Dissolved Silicate Concentrations Research Articles

Tidal variation modulates the dissolved silicate behavior and exchange flux across the semi-enclosed bay‐coastal water continuum, China

Coastal water is the key transition zone for the circulation and transport of nutrients. Their role in transporting nutrients is important to understanding global dissolved silicate (DSi) cycles and sources of nutrients supporting the biological pump and ocean carbon cycle. However, the understanding of controlling DSi exchange flux between the semi-enclosed bay and coastal water was still scarcely due to limitations in continuous observation. In this study, we conducted continuous investigations during spring tide (ST) and neap tide (NT) in 2021 in Shuidong Bay (SDB), China, to explore the impacts of different tidal cycles on DSi in SDB and the fluxes across SDB and South China Sea (SCS) coastal water. The findings demonstrated that there were significant differences in DSi concentrations and nutrients ratios between ST and NT in S1 station (P < 0.05). In addition, the DSi concentrations were 32.01 ± 27.21 μmol/L and 51.48 ± 48.44 μmol/L in ST and NT, respectively. Besides, the net export of DSi from SDB to SCS was 0.18 t throughout the entire early of autumn tidal cycle, suggesting SDB was the source of DSi, and its behavior across the semi-enclosed bay‐coastal water continuum was largely controlled by tidal characteristics (tidal height, flow velocity), water physicochemical parameters (salinity, pH), biological uptake and terrestrial sources input. SDB in ST has higher proportions of DSi: DIN (dissolved inorganic nitrogen) (1.49 ± 1.28) and DSi: DIP (dissolved inorganic phosphorus) (58.6 ± 43.73) compared with NT, DSi: DIN and DSi: DIP for the NT period were 1.45 ± 1.15 and 43.99 ± 28.59, indicating that phosphorus (P) is the limiting trophic factor for SDB. The tidal cycle in SDB would alter the DSi stoichiometry and mitigated the impact of eutrophication caused by terrestrial sources. This study provides new insights in the Si tidal cycling across the semi-enclosed bay‐coastal water continuum, which was implications for understanding DSi biogeochemical process and primary production dynamics in coastal water.

Porewater exchange drives the dissolved silicate export across the wetland‐estuarine continuum

Coastal wetlands are an important hotspot for nutrient cycling and transport from the land to the ocean. Silicon (Si) as a vital biogenic element affects plant growth and health of coastal ecosystems. The understanding of key factors and processes controlling dissolved silicate (DSi) exchange between the wetlands and coastal water has been limited due to the lack of measured data. We carried out intensive investigations of time-series DSi concentrations and porewater exchange across the Sediment-Water Interface (SWI) along a tidal creek with a mangrove-salt marsh gradient during neap and spring tides in 2020. Seasonal observations of surface water in a tidal creek and Zhangjiang Estuary (Fujian Province, China) were conducted from 2017 to 2020. The results showed that there was a net export of DSi from the mangroves to tidal creek with rates of 2.11 and 2.40 mmol m-2 d-1 in neap and spring tides respectively, suggesting the mangroves served as the source of DSi. However, the salt marshes had a net DSi import with one or two orders of magnitude lower than the export from the mangroves. DSi export across the wetland‐estuarine continuum was largely controlled by porewater exchange, groundwater geochemistry (pH, temperature) and plant root uptake. Groundwater in the mangroves has larger ratios of DSi : DIN (dissolved inorganic nitrogen) (2.5 ± 0.6) and DSi : DRP (dissolved reactive phosphorus) (1257 ± 35) compared with surface water. The net export of DSi from mangroves would modify the nutrient stoichiometry and mitigate the effects of reduced river DSi flux caused by damming on coastal ecosystem. This study provides new insights into the wetland Si cycling for sustaining coastal ecosystem health.

A spatiotemporally weighted intelligent method for exploring fine-scale distributions of surface dissolved silicate in coastal seas

The transfer of dissolved silicate (DSi) from land to coastal environments is a crucial part of global biogeochemical cycling. However, the retrieval of coastal DSi distribution is challenging due to the spatiotemporal non-stationarity and nonlinearity of modeling processes and the low resolution of in situ sampling. To explore the coastal DSi changes in a higher spatiotemporal resolution, this study developed a spatiotemporally weighted intelligent method based on a geographically and temporally neural network weighted regression (GTNNWR) model, a Data-Interpolating Empirical Orthogonal Functions (DINEOF) model, and satellite observations. For the first time, the complete surface DSi concentrations of 2182 days at the 500-meter and 1-day resolution in the coastal sea of Zhejiang Province, China, were obtained (Testing R2 = 78.5 %) by using 2901 in situ records with concurrent remote sensing reflectance. The long-term and large-scale distributions of DSi reflected the changes in coastal DSi under the influences of rivers, ocean currents, and biological effects across multiple spatiotemporal scales. Benefiting from the high-resolution modeling, this study found that the surface DSi concentration had at least 2 declines during a diatom bloom process, which can provide crucial signals for the timely monitoring and early warning of diatom blooms and guide the management of eutrophication. It was also indicated that the correlation coefficient between the monthly DSi concentration and the Yangtze River Diluted Water velocities reached −0.462**, quantitatively revealing the significant influence of the terrestrial input. In addition, the daily-scale DSi fluctuations resulting from typhoon transits were finely characterized, which greatly reduces the monitoring cost compared with the field sampling. Therefore, this study developed an effective data-driven-based method to help explore the fine-scale dynamic changes of surface DSi in coastal seas.

Reconstruction of Monthly Surface Nutrient Concentrations in the Yellow and Bohai Seas from 2003–2019 Using Machine Learning

Monitoring the spatiotemporal variability of nutrient concentrations in shelf seas is important for understanding marine primary productivity and ecological problems. However, long time-series and high spatial-resolution nutrient concentration data are difficult to obtain using only on ship-based measurements. In this study, we developed a machine-learning approach to reconstruct monthly sea-surface dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi) concentrations in the Yellow and Bohai seas from 2003–2019. A large amount of in situ measured data were first used to train the machine-learning model and derive a reliable model with input of environmental data (including sea-surface temperature, salinity, chlorophyll-a, and Kd490) and output of DIN, DIP, and DSi concentrations. Then, longitudinal (2003–2019) monthly satellite remote-sensing environmental data were input into the model to reconstruct the surface nutrient concentrations. The results showed that the nutrient concentrations in nearshore (water depth < 40 m) and offshore (water depth > 40 m) waters had opposite seasonal variabilities; the highest (lowest) in summer in nearshore (offshore) waters and the lowest (highest) in winter in nearshore (offshore) waters. However, the DIN:DIP and DIN:DSi in most regions were consistently higher in spring and summer than in autumn and winter, and generally exceeded the Redfield ratio. From 2003–2019, DIN showed an increasing trend in nearshore waters (average 0.14 μmol/L/y), while DSi showed a slight increasing trend in the Changjiang River Estuary (0.06 μmol/L/y) but a decreasing trend in the Yellow River Estuary (–0.03 μmol/L/y), and DIP exhibited no significant trend. Furthermore, surface nutrient concentrations were sensitive to changes in sea-surface temperature and salinity, with distinct responses between nearshore and offshore waters. We believe that our novel machine learning method can be applied to other shelf seas based on sufficient observational data to reconstruct a long time-series and high spatial resolution sea-surface nutrient concentrations.

Underestimation of biogenic silica sinking flux due to dissolution in sediment traps: A case study in the South China Sea

The dissolved silicate concentrations in the supernatant of sediment trap sampling bottles retrieved from deep water (1000-3000 m) at four stations in the South China Sea (SCS) were measured to calculate the underestimated flux of biogenic silica (bSi) in sinking particles due to bSi dissolution. High dissolved silicate concentrations in the supernatant, ranging from 122.6 to 1365.8 μmol/L, indicated significant dissolution of particulate bSi in the sampling bottles. Underestimation of the bSi flux in the SCS by ~2% to ~34% (average: ~10%) due to bSi dissolution was revealed, and the degree of underestimation increased with decreasing total bSi flux. The amount of bSi dissolved within the sampling series at each station was generally positively correlated with dissolution time and to a certain extent influenced by the bSi amount collected by the sampling bottles under a low bSi sinking flux. Apparently stronger bSi dissolution was found at two of the four stations due to relatively high bSi dissolution rates, which were possibly related to a higher reactive surface area of the bSi or bacterial activity. Overall, our results demonstrated that the considerable bSi dissolution in sediment trap sampling bottles should not be ignored, especially in the low-productivity oligotrophic ocean, and bSi flux calibration via measurement of the dissolved silicate in sampling bottles is necessary. To reduce bSi flux underestimation due to the deployment of time-series sediment traps, larger-volume sampling bottles should be avoided in the oligotrophic open ocean, and sinking particle samples should be analyzed as soon as the sediment traps are recovered.

Decadal change in dissolved silicate concentration and flux in the Changjiang (Yangtze) River

Silicon (Si) plays an essential role in the biogeochemistry of rivers. This study explored how damming, eutrophication and climate change alters the abundance and flux of DSi in the Changjiang (Yangtze) River based on long-term observations. The results showed that Three Gorges Reservoir (TGR) could enhance DSi transfer only during low-flow time period, and a downstream DSi retention effect by the TGR was found between the Yichang and Jianli stations in the Changjiang River. This resulted in a DSi loss during March and April in the mainstream from Three Gorges Dam (TGD) to Jianli but a DSi addition during July and October along the main channel of the Changjiang River. Long-term data showed a sharp decrease in DSi abundance at the Cuntan, Hankou and Datong stations between the 1960s and 1980s, but a slight increase in DSi between the 1990s and 2010s at these stations. The decrease in DSi during the 1960s -1980s was primarily the result of a decrease trend of silicate weathering, while a slight DSi increase compared to the temperature/DSi relation after the 1990s was largely due to increased DSi retention in the basin by damming and eutrophication. Eutrophication and damming increase DSi trapping in both the river channel and reservoir systems in the low-flow period and thus enhance the nutrient distortion in the coastal ocean.

Silica removal at sewage treatment plants causes new silica deficiency

The dissolved silicate (DSi) concentration in coastal waters has decreased due to anthropogenic activities. Many studies have indicated that dam construction is a main reason for this reduction. However, recently, dam construction alone has not been sufficient to explain the DSi reduction in some coastal waters. In this study, we focused on silica removal at sewage treatment plants (STPs). DSi and particulate silica (PSi) concentrations were measured in STP influent and effluent waters from September 2020 to September 2021. Dissolution experiments on PSi were also conducted to estimate the fraction of soluble PSi in the STP influent. DSi and PSi were removed by 29.5% and 96.9%, respectively, at the STP. In addition, the soluble PSi in the STP influent accounted for 20.3% of the PSi removed. Therefore, in addition to the DSi removal in STPs, removal of soluble PSi can also cause potential DSi depletion in downstream and coastal waters. In addition to the effect of dams, the silica supply delivered to coastal waters may be further reduced in the future due to the progress of sewage treatment development in coastal areas.

Short-term changes in algal blooms and phytoplankton community after the passage of Super Typhoon Lekima in a temperate and inner sea (Bohai Sea) in China

Extensive multi-species harmful algal blooms (HABs) were triggered by Super Typhoon Lekima in Laizhou Bay (Bohai Sea) in August 2019. After conducting two field cruises before and after the typhoon passage, we employed both high-performance liquid chromatography (HPLC)-pigment and microscopic methods to study the changes in the phytoplankton community and biomass. Following the passage of Lekima, the average surface salinity decreased, while dissolved inorganic nitrogen and dissolved silicate concentrations increased in the study area. The phytoplankton abundance and Chl a significantly increased after the typhoon event. Post-typhoon, the highest abundance values of Pseudo-nitzschia spp., Noctiluca scintillans, and Coscinodiscus spp. reached 106 cells/L and those of Bacillaria paxillifera, Ceratium spp., and Gymnodinium catenatum were in the order of 105 cells/L. HPLC-pigment CHEMTAX analysis showed that the biomass (Chl a) of dinoflagellates, diatoms, cryptophytes, chlorophytes, and haptophytes increased significantly after the typhoon. The increase in Chl a concentration was mainly attributable to large-sized phytoplankton, which are mostly diatoms and dinoflagellates. This study highlights that typhoons may cause HABs by introducing large amounts of freshwater and nutrients and change the phytoplankton community in a temperate and inner bay.

Understanding Competitive Phosphate and Silicate Adsorption on Goethite by Connecting Batch Experiments with Density Functional Theory Calculations.

Despite the biogeochemical importance of phosphate fate and transport in aquatic environments, little is known about how competition with other common aqueous oxyanions affects its retention by mineral surfaces. Here, we examined the competitive uptake of phosphate and silicate on goethite over a wide pH range, using batch measurements supported by DFT calculations. The results show selective adsorption of phosphate at pH < 4 and silicate at pH > 10 with little to no competitive effect. However, between 4 < pH < 10, the total phosphate and silicate loading was found to be almost equal to that of silicate loading from single-component solution, revealing a proportionate competition for surface site types and a competitive effect controlling their mutual retention. DFT-calculated adsorption energies and charge density redistributions for various surface complexes on different charged (101) and (210) facets are consistent with the trends observed in batch measurements, suggesting that the observed behavior reflects the primary controlling influence of goethite surface chemistry at the molecular scale. An important implication is that at the circumneutral pH in most environmental systems, where iron oxyhydroxides comprise much of the reactive interfacial area, unbound phosphate concentrations may be strongly controlled by dissolved silicate concentration, and vice versa.

Nutrient enrichment induced by tropical cyclone Seroja in the southeastern tropical Indian Ocean

Tropical cyclone (TC) passage triggers a complex response from the adjacent ocean, including vertical mixing, leading to biochemical alterations and affecting the surrounding ecosystem’s dynamics. In previous studies, increased nutrient concentrations and primary production were observed along the cyclone track after the storm. TC Seroja was awakened near the equator in the southeastern tropical Indian Ocean, making it interesting to investigate how the ambient ecosystem responds. Hence, we analyzed the sea surface temperature and nutrient changes during the Seroja event using multi-satellite remote sensing and numerical model data in the south of Indonesia and East Timor along the Seroja track between April 2 and 10, 2021. Immediately after the TC Seroja passed, the sea surface temperature cooled to 3 °C around the TC lane. At the same time, the spatial distribution patterns showed the upsurge of some nutrients in response to the passage of TC Seroja; the surface nitrate swells up to 1.5 mmol/m3, while phosphate increased up to 0.2 mmol/m3, and the dissolved silicate concentration enhanced up to 1.0 mmol/m3. The responses recover within 2-7 days. These results indicate that tropical cyclones contribute to nutrient enrichment in oligotrophic areas outside of their usual annual upwelling time, thereby further supporting ecosystem sustainability.

Molecular Traits of Dissolved Organic Matter in the Subterranean Estuary of a High-Energy Beach: Indications of Sources and Sinks

Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.

Spatiotemporal Dissolved Silicate Variation, Sources, and Behavior in the Eutrophic Zhanjiang Bay, China

Dissolved silicate (DSi) is an important nutrient in coastal water, which is used by planktonic diatoms for cell division and growth. In this study, surface water samples were collected in the eutrophic Zhanjiang Bay (ZJB) in 2019, covering a seasonal variation of coastal water and land-based source water discharge. The spatiotemporal DSi distribution, land-based sources flux input and behaviors in ZJB were studied and discussed. The results show that the DSi concentration had significant differences in spatiotemporal scale. The average concentration of DSi in ZJB was 38.00 ± 9.48 μmol·L−1 in spring, 20.23 ± 11.27 μmol·L−1 in summer, 12.48 ± 1.42 μmol·L−1 in autumn and 11.96 ± 4.85 μmol·L−1 in winter. The spatiotemporal DSi distribution showed a decreasing gradient from the top to the mouth of ZJB, which was affected by land source inputs and hydrodynamics. The land-based sources’ input concentration of DSi in ZJB ranged from 0.021 to 0.46 mol·L−1, with an average of 0.14 mol·L−1, and the total annual flux of DSi was 1.06 × 109 mol, comprising up to 8.28%, 41.55% and 50.17% in dry, normal, and wet seasons, respectively. The Suixi River contributed the highest DSi flux proportion in all seasons. The DSi in land-based source water was mainly affected by water flow discharge, diatom uptake and impacts from anthropogenic activities. Compared with dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), the DSi was the limitation nutrient in ZJB. Additionally, the DSi concentration in the coastal water was negatively correlated with salinity. The seasonal DSi/DIN and DSi/DIP ratios in land-based sources discharge water was significantly higher than that in coastal water (p &lt; 0.05). Land-based sources of DSi input played an important role in nutrients composition that sustained diatoms as the dominant species in ZJB.

Comparison of phytoplankton community structure in two tropical estuaries of East Coast of India

Phytoplankton community of two tropical river estuaries of the North-east coast of India was different due to dissolved nutrients concentration in those estuarine waters. The first study site was the Saptamukhi river estuary which is located in the Indian Sundarban (inside mangrove forest) and the second study site was Mahanadi estuary located in Orissa coast (adjacent to industrial and coastal fishing zone). The Saptamukhi estuary was mostly devoid of any anthropogenic influence and here the nutrient source was auto-generated. In Mahanadi estuarine water, the source of the major nutrients was industrial effluent and fishing waste. The Saptamukhi estuarine water had less dissolved phosphorus concentration but huge dissolved nitrogen available for phytoplankton uptake. A sufficient amount of dissolved silicate encouraged the diatom growth over other phytoplankton groups here. Due to the availability of the high amount of dissolved phosphorus and the low amount of dissolved nitrogen in the Mahanadi estuary the phytoplankton community was a mixed population of Bacillariophyceae, Cyanophyceae, Chlorophyceae and Dinophyceae and seasonal Chlorophycean bloom observed during the study period. In both the estuarine water primary productivity was high but the community respiration was higher and the estuaries were heterotrophic. Monsoonal runoff from land considerably changed the community in both estuarine water. Dissolved silicate concentration in both the estuarine water was sufficient for Diatom growth. Dissolve inorganic nitrogen and dissolved inorganic phosphate ratio played a major role for the community change of phytoplankton in two estuarine waters.

Super typhoon induced high silica export from Arakawa River, Japan.

Dissolved silicate (DSi) and particulate silica (PSi) concentrations were measured at Arakawa River and at sewage treatment plants (STP) during October 2018 to October 2019. These included flooding observations after super Typhoon Hagibis. At ordinary water levels, the STP effluents were found to be the largest source of DSi in the river. Although DSi concentrations during the flooding events (165μmolL-1) decreased by about 25% compared to that of ordinary water level (221μmolL-1), PSi was more than sixteen times higher value (301μmolL-1) compared to that of ordinary water level (18μmolL-1). Loading amounts of DSi and PSi (± 1 standard error) were 1.5 × 108 (± 0.1 × 108) and 0.15 × 108 (± 0.02 × 108) mol year-1, respectively, excluding the data of Typhoon Hagibis. Loading amounts during flooding events of DSi and PSi were 1.2 × 108 (± 0.1 × 108) and 2.4 × 108 (± 0.4 × 108) mol 15days-1, respectively. Although the silica loading at ordinary water level was mainly derived from DSi, the silica loading during flooding events was extremely large due to both high level of DSi and PSi; moreover, it was higher than the annual loading amount.

Description and evaluation of the Diat-HadOCC model v1.0: the ocean biogeochemical component of HadGEM2-ES

Abstract. The Diat-HadOCC model (version 1.0) is presented. A simple marine ecosystem model with coupled equations representing the marine carbon cycle, it formed the ocean biogeochemistry sub-model in the Met Office's HadGEM2-ES Earth system model. The equations are presented and described in full, along with the underlying assumptions, and particular attention is given to how they were implemented for the CMIP5 simulations. Results from the CMIP5 historical simulation (particularly those for the simulated 1990s) are shown and compared to data: dissolved nutrients and dissolved inorganic carbon, as well as biological components, productivity and fluxes. Where possible, the amplitude and phase of the predicted seasonal cycle are evaluated. Since the model was developed to explore and predict the effects of climate change on the marine ecosystem and marine carbon cycle, the response of the model to the RCP8.5 future scenario is also shown. While the model simulates the historical and current global annual mean air–sea CO2 flux well and is consistent with other modelling studies about how that flux will change under future scenarios, several of the ecosystem metrics are less well simulated. The total chlorophyll is higher than observations, while the primary productivity is just below the estimated range. In the CMIP5 simulations certain parameter choices meant that the diatoms and the misc-phytoplankton state variables behave more similarly than they should, and the surface dissolved silicate concentration drifts to excessively high levels. The main structural problem with the model is shown to be the iron sub-model.

Nutrient dynamics in the Changjiang and retention effect in the Three Gorges Reservoir

2009 ∼ 2010 was a critical period for the Three Gorges Reservoir (TGR) transition from staged impoundment to full operation. This study reports on the composition of nutrients in the whole Changjiang and retention effect of the reservoir during the transitional period, and the potential impact of the reservoir impoundment on the eutrophication of the TGR and estuary. Water samples were collected in the mainstream and tributaries of Changjiang from August to October 2009 and June 2010 to analyze the concentrations of dissolved and particulate nutrients, and suspended particulate matter (SPM). The Changjiang was enriched with dissolved inorganic nitrogen (DIN), dissolved silicate (DSi) and SPM, but depleted in PO43− and biogenic silica (BSi), with higher DIN:PO43− and DSi:PO43− ratios. DIN and PO43− concentrations were lower in the Jinshajiang, but increased in the Mid-downstream of the Changjiang due to high human activities and nutrients import from major tributaries. DSi concentration varied little along the Changjiang, and was relatively low in the TGR. Particulate inorganic phosphorus (PIP), total particulate phosphorus (TPP) and BSi concentrations were very high in the Jinshajiang, and then dramatically decreased with the SPM in the TGR. The northern tributaries contributed more nutrients to the Changjiang than the southern tributaries. Depending on the nutrients input-output mass balance model, the retention of nutrients in the TGR were higher for TPP and BSi, lower for dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP). Total dissolved nitrogen (TDN), total phosphorus (TP) and DSi retained in the reservoir accounted for 15%, 12% and 1%, respectively of the TDN, TP and DSi fluxes in the Datong station near the estuary. Historical data showed that the impoundment of the TGD greatly contributed to the eutrophication of the reservoir and estuary. Since 2003, algal blooms were likely to occur in the TGR, estuary and adjacent seas, but the outbreak frequency and area of red tides in the East China Sea (ECS) dramatically decreased. Therefore, further study is required to estimate the response of estuarine ecosystem on the dispatching mode of the TGR.

Hypoxia and nutrient dynamics affected by marine aquaculture in a monsoon-regulated tropical coastal lagoon.

The Laoyehai (lagoon) is located at the east coast of Hainan Island in the South China Sea and has been subject to perturbations from human activities, notably marine aquaculture, and has eutrophic surface and hypoxic near-bottom waters. A lack of knowledge of hydrodynamic and biogeochemical processes is a challenge to the sustainable management of lagoon at the ecosystem level in science. Five field campaigns, including three during the southwest monsoon and two in the northeast monsoon periods, were carried out at the Laoyehai in 2008-2011. The aim of this study is to investigate the impacts of dynamic processes of hydrography and human activities on nutrient geochemistry and their relationships to the system eutrophication and hypoxia in the lagoon. In this coastal system, high levels of ammonium relative to nitrate are found, elevated phosphate skews the DIN/DIP relative to the Redfield ratio, and the dissolved silicate concentration is high because of submarine groundwater discharge. The organic fraction in the Laoyehai accounts for a large proportion of the total nutrients associated with the release of wastes from marine aquaculture. The hypoxia of near-bottom waters in the Laoyehai is created and maintained by heterotrophic processes that are fueled by organic matter, which are exacerbated by poor water exchange as a consequence of the geomorphology and weak tidal circulation.

Assessing the Potential of Sponges (Porifera) as Indicators of Ocean Dissolved Si Concentrations

We explore the distribution of sponges along dissolved silica (dSi) concentration gradients to test whether sponge assemblages are related to dSi and to assess the validity of fossil sponges as a palaeoecological tool for inferring dSi concentrations of the past oceans. We extracted sponge records from the publically available Global Biodiversity Information Facility (GBIF) database and linked these records with ocean physiochemical data to evaluate if there is any correspondence between dSi concentrations of the waters sponges inhabit and their distribution. Over 320,000 records of Porifera were available, of which 62,360 met strict quality control criteria. Our analyses was limited to the taxonomic levels of family, order and class. Because dSi concentration is correlated with depth in the modern ocean, we also explored sponge taxa distributions as a function of depth. We observe that while some sponge taxa appear to have dSi preferences (e.g., class Hexactinellida occurs mostly at high dSi), the overall distribution of sponge orders and families along dSi gradients is not sufficiently differentiated to unambiguously relate dSi concentrations to sponge taxa assemblages. We also observe that sponge taxa tend to be similarly distributed along a depth gradient. In other words, both dSi and/or another variable that depth is a surrogate for, may play a role in controlling sponge spatial distribution and the challenge is to distinguish between the two. We conclude that inferences about palaeo-dSi concentrations drawn from the abundance of sponges in the stratigraphic records must be treated cautiously as these animals are adapted to a great range of dSi conditions and likely other underlying variables that are related to depth. Our analysis provides a quantification of the dSi ranges of common sponge taxa, expands on previous knowledge related to their bathymetry preferences and suggest that sponge taxa assemblages are not related to particular dSi conditions. (Less)

Coupling of carbon and silicon geochemical cycles in rivers and lakes.

Carbon (C) and silicon (Si) biogeochemical cycles are important factors in the regulation of atmospheric CO2 concentrations and hence climate change. Theoretically, these elements are linked by chemical weathering and organism stoichiometry, but this coupling has not been investigated in freshwaters. Here we compiled data from global rivers and lakes in the United States of America and the United Kingdom, in order to characterize the stoichiometry between the biogeochemical cycles of C and Si. In rivers this coupling is confirmed by a significant relationship between HCO3−/Na+ and DSi/Na+, and DSi:HCO3− ratio can reflect the mineral source of chemical weathering. In lakes, however, these characteristic ratios of chemical weathering are altered by algal activity. The lacustrine Si:C atomic ratio is negative feedback regulation by phytoplankton, which may result in this ratio in algal assemblages similar to that in water column. And this regulation suggests lacustrine photosynthetic C fixation in this equilibrium state is quantitative and depends on the DSi concentration. These findings provide new insights into the role of freshwaters in global C and Si biogeochemical cycles.

闽江口潮滩湿地不同植被带土壤及间隙水中硅的分布特征

PDF HTML阅读 XML下载 导出引用 引用提醒 闽江口潮滩湿地不同植被带土壤及间隙水中硅的分布特征 DOI: 10.5846/stxb201501140109 作者: 作者单位: 福建师范大学地理研究所,浙江大学环境与资源学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（41401114）；福建省教育厅项目（JA14082） Changes in the distribution of silica in the porewaters and sediments of the intertidal zone with different plant communities in the Min River Estuary Author: Affiliation: Key Laboratory of Humid Sub-tropical Eco-geographical Process of Ministry of Education,Research Centre of Wetlands in Subtropical Region,Fujian Normal University, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:以闽江口潮滩湿地为研究对象，由岸及海方向对芦苇、短叶茳芏、互花米草（潮沟内）和互花米草（潮沟外）4种湿地土壤生物硅和土壤间隙水氮硅营养盐含量及其随深度变化的特征进行为期1a的季度观测。结果显示：互花米草（外）、短叶茳芏、芦苇和互花米草（内）带湿地土壤生物硅的年均含量依次降低，分别为14.33、10.40、9.98、7.50 mg/g；互花米草（外）、互花米草（内）、短叶茳芏和芦苇带湿地土壤间隙水活性硅酸盐年均含量依次降低，分别为407、359、344、323 μmol/L；湿地各植被带土壤及间隙水含硅量均呈现夏秋季节高于冬春季节的趋势。统计分析表明：间隙水活性硅酸盐与土壤生物硅含量、距潮沟的距离之间的正相关性均比较显著（P<0.05），温度对土壤中硅含量的影响也有一定的正相关性，说明湿地植被、温度和潮汐作用是影响闽江口湿地硅分布的重要因素。与土著种对比，互花米草入侵在一定程度上改变了闽江口潮滩湿地土壤硅分布的格局。 Abstract:Silicon (Si) is an essential nutrient for many plants as well as an important element that controls the functions of terrestrial and aquatic ecosystems. Wetland ecosystems are considered to be very important for Si transport and transformation in nutrient biogeochemical cycles. However, information on Si cycling in wetland ecosystems is scarce. The Shanyutan wetland is the largest tidal wetland in the estuary of the Min River, southeast China. In this study, the Si distribution pattern and its influencing factors were determined by seasonally measuring the levels of dissolved silicates (DSis) in the porewater and biogenic silica (BSi) in the sediments of Phragmites australis wetland (PAW), Cyperus malaccensis wetland (CMW), and Spartina alterniflora wetland (SAW) from October 2012 to September 2013. For better comparison, three sampling sites were added in SAW since January 2013; these sites were isolated by a 30m wide tidal creek. The sampling sites near the land were marked as S. alterniflora wetland (inside; SAWI), whereas those near the sea were marked as S. alterniflora wetland (outside; SAWO). The results indicated that the average contents of BSi were 14.33 mg/g in SAWO, 10.40 mg/g in CMW, 9.98 mg/g in PAW, and 7.50 mg/g in SAWI. The average concentrations of DSis in the sediment porewater were 407 μmol/L in SAWO, 359 μmol/L in SAWI, 344 μmol/L in CMW, and 323 μmol/L in PAW. Generally, the contents of BSi in the sediments and DSis in the porewater were higher in summer and autumn, whereas they were lower in spring and winter. Statistical analysis revealed significantly positive correlations between DSi concentration in the porewater and BSi content in the sediments or the distance from the shoreside to the tidal creek (P<0.05). BSi content in the sediments was slightly positively correlated with temperature. Taken together, our findings suggest that the distribution of Si in the wetlands of the Min River estuary was correlated with vegetation, temperature, and tidal action. Unlike that of native species, the invasion of S. alterniflora, to some extent, altered the distribution patterns of Si in the wetlands. 参考文献 相似文献 引证文献