Water Depth Gradient Research Articles

Influence of Bed Variations on Linear Wave Propagation beyond the Mild Slope Condition

As water waves travel from deep to shallow waters, they experience increased nonlinearity and decreased dispersion due to the reduced water depth. While the impact of bed slope on wave propagation celerity is documented, it is often overlooked in commonly used depth-integrated wave models. This study uses the WKB approximation and solves higher-order slope-related terms to analyze the influence of varying depth, including the gradient and laplacian of water depth. One result is an extended longwave for linear wave reflection and transmission on a ramp to deeper waters. The main outcome and focus of this work, however, is a new, simple analytical expression for linear dispersion that includes bed variations. The results are applied to two cases: wind-generated water wave propagation in the nearshore, emphasizing corrections to bathymetry inversion methods, and tsunami propagation over the continental slope, highlighting the limitations of neglecting slope on dispersion and the significant role of the Laplacian of water depth.

Sediment carbon storage in subtidal beds of the invasive seagrass Halophila stipulacea along an extreme water depth gradient, St. Thomas, U.S. Virgin Islands

Blue carbon ecosystems such as mangroves, salt marshes, and seagrass beds are found globally and are fundamental to fisheries production, storm surge protection, and carbon sequestration. The contribution of seagrass ecosystems to global carbon stocks is still not well understood, including in the United States Virgin Islands (USVI). No study has been published to-date assessing the sediment carbon density (SCD) in seagrass beds in the USVI. This study focused on the carbon storage ability of the invasive species, Halophila stipulacea, which is compact in size compared to common native seagrasses and has spread rapidly to become a dominant seagrass in the USVI. This species forms dense mats across a wide depth range (<1 m to 50 m) typically uninhabitable to its native counterparts (Syringodium filiforme and Thalassia testudinum). Several biotic and abiotic factors influence the carbon storage ability of seagrass, yet little is known about carbon storage sequestration along a depth gradient for H. stipulacea. This study provides the first assessment of the biological characteristics (shoot density, leaf area, leaf height, and percent cover) and carbon storage ability of H. stipulacea across a depth gradient (shallow: 5–10 m; medium: 15–20 m; deep: 25–30 m) at two sites in St. Thomas, USVI. Mean sediment carbon density (SCD) values per core reported for H. stipulacea in this study ranged from 3.88 to 15.67mgC/cm3; these were comparable to regional and global seagrass studies. Biological characteristics were not an accurate predictor of SCD. A significant interaction between water depth and site was found to affect mean SCD of H. stipulacea beds. It is likely that site-specific factors most likely account for variations seen within the data. Although carbon values in this study compared to values reported in the literature, other factors such as land use, proximity to carbon sources, sediment microbial community, and water current patterns may be driving SCD values. These findings highlight the need for site and species-specific carbon storage assessments on local to regional scales to accurately estimate current and forecasted blue carbon stocks.

The large‐scale restoration of fire and water regimes in Everglades National Park reveal little change in plant diversity along an elevational gradient

Quantifying the response of plant diversity to large‐scale restoration is essential for measuring management success. One of the world's largest restoration efforts began in 2000 in the Everglades ecosystem in Florida, United States, through the Comprehensive Everglades Restoration Plan. In coordination with ongoing fire management, this restoration effort aims to restore natural hydrologic and fire regimes in this dynamic ecosystem. Restored water and fire regimes in the Everglades interact along an elevational gradient between seasonally inundated marl prairie and frequently burned pine rockland on Long Pine Key in Everglades National Park. To determine the impact of management on plant community richness and composition, we resampled transects spanning the elevational gradient between marl prairie and pine rockland initially sampled prior to the implementation of current restoration efforts (circa 1997–1999). We measured percent plant cover and developed generalized linear mixed models to determine the effects of fire frequency, average water depth, and elevation on plant species richness and composition across the two time periods. Additionally, we used species‐level random effects to examine how individual species respond to each environmental variable. We failed to detect any systematic shifts in plant composition in response to fire and water management. However, we found that complex interactions between fire and water structure plant composition and maximize species richness along gradients of water depth and elevation, with low, moderate, and high burn frequencies increasing richness in wet, moderately wet, and dry sites, respectively.

Water‐level fluctuations and nutrient enrichment interact to alter ecosystem structure in distinct ways at different water depths

Abstract Aquatic ecosystems are often impacted by several human‐derived pressures simultaneously. A key challenge for ecologists is to develop mechanistic understanding of the cumulative effects of multiple stressors on freshwater biodiversity. We established an outdoor mesocosm experiment to examine the individual and combined effects of two key stressors on lake ecosystems globally—nutrient enrichment and amplified water‐level fluctuations—on benthic algal and macroinvertebrate assemblages along a gradient in littoral water depth. We found that nutrient enrichment and water‐level fluctuations not only altered the structure of littoral communities when acting individually, but also interacted to dynamically alter biomass and community structure in different ways at different depths. As expected, enrichment increased benthic algal biomass significantly in very shallow waters. Yet, when enrichment co‐occurred with water‐level fluctuations, this pattern was reversed, with algal biomass increasing significantly in deeper waters, but not in the shallows that were exposed to fluctuating water levels. This pattern occurred across all algal groups that we examined except for green algae. Green algae biomass increased with nutrient enrichment, was greatest in the shallows, and remained unaffected by fluctuating water levels. Water‐level fluctuations modified the functional feeding group structure of benthic macroinvertebrate communities, reducing relative densities of collector‐gatherers and increasing those of predators. However, these changes occurred only in the shallow‐most regions exposed directly to fluctuating water levels. Our results reveal how two globally important stressors of standing water systems can interact in different ways in different ecological contexts and show how stressors can combine to have unanticipated impacts that vary in space. Our findings underscore the potentially highly multifaceted and complex nature of stressor combinations that present a major and growing challenge to ecosystem management, particularly in light of accelerating global environmental change.

Depth-dependent transformation of ZnO and Ag nanoparticles in sulfate-reducing sediments tracked using scanning transmission electron microscopy

The transformation of Ag and ZnO nanoparticles was investigated along pore water depth gradients in sulfate-reducing sediments. This work highlights the benefits of tailored experimental strategies to study nanoparticle environmental transformations.

Nitrogen isotopes and geochemistry of the basal Datangpo Formation: Contrasting redox conditions in the upper and lower water columns during the Cryogenian interglaciation period

The late Neoproterozoic witnessed series of remarkable geological evolutions, including the Sturtian and Marinoan glaciation events during the Cryogenian. A stratified oceanic redox structure has been proposed for this period, and the dynamics of the deep water redox conditions have been well illustrated by trace metals and their isotopes. However, variations in the redox conditions of the upper water column have received much less attention. Sedimentary nitrogen isotopes (δ15N) are critical for reconstructing the redox conditions in the upper water columns of ancient oceans. Existing research on the N cycle during the Cryogenian period is scarce, and the majority of studied samples were collected from shallow and medium water depth facies. To address this paucity of studies, we conducted a high-resolution N isotope study in combination with Fe speciation and Mo concentration data from two deep-water sections of black shales in the basal Datangpo Formation in the Nanhua Basin, a typical organic-rich stratum deposited during the Cryogenian interglaciation period. While the trace metal proxies suggest a fluctuating euxinic–ferruginous bottom water column for the lower interval and a stable euxinic bottom water column for the upper interval, the redox conditions of the upper water column were relatively stable, as indicated by minimal changes in the recorded δ15N values. Furthermore, the δ15N values of black shales in the basal Datangpo Formation of the Nanhua Basin have a limited range (3.1‰ − 5.8‰, average of 4.3‰, standard deviation (SD) = 0.5‰, n = 104) and show no water-depth gradient, indicating the presence of a relatively large and stable nitrate reservoir and a deep redox chemocline in the Nanhua Basin. The characteristic of the nitrate reservoir reflected by these δ15N signals can be roughly analogized to the global interglacial ocean, although only the counterpart northeast Svalbard can be used for comparison. Therefore, the nitrate availability in the marine water column may have had little effect on animal radiation during the Cryogenian interglacial period. These findings help to clarify the N cycle in the Cryogenian Nanhua Basin and shed light on its significance on bioevolution.

Response of a submerged macrophyte (Vallisneria natans) to water depth gradients and sediment nutrient concentrations

Submerged plants constitute a vital component of shallow lake ecosystems, where water depth and sediment nitrogen‑phosphorus content are two key factors influencing their growth. This study focuses on Vallisneria natans and investigates the morphological and physiological changes of V. natans under the interaction of three water depth gradients and two different sediment nutrient levels. It explores the mechanisms through which varying sediment nutrient conditions under different water depths affect the growth of V. natans. The results indicate that both independent and interactive effects of water depth and sediment nutrient status significantly impact the morphology, antioxidant enzyme activity, and photosynthetic pigment content of V. natans, with water depth having a greater influence. To adapt to increased water depth-induced light stress, V. natans responds morphologically by increasing leaf length, leaf width, and decreasing maximum root length. Physiologically, it enhances its antioxidant regulation capacity and photosynthetic efficiency by increasing antioxidant enzyme activity, root vitality, and photosynthetic pigment content to counter weak light stress. However, these adaptations are insufficient to cope with excessively deep waters (200 cm). Sediment nutrient levels primarily control the growth of V. natans by affecting its root system. When sediment nitrogen and phosphorus content is lower, V. natans exhibits greater total root volume and surface area to enhance nutrient absorption efficiency. Water depth not only directly influences the growth of submerged plants but may also impact the migration and transformation of phosphorus in sediments, further exacerbating its effects on the growth of these plants, thus accelerating the regime shift of shallow lakes. Therefore, this study reveals V. natans' response strategies to varying water depths and sediment nutrient levels, determining suitable water levels and sediment nutrient conditions for its growth. These research findings provide a scientific basis for water level management and ecological restoration of submerged aquatic plants in shallow lakes.

Lower Eocene carbonate ramp clinoforms of the southern Tethys; Zagros Foreland Basin, SW Iran: Sequence stratigraphy architecture, basin physiography and carbonate factory controlling parameters

AbstractExcellent cliff exposures in the Khush‐Ab and Chenareh anticlines (Zagros Foreland Basin, SW Iran) offer a good opportunity to document stratal geometries, paleofacies heterogeneity, depositional architecture and depositional cycles of the Lower Eocene sedimentary basin system. These unique outcrops containing six logged sections have been studied across a large‐scale transect, covering ca. 10 km of continental (Kashkan Formation), carbonate‐dominated platforms (Taleh‐Zang Formation) and submarine fan to basin‐floor settings (Amiran and Pabdeh formations). Field observations of the bedding geometries revealed a set of NE–SW oriented carbonate ramp clinoforms (clinoformal units 1–4) with sigmoidal cross‐sectional shapes and an internal fore‐stepping architecture. Based on detailed facies analysis, six facies associations (FA) were identified (FA.I to FA.VI), which are interpreted to have been deposited laterally in the continental, proximal to distal and deep‐water settings of a distally steepened carbonate ramp. According to the stratal stacking pattern, bounding surface, facies architecture and internal makeup of carbonate clinoforms, four H‐F‐cycles (cycle I–IV) corresponding to a lower hierarchical rank (fourth‐order cycle) were recognized and nested within the regressive stacking pattern (HST) of a higher hierarchical rank (third‐order sequence). These H‐F‐cycles are arranged in three segments (bottomset, foreset and topset) of each clinoformal unit. Higher rank transgressive blocks (TST) discriminated each clinoformal unit by up‐deepening sets of the H‐F‐cycle V. In total, five third‐order depositional sequences were identified. The sequence, stratigraphic framework and internal makeup of this carbonate platform indicate that these carbonate sloping successions are the type of accretionary carbonate ramp clinoforms that display an ascending ramp‐slope break trajectory. Evolutionary episodes of biogenic communities, climatic change, local tectonic movements, physical processes (e.g. waves and storms) and water depth gradient are major forcing parameters that controlled the carbonate factory and depositional geometry of this Lower Eocene succession; however, carbonate‐producing organisms and eustatic sea‐level fluctuations played the first role, and local tectonic movements in response to tectonic activities of the Zagros Foreland Basin played the second role. As a result, Taleh‐Zang carbonate platforms are rich in diverse assemblages of LBFs without fragments of coral and red algae, which appear to be a consequence of a hothouse state that diminishes the global thermal gradient, weakens pycnoclines and thereby limits the turbulence.

Free-living nematode assemblages along a water-depth gradient in the Perdido belt, northwestern Gulf of Mexico

This study investigated nematode assemblages in the Perdido belt, northwestern Gulf of Mexico (GoM) in terms of species composition, distribution, density, and diversity. We examined 27 sediment samples collected at depths of 44-3466 m. The nematodes were identified at the lowest level possible and classified into four trophic groups based on their buccal cavity. Additionally, their sex and stage of life were determined. The sediment composition was determined, as well as the percentage of organic carbon (OC). The density per site fluctuated between 0.02 and 13.30 ind/10 cm2. A total of 1768 nematodes were identified, belonging to 9 orders, 35 families, 96 genera, and 153 species, of which 9 genera and 15 species are new records for the GoM. The families Xyalidae and Comesomatidae were the most diverse genera (9 and 8, respectively). Sabatieria sp. 1 was the most abundant and widely distributed species. The number of species varied at different depths (range 1-60 species) and Pielou evenness (0.7-1). Most of the organisms collected were adult females. The non-selective deposit feeders were the most abundant (49%), followed by epigrowth feeders (20%). Most of the sites consisted of sand (range 10.1-70.2%). The percentage of OC varied at different depths (0.1-1.4%). No significant correlation was found between species richness and depth. The species composition changes with depth, density and most of the trophic groups tend to decrease with increasing depth. The dominant species are more related to the percentage of sands and silts than to clays and OC.

Diversity, distribution, and conservation of the Trichoptera and their habitats in north‐eastern Algeria

Abstract Community patterns are generated by multiple processes and constraints generated at different spatial and temporal scales. A large‐scale study was carried out on the Trichoptera of four Algerian regions with contrasting climates, vegetation cover, and human pressures. Overall, 22 identified taxa were recorded, including nine families and 14 genera. The genus Hydropsyche was represented by eight species, with Hydropsyche maroccana and Hydropsyche lobata being the most abundant species. With one exception, localities in the subhumid Collo and the high‐altitude Batna were the most species rich, whereas localities in the semi‐arid Oum el Bouaghi were the most species poor. Nonmetric multidimensional scaling (NMDS) separated the caddisflies recorded into four assemblages organized by a set of constraining environmental factors (land use, altitude, water depth, helophyte cover, and flow velocity gradients) acting at different scales. As inadequate knowledge of insect biodiversity is a major limitation to their conservation, this article discusses how these results might help predict caddisfly communities in adjacent regions and enhance conservation efforts.

Trace metals as a redox proxy in Arabian Sea sediments in and below the oxygen minimum zone

Sedimentary trace metals are widely used to reconstruct past bottom water redox conditions. The calibration of trace metals as a redox proxy for oxygen minimum zones (OMZs) can still be improved. Here, we combine pore water and solid phase Mo, U, Re and V profiles with Fe and Mn data for ten sites along a bottom water oxygen (2 to 83 μmol L−1 O2) and water depth gradient (885 to 3010 m) in and below the perennial OMZ in the northern Arabian Sea (Murray Ridge). Trends in sedimentary Mo, U and Re contents generally follow ambient bottom water redox conditions, with the highest enrichments in OMZ sediments, supporting the validity of these trace metals as redox proxies. Vanadium and Fe are exclusively enriched in the sediments of the most anoxic OMZ site and do not capture further redox changes in and below the OMZ. We attribute the absence of a redox trend in sedimentary Fe content to the mildly reducing conditions in the sediments, with little FeS formation and benthic release of Fe. Manganese, in contrast, is depleted in the OMZ sediments and enriched in sediments below the OMZ, in accordance with loss from OMZ sediments and transfer of Mn to deeper sites (“Mn shuttling”). Manganese oxides are likely a key carrier of Mo and V to the sediments, especially below the OMZ, while diffusion across the sediment-water interface supplies U. Sedimentary Mo, U and V contents in the present-day Arabian Sea OMZ are generally lower than observed in other perennial OMZs. This may be related to a lower input of organic matter in this part of the Arabian Sea when compared to other OMZs, and hence, less anaerobic degradation of organic matter and less authigenic fixation of metals, even at the same bottom water oxygen concentrations. Our results have implications for the detection of OMZs in the geological record, implying that thresholds in trace metal concentrations for perennial OMZs may be lower than previously considered. Comparison of our Mo, U, Re and V data to trace metal records from 15 to 200 ka for the same Arabian Sea region suggests that the OMZ was periodically wider and more reducing in the past.

Introgression from extinct species facilitates adaptation to its vacated niche.

Anthropogenic disturbances of ecosystems are causing a loss of biodiversity at an unprecedented rate. Species extinctions often leave ecological niches underutilized, and their colonization by other species may require new adaptation. In Lake Constance, on the borders of Germany, Austria and Switzerland, an endemic profundal whitefish species went extinct during a period of anthropogenic eutrophication. In the process of extinction, the deep-water species hybridized with three surviving whitefish species of Lake Constance, resulting in introgression of genetic variation that is potentially adaptive in deep-water habitats. Here, we sampled a water depth gradient across a known spawning ground of one of these surviving species, Coregonus macrophthalmus, and caught spawning individuals at greater depths (down to 90 m) than historically recorded. We sequenced a total of 96 whole genomes, 11-17 for each of six different spawning depth populations (4, 12, 20, 40, 60 and 90 m), to document genomic intraspecific differentiation along a water depth gradient. We identified 52 genomic regions that are potentially under divergent selection between the deepest (90 m) and all shallower (4-60 m) spawning habitats. At 12 (23.1%) of these 52 loci, the allele frequency pattern across historical and contemporary populations suggests that introgression from the extinct species potentially facilitates ongoing adaptation to deep water. Our results are consistent with the syngameon hypothesis, proposing that hybridization between members of an adaptive radiation can promote further niche expansion and diversification. Furthermore, our findings demonstrate that introgression from extinct into extant species can be a source of evolvability, enabling rapid adaptation to environmental change, and may contribute to the ecological recovery of ecosystem functions after extinctions.

Depth-related patterns and regional diversity of free-living nematodes in the deep-sea Southwestern Gulf of Mexico

Diversity of free-living marine nematode assemblages in the Southwestern Gulf of Mexico (SW GoM) is scarcely studied. Here, we aimed (1) to analyze the influence of a water depth gradient on the species richness, feeding type and taxonomic composition of assemblages; and (2) to document the regional diversity of free-living nematodes in the SW GoM. We sampled 63 sites along a water depth gradient from 186 to 3774 m during four oceanographic cruises. We found clear variations along the depth gradient of bottom water (temperature, salinity, and dissolved oxygen) and sediment variables (grain size and organic content). We identified 1881 nematodes belonging to 108 genera, 33 families, ten orders, and two classes. The abundance and species richness decreased with water depth. However, the expected number of species for a same level of abundance did not change along the depth gradient likely because the scarcity of food was compensated by reduced environmental stress (e.g., higher oxygen content and physical stability). Microbial sucker was the most abundant feeding type indicating the important role of sediment bacteria in the nematode’s diet. Species composition varied along the depth gradient with dominance in the upper slope sites of species of Comesomatidae tolerant to reduced sediments (e.g., Dorylaimopsis sp., Sabatieria spp., and Setosabatieria hilarula). Many congeneric species typical of deep sea were restricted to the deepest sites such as Acantholaimus spp., Ledovitia spp., Desmoscolex spp., and Halalaimus spp. The nematode regional diversity of SW GoM was 154 species, but the Chao 1 estimator indicated a richness of about 194 species and a highest limit of 246 species. The accumulation curves of richness were non-asymptotic suggesting a substantial fraction of undiscovered richness. Our study increased the free-living nematode fauna of GoM in 144 species (76%) respect to Hope’s list (2009). The large diversity of nematodes stands out the necessity of further studies to unravel the environmental drivers of α- and β-diversities and highlights the potential of this taxon for monitoring the deep sea of the Gulf of Mexico.

Lake Sediments From Littoral and Profundal Zones are Heterogeneous but Equivalent Sources of Methane Produced by Distinct Methanogenic Communities—A Case Study From Lake Remoray

AbstractLake ecosystems contribute significantly to atmospheric methane and are likely to become even bigger methane emitters with the global spread of hypoxia/anoxia in freshwater ecosystems. Here we characterized the spatial heterogeneity of methane production potential, methane concentration, archeal and bacterial communities across Lake Remoray sediment during the summer period when hypoxic conditions settle in the deepest part of the water column. It was hypothesized that methane concentration and production would be higher in the deeper part of the lake, our results showed that some littoral areas exhibited similar or higher values than the deepest area. The full 16S rRNA gene sequencing dataset counted 41 OTUs affiliated with methanogenic species in abundances that depended more on sampling‐site location than on the water depth gradient. The methanogenic co‐occurrence network revealed the existence of five distinct sub‐communities, suggesting the presence of different methanogenic niches across Lake Remoray. The variation in abundance of the two larger methanogenic sub‐communities was significantly related to methanogenesis potential and sediment methane concentration across‐lake but further studies investigating their real activities would provide additional insights. In a globally changing environment (temperature, eutrophication, …) a better understanding of the functional specificities and characteristics of the potential of methane cycle actors would allow us to better predict their future implications for greenhouse gas production and mitigation.

IMPACT OF ECOHYDROLOGICAL INTERVENTIONS ON MACROINVERTEBRATE DIVERSITY ASSOCIATED WITH PHYSICOCHEMICAL PARAMETERS ON THE WETLANDS ALONG THE SOUTHEASTERN SHORE OF LAKE TANA, ETHIOPIA

Biological indicators have the improvement of monitoring ecological conditions and to measure of anthropogenic impacts on aquatic ecosystems. This indicates that appropriate wetland management mitigation measures that should be implemented in degraded wetlands to conserve biodiversity and improve water quality. The aim of the study was to investigate the impacts of ecohydrological intervention in relation to macroinvertebrate diversity and relate macroinvertebrate diversity with physicochemical parameters of the wetland in Lake Tana shore intervention site. Sampling stations were selected according to vegetation gradient. The sampling procedure employed using quadrats along transects following the main ecological gradient of water depth and vegetation structure and growth form. Four quadrats at every 25m distance were laid at each transect. A total of 24 quadrats of 1m x1m size were taken. Collected samples of each quadrat were pooled to represent each sampling site. One way ANOVA was further done to test any significant variation in macroinvertebrate and physiochemical attributes among stations. The Hilsenhoff Family-level Biotic Index (HFBI) was calculated for macroinvertebrate families. The HFBI index was analyzed and identified 975 macroinvertebrate individuals belonging to 15 families which were collected from all sampling sites during the study period, and the most dominant family were Hydrophilidae Coenagrionidae and Physidae. Based on the Shannon diversity index, the Gumetirs wetland was grouped under good ecological condition as compared to the remaining two reference wetlands. There was a significant variation in abundance and biodiversity of macroinvertebrates among sampling sites and better conditions exist at the intervention site. The correlation analysis between physicochemical parameters and macro-invertebrate abundance showed that organic carbon and temperature are most likely positively correlated with macroinvertebrate abundance. Thus, high macroinvertebrate diversity indicated that wetlands found in a good ecological condition.

A record of Holocene climate changes in central Asia derived from diatom-inferred water-level variations in Lake Kalakuli (Eastern Pamirs, western China)

The unique geographical and climatic settings of the eastern Pamirs make this region sensitive to the Westerlies and global climate change. Holocene fluctuations in water-level of Lake Kalakuli, a proglacial lake located to the northwest of the Muztag Ata glacier, were reconstructed based on diatoms from a ∼15 m long sediment core spanning the last ∼9,900 years. To establish how diatom species distribute in relation to water depth in Lake Kalakuli, a dataset of 45 surface sediment samples was retrieved from different water depth. Statistical analyses such as cluster analysis (TWINSPAN) and redundancy analysis (RDA) were used to demonstrate that the water depth gradient is the main environmental gradient driving the distribution of these diatom assemblages. A diatom-water depth transfer function, was then developed using a weighted averaging partial least squares component 2 model (R2 = 0.89, RMSEP = 1.85 m) and applied to the Holocene diatom sequence from Lake Kalakuli. Due to the large residual errors in the model only the general trends in water level are proposed. Effective moisture increased rapidly during the early Holocene, as the water depth reached a high level from the lowest level within about two thousand years. Only small amplitude fluctuations were recorded during the mid- and late Holocene until the last few hundred years when a marked increase occurred. Changes in summer insolation over the northern hemisphere drove the advances and retreats of the Muztag Ata glacier, which in turn controlled the fluctuations of water level in this lake. The diatom-derived paleoclimatic trend from Lake Kalakuli is consistent with the Holocene climate evolution in the Westerlies-dominated area of Central Asia.

Differential Growth Responses of Alternanthera philoxeroides as Affected by Submergence Depths.

Global climate change has resulted in an increase in intensity and frequency of flooding, plants living in lowlands, and shore areas have to confront submergence caused by flooding, submergence-tolerant plants usually respond by adopting either escape or quiescence strategies. While certain plants exhibit a changeover from escape strategy upon partial submergence to quiescence strategy under complete shallow submergence, it remains unknown whether plants completely submerged at different water depths would adjust their strategies to cope with the change in submergence depth. Alternanthera philoxeroides is an ideal species to explore this adjustment as it is widely distributed in flood-disturbed habitats and exhibits an escape strategy when completely submerged in shallow waters. We investigated the responses of A. philoxeroides in terms of morphology, anatomy, and non-structural carbohydrate metabolism by conducting experiments using a series of submergence depths (0, 2, 5, and 9 m). During the submergence treatment, environmental factors such as light, dissolved oxygen, and temperature for submerged plants were kept constant. The results showed that A. philoxeroides plants submerged at depth of 2 m presented an escape strategy via fast stem elongation, extensive pith cavity development, and small biomass loss. However, the retarded stem elongation, reduced pith cavity transverse area, and increased biomass loss along the water depth gradient indicated that A. philoxeroides altered its growth response as water depth increased from 2 to 9 m. It is found that the changeover of response strategies occurred at higher submergence depths (5–9 m). Based on the results of our experiments, we demonstrated that water depth played an important role in driving the change in strategy. The water-depth-dependent growth performance of A. philoxeroides would benefit the species in habit exploration and exploitation. Further studies should focus on the performances of plants when submerged at varied water depths with different light climates and dissolved oxygen content, and how water depths drive the response behaviors of the submerged plants.

A modern snapshot of the isotopic composition of lacustrine biogenic carbonates – records of seasonal water temperature variability

Abstract. Carbonate shells and encrustations from lacustrine organisms provide proxy records of past environmental and climatic changes. The carbon isotopic composition (δ13C) of such carbonates depends on the δ13C of dissolved inorganic carbon (DIC). Their oxygen isotopic composition (δ18O) is controlled by the δ18O of the lake water and by water temperature during carbonate precipitation. Lake water δ18O, in turn, reflects the δ18O of atmospheric precipitation in the catchment area, water residence time and mixing, and evaporation. A paleoclimatic interpretation of carbonate isotope records requires a site-specific calibration based on an understanding of these local conditions. For this study, samples of different biogenic carbonate components and water were collected in the littoral zone of Lake Locknesjön, central Sweden (62.99∘ N, 14.85∘ E, 328 ma.s.l.) along a water depth gradient from 1 to 8 m. Carbonate samples of living organisms and subfossil remains in surface sediments were taken from the calcifying alga Chara hispida, from bivalve mollusks of the genus Pisidium, and from adult and juvenile instars of two ostracod species, Candona candida and Candona neglecta. Our results show that neither the isotopic composition of carbonates nor the δ18O of water vary significantly with water depth, indicating a well-mixed epilimnion. The mean δ13C of Chara hispida encrustations is 4 ‰ higher than the other carbonates. This is due to fractionation related to photosynthesis, which preferentially incorporates 12C into the organic matter and increases the δ13C of the encrustations. A small effect of photosynthetic 13C enrichment in DIC is seen in contemporaneously formed valves of juvenile ostracods. The largest differences in the mean carbonate δ18O between species are caused by vital offsets, i.e., the species-specific deviations from the δ18O of inorganic carbonate which would have been precipitated in isotopic equilibrium with the water. After subtraction of these offsets, the remaining differences in the mean carbonate δ18O between species can mainly be attributed to seasonal water temperature changes. The lowest δ18O values are observed in Chara hispida encrustations, which form during the summer months when photosynthesis is most intense. Adult ostracods, which calcify their valves during the cold season, display the highest δ18O values. The seasonal and interannual variability in lake water δ18O is small (∼ 0.5 ‰) due to the long water residence time in the lake. Seasonal changes in the temperature-dependent fractionation are therefore the dominant cause of carbonate δ18O differences between species when vital offsets are corrected. Temperature reconstructions based on paleotemperature equations for equilibrium carbonate precipitation using the mean δ18O of each species and the mean δ18O of lake water are well in agreement with the observed seasonal water temperature range. The high carbonate δ18O variability of samples within a species, on the other hand, leads to a large scatter in the reconstructed temperatures based on individual samples. This implies that care must be taken to obtain a representative sample size for paleotemperature reconstructions.

Effects of drawdown on growth and reproduction of submerged macrophyte Vallisneria spinulosa

Water regime plays a determinant role in plant community development and patterns of plant zonation in wetlands. The waters located in the middle reach of the Yangtze River (China) have experienced a large decrease in wetland area from human activities for a long time, especially after the hydrological drought events in recent years. This outdoor study was conducted to clarify the morphological responses and reproductive strategy of Vallisneria spinulosa to water depth gradients (0.6 m, 1.0 m, 1.5 m) and to drawdown (down to 0.3 m). In static water, water depth of 1.0 m was the best condition for V. spinulosa to grow, manifested as high biomass of vegetative parts, tubers and sexual structures. However, V. spinulosa growing in water depth of 1.5 m was able to adapt to the decline in water depth in autumn, expressed as increased ramet number, stolon weight and belowground biomass. The low water depth of 0.6 m produced small plants with slight vegetative biomass and drawdown produced more tuber biomass. The drawdown caused a decline in vegetative ratio of V. spinulosa. Moreover, it caused an increase in clonal ratio at water levels of 1.5 m and 0.6 m, and a decline in sexual ratio at water levels of 1.5 m and 1.0 m. The results show that under the condition of relatively large fluctuations in water level, V. spinulosa will reduce the allocation of sexual biomass and increase the allocation of clonal biomass in order to cope with environmental changes so that it can proliferate better. Thus, water level changes have a great influence on the growth and reproduction of V. spinulosa.

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.