Sediment Moisture Research Articles

Unravelling CH4 and N2O dynamics in tidal wetlands using natural abundance isotopes and functional genes

Tidal wetlands are one of the major sources of CH4 and N2O in natural systems to the atmosphere; yet we still lack insights into the impact of their biogeochemical dynamics on the emissions of these greenhouse gases (GHGs). Here, we investigated the CH4 and N2O sources in four tidal wetlands ranging from freshwater to polyhaline with a focus on their production pathways. By using natural abundance isotopes and functional marker genes, we found that salinity level, sediment moisture content, quantity and quality of organic carbon (OC) and nutrients were major drivers of the wetland CH4 and N2O emissions. As the salinity levels decreased in the tidal wetlands, both the labile nature and concentration of nutrients and OC increased. These conditions favored methanogenesis as indicated in the abundance and expression of mcrA and the CH4 emissions from the freshwater wetland. Conversely, higher salinity depressed organic matter decomposition rates and microbial activities, causing much lower CH4 production in the saline wetlands. Isotope mapping revealed that denitrification contributed mainly to wetland N2O emissions (80–90%), reflected in the strong expression of denitrifier marker genes nirS, nirK, and nosZ and low nosZ:nir ratio. Nitrification played an important role in wetland N2O emissions at high NH4+ and low salinity levels. This condition was the case in the freshwater wetland – the strongest N2O emitter – where we found the highest NH4+ concentrations and the most abundant and expressed nitrifier marker genes amoA AOA and amoA AOB. Methanogen and denitrifier marker genes were more abundant and expressed at the surface layer compared to the subsurface layer, implying the presence of methane and denitrification paradoxes in the tidal wetlands. This study paves a way for the coupling of isotope and functional gene analyses to deeply explore GHG formation pathways and responsible microbial activities in dynamic wetland systems.

Microorganisms Directly Affected Sediment Carbon–Nitrogen Coupling in Two Constructed Wetlands

Clarifying the carbon–nitrogen coupling pattern in wetlands is crucial for understanding the driving mechanism of wetland carbon sequestration. However, the impacts of plants and environmental factors on the coupling of carbon–nitrogen in wetland sediments are still unclear. Sediment samples from plant (Typha angustifolia and Phragmites australis)-covered habitats and bare land were collected in two constructed wetlands in northern China. The contents of different forms of carbon and nitrogen in sediments and plants, and the sediment microbial community were detected. It was found that the sediment carbon to nitrogen (C/N) ratios did not differ significantly in the bare sites of different wetlands, but did in the plant-covered sites, which highlighted the different role of plants in shifting the carbon–nitrogen coupling in different constructed wetlands. The effects of plants on the sediment carbon–nitrogen coupling differed in two constructed wetlands, so the structural equation model was used and found that sediment microorganisms directly affected sediment C/N ratios, while water and sediment physicochemical properties indirectly affected sediment C/N ratios by altering sediment microbial functions. Multiple linear regression models showed that water pH, sediment moisture content, water dissolved oxygen, and water depth had a greater influence on the carbon metabolism potential of the sediment microbial community, while sediment moisture content had the greatest impact on the sediment microbial nitrogen metabolism potential. The study indicates that variations in environmental conditions could alter the influence of plants on the carbon and nitrogen cycles of wetland sediments. Water environmental factors mainly affect microbial carbon metabolism functions, while soil physicochemical factors, especially water content, affect microbial carbon and nitrogen metabolism functions.

The Effects of Rainfall Events on the Composition and Diversity of Microplastics on Beaches in Xiamen City on a Short-Term Scale.

Coastal beaches are vulnerable to microplastic pollution originating primarily from terrestrial and marine sources or the in situ weathering of plastic waste. The present study investigates the effects of rainfall events on the composition and diversity of microplastics on beaches in Xiamen City on a short-term scale. In the results, the quantity of microplastics in beach sediments was 245.83 ± 11.61 items·kg-1 (mean ± standard error). The abundance of microplastics did not differ after each rainfall event but significantly decreased after multiple rainfall events. When the diversity of microplastics in the coastal area was evaluated, the Shannon-Wiener index and Pielou's index also decreased from 3.12 and 0.64 to 2.99 and 0.62, respectively, after multiple rainfall events. Rainfall had varying effects on microplastics depending on their size and shape, with particles smaller than 500 μm experiencing pronounced reductions. There was a significant negative correlation between the abundance of microplastics and the grain size of sand, but a positive correlation with sediment moisture content. We encourage the consideration of the potential impact of rainfall events during sample collection to ensure the reliability of the data. We also recommend using diversity indexes to help in understanding the influence of physical processes on microplastic distribution and their mechanisms.

Reducing operational risks during suspensions separation in rare metals technology

The work presents the results of studies of the solid-liquid separation processes of different mineralogical and granulometric compositions, different viscosity and concentrations of uranium, zirconium, nitric acid, soda, sodium nitrate. It has been shown that the process of industrial filtration is impossible for finely dispersed phases due to the low separation rate. Requirements for the purity of technological solutions of uranium and zirconium for the extraction process are limited by the technological ability of the filter fabric to retain the smallest particles, including chemical compounds with a particle size of 1–5 μm. The control filtration process is only possible under pressure, for which a new disk press filter was created, which reduced the concentration of the solid phase in the filtrate to 0.06 g/L. For difficult-to-filter suspensions, a screw centrifuge was developed, which ensured minimum sediment moisture after separating the finely dispersed phase from a solution of soda and sodium silicate and created conditions for maximum extraction of zirconium and hafnium from the solid phase. The separation of water-soluble sodium silicate ensured the stability of the process of extraction separation of zirconium and hafnium and made it possible to isolate silicon oxide.

Biochar from Raw Materials Arundo Spp; Effect on Corn Plant Growth and Production

Biochar is widely used for water and soil remediation due to its local availability and low production costs. However, its effectiveness depends on the physicochemical properties related to the raw material and pyrolysis temperature, as well as the environmental conditions where it is used. Additionally, biochar is susceptible to natural aging caused by changes in soil or sediment moisture, which can alter its redox properties and interactions with contaminants such as arsenic. Therefore, we are interested in conducting fundamental research on raw materials from grass types and their influence on the growth and production of QPM corn plants. In this research, we carried out laboratory content tests and field experiments. The treatments were: p0 control, p1 = 3 tons ha-1 biochar, p2 = 6 tons ha-1 biochar, p3 = 9 tons ha-1, p4 = 12 tons ha-1, p5 = 15 tons ha-1. The experimental results showed that the administration of biochar made from Arundo Spp. 12 tons Ha-1 can increase the effective growth and production of corn plants

Preliminary Determination of Organic Carbon Content in Seagrass Bed at Ao Pae, Rayong Province, Thailand

Seagrass meadows play crucial roles in delivering ecosystem services to various organisms and environmental systems, particularly in efficiently capturing and storing carbon. Thus, seagrasses are included in plans to address climate change challenges. However, there is limited data on carbon sequestration by seagrass in Thailand, especially in localized areas. Therefore, this study aimed to assess the carbon storage in both sediments and seagrass at Ao Pae, Rayong Province. The analysis focused on sediment depth and the impact of moisture content. Sediment and seagrass samples were collected during the low tide season during May 2022. The sampling area was divided into two zones: one with seagrass presence (9 stations) and another where seagrass was absent (4 stations). Sediment was collected from a depth of 60 cm, divided into six layers of 10 cm each. Results indicated that the highest organic carbon content was observed in sediment at depths of 0-10 cm (22.82±2.08%). Additionally, the organic carbon content showed a significant correlation with sediment moisture content in the area with seagrass presence (p<0.05). Furthermore, the mean belowground organic carbon (1.93±0.29%) exceeded the aboveground carbon (1.66±0.28%), and sediments from stations with seagrass had higher organic carbon content compared to those without seagrass. These findings underscore the potential role of seagrass as a significant carbon sequester in Ao Pae, Rayong Province.

Environmental factors and prey availability determine wintering habitat use by shorebirds along the west coast of India.

The spatial and temporal factors governing shorebird habitat use are crucial for managing coastal habitats and halting the decline of shorebird populations in the Central Asian Flyway. Therefore, we investigated how environmental variables and prey availability influenced shorebird distribution and abundance across habitats and seasons along the west coast of India between January 2015 and December 2016. Systematic sampling design was followed to collect data on shorebirds, sediment and hydrological properties (organic matter and physico-chemical properties), and prey availability across habitats over different seasons. Prey density and environmental variables such as salinity, organic matter, and sediment moisture content differed significantly across habitats and seasons. Prey density was influenced by salinity, with higher densities in mudflats (>1000 individuals per m2) during the winter season. Prey density decreased considerably in all coastal habitats during the departure (748.45 ± 320.93) and breeding seasons (537.55 ± 342.29). The fluctuations in prey density and salinity levels had a strong influence on distribution and abundance of shorebirds. These findings highlight the importance of mudflats as a foraging ground during the winter and freshwater inflow in maintaining salinity in estuarine habitats. Therefore, any alteration of river flow (e.g., construction of dams and check dams) or mudflats (e.g., mangrove plantations) would have a serious impact on the availability of prey organisms and the population of shorebirds that depend on them along the Central Asian Flyway.

Greenhouse gas dynamics in river networks fragmented by drying and damming

Abstract River fragmentation by drying and damming is occurring more frequently in the Anthropocene era, yet there is limited knowledge of how this fragmentation influences greenhouse gas (GHG) fluxes in river networks. River networks have the potential to be important sources of GHGs to the atmosphere through both similar and dissimilar mechanisms associated with temporary (drying) and permanent (damming) fragmentation. We conducted a review of the literature and found 49, 43 and six studies about GHGs (CO2, CH4 and N2O) in rivers impacted by damming, drying and their interaction, respectively. We found research lacking in non‐arid climates and in small water‐retention structures for studies regarding drying and damming, respectively. The major factors directly influencing GHG fluxes in river networks impacted by drying were sediment moisture, temperature, organic matter content and texture. In networks impacted by damming, the most influential factors were water temperature, dissolved oxygen, and phytoplankton Chlorophyll‐a. Based on our literature review and meta‐ecosystem theory, we propose that the spatial distribution of fragmentation strongly influences GHG fluxes at the river‐network scale. The actionable future research directions identified here will help to improve our understanding of the effects of fragmentation by drying and damming on GHG fluxes, with the potential to inform river management and climate change mitigation strategies.

Pyrolysis temperature and biochar redox activity on arsenic availability and speciation in a sediment

Biochar is widely used for water and soil remediation in part because of its local availability and low production cost. However, its effectiveness depends on physicochemical properties related to its feedstock and pyrolysis temperature, as well as the environmental conditions of its use site. Furthermore, biochar is susceptible to natural aging caused by changes in soil or sediment moisture, which can alter its redox properties and interactions with contaminants such as arsenic (As). In this study, we investigated the effect of pyrolysis temperature and biochar application on the release and transformations of As in contaminated sediments subjected to redox fluctuations. Biochar application and pyrolysis temperature played an important role in As species availability, As methylation, and dissolved organic carbon concentration. Furthermore, successive flooding cycles that induced reductive conditions in sediments increased the As content in the solution by up to seven times. In the solid phase, the application of biochar and the flooding cycle altered the spatial distribution and speciation of carbon, iron (Fe) and As. In general, the application of biochar decreased the reduction of Fe(III) and As(V) after the first cycle of flooding. Our results demonstrate that the flooding cycle plays an important role in the reoxidation of biochar to the point of enhancing the immobilization of As.

Nitrogen mineralization and immobilization in surface sediments of coastal reclaimed aquaculture ecosystems

Sediment nitrogen (N) mineralization and immobilization are two crucial processes driven by microorganisms, which may play significant roles in the regulation of water quality in aquaculture ecosystems. However, limited information is available about the quantitative importance of sedimentary N mineralization and immobilization in coastal aquaculture systems. Here, a combination of incubation experiments with a 15N isotope dilution technique were employed, aiming to quantify N mineralization and immobilization processes in surface sediments (0–5 cm) of three types of aquaculture ecosystems (seabass, white shrimp, and green crab ponds) reclaimed within the western bank of the Pearl River Estuary. Our results showed that no significant difference in sediment N mineralization and immobilization rates, microbial abundances, and organic matter among different aquaculture types on small-scale range. Meanwhile, prolonged pond-drying significant reduced sediment N mineralization and immobilization rates, bacterial abundances, organic matter, moisture content, ferrous ion (Fe2+), Fe2+/Fe3+, and ammonium (NH4+), while not strongly altered sediment percentage of NH4+ mineralized per day (PAM), relative ammonium immobilization (RAI), fungal abundances, TOC/TN, nitrate (NO3−), and δ13Corg. N mineralization and immobilization rates were both significantly related to overlying water NO3−, as well as sediment moisture content, bulk density, organic matter, Fe2+, and microbial abundances. In addition, the total mineralized and immobilized N in aquaculture surface sediments from the Guangdong-Hong Kong-Macao Greater Bay Area were estimated to be approximately 4.55×104 and 3.68×104 t N yr-1, respectively. Higher N mineralization relative to N immobilized fluxes indicated that the sediment serves as an important source of eutrophication in reclaimed aquaculture system of coastal wetlands.

Complex Dose Rate Calculations in Luminescence Dating of Lacustrine and Palustrine Sediments from Niederweningen, Northern Switzerland

This study focusses on dose rate determination in complex settings in two drill cores from the site of Niederweningen, northern Switzerland. A crosscheck with a certified standard material and neutron activation analyses (NAA) reveals an overall good performance of high-resolution gamma spectrometry (HR-GS) when determining dose rate-relevant elements. A second focus is on average water content during burial, by comparing measured sediment moisture with water uptake capability. Furthermore, layer models are used to investigate the impact of inhomogeneous stratification on dose rate. Finally, different scenarios to correct for radioactive disequilibrium in the uranium decay chain are investigated. While most of the applied corrections appear to have only a minor to moderate effect on age calculation, the results for one core are contradictory. Possibly, some of the applied correction scenarios are not reflecting the complex natural setting sufficiently, in particular average sediment moisture during burial and the timing of radioactive disequilibrium might be incorrectly estimated. While deposition in one core can be quite securely attributed to the period 100–70 ka, assigning the sediment sequence in the other core to the time between ca. 130 ka and 90 ka remains to some extent insecure.

High resolution spatiotemporal patterns of flow at the landscape scale in montane non‐perennial streams

AbstractIntermittent and ephemeral streams in dryland environments support diverse assemblages of aquatic and terrestrial life. Understanding when and where water flows provide insights into the availability of water, its response to external controlling factors, and potential sensitivity to climate change and a host of human activities. Knowledge regarding the timing of drying/wetting cycles can also be useful to map critical habitats for species and ecosystems that rely on these temporary water sources. However, identifying the locations and monitoring the timing of streamflow and channel sediment moisture remains a challenging endeavor. In this paper, we analyzed daily conductivity from 37 sensors distributed along 10 streams across an arid mountain front in Arizona (United States) to assess spatiotemporal patterns in flow permanence, defined as the timing and extent of water in streams. Conductivity sensors provide information on surface flow and sediment moisture, supporting a stream classification based on seasonal flow dynamics. Our results provide insight into flow responses to seasonal rainfall, highlighting stream reaches very reactive to rainfall versus those demonstrating more stable streamflow. The strength of stream responses to precipitation are explored in the context of surficial geology. In summary, conductivity data can be used to map potential stream habitat for water‐dependent species in both space and time, while also providing the basis upon which sensitivity to ongoing climate change can be evaluated.

Hatching Success of Caretta caretta on a Mediterranean Volcanic Beach: Impacts from Environmental Parameters and Substrate Composition

Visconti, G.; Scopelliti, G.; Caldareri, F.; Agate, M.; Cambera, I.; Sulli, A., and Arculeo, M., 2022. Hatching success of Caretta caretta on a Mediterranean volcanic beach: Impacts from environmental parameters and substrate composition. Journal of Coastal Research, 38(3), 603–612. Coconut Creek (Florida), ISSN 0749-0208. New data on the potential impact of environmental parameters and the mineralogical nature of the substrate on the hatching success of the loggerhead sea turtle Caretta caretta on a volcanic beach on Linosa Island in the central Mediterranean Sea are reported. During 2 years of investigation (2018–19), five nests were observed. The temperature and moisture of the nests were determined along with the grain size and mineralogical nature of the substrate. The temperature values of the nests recorded at depths of 5 and 35 cm showed an average temperature incubation of 32.2°C ± 1.4°C and 32.8°C ± 1.6°C, respectively, with peaks close to 37°C. The sedimentological analysis carried out on two cores revealed the prevalence of large sandy particle and sediment moisture of about 35% in the deeper layers. A total of 214 laid eggs were recorded in 2018 with only 44 emerged turtles, whereas 45 turtles emerged from 199 laid eggs in 2019. Both years showed a low hatching success (25%–30%) and a high frequency of anomalies (>60%), which were present in individuals as nonmodal scutes or bifid limbs. Results suggest that environmental parameters such as temperature, sand moisture, and the geological nature of the substrate seem to play an important role in the reproductive success of this species. Some aspects for improving the nesting site and egg development conditions are also discussed.

Variations in the mineral structures dominating solute mobilization during clay compaction

Microbial-mediated redox reactions typically control the solute mobilization in groundwater systems, in which the clay aquitard is usually a triggering factor of the release of the dissolved organic carbon, ammonium, and iron into the aquifer during pore water discharge and clay compaction. However, solute mobilization inside the aquitard during clay compaction is generally mistaken for a similar water–rock interaction process in aquifers controlled by microbial-mediated redox reactions. Through a simulation experiment on argillaceous sediment compaction, we tested the currently accepted solute mobilization mechanism and determined that the variation in the mineral structure dominates it. The variation in the mineral structure occurs in low symmetry minerals, such as clay minerals, and is controlled by the sediment moisture content, the liquid-plastic limit, and the effective stress. When the sediment moisture content decreases to below the plastic limit through pore water discharge and compaction, sudden changes in the mineral structure occur, releasing iron and capturing ammonium through variations in the relative position of the Si-O tetrahedrons and isomorphism of similar atoms without the participation of microbial-mediated redox reactions. These results suggest that the biochemical reactivities of organic carbon, iron, and ammonium are sometimes overestimated, i.e., when the role of the physical processes is ignored, in solute mobilization during clay compaction, which warrants more attention and investigation.

Small rain events during drought alter sediment dissolved organic carbon leaching and respiration in intermittent stream sediments

With climate change, streams and rivers are at increased risk of droughts and flow intermittency. The full implications of these conditions for fluvial carbon (C) processing and stream-atmosphere CO2 emissions are not well understood. We performed a controlled drought experiment in outdoor hyporheic flumes. We simulated small rain events that increase sediment moisture content, but do not cause streamflow in order to investigate how these events affect streambed dissolved organic C dynamics, biofilm respiration and enzyme activity, and bacterial community composition. Flumes were subject to a non-flow phase of one month with small rain events with varying frequency (weekly, 3 × weekly, and no rain). Sediment was sampled at the surface and from the hyporheic zone at the end of the non-flow phase. We quantified microbial respiration of the dry sediments and sediment DOC leaching after simulated flow resumption. We found that, at the surface, more frequent rain events significantly increased microbial respiration from 12.6 ± 0.25 µg CO2 g−1 DW h−1 to 26.5 ± 11.3 µg CO2 g−1 DW h−1 between the control and 3 × weekly rain events. The average amount of DOC leached from surface sediments during flow resumption was reduced by 0.813 ± 0.62 mg L−1 with more frequent rain events. More frequent rain events also resulted in the leaching of fresher DOM with increased tryptophan fluorescence and a higher BIX. This, along with higher glucosidase activity in the biofilms, indicates higher OC processing during the drought period with more frequent rain events. Small rain events also enhanced Shannon diversity of microbial communities, with a stronger presence of ‘terrestrial-like’ bacterial clades. We propose that rain events during drought, even those of small size, are highly relevant for fluvial organic C processing during the dry phase. Future research should explicitly consider small rain events when investigating C fluxes in intermittent streams to fully understand the C processing in these systems with climate change. We conclude that small rain events impact DOM dynamics during reflow and likely impact the cascading C processing in the downstream river network.

Concurrent effects of flooding regimes and floodwater quality on sediment properties in a Yangtze River-connected floodplain wetland: Insights from field investigations during 2011–2020

Changes in flood regimes, floodwater quality, and macrophyte types may affect sediment characteristics post-flooding. However, few studies have attempted to unravel their complex influences in floodplain wetlands. From 2011 to 2020, the physical and chemical properties of surface layer sediment pre- and post-flooding was investigated through field surveys in the Dongting Lake wetland. Results indicated that the pre-flooding soil total phosphorus (TP) and total nitrogen (TN) exhibited an increasing trend during 2011–2020. Soil TP increased post-flooding relative to that pre-flooding. The changes in TN, sediment organic matter (SOM), sediment moisture content (SMC), and sediment bulk density (SBD) fluctuated over the years. The best-fitting multi-regression model demonstrated that the changes in sediment variables post-flooding showed a parabolic trajectory along the inundation duration (ID), except for SMC. Changes in soil properties post-flooding were negatively correlated with ID for sediment with a low IDs (<148 days). Meanwhile, changes in soil properties post flooding were positively correlated with ID for sediment with a high IDs (>193 days). Changes in SBD and SOM post-flooding were positively influenced by the TP content in the floodwater. These findings indicate that changes in the flooding regime, and water quality generated by anthropogenic disturbances such as the Three Gorges Dam significantly affect sediment properties, and subsequently influence the ecological functions of the Dongting Lake wetland.

Effects of simulated rainfall enhancement on sediment CO2 flux in dry lakebed of Barkol Lake, China

Understanding the responses of lake sediment carbon process to climate change is an important part of a comprehensive understanding of lake carbon budget. To explore the effects of future rainfall increase on sediment carbon flux, undisturbed sediment samples were collected from the bottom of dry lake Barikun in Hami, Xinjiang for the incubation experiment. Based on the increase rate of precipitation (4 mm·10 a-1) and the distribution characteristics of rainfall in the plant growing season in Hami, Xinjiang since 1960, five rainfall treatments were set (86 mm, T0; 94 mm, T1; 102 mm, T2; 110 mm, T3; 126 mm, T4) based on the rainfall in growing season of 2016 (86 mm). We analyzed the effects of rainfall increase on sediment CO2 flux. Results showed that compared with that before rainfall, the sediment CO2 flux increased after 1 day of rainfall in the study area. Compared with that during May to July, the CO2 flux of sediments in August to October decreased. There was no variation of CO2 accumulative emission among the T0-T3 treatments from May to October. However, the average CO2 emission rate under the T3 treatment (0.22 μmol·m-2·s-1) was significantly higher than that under the T4 treatment (0.14 μmol·m-2·s-1). All treatments showed CO2 sink at the first day of rainfall (1 d), with T4 treatment (-0.13 μmol·m-2·s-1) having the highest "carbon sink" capacity. After 1 day, the CO2 sink converted to CO2 source under the five rainfall treatments, with the CO2 emission rate under T3 treatment (0.34 μmol·m-2·s-1) being significantly higher than those under other treatments. Compared with May, the CO2 emission fluxes of T2-T4 treatments were significantly higher than those at the time from August to October. Under the condition with relatively stable temperature, the CO2 flux of sediments was significantly correlated with the sediment moisture and air humidity. In the next 60 years, the continuous increase of future rainfall may be an important factor promoting CO2 emission from lake sediment in arid regions, and thus affecting global warming.

Retrieval of Tidal Flat Elevation Based on Remotely Sensed Moisture Approach

Due to challenging conditions of field survey techniques, it is difficult to measure the topography of tidal flats, an important parameter to understanding the evolution and dynamics of the constantly changing zone. This study used remotely sensed sediment moisture estimates to retrieve tidal flat elevation. The method is based on the observation that the intertidal zone is gradually exposed from land to sea at low tide, meaning that higher elevations contain less moisture. Here, we investigate the nature of the relationship between reflectance and moisture content from Landsat Enhanced Thematic Mapper Plus images and the study areas as a proxy for mapping the elevation of an exposed tidal flat surface. Statistical analysis confirmed a negative correlation between moisture and elevation; however, the correlation coefficient was relatively weak, and the slope of the intersecting tidal creek was found to be a crucial factor affecting this relationship. After segmenting the slope to correspond to areas of tidal flat and nontidal flat surfaces, the correlation coefficient of the moisture and elevation increased significantly. A retrieval model was then developed to generate the tidal flat elevations of different slope grades. After verification, the retrieval accuracy of the model was up to 17.3 cm. This research study demonstrated that the remotely sensed moisture method is suitable for monitoring the surface elevation of tidal flats.

Study on the dinoflagellate cysts in ballast tank sediments of international vessels in Chinese shipyards

The problem of aquatic invasive species caused by discharge of ballast water and sediments from ships’ ballast tanks has become extremely prominent. Seventeen sediment samples taken from ballast tanks of different ships docked in two Chinese shipyards were examined to identify the variety of resting dinoflagellate cysts. Twenty-two dinoflagellate cyst taxa were identified in these samples, including 11 photosynthetic and eleven heterotrophic species. These species represent 10 genera with the dominating assemblages of Alexandrium minutum, Scrippsiella acuminata, Lingulodinium polyedra, Protoperidinium sp. and Protoperidinium conicum. The total abundance of the dinoflagellate cysts ranged from 36 to 448 cysts g−1 dry weight, which demonstrated a wide range of diversity for different ships. It was observed that the number of taxa and concentrations of cysts in ballast tank sediments were slightly greater for ships performing short voyage than ships performing longer voyage. The compositions of dinoflagellate cysts in sediments from ships sailing diverse routes were more variable than those sailing same routes. Sediment moisture content proved to be well correlated to the total cyst abundance (r = 0.7422, P < 0.01). Furthermore, nine toxic and harmful species were observed from all sediment samples, which indicated a wide range of distribution and potential risk of harmful algal blooms if being discharged to Chinese waters. As a result, full attention should be drawn to the studies on dinoflagellate cysts in the ballast tank sediments from ships arriving at China, this is of great significance for preventing introduction of toxic and harmful dinoflagellate cysts and protecting native marine biodiversity.

Biogeochemical characteristics and hydroperiod affect carbon dioxide flux rates from exposed high‐elevation pond sediments

AbstractWhile inundated, small ponds (&lt; 1000 m2 area) account for disproportionately large contributions of CO2 efflux to the global carbon budget and also store carbon in anoxic sediments. However, pond hydrology is shifting toward increasingly dry conditions in alpine and temperate zones, which might lead to increased exposure of shallow pond sediments. We analyzed sediment CO2 efflux rates in dried sediments of multiple ponds of varying hydrology and sediment characteristics at montane and subalpine elevations near the Rocky Mountain Biological Laboratory in Colorado. Average CO2 efflux rates from exposed sediments, 331.5 ± 11.5 mmol m−2 d−1 at the montane sites and 142.8 ± 45.1 mmol m−2 d−1 at the subalpine sites, were 10 times higher than average CO2 efflux rates from pond water. Principal components analysis to reduce dimensionality of sediment characteristics revealed that random inter‐pond differences rather than exposure timing or hydroperiod drove variation among sediments. In linear mixed effects models of CO2 flux rates, significant predictors included sediment moisture and temperature, pH, total organic carbon, and organic matter content at all pond hydroperiod classifications and sites. However, the sediment characteristics explaining the most variance differed among sites and hydroperiods and included nitrate concentrations, pH, bulk density, and temperature. We conclude that pond sediments are heterogeneous both within and among ponds in close proximity, and drivers of relatively high CO2 efflux rates differ among pond hydroperiods and elevations. This work emphasizes that local differences can impact predictions of CO2 flux from lentic sediments which are becoming increasingly exposed.