, ETHIOPIA

Lake Tana watershed wetlands have critical functions of nutrient and sediment retention, biodiversity and hydrological functions. Although ecohydrological solutions have been proposed, a comprehensive study on the impacts of ecohydrology intervention in relation to biological aspects is limited. Information regarding the ecological impacts of ecohydrology based intervention approach in the wetland ecosystem plays an indispensable role in increasing biodiversity and improves water quality. Thus, the aim of this study was to investigate the impacts of ecohydrological intervention in relation to macrophytes, phytoplankton and zooplankton of the wetlands in Lake Tana shore. Quantitative characteristics of macrophytes and plankton sampling were conducted using transect and quadrat method along vegetation and water depth gradients. The biological data were measured on each fixed sampling point. Macrophytes, phytoplankton and zooplankton were significantly different (p<0.05) across the sampling sites. Low disturbed wetland supports higher abundance and diversity of macrophytes, phytoplankton and zooplankton than moderately and highly disturbed wetlands. Overall results of the study confirmed that biological abundance and diversity were associated with the level of disturbance of wetlands. Therefore, emphasis should be given to the factors which may lead to the degradation of wetland resources within and its catchment. Immediate ecohydrology based intervention approaches should be implemented to conserve and minimize further degradation of wetlands along the shore of Lake Tana.

What is the mechanism of submerged macrophyte biodiversity affecting biomass productivity along water depth: Niche complementarity or selection effect?

The impact of biodiversity on ecosystem functions and services remains a hot topic in ecology, especially under increasing biodiversity loss. Studies on the relationship between biodiversity and ecosystem functioning have identified two regulatory mechanisms: i.e., “niche complementarity” and “selection effect”. However, the relative importance of these mechanisms within communities and how they change along ecological gradients remain unclear. Moreover, most studies have focused on relatively stable ecosystems (e.g., forest, grassland), with limited attention paid to ecosystems with obvious seasonal changes, e.g., freshwater ecosystems. In this study we conducted a seasonal survey of submerged macrophyte communities to clarify the mechanisms of biodiversity on biomass productivity accounting for seasons and water depth. Our results showed that (1) seasonal variations of community indicators exhibited different trends along the water depth gradient. Biomass productivity showed high seasonal variation at intermediate depth while temporal beta diversity showed the opposite pattern. (2) Water depth not only inhibited biomass productivity directly but also indirectly by reducing species richness and enhancing temporal beta diversity. (3) The positive effect of species richness on biomass productivity was lower than the negative effect of evenness (path coefficients: 0.13 vs. 0.27), indicating that “selection effect” played a more important role in the relationship. (4) In shallow areas, the mechanism underlying the effects of biodiversity on biomass productivity was more from “selection effect”, whereas, in deep areas, the effects were related to both “niche complementarity” and “selection effect”. Our findings suggest that “niche complementarity” and “selection effect” are two complementary mechanisms underpinning the relationship between biodiversity and biomass productivity, and their relative importance varies with specific ecological gradients. Our results also provide a reference for studies on other freshwater organisms with analogous distribution patterns as submerged macrophytes with water depth. Scale dependence of the above relationship should be considered in future studies.

Molecular evolution and depth-related adaptations of rhodopsin in the adaptive radiation of cichlid fishes in Lake Tanganyika.

The visual sensory system is essential for animals to perceive their environment and is thus under strong selection. In aquatic environments, light intensity and spectrum differ primarily along a depth gradient. Rhodopsin (RH1) is the only opsin responsible for dim‐light vision in vertebrates and has been shown to evolve in response to the respective light conditions, including along a water depth gradient in fishes. In this study, we examined the diversity and sequence evolution of RH1 in virtually the entire adaptive radiation of cichlid fishes in Lake Tanganyika, focusing on adaptations to the environmental light with respect to depth. We show that Tanganyikan cichlid genomes contain a single copy of RH1. The 76 variable amino acid sites detected in RH1 across the radiation were not uniformly distributed along the protein sequence, and 31 of these variable sites show signals of positive selection. Moreover, the amino acid substitutions at 15 positively selected sites appeared to be depth‐related, including three key tuning sites that directly mediate shifts in the peak spectral sensitivity, one site involved in protein stability and 11 sites that may be functionally important on the basis of their physicochemical properties. Among the strongest candidate sites for deep‐water adaptations are two known key tuning sites (positions 292 and 299) and three newly identified variable sites (37, 104 and 290). Our study, which is the first comprehensive analysis of RH1 evolution in a massive adaptive radiation of cichlid fishes, provides novel insights into the evolution of RH1 in a freshwater environment.

The effect of trait‐based diversity on productivity results mainly from intraspecific trait variability in the macrophyte community

Abstract Trait‐based methods are key to understanding the biodiversity–productivity relationship (BPR) of macrophyte communities. Community‐weighted mean traits (i.e., community trait structure) have been proven to have more influence on macrophyte community productivity than species richness. However, the underlying mechanism by which community trait structure variation affects macrophyte community productivity along an environmental gradient is still not well understood. A mesocosm experiment was used to investigate how community trait structure shapes macrophyte community productivity along the water depth gradient. Three submerged macrophyte species (Myriophyllum spicatum, Vallisneria natans and Potamogeton malaianus) were assembled into all possible combinations (one, two and three species per community) at three different water depths (1, 2.5 and 4 m). We fitted the relationships between community‐weighted mean traits and community productivity, and between species richness and community productivity as a comparison. We compared functional traits under different water depths and species composition at the species and community levels and disentangled the community trait structure into species turnover and intraspecific trait variability effects. The results showed that community trait structure had various influences on macrophyte community productivity, which was based on traits per se. However, species richness had a non‐significant impact on macrophyte community productivity. Water depth had a significant impact on most traits of the three species at the species level, whereas both water depth and species composition had significant impacts on community trait structure which was mainly affected by intraspecific trait variability along the water depth gradient. Our findings highlight the importance of considering intraspecific trait variability and species turnover under water depth gradients to understand the relationship between trait‐based biodiversity and productivity.

Environmental DNA Monitoring of Biodiversity Hotspots in Danish Marine Waters

We investigated the use of eDNA metabarcoding for supplementing traditional diver-based monitoring of biodiversity of marine boulder reefs within the photic zone. The applied sampling design made it possible to evaluate the usefulness of eDNA monitoring as a supplement for traditional monitoring. Specifically, this study aimed to (1) assess the local influence of boulder reefs on biodiversity across the North Sea to Baltic Sea transition zone and (2) investigate the importance of environmental gradients for patterns in community structure. On samples collected during August 2020, we compared the composition and abundance of species associated with nine reefs, representing an environmental gradient of salinity (16–33 psu), water temperature (16–21°C) and water depth (6–29 m). At each reef site, water was sampled near the bottom just above the reef and on average 2.6 km upstream and downstream (location) and sequenced with metabarcoding using COI, 18S and 12S rDNA primers. eDNA identified 400 species, diver-based observations identified 184 with an overlap of 70 species (12%) and 81 genera (18%). While eDNA identified many infaunal species, it did not detect several macroalgal species which dominated in the diver-based observations. Multivariate analysis of eDNA and diver-based community structure both distinguished between reef communities, with a significant match between patterns observed by the two methods (r = 0.37, p = 0.02). Furthermore, the eDNA approach made it possible to identify significant differences in species composition between upstream, above-reef and downstream locations, suggesting that eDNA leaves a local footprint in benthic habitats. Patterns in both eDNA and diver-based species composition and richness were significantly related to geographical distance, salinity, water temperature and water depth. Despite of low detection of macroalgae, the eDNA sampling provided a substantial supplement to traditional diver-based monitoring of biodiversity around benthic hotspots in the Danish marine waters and therefore we recommend to add eDNA methods to conventional monitoring programs in the future.

Interplay between eutrophication and climate warming on bacterial communities in coastal sediments differs depending on water depth and oxygen history

Coastal aquatic systems suffer from nutrient enrichment, which results in accelerated eutrophication effects due to increased microbial metabolic rates. Climate change related prolonged warming will likely accelerate existing eutrophication effects, including low oxygen concentrations. However, how the interplay between these environmental changes will alter coastal ecosystems is poorly understood. In this study, we compared 16S rRNA gene amplicon based bacterial communities in coastal sediments of a Baltic Sea basin in November 2013 and 2017 at three sites along a water depth gradient with varying bottom water oxygen histories. The shallow site showed changes of only 1.1% in relative abundance of bacterial populations in 2017 compared to 2013, while the deep oxygen-deficient site showed up to 11% changes in relative abundance including an increase of sulfate-reducing bacteria along with a 36% increase in organic matter content. The data suggested that bacterial communities in shallow sediments were more resilient to seasonal oxygen decline, while bacterial communities in sediments subjected to long-term hypoxia seemed to be sensitive to oxygen changes and were likely to be under hypoxic/anoxic conditions in the future. Our data demonstrate that future climate changes will likely fuel eutrophication related spread of low oxygen zones.

Ecosystem functioning is linked to microbial evenness and community composition along depth gradient in a semiarid lake

Lake level or water depth are key physical variables known to respond dramatically to climate change, especially in arid regions, and their fluctuations exert substantial influences on lake biodiversity and ecosystem functioning. However, it is unclear how multiple ecosystem functions (i.e., ecosystem multifunctionality, EMF) respond to changes in water depth and how aquatic community attributes, such as species richness, evenness and community compositions, are linked to EMF along water depth gradients. Lake Hulun, a representative of semiarid-region lake in China, has experienced serious lake area shrinkage over the past 20 years. Here, we explored the water-depth patterns of three microbial taxonomic groups of bacteria, archaea and fungi and nine ecosystem functions related to nutrient cycling in Lake Hulun. We further examined the relative importance of different community attributes on EMF variations. We found that the community compositions of bacteria, archaea and fungi showed consistent water-depth decay patterns, and EMF and most individual ecosystem functions involved in C, N, P and S cycling increased with water depth. Further, EMF was predominantly mediated by microbial evenness and community composition, but not species richness, as predicted by the traditional theory of biodiversity-ecosystem functioning relationships. In addition, water depth indirectly affected the relationships between the microbial community and EMF via sediment nutrient contents. These findings indicate that the water depth changes under climate change could substantially alter ecosystem structure and functioning in arid regions. We further emphasize the necessity of including multiple community attributes in biodiversity-EMF relationship research to clarify the biotic and abiotic forces underlying EMF variations.

Rapid adaptive responses of rosette-type macrophyte Vallisneria natans juveniles to varying water depths: The role of leaf trait plasticity.

Rosette‐type submerged macrophytes are widely distributed across a range of water depths in shallow lakes and play a key role in maintaining ecosystem structures and functions. However, little is known about the rapid adaptive responses of such macrophytes to variations in water depth, especially at the juvenile stage. Here, we conducted a short‐term in situ mesocosm experiment, in which the juveniles of Vallisneria natans were exposed to a water depth gradient ranging from 20 to 360 cm. Twenty‐two leaf‐related traits were examined after 4 weeks of growth in a shallow lake. Most (18) traits of V. natans generally showed high plasticity in relation to water depth. Specifically, juveniles allocated more biomass to leaves and had higher specific leaf area, leaf length‐to‐width ratio, chlorophyll content, and carotenoids content in deep waters, displaying trait syndrome associated with high resource acquisition. In contrast, V. natans juveniles in shallow waters had higher leaf dry matter content, leaf soluble carbohydrate content, carotenoids per unit chlorophyll, and peroxidase activity, pertaining to resource conservation. Notably, underwater light intensity was found to be the key factor explaining the trait plasticity along the water depth gradient, and 1.30 mol photons m−2 d−1 (at 270 cm) could be the optimal irradiance level based on the total biomass of V. natans juveniles. The present study highlights the significance of leaf trait plasticity for rosette‐type macrophytes in response to variations in water depth and sheds new light on the differences between trade‐offs in deep‐ and shallow‐water areas.

Benthic microbial diversity trends in response to heavy metals in an oxygen-deficient eutrophic bay of the Humboldt current system offshore the Atacama Desert

Mejillones Bay is a coastal ecosystem situated in an oxygen-deficient upwelling area impacted by mining activities in the coastal desert region of northern Chile, where conspicuous microbial life develops in the sediments. Herein, heavy metal (loid)s (HMs) such as Cu, Pb, As, Zn, Al, Fe, Cd, Mo, Ni and V as well as benthic microbial communities were studied using spectrometry and iTag-16 S rRNA sequencing. Samples were taken from two contrasting sedimentary localities in the Bay named Punta Rieles (PR) and Punta Chacaya (PC) within 10–50 m water-depth gradient. PR sediments were organic matter rich (21.1% of TOM at 50 m) and overlaid with low-oxygen waters (<0.06 ml O2/L bottom layer) compared with PC. In general, HMs like Al, Ni, Cd, As and Pb tended to increase in concentration with depth in PR, while the opposite pattern was observed in PC. In addition, PR presented a higher number of unique families (72) compared to PC (35). Among the top ten microbial families, Desulfobulbaceae (4.6% vs. 3.2%), Flavobacteriaceae (2.8% vs. 2.3%) and Anaerolineaceae (3.3% vs. 2.3%) dominated in PR, meanwhile Actinomarinales_Unclassified (8.1% vs. 4.2%) and Sandaracinaceae (4.4% vs. 2.0%) were more abundant in PC. Multivariate analyses confirmed that water depth-related variation was a good proxy for oxygen conditions and metal concentrations, explaining the structure of benthic microbial assemblages. Cd, Ni, As and Pb showed uniformly positive associations with communities that represented the keystone taxa in the co-occurrence network, including Anaerolineaceae, Thiotrichaceae, Desulfobulbaceae, Desulfarculaceae and Bacteroidales_unclassified communities. Collectively, these findings provide new insights for establishing the ecological interconnections of benthic microorganisms in response to metal contamination in a coastal upwelling environment.

The response mechanism of Hydrilla verticillata and leaf epiphytic biofilms to depth and nutrient removal.

The mechanism of morphological and physiological regulation of submerged aquatic plants (Hydrilla verticillata) is influenced by spatial and environmental changes related to water depth gradients. In the present study, changes in the aquatic microcosm were explored at the depth gradients of 0.3 m, 0.6 m, 0.9 m, 1.2 m, and 1.5 m, and the depth was recognized as a critical factor for improving water quality, especially for the removal of total phosphorus (TP) and recalcitrant protein-like molecules. At 0.9 m, the removal rates of TP and protein-like substances reached 78% and 18.67%, respectively, 1.76 times and 1.28 times the rates at 0.3 m. The maximum shoot/root growth and chlorophyll (a + b) suggest photosynthesis inhibition is minimal at 1.2 m. Fluctuations in enzyme activities imply an antioxidant response to lipid peroxidation damage under different oxidative stress. The adjusted activities of glutamine synthetase (GS) and alkaline phosphatase (APA) were an adaptive nutrient utilization strategy to different water depths. Microbiological diversity analysis of biofilms indicates that community structure changes in response to water depth. Considering the growth status and nutrient removal effects, the results indicate that the optimal planting depth for H. verticillata is 0.9-1.2 m. These findings contribute to understanding water purification mechanisms in depth gradients, and support the effective rebuilding and management of submerged macrophyte communities in natural shallow lakes.

MPS‐Based Model to Solve One‐Dimensional Shallow Water Equations

AbstractHere, a moving particle simulation method is presented to spatially integrate the cross‐sectional average shallow water equations using a prediction‐correction procedure for the time discretization. A density‐ratio equation is derived for the water depth computation according to the particle number density concept. The newly derived equation does not miscalculate the water depth in case an incorrect searching radius parameter is adopted, unlike the typical volume‐summation formula in meshless shallow water flows. A new one‐dimensional Spiky kernel function is developed to satisfy the unity condition employed in the Newton‐Raphson iteration to calculate the water depth. Dynamic stabilization is adopted to capture shockwave problems, a case‐independent technique based on the inelastic collision with unequal masses. The convective flux term is eliminated under the Lagrangian framework, and so the momentum is adequately conserved without the need for any special treatment. The proposed scheme maintains the exact C‐property, meaning that the water depth gradient and bed slope are hydrostatically well balanced within a discretized solution domain. Compared to analytical solutions and experimental data, the results of this study reveal that the present model is a robust numerical solver without unphysical oscillations. It can capture various shock problems, including steep gradient shock‐front, discontinuous wet bed, transcritical flow regimes, and irregular bed topography. The present model can also simulate the dry‐wet flow transition influenced by friction without experiencing any divisions by zero, negative values, or unphysical perturbations. This advantage is basically due to the water particles' absence and presence for the dry and wet regions, respectively.

A well-balanced lattice Boltzmann model for the depth-averaged advection–diffusion equation with variable water depth

A well-balanced lattice Boltzmann model for the depth-averaged advection–diffusion equation is developed to solve the solute transport problem in shallow water flows with obvious gradient changes in water depth. The flow field is simulated by the lattice Boltzmann model for shallow water equations with the central moments (CMs) collision operator and D2Q9 lattice pattern. In this model, the diffusion term of the depth-averaged advection–diffusion equation is divided into two parts by the product rule, one part of which is combined into the advection term to produce the pseudo velocity. The relaxation time is independent of the diffusion coefficient, and the remaining value of (3+3)∕6 derived from the Chapmann–Enskog expansion reduces the third-order error. According to the different treatments of the water depth gradient in the equilibrium distribution function, center scheme and linked scheme are developed. Both schemes can ensure the conservation of the solute mass. Numerical tests show that these two schemes give predictions in good agreement, but the linked scheme works better at the breakpoint. Comparisons with the reference indicate that this model is advantageous with respect to both stability and efficiency.

Proposal of new indexes for development characteristics of storm surge considering topographical features and tides

ABSTRACT We focused on the effects of tides on storm surge deviations. According to previous studies, storm surge deviations depend on when the typhoon strikes in the tidal phase because the water depth of a bay varies with tides. The previous studies concluded that the dependence can be explained by the conventional formula. However, the results of our numerical simulations could not be explained quantitatively by the conventional formula and require a development mechanism for storm surges that are not expressed in the formula, to explain the dependency. Therefore, we focused on the water depth gradient, and confirmed from the numerical simulation results that the depth gradient greatly influenced the development characteristics of storm surges. Moreover, a new formula considering the depth gradient was derived, and the tidal dependence of storm surge deviations was investigated. Based on the new formula, two indexes that indicate the effect of the water depth gradient and the effect of tides on storm surges were derived. We examined the indexes to clearly understand the development characteristics of storm surges. Finally, the validity of the derived indexes was investigated by comparing the computed surge heights based on the indexes with the results of the numerical simulations.

Effective suppression of established invasive Phragmites australis leads to secondary invasion in a coastal marsh

AbstractInvasive species negatively impact vegetation communities. Invasive European common reed [Phragmites australis (Cav.) Trin. ex Steud. ssp. australis] is rapidly spreading throughout North American wetlands. As such, the suppression of P. australis populations is a goal of many managers, as its removal should provide an opportunity to restore native vegetation communities. In Ontario, managers applied a glyphosate-based herbicide to more than 400 ha of P. australis in ecologically significant coastal marshes, representing the first time this tool was used over standing water to suppress an invasive species in Canada. Using a before–after–control–impact monitoring design, we evaluated the efficacy of glyphosate-based herbicide at removing P. australis along a water-depth gradient and assessed the recovery of the vegetation community for 2 yr after treatment in relation to reference conditions. We found that herbicide suppressed more than 99% of P. australis 1 yr after treatment and worked effectively along the entire water-depth gradient (10 to 48 cm). However, the post-treatment vegetation community remains distinctive from reference marsh 2 yr after treatment. In many plots where P. australis was removed, nonnative European frog-bit (Hydrocharis morsus-ranae L.) is now dominant, likely aided by high lake-water levels.

Does evolutionary relatedness predict ecological similarity?

AbstractA fundamental question in paleobiology is whether ecology is correlated with evolutionary history. By combining time-calibrated phylogenetic trees with genus occurrence data through time, we can understand how environmental preferences are distributed on a tree and evaluate support for models of ecological similarity. Exploring parameters that lend support to each evolutionary model will help address questions that lie at the nexus of the evolutionary and ecological sciences. We calculated ecological difference and phylogenetic distance between species pairs for 83 taxa used in recent phylogenetic revisions of the brachiopod order Strophomenida. Ecological difference was calculated as the pairwise distance along gradients of water depth, carbonate, and latitudinal affinity. Phylogenetic distance was calculated as the pairwise branch length between tips of the tree. Our results show no relationship between ecological affinity and phylogeny. Instead results suggest an ecological burst during the initial radiation of the clade. This pattern likely reflects scaling at the largest macroevolutionary and macroecological scales preserved in the fossil record. Hierarchical scaling of ecological and evolutionary processes is complex, but phylogenetic paleoecology is an avenue for better evaluating these questions.

Wave-current interactions during extreme weather conditions in southwest of Bohai Bay, China

Wave-current interactions widely occur in shallow waters. However, the dominant factors for the modulation differ over different coastal regions and the relevant generation mechanisms have not been fully addressed. In this study, a Delft3D wave-current coupled model is applied to the southwestern Bohai Bay, China, which is a shallow-water coast with a mild slope, to estimate the local wave-current interactions during extreme weathers. The results indicate that the wave set-up is strongly influenced by local bottom slope, water depth and radiation stress gradient. The mean current velocity is decreased by wave by up to 15% due to enhanced bottom roughness, and the impact of momentum transfer from wave to current is non-negligible. Both tidally-varying depth and current are able to modulate significant wave height (SWH), and their modulating abilities are increased and decreased onshore respectively. Furthermore, the sensitivity studies suggest that the depth-induced SWH modulation is primarily due to the shoaling effect which is related to bottom slope. The wind speed plays an important role on the depth- and current-induced SWH modulation, as it could affect the state of wind-wave and wind input energy. Whereas, wind direction change from onshore to offshore only results in remarkable changes of current-induced modulation.

Environmental vs. spatial drivers of submerged macrophyte community assembly in different seasons and water depths in a mesotrophic bay of Erhai Lake, China

The relative importance of environmental and spatial drivers in shaping local plant diversity patterns remains unclear. Here, we investigated the environmental and spatial drivers regulating species composition of the submerged macrophyte community in Erhai Lake, China, over different seasons and water-depth gradients. Results showed that environmental and spatial drivers significantly affected species composition of the submerged macrophyte community at the local and fine scale, with shared environmental and spatial drivers explaining 28.3%–38.4% of the variation in species composition for all four seasons. The unique spatial drivers demonstrated a relatively stable contribution (11.8%–16.0%) over the four seasons, but the contributions from the unique environmental drivers showed seasonal variability, being high in summer and winter. In shallow water (0–3 m), spatial drivers were more important in the process of community assembly; in deep water (3–6 m), however, environmental and spatial drivers both significantly affected community assembly. The different water areas also showed strong seasonal changes in the explanatory power of environmental and spatial drivers of submerged macrophyte community variation. Thus, future studies should consider sampling macrophytes over different seasons and at different water depths to better trace the indicators regulating submerged macrophyte community assembly in lakes.

Taxonomic dependency of beta diversity components in benthic communities of bacteria, diatoms and chironomids along a water-depth gradient

Community variation (i.e., beta diversity) along geographical gradients is a well-known ecological pattern, but the corresponding variation in beta diversity components (e.g., species turnover and nestedness) and underlying drivers remain poorly understood. Based on two alternative approaches (that is, the beta diversity partitioning proposed by Baselga and the Local Contributions to Beta Diversity (LCBD) partitioning proposed by Legendre), we examined the patterns of beta diversity components of lacustrine benthos, from bacteria to diatoms and chironomids, in the surface sediments along a 100-m water-depth gradient in Lugu Lake. We further quantified the relative importance of spatial, environmental and biotic variables in explaining water-depth patterns in beta diversity. Based on the Baselga's framework, there was a taxonomic dependency for the patterns of beta diversity components with water-depth, showing a significant species turnover pattern for bacteria, while diatoms and chironomids showed significant nestedness. This dependency was also evident in the patterns of community uniqueness with water-depth because based on Legendre's framework, the LCBD decreased with water depth for bacteria whereas increased with depth for diatoms. The total beta diversity and species turnover of bacteria could be explained by the pure effects of spatial, environmental and biotic variables. A total of 26.8% and 23.6% of the nestedness component of diatoms and chironomids was explained by environmental variables, respectively, while species turnover was mostly related to spatial variables. Bacteria total LCBD and species replacement were driven only by environmental variables. For diatoms and chironomids, however, most of the total LCBD and its two components were explained by spatial variables, and biotic variables were most important for the diatom replacement component. Our findings provide insights into the mechanisms responsible for community organizations along water-depth gradients from the perspective of beta diversity components.

The influence of flow rates and water depth gradients on the growth process of submerged macrophytes and the biomass composition of the phytoplankton assemblage in eutrophic water: an analysis based on submerged macrophytes photosynthesis parameters.

Submerged macrophytes and phytoplankton assemblage play significant roles in the functioning of aquatic ecosystems. An experiment was carried out in Beijing in order to further evaluate the environmental factors that affect the growth of submerged macrophytes and phytoplankton assemblage. Submerged macrophytes (i.e., Vallisneria natans, Hydrilla verticillata, and Ceratophyllum demersum) constructed the growth system with some controllable influencing factors (i.e., the flow rate and water depth gradient). The flow rates were set separately as 4L/h (1#), 6L/h (2#), and 12L/h (3#), while the water depth gradient was of 0.5-1.7m in eutrophic water. Generally, all macrophytes could grow normally in the experiment, and the system could maintain and improve the effluent quality. The average removal rates of NH3-N, COD, NO3-N, TN, and TP were about 90%, 33%, 65%, 45%, and 40%, respectively. Seen from the results of the water depth gradient experiments, it is indicated that Vallisneria natans grows better in shallow water (0.5m) and moderate shallow water (0.7m) area, with an average relative growth rate (ARGA) of 57%. Hydrilla verticillata and Ceratophyllum demersum grow better in moderate deep water (1.2m) and deep water (1.7m) area (ARGA of 66% and 64%, respectively). Results of the flow rate experiments showed that the moderate flow rate (6L/h) was the best for those three macrophytes' growth. As the fitting results of the rapid light curves (RLCs) showed that the utilization of light and the tolerance to strong light were different for these macrophytes, if they are ranked in the order of the utilization and the tolerance from strong to weak, they are Hydrilla verticillata, Ceratophyllum demersum, and Vallisneria natans. Microbial analyses indicated that the overall system diversity of the experimental groups have been improved after cultivation of macrophytes. However, the accumulated Cyanobacteria caused by the low flow rate (1#) would lead to the suppression of microbial organics decomposition and nutrient metabolism in the macrophytes. To sum up, the results of this study provided theoretical guidance and technical support for the restoration of submerged macrophytes in eutrophic water.