Silica retention in the Three Gorges Reservoir

A mass balance of dissolved silica (DSi) based on daily measurements at the inflow and outflow of the Three Gorges Reservoir (TGR) in 2007 and a more precise budget, with inflow, outflow, primary production, biogenic silica (BSi) settlement, dissolution of BSi in the water column and flux of DSi at the sediment–water interface in the dry season (April) of 2007 were developed. We address the following question: How much does the Three Gorges Dam (TGD) affect silica transport in the TGR of the Changjiang River (Yangtze River)? The DSi varied from 71.1 to 141 μmol/l with an average of 108 μmol/l, and it ranged between 68.1 and 136 μmol/l, with an average of 107 μmol/l in inflow and outflow, respectively, in the TGR in 2007. The linear relationship of DSi between inflow and outflow water is significant (r = 0.87, n = 362, p < 0.01). Along the main stream of the TGR, the DSi concentration decreases with an average concentration of 84.0 μmol/l in the dry season. However, the stratification of DSi was not obvious in the main channel of the TGR in the dry season. The BSi is within the range of 0.04–5.00 μmol/l, with an average concentration of 2.1 μmol/l in the main channel of the TGR, while it is much higher in Xiangxi Bay (1.30–47.7 μmol/l, 13.1 μmol/l) than in the main stream of the TGR and the other bays. After the third filling of the TGR, approximately 3.8% of the DSi was retained by the TGR based on a 12-month monitoring scheme in 2007, which would slightly reduce nutrient fluxes of the Changjiang River to the East China Sea (2%). DSi was lost during January to June and November, whereas the additions of DSi were found during the other months in 2007. The budget results also indicate that there is a slight retention of DSi. The retention of DSi in the reservoir is approximately 2.9%, while BSi is approximately 44%. Compared with the total silica load, the retention of DSi and BSi in the reservoir is only 5.0% in the dry season. With its present storage capacity, the reservoir does not play an important role as a silica sink in the channel of the TGR. The DSi load is significantly related to discharge both in inflow and outflow waters (p < 0.01). DSi retention, to some extent, is the runoff change due to impoundment.     

Changes in amorphous silica sequestration with eutrophication of riverine impoundments

The effect of eutrophication on particulate amorphous silica (ASi) sequestration was isolated and quantified in Lake St. Croix and Lake Pepin, two natural, human-impacted impoundments of the upper Mississippi River. In contrast to impoundments behind engineered dams, where silica (Si) fluxes may be changed by various aspects of dam construction, these two riverine lakes have long (9,000+ years) sedimentary sequences that record the entire span of cultural eutrophication and the resulting silica sequestration. The concentrations of dissolved silicate (DSi) and ASi in the lake inflows were measured for 1 year to obtain the total flux of bioavailable silica (TSi_b = DSi + ASi) to each impoundment. Historical rates of Si sequestration in each lake were determined using ASi burial in multiple sediment cores and modeled estimates of historical TSi_b fluxes. The Si trapping efficiency of each lake was found to have increased exponentially with cultural eutrophication (estimated two- to fivefold increase in Lake St. Croix and 9- to 16-fold increase in Lake Pepin over the last 100 years), indicating the degree to which eutrophication of impoundments can reduce silica export to downstream coastal and marine ecosystems. Because these two lakes presently exhibit different degrees of eutrophication, together they depict a relationship between phosphorus concentration and Si trapping efficiency that may be applied to other impoundments, including human-made reservoirs.     

Nitrate uptake dynamics of surface transient storage in stream channels and fluvial wetlands

River systems are important regulators of anthropogenic nitrogen flux between land and ocean. Nitrogen dynamics in small headwater streams have been extensively measured, whereas less is known about contributions of other components of stream networks to nitrogen removal, including larger streams or fluvial wetlands. Here, we quantified nitrate reaction rates in higher-order stream channels and in surface transient storage (STS) zones (sub-systems with greater water residence time than the main channel) of the Ipswich River watershed, a temperate basin characterized by suburban development. We characterized uptake in STS both within higher-order stream channels and in fluvial wetlands that remain connected to advective fluxes but not constrained within channels. We compare reaction rates in these systems to those previously measured in headwater streams in the same basin. We found that (1) nitrate reaction rates (as uptake velocity, υ_f) in higher-order streams (n = 2) differed from each other but were consistent with previous estimates from headwater streams, (2) nitrate reaction rates in STS zones within higher-order stream channels (n = 2) were higher than rates estimated at the whole-stream scale, (3) ambient nitrate reaction rates in fluvial wetland STS (n = 7) were high but comparable to headwater streams with low nitrate concentration, (4) nitrate reaction rates were higher in fluvial wetland STS compared to headwater stream channels at elevated nitrate concentration, and (5) efficiency loss (EL) similar to that found in headwater streams was also apparent in fluvial wetlands. These results indicate that STS are potential hotspots of biogeochemical activity and should be explicitly integrated into network scale biogeochemical models. Further, experimental evidence of EL in fluvial wetlands suggests that the effectiveness of STS to retain N may decline if N loading increases.     

Lack of steady-state in the global biogeochemical Si cycle: emerging evidence from lake Si sequestration

Weathering of silicate minerals releases dissolved silicate (DSi) to the soil-vegetation system. Accumulation and recycling of this DSi by terrestrial ecosystems creates a pool of reactive Si on the continents that buffers DSi export to the ocean. Human perturbations to the functioning of the buffer have been a recent research focus, yet a common assumption is that the continental Si cycle is at steady-state. However, we have no good idea of the timescales of ecosystem Si pool equilibration with their environments. A review of modelling and geochemical considerations suggests the modern continental Si cycle is in fact characterised in the long-term by an active accumulation of reactive Si, at least partially attributable to lakes and reservoirs. These lentic systems accumulate Si via biological conversion of DSi to biogenic silica (BSi). An analysis of new and published data for nearly 700 systems is presented to assess their contribution to the accumulating continental pool. Surface sediment BSi concentrations (n = 692) vary between zero and >60 % SiO₂ by weight, apparently independently of lake size, location or water chemistry. Using sediment core BSi accumulation rates (n = 109), still no relationships are found with lake or catchment parameters. However, issues associated with single-core accumulation rates should in any case preclude their use in elemental accumulation calculations. Based on lake/reservoir mass-balances (n = 34), our best global-scale estimate of combined lake and reservoir Si retention is 1.53 TMol year^?1, or 21–27 % of river DSi export. Again, no scalable relationships are apparent, suggesting Si retention is a complex process that varies from catchment to catchment. The lake Si sink has implications for estimation of weathering flux generation from river chemistry. The size of the total continental Si pool is poorly constrained, as is its accumulation rate, but lakes clearly contribute substantially. A corollary to this emerging understanding is that the flux and isotopic composition of DSi delivered to the ocean has likely varied over time, partly mediated by a fluctuating continental pool, including in lakes.     

Decreased Silica Land–sea Fluxes through Damming in the Baltic Sea Catchment – Significance of Particle Trapping and Hydrological Alterations

We tested the hypothesis that reservoirs with low water residence time and autochthonous production influence river biogeochemistry in eutrophied river systems draining cultivated watersheds. The effect of a single artificial water reservoir and consecutive reservoirs on silica (Si) river fluxes is exemplified by the moderately dammed Vistula River and the heavily regulated Daugava River that are compared with the practically undammed Oder River. The sum of the discharge weighted annual mean biogenic silica (BSi) and dissolved silicate (DSi) concentrations in the rivers Oder, Vistula and Daugava were about 160 μ M (40 + 120 μ M), 150 μ M (20 + 130 μ M) and 88 μ M (6 + 82 μ M), respectively. Assuming BSi and DSi concentrations as observed in the Oder River as typical for eutrophied but undammed rivers, complete trapping of this BSi could have lowered Si fluxes to the Baltic Sea from rivers with cultivated watersheds by 25%. The superimposed effect of hydrological alterations on reduced Si land–sea fluxes is demonstrated by studies in the boreal/subarctic and oligotrophic rivers Kalixälven and Lueälven. The DSi yield of the heavily dammed Luleälven (793 kg km^?2 yr^?1) constituted only 63% of that was found in the unregulated Kalixälven (1261 kg km^?2 yr^?1), despite the specific runoff of the Luleälven (672 mm m^?2 yr^?1) being 19% higher than that of theKalixälven (563 mm m^?2 yr^?1); runoff normalized DSi yield of the former, regulated watershed, was only half the DSi yield of the latter, unperturbed watershed. Based on these findings, it is hypothesized here that perturbed surface water–groundwater interactions are the major reasons for the reduced annual fluctuations in DSi concentrations as also seen in the heavily dammed and eutrophic river systems such as the Daugava and Danube.     

Dynamics of dissolved and biogenic silica in the freshwater reaches of a macrotidal estuary (The Scheldt, Belgium)

Temporal evolution of dissolved and biogenic silica concentrations along the Scheldt tidal river and in its tributaries was investigated during 1 year in 2003. In the tributaries, dissolved silica (DSi) concentrations remained high and biogenic silica (BSi) concentrations were low throughout the year. In the tidal river during summer, DSi was completely consumed and BSi concentrations increased. Overall, most of the BSi was associated with living diatoms during the productive period in the tidal river. Nevertheless, the detrital BSi was a significant fraction of the total BSi pool, of which less than 10% could be attributed to phytoliths. The tidal river was divided into two zones for budgeting purposes. The highest productivity was observed in the zone that received the highest water discharge, as higher riverine DSi input fluxes induced presumably a less restrictive DSi limitation, but the discharge pattern could not explain all by itself the variations in DSi consumption. Silica uptake and retention in the tidal river were important at the seasonal time-scale: from May to September, 48% of the riverine DSi was consumed and 65% of the produced BSi was deposited, leading to a silica (DSi + BSi) retention in the tidal river of 30%. However, when annual fluxes were considered, DSi uptake in the tidal river amounted to 14% of the DSi inputs and only 6% of the riverine silica (DSi + BSi) was retained in the tidal river.     

Increase in silicon concentrations and release from suspended solids and bottom sediments in Lake Kasumigaura, Japan

Increasing trends of dissolved Si measured by a colorimetric method and ICP (DSi_col and DSi_ICP, respectively) and total Si concentrations were detected at the center of Lake Kasumigaura during 1980–2006 (mean DSi_col concentration in the 1980s and 2000s was 1.3 and 4.0 mg l⁻¹, respectively). The observation of such trends is rare; therefore, the elucidation of the causes could be useful to understanding silicon dynamics in inland waters. Based on statistical analysis, we found that the increases in DSi_col and lithogenic Si accounted for most of the total Si increase (44 and 45%, respectively) and that biogenic Si, consisting of diatom frustules, also increased with them. Increases in DSi_ICP concentration were not detected near the mouth of the inflowing rivers, suggesting that the increase was caused by in-lake processes. Because the increase in suspended solids (SS) caused by sediment resuspension had been observed in the lake for the same period, we assumed that the Si release from SS containing diatom frustules contributed to the increase. The results of the laboratory experiments in which surface sediments were stirred in lake waters showed that the change in DSi_col concentration depended mainly on SS concentration, water temperature and the elapsed time of diatom frustules dissolution. An estimation of the released amount of Si from SS using the sediment resuspension model was (1.0–2.7) × 10⁹ g year⁻¹ in the 2000s, which was about 30–90% of the increase in the DSi_col outflow of 3.0 × 10⁹ g year⁻¹ from the 1980s to the 2000s. We also determined the Si release rates from bottom sediments through laboratory experiments. The Si amount released from bottom sediments in the lake in the 2000s was estimated to be 4.3 × 10⁹ g year⁻¹, which was about 2–4 times higher than the estimated Si amount released from SS. These findings suggest that the sediment resuspension might be the cause of the latest DSi increase.     

Silica fluxes and trapping in two contrasting natural impoundments of the upper Mississippi River

Human activities have altered riverine silica cycling and diminished the supply of silica to the oceans, but few rivers have been intensively monitored to evaluate the magnitude of these changes. In this study we measured dissolved silica (DSi) and amorphous silica (ASi) fluxes into and out of two large, culturally-impacted natural impoundments of the upper Mississippi River, Lakes St. Croix and Pepin, USA. ASi sedimentation rates and sediment–water fluxes of DSi were calculated for each lake, and a mass-balance approach was used to determine in-lake ASi production. ASi from terrestrial phytoliths in the lake sediments was determined to be only partially available to biotic recycling, and in-lake ASi dissolution was small relative to the total silica budgets. The river reaches upstream of the two lakes were found to have abundant DSi, and riverine diatom production was found to contribute significant amounts of ASi to each lake. The average total phosphorus concentration in Lake Pepin is four times that in Lake St. Croix but ASi production in Lake Pepin is only 2.3 times higher than in Lake St. Croix, indicating that diatom growth in Pepin is limited by factors such as turbidity. Lake St. Croix currently traps about 10% of the inflowing total bioavailable silica (TSi_b = DSi + ASi) while Lake Pepin traps closer to 20% of its inflowing TSi_b, clearly demonstrating the importance of silica retention in lakes and reservoirs along the land–ocean continuum.     

Water column dissolved silica concentration limits microphytobenthic primary production in intertidal sediments

Primary production of microphytobenthos (MPB) contributes significantly to the total production in shallow coastal environments. MPB is a diverse community in which diatoms are usually the main microalgal group. Diatoms require N, P, and other nutrients as with other autotrophs, but in addition require silicate to create their outer cell wall. Therefore, dissolved silica (DSi) might be a potential limiting factor for benthic primary production in areas with reduced freshwater input. To test this hypothesis, a microcosm experiment was conducted using intact sediment cores collected from an intertidal mudflat in the Bay of Cádiz and supplied with increasing concentrations of DSi (0, 5, 10, 25, and 45 μmol · L^?1). After 7 d of enrichment, we determined chlorophyll a and c (Chl a, c) contents, metabolic rates (Net [P_n] and Areal Gross [P_g^A] Production and Light [R_L] and Dark [R_D] Respiration), as well as fluxes of inorganic nutrients across the sediment‐water interface. Chl a and c contents increased significantly with respect to the initial conditions but no differences between treatments were found. Both P_n and P_g^A showed a saturating‐like pattern with silicate concentration, reaching maximum rates at a DSi concentration of 45 μmol · L^?1. The addition of DSi also resulted in an increase of DSi and ammonium uptake by the sediment, which was significantly higher in light than in darkness. Our results clearly show that water column DSi concentrations have a direct impact on benthic primary production, also controlling other related processes such as inorganic nutrient fluxes.     

Biogeochemical processes at the sediment–water interface, Bombah Broadwater, Myall Lakes

Myall Lakes has experienced algal blooms in recent years which threaten water quality. Biomarkers, benthic fluxes measured with chambers, and pore water metabolites were used to identify the nature and reactivity of organic matter (OM) in the sediments of Bombah Broadwater (BB), and the processes controlling sediment-nutrient release into the overlying waters. The OM in the sediments was principally from algal sources although terrestrial OM was found near the Myall River. Terrestrial faecal matter was identified in muddy sediments and was probably sourced via runoff from farm lands. The reactive OM which released nutrients into the overlying waters was from diatoms, dinoflagellates and probably cyanobacteria. Microcystis filaments were observed in surface sediments. OM degradation rates varied between 5.3 and 47.1 mmol m^?2 day^?1 (64–565 mg m^?2 day^?1), were highest in the muddy sediments and sulphate reduction rates accounted for 20–40% of the OM degraded. Diatoms, being heavy sink rapidly, and are an important vector to transport catchment N and P to sites of denitrification and P-trapping in the sediments. Denitrification rates (mean ～4 mmol N m^?2 day^?1), up to 7 mmol N m^?2 day^?1 (105 mg N m^?2 day^?1) were measured, and denitrification efficiencies were highest (mean = 86 ± 4%) in the sandy sediments (～20% of the area of BB), but lower in the muddy sediments (mean = 63 ± 15%). These differences probably result from higher OM loads and anaerobic respiration in muddy sediments. Most DIP (>70%) from OM degradation was not released into overlying waters but remained trapped in surface sediments. Biophysical (advective) processes were responsible for the measured metabolite (O₂, CO₂, DSi, DIN and DIP) fluxes across the sediment–water interface.     

Human activities directly alter watershed dissolved silica fluxes

Controls on chemical weathering, such as bedrock geology, runoff, and temperature, are considered to be the primary drivers of Si transport from the continents to the oceans. However, recent work has highlighted terrestrial vegetation as an important control over Si cycling. Here we show that at the regional scale (Southern New England, USA), land use/land cover (LULC) is an important variable controlling the net transport of Si from the land to the sea, accounting for at least 40% of dissolved Si (DSi) fluxes. A multiple linear regression model using average DSi fluxes from 25 rivers (>2,300 observations) shows the percent forest cover, as well as development and agricultural land use, to be significant (p < 0.05) drivers of DSi flux. This was true regardless of watershed size and lithology. Furthermore, forest cover is significantly negatively correlated, while development is significantly positively correlated, with Si concentrations and fluxes. We hypothesize that these relationships are due to several mechanisms, specifically the ability of terrestrial vegetation to store large amounts of Si within its biomass, the altered watershed hydrology that accompanies LULC change, and the capability of urban regions to serve as sources of Si to aquatic systems. Thus, we conclude that anthropogenic activities may be directly perturbing the global Si cycle through land use change and we offer a conceptual model which highlights a new approach to understanding the non-geochemical controls on Si fluxes.     

Seasonal and spatial variations of methane emissions from coastal marshes in the northern Yellow River estuary, China

Aims and methods

To evaluate the seasonal and spatial variations of methane (CH₄) emissions and understand the controlling factors, we measured CH₄ fluxes and their environmental variables for the first time by a static chamber technique in high Suaeda salsa marsh (HSM), middle S. salsa marsh (MSM), low S. salsa marsh (LSM) and bare flat (BF) in the northern Yellow River estuary throughout a year.

Results

CH₄ emissions from coastal marsh varied throughout different times of the day and significant differences were observed in some sampling periods (p?<?0.05). Over all sampling periods, CH₄ fluxes averaged between ?0.392 mgCH₄ m^?2?h^?1 and 0.495 mgCH₄ m^?2?h^?1, and emissions occurred during spring (0.008 mgCH₄ m^?2?h^?1) and autumn (0.068 mgCH₄ m^?2?h^?1) while sinks were observed during summer (?0.110 mgCH₄ m^?2?h^?1) and winter (?0.009 mgCH₄ m^?2?h^?1). CH₄ fluxes from the four marshes were not significantly different (p?>?0.05), and emissions occurred in LSM (0.026 mgCH₄ m^?2?h^?1) and BF (0.055 mgCH₄ m^?2?h^?1) while sinks were observed in HSM (?0.035 mgCH₄ m^?2?h^?1) and MSM (?0.022 mgCH₄ m^?2?h^?1). The annual average CH₄ flux from the intertidal zone was 0.002 mgCH₄ m^?2?h^?1, indicating that coastal marsh acted as a weak CH₄ source. Temporal variations of CH₄ emission were related to the interactions of abiotic factors (temperatures, soil moisture and salinity) and the variations of limited C and mineral N in sediments, while spatial variations were mainly affected by the vegetation composition at spatial scale.

Conclusions

This study observed a large spatial variation of CH₄ fluxes across the coastal marsh of the Yellow River estuary (CV?=?7856.25 %), suggesting that the need to increase the spatial replicates at fine scales before the regional CH₄ budget was evaluated precisely. With increasing exogenous nitrogen loading to the Yellow River estuary, the magnitude of CH₄ emission might be enhanced, which should also be paid more attentions as the annual CH₄ inventory was assessed accurately.     

Tree species impact the terrestrial cycle of silicon through various uptakes

The quantification of silicon (Si) uptake by tree species is a mandatory step to study the role of forest vegetations in the global cycle of Si. Forest tree species can impact the hydrological output of dissolved Si (DSi) through root induced weathering of silicates but also through Si uptake and restitution via litterfall. Here, monospecific stands of Douglas fir, Norway spruce, Black pine, European beech and oak established in identical soil and climate conditions were used to quantify Si uptake, immobilization and restitution. We measured the Si contents in various compartments of the soil–tree system and we further studied the impact of the recycling of Si by forest trees on the DSi pool. Si is mainly accumulated in leaves and needles in comparison with other tree compartments (branches, stembark and stemwood). The immobilization of Si in tree biomass represents less than 15% of the total Si uptake. Annual Si uptake by oak and European beech stands is 18.5 and 23.3 kg ha^?1 year^?1, respectively. Black pine has a very low annual Si uptake (2.3 kg ha^?1 year^?1) in comparison with Douglas fir (30.6 kg ha^?1 year^?1) and Norway spruce (43.5 kg ha^?1 year^?1). The recycling of Si by forest trees plays a major role in the continental Si cycle since tree species greatly influence the uptake and restitution of Si. Moreover, we remark that the annual tree uptake is negatively correlated with the annual DSi output at 60 cm depth. The land–ocean fluxes of DSi are certainly influenced by geochemical processes such as weathering of primary minerals and formation of secondary minerals but also by biological processes such as root uptake.     

Seasonal variations of methane fluxes from an unvegetated tidal freshwater mudflat (Hammersmith Creek, GA)

Wetlands are estimated to contribute nearly 40 % of global annual methane (CH₄) emissions to the atmosphere. However, because CH₄ fluxes from these systems vary spatially, seasonally, and by wetland type, there is a large uncertainty associated with scaling up the CH₄ flux from these environments. We monitored seasonal patterns of CH₄ cycling from tidal mudflat wetland sediments adjacent to a vegetated freshwater wetland in coastal Georgia between 2008 and 2009. CH₄ emissions were significantly correlated with CH₄ production and sediment saturation state with respect to CH₄ but not with temperature. CH₄ cycling displayed distinct seasonal patterns. Winter months were characterized by low CH₄ production and emissions. During the spring, summer and fall, CH₄ fluxes exceeded CH₄ production in the top 40 cm. Comparison of CH₄ sources and sinks in conjunction with the interpretation of CH₄ concentration profiles using a 1D reactive transport model indicated that CH₄ delivered via lateral tidal pumping likely provided additional CH₄ to the upper sediment column. Seasonally high CH₄ ebullition rates reflected increased CH₄ production and decreased CH₄ solubility. The annual CH₄ flux was estimated to be on the order of 10 mol CH₄ m^?2 y^?1 which is 2–4 times the global average for wetland CH₄ emissions. Thus, even though tidal freshwater mudflats are of limited spatial extent, these environments may serve as globally significant sources of CH₄ to the atmosphere. This study highlights the importance of these dynamic environments to the global CH₄ cycle and their relevance to climate change.     

Sediment-water fluxes of dissolved inorganic carbon, O2, nutrients, and N2 from the hypoxic region of the Louisiana continental shelf

Globally, hypoxic areas (<63?mmol O₂ m^?3) in coastal waters are increasing in number and spatial extent. One of the largest coastal hypoxic regions has been observed during the summer in the bottom-water of the Louisiana continental shelf. The shelf receives the sediments, organic matter, and nutrients exported from the Mississippi River watershed, and much of this material is ultimately deposited to the sea floor. Hence, quantifying the rates of sediment-water dissolved inorganic carbon (DIC), oxygen (O₂), and nutrient fluxes is important for understanding how these processes relate to the development and maintenance of hypoxia. In this study, the sediment-water fluxes of DIC, O₂, nutrients, and N₂ (denitrification) were measured on the Louisiana shelf during six cruises from 2005 to 2007. On each cruise, three to four sites were occupied in or directly adjacent to the region of the shelf that experiences hypoxia. DIC fluxes, a proxy for total sediment respiration, ranged from 7.9 to 21.4?mmol?m^?2 day^?1 but did not vary significantly either spatially or as a function of bottom-water O₂ concentration. Overall, sediment respiration and nutrient flux rates were small in comparison to water-column respiration and phytoplankton nutrient demand. Nitrate fluxes were correlated with bottom-water O₂ concentrations (r?=?0.69), and there was evidence that decreasing O₂ concentrations inhibited coupled nitrification-denitrification. Denitrification rates averaged 1.4?mmol?N?m^?2 day^?1. Scaled to the area of the shelf, the denitrification sink represented approximately 39% of the N load from the Mississippi River watershed. The sediment-water fluxes reported from this study add substantial information on the spatial and temporal patterns in carbon, O₂, and nutrient cycling available for the Louisiana continental shelf and, thus, improve the understanding of this system.     

Porewater nitrate profiles in sandy sediments hosting submarine groundwater discharge described by an advection�Cdispersion-reaction model

In order to separate the effects of reaction from those of transport on vertical porewater concentration profiles of nitrate at an intertidal groundwater seepage site (Ria Formosa, Portugal), a free-boundary solution of an Advection?CDispersion-Reaction (ADR) model was used to describe the shape of NO₃ ^? concentration profiles collected in situ. The model includes three sequential reaction layers, postulated with basis on the local distribution of the benthic organic C:N ratio and major identifiable changes in concentration gradients with depth. The advective nature of the system was used to propose a mass balance simplification to the constitutive equations permitting a free-boundary solution, which in turn allowed prediction of sediment?Cwater fluxes of NO₃ ^?. Sensitivity analysis confirmed that in similarly advective-dominated environments, both the porewater concentration distribution and the interfacial fluxes are strongly dependant on seepage rate and benthic reactivity. The model fitted the measured profiles with high correlation (usually higher than 90%), and model-derived sediment?Cwater NO₃ ^? fluxes were in good agreement to fluxes measured in situ with Lee-type seepage meters (0.9948 slope, R² = 0.8672, n = 8). Nitrate oxidation and reduction rates extracted from model fits to the data (10^?2?C10⁰ mmol m^?2 h^?1) agreed with literature values. Because dispersive effects are not included in direct mass balances based on the porewater concentrations, the model presented here increases the accuracy of apparent reaction rate estimates and geochemical zonation at Submarine Groundwater Discharge (SGD) sites. The results establish the importance of sandy sediments as reactive interfaces, able to modulate mass transfer of continental-derived contaminants into coastal ecosystems. We suggest that tools such as the one described here might be used to advantage in preparing further experimental studies to elucidate how benthic reactivity affects nitrate distribution and fluxes in sediments affected by SGD.     

Proving of a Mathematical Model of Cell Calculation Based on Apparent Diffusion Coefficient

OBJECTIVES: Recently, Atuegwu et al. proposed a mathematical model based on ADC_mean and ADC_min to calculation of cellularity. Our purpose was to compare the calculated cellularity according to the formula with the estimated cell count by histopathology in different tumors. METHODS: For this study, we re-analyzed our previous data regarding associations between ADC parameters and histopathological findings. Overall, 134 patients with different tumors were acquired for the analysis. For all tumors, the number of tumor cells was calculated according to Atuegwu et al. 2013. We performed a correlation analysis between the calculated and estimated cellularity. Thereby, Pearson's correlation coefficient was used and P < .05 was taken to indicate statistical significance in all instances. RESULTS: The estimated and calculated cellularity correlated well together in HNSCC (r = 0.701, P = .016) and lymphomas (r = 0.661, P = .001), and moderately in rectal cancer (r = 0.510, P = .036). There were no statistically significant correlations between the estimated and calculated cellularity in uterine cervical cancer, meningiomas, and in thyroid cancer. CONCLUSION: The proposed formula for cellularity calculation does not apply for all tumors. It may be used for HNSCC, cerebral lymphomas and rectal cancer, but not for uterine cervical cancer, meningioma, and thyroid cancer. Furthermore, its usefulness should be proved for other tumors.     

High precipitation causes large fluxes of dissolved organic carbon and nitrogen in a subtropical montane <Emphasis Type="Italic">Chamaecyparis</Emphasis> forest in Taiwan

Fluxes of dissolved organic carbon (DOC) and nitrogen (DON) may play an important role for losses of C and N from the soils of forest ecosystems, especially under conditions of high precipitation. We studied DOC and DON fluxes and concentrations in relation to precipitation intensity in a subtropical montane Chamaecyparis obtusa var. formosana forest in Taiwan. Our objective was, to quantify DOC and DON fluxes and to understand the role of high precipitation for DOC and DON export in this ecosystem. From 2005 to 2008 we sampled bulk precipitation, throughfall, forest floor percolates and seepage (60 cm) and analyzed DOC, DON and mineral N concentrations. Average DOC fluxes in the soil were extremely high (962 and 478 kg C ha^?1 year^?1 in forest floor percolates and seepage, respectively) while DON fluxes were similar to other (sub)tropical ecosystems (16 and 8 kg N ha^?1 year^?1, respectively). Total N fluxes in the soil were dominated by DON. Dissolved organic C and N concentrations in forest floor percolates were independent of the water flux. No dilution effect was visible. Instead, the pool size of potentially soluble DOC and DON was variable as indicated by different DOC and DON concentrations in forest floor percolates at similar precipitation amounts. Therefore, we hypothesized, that these pools are not likely to be depleted in the long term. The relationship between water fluxes in bulk precipitation and DOC and DON fluxes in forest floor percolates was positive (DOC r = 0.908, DON r = 0.842, respectively, Spearman rank correlation). We concluded, that precipitation is an important driver for DOC and DON losses from this subtropical montane forest and that these DOC losses play an important role in the soil C cycle of this ecosystem. Moreover, we found that the linear relationship between bulk precipitation and DOC and DON fluxes in forest floor percolates of temperate ecosystems does not hold when incorporating additional data on these fluxes from (subtropical) ecosystems.     

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO₂ supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH₄) flux, direct CO₂ and CH₄ fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO₂ concentrations ranged from 6,491 to 14,976 ??atm and directly-measured stream CO₂ outgassing flux was 5,994 ± 677 g C m^?2 y^?1 of stream surface. Stream pCH₄ concentrations ranged from 291 to 438 ??atm and measured stream CH₄ outgassing flux was 987 ± 221 g C m^?2 y^?1. Despite high flux rates from the stream surface, the small area of stream itself (970 m², or 0.007% of watershed area) led to small directly-measured annual fluxes of CO₂ (0.44 ± 0.05 g C m² y^?1) and CH₄ (0.07 ± 0.02 g C m² y^?1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m^?2 y^?1), DOC (0.16 ± 0.03 g C m^?2 y^?1) and coarse plus fine particulate C (0.001 ± 0.001 g C m^?2 y^?1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m^?2 y^?1 as CO₂ outgassing, 11.29 g C m^?2 y^?1 as fluvial DIC and 0.64 g C m^?2 y^?1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40?C50 g C m^?2 y^?1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m^?2 y^?1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m^?2 (±1.5 Mg C ha^?1) that is exchanged annually across Amazon tropical forest canopies.