Manganese Fluxes Research Articles

Winter entrainment drives the mixed layer supply of manganese in the Southern Ocean

AbstractDespite the subnanomolar dissolved manganese concentrations that can co‐limit Southern Ocean primary production, their physical supply mechanisms during winter, for biological consumption in spring and summer have not yet been explored. During austral winter and spring 2019, two cruises were conducted in the Atlantic sector of the Southern Ocean, to determine the distribution and surface water supply mechanisms of dissolved manganese in the upper water column. The supply mechanisms were used to calculate the total flux of dissolved manganese to productive surface waters and were compared to biological consumption estimates. Mean dissolved manganese concentrations in the upper water column (< 500 m) during winter and spring were comparably low (≤ 0.34 nmol kg−1; p > 0.05), with seasonal mixed layer reservoir sizes averaging 65.21 ± 12.93 and 21.64 ± 19.32 μmol m−2, respectively. Winter entrainment contributed 89.33–99.99% (average 97.26% ± 5.28%) of the total dissolved manganese flux, while diapycnal diffusion contributed 0.52–10.58% (average 4.92% ± 5.14%), was identified as the dominant mechanisms for transporting dissolved manganese into the mixed layer in the subantarctic zone, polar frontal zone, and antarctic zone. Here, the winter physical supply rates were higher than the estimated consumption rates during spring, meeting phytoplankton biological demands. Whereas in the subtropical zone, the supply rates were lower than the consumption rates, indicating the presence of additional supply mechanisms such as coastal upwelling, which may help to meet the biological demands in this region.

Metal ion fluxes controlling amphibian fertilization.

Mammalian oocytes undergo major changes in zinc content and localization in order to be fertilized, the most striking being the rapid exocytosis of over ten billion zinc ions, known as zinc sparks. Here we report that fertilization of amphibian Xenopus laevis eggs also initiates a zinc spark that progresses across the cell surface in coordination with dynamic calcium waves. This zinc exocytosis is accompanied by a newly recognized loss of intracellular manganese: synchrotron-based X-ray fluorescence and analytical electron microscopy reveal that zinc and manganese are sequestered in a system of cortical granules that are abundant at the animal pole. Through Electron-Nuclear Double-Resonance (ENDOR) studies, we rule out Mn2+ complexation with phosphate or nitrogenous ligands in intact eggs but the data are consistent with a carboxylate coordination environment. Our observations suggest that zinc and manganese fluxes are a conserved feature of fertilization in vertebrates and that they function as part of a physiological block to polyspermy.

Sedimentary cycling and benthic fluxes of manganese, cobalt, nickel, copper, zinc and cadmium in the Peruvian oxygen minimum zone

Marine sediments are an important source and sink of bio-essential trace metals to the ocean. However, the different mechanisms leading to trace metal release or burial are not fully understood and the associated fluxes are not well quantified. Here, we present sediment, pore water, sequential extraction and benthic flux data of Mn, Co, Ni, Cu, Zn and Cd along a latitudinal depth transect across the Peruvian oxygen minimum zone at 12°S. Sediments are depleted in Mn and Co compared to the lithogenic background. Diffusive Mn fluxes from the sediments into the bottom water (−26 to −550 μmol m−2 y−1) are largely consistent with the rate of Mn loss from the solid phase (−100 to −1160 μmol m−2 yr−1) suggesting that 50% or more of the sedimentary Mn depletion is attributed to benthic efflux. In contrast, benthic Co fluxes (~ −3 μmol m−2 yr−1) are lower than the rate of Co loss from the solid phase (up to −120 μmol m−2 yr−1), implying Co dissolution in the water column. The trace metals Ni, Cu, Zn and Cd are enriched within the sediments with respect to the lithogenic background. Uptake of Ni by phytoplankton in the photic zone and delivery with organic matter to the sediment surface can account for up to 100% of the excess Ni accumulation (87 to 180 μmol m−2 y−1) in shelf sediments near the coast, whereas at greater water depth additional scavenging by Mn- and Fe-oxides may contribute to Ni accumulation. Up to 20% of excess Cu (33 to 590 μmol m−2 y−1) and generally less than 20% of excess Zn (58 to 2170 μmol m−2 y−1) and Cd (6 to 260 μmol m−2 y−1) can be explained by delivery with fresh organic matter. Sequential extraction data suggest that the discrepancies between the known sources of Cd (and Cu) and their excess accumulation may be driven by the delivery of allochthonous sulphide minerals precipitated from the water column. Additionally, Cu may be scavenged by downward sinking organic material. In contrast, precipitation of Zn sulphide chiefly takes place in the sediment. Diffusive Zn fluxes into the sediment (21 to 1990 μmol m−2 y−1) match the excess Zn accumulation suggesting that Zn delivery is mediated by molecular diffusion from bottom waters. Considering the diverse behavioural pattern of trace metals observed in this study, we argue that declining oxygen and increasing hydrogen sulphide concentrations in a future ocean will modify trace metal fluxes at the seafloor and the trace metal stoichiometry of seawater.

Impact of Phytophthora agathidicida infection on canopy and forest floor plant nutrient concentrations and fluxes in a kauri-dominated forest.

Kauri dieback, caused by Phytophthora agathidicida, is a biotic disturbance that poses a recent threat to the survival of kauri (Agathis australis) forests in New Zealand. Previous studies have shown that throughfall and stemflow play an important role in the kauri forests’ internal nutrient cycle. However, the effects of P. agathidicida infection on canopy and forest floor nutrient fluxes in kauri forests remain unknown. Here, we measured throughfall, stemflow and forest floor water yield, nutrient (potassium, calcium, magnesium, manganese, silicon, sulfur, sodium, iron) concentrations and fluxes of ten kauri trees differing in soil P. agathidicida DNA concentration, and health status. We did not observe an effect of soil P. agathidicida DNA concentration on throughfall, stemflow, and forest floor water yield. Throughfall and forest floor nutrient concentrations and fluxes decreased (up to 50%) with increasing soil P. agathidicida DNA concentration. We found significant effects on potassium and manganese fluxes in throughfall; calcium and silicon fluxes in forest floor leachate. A decline in canopy and forest floor nutrient fluxes may result in soil nutrient imbalances, which in turn may negatively impact forest productivity and may increase the susceptibility of trees to future pathogen infection in these ecologically unique kauri forests. Given our findings and the increasing spread of Phytophthora species worldwide, research on the underlying physiological mechanisms linking dieback and plant–soil nutrient fluxes is critical.

Seasonal and Inter Annual Variations between Concentrations and Fluxes of Dissolved and Particulate Organic Carbon, Iron and Manganese from the Northern Dvina River to the White Sea

The paper presents new data on concentrations and fluxes of organic carbon, iron, and manganese in waters of the mouth area of the Northern Dvina River. The work was conducted within the project “Northern Dvina River Marginal Filter Observatory” from May 2015 to May 2019 with monthly collection of water samples. Systematic determinations of the concentrations of the main carrier elements of many trace elements in dissolved and particulate forms were used to establish the average weighted of Corg, Fe, and Mn concentrations. The results of 4 years of systematic investigations established the main regularities of seasonal variations of dissolved and particulate forms of element concentrations and the relationships between. Significant variations were observed not only between concentrations but also between fluxes of elements in both forms. However, more than half the annual fluxes of all elements in all years studied was transported to the sea in the spring flood period. The results also demonstrated significant variations between element concentrations and their fluxes in different years. This is unsurprising, since even during the 4 years of investigations, the difference between water runoff and SPM fluxes in different years was up to, respectively, 1.4 and 2.6 times.

Iron and manganese fluxes across the sediment-water interface in a drinking water reservoir

The development of low dissolved oxygen (DO) concentrations in the hypolimnion of drinking water reservoirs during thermal stratification can lead to the reduction of oxidized, insoluble iron (Fe) and manganese (Mn) in sediments to soluble forms, which are then released into the water column. As metals degrade drinking water quality, robust measurements of metal fluxes under changing oxygen conditions are critical for optimizing water treatment. In this study, we conducted benthic flux chamber experiments in summer 2018 to directly quantify Fe and Mn fluxes at the sediment-water interface under different DO and redox conditions of a eutrophic drinking water reservoir with an oxygenation system (Falling Creek Reservoir, Vinton, VA, USA). Throughout the experiments, we monitored DO, oxidation-reduction potential (ORP), water temperature, and pH in the chambers and compared the metal fluxes in the chambers with time-series of fluxes calculated using a hypolimnetic mass balance method. Our results showed that metal fluxes were highly variable during the monitoring period and were sensitive to redox conditions in the water column at the sediment-water interface. The time-series changes in fluxes and relationship to redox conditions are suggestive of “hot moments”, short time periods of intense biogeochemical cycling. Although the metal concentrations and fluxes are specific to this site, the approaches for examining relationships between metals, oxygen concentrations and overall redox conditions can be applied by water utilities to improve water quality management of Fe and Mn.

The solute carriers ZIP8 and ZIP14 regulate manganese accumulation in brain microvascular endothelial cells and control brain manganese levels

Manganese supports numerous neuronal functions but in excess is neurotoxic. Consequently, regulation of manganese flux at the blood-brain barrier (BBB) is critical to brain homeostasis. However, the molecular pathways supporting the transcellular trafficking of divalent manganese ions within the microvascular capillary endothelial cells (BMVECs) that constitute the BBB have not been examined. In this study, we have determined that ZIP8 and ZIP14 (Zrt- and Irt-like proteins 8 and 14) support Mn2+ uptake by BMVECs and that neither DMT1 nor an endocytosis-dependent pathway play any significant role in Mn2+ uptake. Specifically, siRNA-mediated knockdown of ZIP8 and ZIP14 coincided with a decrease in manganese uptake, and kinetic analyses revealed that manganese uptake depends on pH and bicarbonate and is up-regulated by lipopolysaccharide, all biochemical markers of ZIP8 or ZIP14 activity. Mn2+ uptake also was associated with cell-surface membrane presentation of ZIP8 and ZIP14, as indicated by membrane protein biotinylation. Importantly, surface ZIP8 and ZIP14 biotinylation and Mn2+-uptake experiments together revealed that these transporters support manganese uptake at both the apical, blood and basal, brain sides of BMVECs. This indicated that in the BMVECs of the BBB, these two transporters support a bidirectional Mn2+ flux. We conclude that BMVECs play a critical role in controlling manganese homeostasis in the brain.

Dissolved organic matter and trace element variability in a blackwater-fed bay following precipitation

Dissolved organic matter (DOM) often forms complexes with trace metals. The main objective of this study was to identify the sources of trace elements to a coastal bay that is fed by two blackwater rivers using DOM compositions. Surface water samples from twelve sites in Weeks Bay, Alabama were collected during four field trips and a bottom sediment sample was collected during one of the trips. Spectroscopic measurements in tandem with parallel factor analysis and multivariate statistics were used to derive DOM compositions and inductively coupled plasma mass spectrometry was used for determining trace metal concentrations. DOM chemistry and trace element concentrations together with precipitation and discharge, watershed land use and land cover data, and physicochemical parameters were used to determine the source of trace elements in the adjoining areas of the watershed to the bay and finally settling into the bay sediments. Arsenic, copper, and uranium fluxes increased while fluxes of mercury, zinc, manganese, and iron decreased following precipitation events. Concentration of all trace elements analyzed were 10,000 times higer in the bay botton sediments than their concentrations in the water colunmn. Microbially reprocessed and soil derived DOM components increased along with the trace elements and nutrients in the bay as a result of precipitation whereas concentration of terrestrial humic-like DOM component remained the same during both dry (spring) and wet (summer and fall) periods. Principal component analysis revealed significant correlations between microbially reprocessed DOM and the trace elements including arsenic, copper and uranium indicating urban, pasture, and agricultural areas in the watershed were the sources for these trace metals. On the other hand, multiple sources were identified for manganese, zinc, and iron. Overall it was found that DOM compositions are useful for inferring the possible sources and mobility of trace elements in aquatic systems.

Genesis of manganese ore occurrences of the Olkhon Terrane


 Geological and mineralogical data are reported on the manganese occurrences of the Olkhon terrane (Western Baikal region), which are localized in metadolerites of the Ustkrestovsky Complex, high-temperature mafic hornfels, granites, calcitic marbles and calciphyres, and occasionally are developed as separate veins in gneiss granites or small lenses in quartzites. Most of them are made up of high-temperature mineral assemblages (Opx + Cpx + Pl + Ilm ± Grt± Bt ± Amp), the main manganese carriers in which are ferrorhodonite (33–36 wt % MnO), orthopyroxene (6–12 wt % MnO), and ilmenite (3–16 wt % MnO). Obtained data are in conflict with traditional concepts that these rocks are gondites (manganese-rich metamorphosed sediments) or that manganese flux in carbonate sediments was related to the volcanic activity that occurred simultaneously with sedimentation at about 500 Ma. The diversity of manganese occurrences was produced by metasomatic processes that occurred almost simultaneously with regional metamorphism and emplacement of subalkaline mafic bodies during collisional tectonogenesis (about 470 Ma).

Genesis of Manganese Ore Occurrences of the Olkhon Terrane the Western Baikal Region

Geological and mineralogical data are reported on the manganese occurrences of the Olkhon terrane (Western Baikal region), which are localized in metadolerites of the Ustkrestovsky Complex, high-temperature mafic hornfels, granites, calcitic marbles and calciphyres, and occasionally are developed as separate veins in gneiss granites or small lenses in quartzites. Most of them are made up of high-temperature mineral assemblages (Opx + Cpx + Pl + Ilm ± Grt ± Bt ± Amp), the main manganese carriers in which are ferrorhodonite (33–36 wt % MnO), orthopyroxene (6–12 wt % MnO), and ilmenite (3–16 wt % MnO). Obtained data are in conflict with traditional concepts that these rocks are gondites (manganese-rich metamorphosed sediments) or that manganese flux in carbonate sediments was related to the volcanic activity that occurred simultaneously with sedimentation at about 500 Ma. The diversity of manganese occurrences was produced by metasomatic processes that occurred almost simultaneously with regional metamorphism and emplacement of subalkaline mafic bodies during collisional tectonogenesis (about 470 Ma).

Growth of the metastable zinc-blende MnTe films on highly dissimilar perovskite SrTiO3 (001) substrates by molecular beam epitaxy

The systematic growth dependent on the growth temperature and telluride and manganese flux ratio of MnTe films on perovskite SrTiO3 (001) is addressed in this article. Metastable zinc-blende (ZB) MnTe single crystal films with high-crystalline quality and atomically flat surfaces are epitaxially deposited abiding by a two-dimensional growth regime. Epitaxial relationships are summarized as: (111)ZB-MnTe || (002)STO and [110]ZB-MnTe || [010]STO, or equivalent directions. Finally, compliant epitaxial mechanism, i.e. the formation of an ultrathin compliment substrate revealed by the evolution of inter-atomic distances deduced from electron diffraction, X-ray diffraction and electron microscopy, is proposed to account for the growth of the ZB-MnTe films on SrTiO3(001).

On Interdiffusion in FeNiCoCrMn High Entropy Alloy

Interdiffusion was studied in FCC FeNiCoCrMn high entropy alloy (HEA) system with the help of two quinary diffusion couples annealed at 1000 °C for 100 hours. The terminal alloys of the two couples were selected based upon the knowledge of binary thermodynamic interactions so as to have enhancement or reduction of interdiffusion of particular components. Interdiffusion fluxes of nickel and manganese, which have highest negative binary enthalpy of mixing, were observed to be enhanced up the gradients of each other and reduced down the gradients of each other. Regions of uphill interdiffusion observed for chromium and iron and presence of a zero flux plane observed for iron in one of the diffusion couples indicate the existence of strong diffusional interactions in this HEA. Quinary interdiffusion coefficients were also calculated at various compositions of the FeNiCoCrMn system based upon Manning’s model, utilizing the knowledge of tracer diffusivities of constituent elements and thermodynamic factors. The calculated cross interdiffusion coefficients were shown to be consistent with the diffusional interactions observed in the two diffusion couples. Nickel and Manganese, which are slowest and fastest diffusing species in the FeNiCoCrMn HEA and, which also possess highly negative binary enthalpy of mixing were observed to play particularly significant role in determining the diffusional interactions in this HEA system. Validity of the interdiffusion coefficients evaluated by Manning’s approach was established by regenerating the concentration profiles of the experimental diffusion couples based on transfer matrix method (TMM).

Exchange of manganese compounds between bottom sediments and water: 2. Manganese flux from bed into water (a brief review of studies)

The formation mechanisms of dissolved Mn flux from bottom sediments into water are considered. Data that characterize the processes of Mn compound transformations and factors that govern those processes are given. Data on the magnitudes of Mn fluxes from bed into water and processes that govern them are generalized.

Kinetics of manganese adsorption, desorption, and oxidation in coastal marine sediments

Ejection of excavated manganese (Mn)‐laden particles from anoxic subsurface sediment onto an oxic sediment‐water interface by infauna and by other physical disturbances triggers desorption and oxidation of reduced manganese species. These competing reactions determine whether adsorbed Mn(II) will be desorbed and escape into the water column or be oxidized to Mn(IV) and retained as an insoluble component of the sediment. Consequently, the net flux of Mn(II) to the water column depends on the relative rates of the two reactions. We determined the rates of Mn(II) adsorption, desorption, and oxidation by incubating slurries of natural anoxic sediment at 25°C and monitoring the concentration of manganese in the aqueous phase of the slurry. The kinetics of adsorption, desorption, and oxidation were fitted to first‐order rate laws. The first‐order rate constants for adsorption ( = 7.0 ± 3.4 h−1) and desorption ( = 12.7 ± 3.3 h−1) are more than two orders of magnitude greater than the first‐order rate constant for oxidation ( = 0.020 ± 0.006 h−1). The combination of rapid desorption kinetics and slow oxidation kinetics allows dissolved manganese to be dispersed into the bottom water by turbulent diffusion and escape local reoxidation and retention at the sediment‐water interface. Thus, advection of anoxic sediment to the sediment‐water interface contributes to the overall flux of manganese from the sediment to the water column.

Benthic manganese fluxes along the Oregon–California continental shelf and slope

Here we examine the factors that influence the manganese (Mn) benthic flux from eastern North Pacific marine sediments, with a primary emphasis on continental shelf locations off Oregon and California and studies that involve the use of in situ benthic chambers. Typical shelf-to-shallow margin (<∼350m) sites have benthic Mn efflux rates that average ∼8±5μmolm−2d−1. In contrast, for the Eel River continental shelf region the benthic Mn efflux can be an order of magnitude higher than other shelf settings with benthic effluxes exceeding ∼50μmolm−2d−1. Based on prior work and some new results, continental margin and slope sites (350–∼4000m) have benthic Mn efflux rates that average ∼1±1μmolm−2d−1. The combination of the benthic flux and Mn solid-phase data, indicate that for the continental shelf off the Umpqua and Eel Rivers, approximately 15±10% of the total Mn that is delivered to the seafloor is remobilized. The compiled data set shows that the benthic Mn efflux co-varies with the organic carbon oxidation rate with a Mn to organic carbon oxidation (Cox) ratio of ∼0.8mmol Mn mol−1. Although this ratio can be as high as ∼5 for some Eel River sites, the generally close correspondence between Mn and organic carbon implies that the organic carbon oxidation rate exerts some primary control over the rate of the Mn efflux. The amount of organic carbon oxidized by Mn-oxides, however, represents a small fraction (i.e., generally <1%) of the total organic carbon oxidized in these seafloor sediments.

Update on a Pharmacokinetic-Centric Alternative Tier II Program for MMT-Part II: Physiologically Based Pharmacokinetic Modeling and Manganese Risk Assessment.

Recently, a variety of physiologically based pharmacokinetic (PBPK) models have been developed for the essential element manganese. This paper reviews the development of PBPK models (e.g., adult, pregnant, lactating, and neonatal rats, nonhuman primates, and adult, pregnant, lactating, and neonatal humans) and relevant risk assessment applications. Each PBPK model incorporates critical features including dose-dependent saturable tissue capacities and asymmetrical diffusional flux of manganese into brain and other tissues. Varied influx and efflux diffusion rate and binding constants for different brain regions account for the differential increases in regional brain manganese concentrations observed experimentally. We also present novel PBPK simulations to predict manganese tissue concentrations in fetal, neonatal, pregnant, or aged individuals, as well as individuals with liver disease or chronic manganese inhalation. The results of these simulations could help guide risk assessors in the application of uncertainty factors as they establish exposure guidelines for the general public or workers.

Radium-based pore water fluxes of silica, alkalinity, manganese, DOC, and uranium: A decade of studies in the German Wadden Sea

A decade of studies of metal and nutrient inputs to the back-barrier area of Spiekeroog Island, NW German Wadden Sea, have concluded that pore water discharge provides a significant source of the enrichments of many components measured in the tidal channels during low tide. In this paper we add studies of radium isotopes to help quantify fluxes into and out of this system. Activities of radium isotopes in surface water from tidal channels in the back-barrier area exhibit pronounced changes in concert with the tide, with highest activities occurring near low tide. Other dissolved components: silica, total alkalinity (TA), manganese, and dissolved organic carbon (DOC) exhibit similar changes, with patterns matching the Ra isotopes. Uranium follows a reverse pattern with highest concentrations at high tide. Here we use radium isotope measurements in water column and pore water samples to estimate the fluxes of pore waters that enter the tidal channels during low tide. Using a flushing time of 4 days and the average activities of 224Ra, 223Ra, and 228Ra measured in the back-barrier surface and pore waters, we construct a balance of these isotopes, which is sustained by a deep pore water flux of (2–4)×108L per tidal cycle. This flux transports Ra and the other enriched components to the tidal channels and causes the observed low tide enrichments. An independent estimate of pore water recharge is based on the depletion of U in the tidal channels. The U-based recharge is about two times greater than the Ra-based discharge; however, other sinks of U could reduce the recharge estimate. The pore waters have wide ranges of enrichment in silica, alkalinity, manganese, DOC, and depletion of U with depth. We estimate concentrations of these components in pore water from the depth expected to contribute the majority of the pore water flux, 3.5m, to determine fluxes of these components to the tidal channels. Samples from this depth have minimum concentrations of silica, alkalinity, manganese, and DOC. We also estimate the exports of these components (and import of U) due to mixing based on average measured concentrations in the tidal creeks and the 4-day flushing time. A comparison of these estimates reveals that the exports (negative in the case of U) equal or exceed the pore water fluxes. By using values slightly higher than the minimum concentrations at 3.5m to calculate inputs, the two estimates could be forced to match. We conclude that pore water drainage is the major factor regulating fluxes of Ra isotopes, silica, alkalinity, manganese, DOC, and uranium in this system.

Manganese Flux Across the Blood–Brain Barrier

Manganese (Mn) is essential for brain growth and metabolism, but in excess can be a neurotoxicant. The chemical form (species) of Mn influences its kinetics and toxicity. Significant Mn species entering the brain are the Mn(2+) ion and Mn citrate which, along with Mn transferrin, enter the brain by carrier-mediated processes. Although the divalent metal transporter (DMT-1) was suggested to be a candidate for brain Mn uptake, brain Mn influx was not different in Belgrade rats, which do not express functional DMT-1, compared to controls. Brain Mn influx was not sodium dependent or dependent on ATP hydrolysis, but was reduced by mitochondrial energy inhibitors. Mn and Fe do not appear to compete for brain uptake. Brain Mn uptake appears to be mediated by a Ca uptake mechanism, thought to not be a p-type ATPase, but a store-operated calcium channel. Efflux of Mn from the brain was found to be slower than markers used as membrane impermeable reference compounds, suggesting diffusion mediates brain Mn efflux. Owing to carrier-mediated brain Mn influx and diffusion-mediated efflux, slow brain Mn clearance and brain Mn accumulation with repeated excess exposure would be predicted, and have been reported. This may render the brain susceptible to Mn-induced neurotoxicity from excessive Mn exposure.

The effect of induced anoxia and reoxygenation on benthic fluxes of organic carbon, phosphate, iron, and manganese

Eutrophication causes seasonally anoxic bottom waters in coastal environments, but we lack information on effects of onset of anoxia and subsequent reoxygenation on benthic fluxes of redox-sensitive minerals and associated organic carbon (OC). As the first study, we determined the effect of inducing anoxia and subsequently restoring oxic conditions in mesocosms with surface sediment and water from a coastal environment. These concentration changes were compared with those in an oxygenated control. We determined water column concentrations of dissolved organic carbon (DOC), particulate organic carbon (POC), iron, manganese, and phosphate. Benthic fluxes of DOC, POC, and iron increased at the onset of anoxia in oxygen-depleted treatments. DOC and iron concentrations increased concomitantly towards maxima, which may have indicated reductive dissolution of FeOOH and release of associated OC. The subsequent concomitant concentration decreases may have been the result of coprecipitation of OC with iron-containing minerals. In contrast, the phosphate-concentration increase occurred several days after the onset of anoxia and the manganese concentration was not affected by the onset of anoxia. Restoring oxic conditions resulted in a decrease in DOC, POC, and phosphate concentrations, which may indicate coprecipitation of OC with phosphate-containing minerals. The high DOC fluxes at the onset of anoxia indicate that redox oscillations may be important in OC degradation. Further, our results indicate a close coupling between OC cycling and dissolution/precipitation of iron-containing minerals in intermittently anoxic sediments.

Anthropogenic and seasonal influences on the dynamics of selected heavy metals in Lake Naivasha, Kenya

AbstractLake Naivasha is a freshwater lake in the Eastern Rift Valley of Kenya (0°45′S and 36°20′E). It has no surface outlet and is perceived to be under anthropogenic stress. Being situated at the basin of the rift valley, the lake acts as a sink for wastes from the town of Naivasha and the surrounding horticultural industry.Flux experiments were conducted to investigate the dynamics of heavy metals between the sediment–water interface in Lake Naivasha. In situ benthic flux experiments were conducted at two sites, one near the municipal wastewater inflow to the lake (site SS), and one at the papyrus field near the horticultural farms (site SH). Sediment samples from the exposed riparian land were collected during the dry season after the lake has receded, and the fluxes of selected metals were determined in the laboratory under simulated conditions. Aluminium in situ benthic flux at site SS averaged 7 mmol m−2 h−1, and was correlated positively with pH (Pearson correlation coefficient (r) = 0.89). While the in situ benthic flux of aluminium at site SH averaged 1 mmol m−2 h−1. In situ benthic fluxes of copper and manganese were predominantly positive at site SS, but not at site SH. The papyrus field at site SH played an important role in buffering of the lake in regard to the selected metals investigated in this study. Redox‐sensitive metals were precipitated in the benthic flux experiment for this site.