An experimental study on the speciation of dissolved zinc,cadmium, lead and copper in river rhine and north sea water,by differential pulsed anodic stripping voltammetry

Dissolved electroactive concentrations of zinc, cadmium, lead and copper in river Rhine and North Sea samples have been measured at natural and lower pH values by differential pulsed anodic stripping voltammetry using a Kemula-type hanging mercury drop electrode. Average concentrations detected in North Sea samples at salinities $\text{? 32‰ S}$ and their range are (in μgl^?1): 3.9 (2.0–7.5) for zinc, 0.23 (0.13–0.31) for cadmium, 0.3 (0.1–0.6) for lead and 0.3 (0.25–0.60) for copper (pH 8.1). The APDC-MIBK extracting/concentrating method, followed by AAS measurement applied to the same samples, resulted in 3.9 (2.0–7.5) for zinc, 0.11 (0.01–0.27) for cadmium, 0.5 (0.2–0.9) for lead and 1.6 (0.7–3.2) for copper. A fraction of the electroactive concentrations at pH 2.7 (6.1 for Zn) is electroactive at pH 8.1. The fractions are 100% for Cd, 20% for Cu, 13% for Pb and 40% for Zn. The remaining fractions are partly composed of organically bound species in solution. The low value for lead may be caused by the presence of particulate lead that is dissolved at low pH.Ionic copper and lead species, added separately to seawater at pH 8.1 are removed from the electroactive form, and taken up in (organic) complexes in the same ratio (at least for copper) as the species already present. Added ionic zinc is not removed within the time scale of the measurements (30 min). North Sea water at the natural pH has a complexing capacity, probably due to the presence of dissolved organic compounds, in a concentration equivalent to 3.10^?7M copper. The complexing capacity is zero at pH 2.7. The usual method of standard addition for the determination of electroactive copper and lead concentrations may lead to erroneous results in samples where complexation of this type occurs.     

Copper accumulation by two bivalve molluscs: Salinity effect is independent of cupric ion activity

Oysters (Crassostrea virginica) and soft shell clams (Mya arenaria) were exposed to two different total copper concentrations (10 μg liter⁻¹ and 30 μg liter⁻¹) and two different cupric ion activities (10⁻¹⁰ m and 10⁻⁹ m) under high salinity and low salinity conditions. Cupric ion activities were adjusted by adding nitrilotriacetic acid (NTA) to the experimental media.Copper accumulation by both species was inversely related to salinity and positively associated with total copper and cupric ion activity. This salinity effect was proportionately greater for lower copper concentrations and activities and it was concluded that salinity would, therefore, be an important influence on metal accumulation in the natural environment. The salinity effect was still apparent when the cupric ion activity was held constant indicating that the effect is, at least in part, independent of the copper speciation in the medium.     

Complexation of copper and nickel in the dissolved phase of marine sediment slurries

Speciation of copper and nickel in the water phase of incubated marine slurries under aerobic conditions was performed with MnO₂ and Sep—Pak C₁₈ cartridges. Changes in time during the incubations of concentrations of dissolved organic carbon (DOC), dissolved copper and nickel and inorganic nitrogen were followed. The influence of organic complexation on the dissolved concentrations of copper and nickel was investigated as well as competition between copper and nickel for dissolved organic ligands.Two pools of dissolved organic ligands could be distinguished. With the MnO₂ method a relatively strong ligand group was determined that was subjected to degradation. The conditional stability constant for copper with the relatively strong ligand was 10^11.1. The conditional stability constant for the relatively strong nickel ligand was difficult to determine due to saturation of the ligand sites; it was found to be around 10¹⁰. However, it could not be ascertained whether nickel was reversibly com-plexed with the organic ligands.With Sep—Pak a relatively weak Hgand group was detected that was probably more resistant to degradation. The conditional stability constant of the weaker ligand could not be estimated, an approximation revealed that it was weaker than the ligand group determined with the MnO₂ method. For copper the difference between binding strength of the ligand groups was at least 100, for nickel the difference was less.Competition between copper and nickel for the ligands could not be detected. Only during the first day of the experiment, when the system was not in equilibrium was competition suspected. However, the replacement of nickel by copper from the ligand sites was not straightforward and could not be accounted for by our model.The concentration of total dissolved copper during the first week of the experiment was found to be controlled on the one hand by release from the sediment of copper already associated with dissolved organic matter (DOM) and on the other hand by concentration of the strongest ligand. The calculated free copper concentration increased from 10⁻¹² to 10⁻⁹mol l⁻¹ due to the oxidation of the strongest ligand. After saturation of the strongest ligand the relatively weak ligand controlled the free copper concentration. A continuing release of copper from the sediment by degradation of particulate organic matter (POM) will not increase the free copper concentration until the ligand sites of the weaker ligands get saturated.The total dissolved nickel concentration seemed only to be determined by the sum of the concentrations of the organic ligands. A degradation of ligands resulted in a decrease of the total dissolved nickel concentration. The calculated free nickel concentration did not change with time.     

Chemical studies of copper-organic complexes isolated from estuarine waters using C18 reverse-phase liquid chromatography

Dissolved organic matter (DOM) and dissolved copper-organic complexes were isolated from the estuarine waters of Narragansett Bay, RI, using reverse-phase liquid chromatography employing C₁₈ Sep-Pak cartridges (Waters Associates). The cartridges were found to have a constant retention efficiency for processing ? 1-l volumes of seawater. Fractionation of the isolated material, by sequential elution of the Sep-Pak with water: methanol mixtures of increasing organic solvent concentration, yielded a fraction of the organic matter with a specific copper activity six times greater than the overall activity for the isolated DOM. Analysis of this fraction by high performance liquid chromatography suggested that the organic components are of intermediate polarity and have appreciable aromatic character.An investigation of the protonation characteristics of the isolated complexes indicated that most of the copper is associated with a broad range of acidic sites on the DOM. Analysis by electron paramagnetic resonance spectroscopy confirmed the organic association of the isolated copper and also suggested the presence of several types of binding sites which probably involve oxygen donor ligands.Studies of the exchange of ⁶⁴Cu with these binding sites on the isolated DOM indicated that 70% of the sites undergo rapid exchange with copper in seawater while 20% of these sites did not exchange in a 24-h time period.     

Effects of salinity, temperature and food type on the uptake and elimination rates of cd, cr, and zn in the asiatic clamcorbicula fluminea

Laboratory radiotracer experiments were conducted to determine assimilation efficiencies (AE) from ingested algal food and oxic sediment particles, uptake rates from the dissolved phase, and the efflux rates of Cd, Cr and Zn in the Asiatic clamCorbicula fluminea. Among three elements, AE from both algal and sediment food was greatest for Cd, followed by Zn and Cr. The AEs of tested elements from algal food(Phaeodactylum tricornutum) were consistently higher than those from sediments at a given salinity and temperature. The influence of salinity (0, 4 and 8 psu) and temperature (5, 13 and 21¼) on the metal AEs was not evident for most tested elements, except Cd AEs from sediment. The rate constant of metal uptake from the dissolved phase (k _u was greatest for Cd, followed by Zn and Cr in freshwater media. However, in saline water, thek _u of Zn were greater than those of Cd. The influx rate of all tested metals increased with temperature. The efflux rate constant was greatest for Cr (0.02 d^-1), followed by Zn (0.010~0.017 d^-1) and Cd (0.006 d^-1). The efflux rate constant for Zn in clam tissues depurated in 0 psu (0.017 d^-1) was faster than that in 8 psu (0.010 d^-1). Overall results showed that the variation of salinity and temperature in estuarine systems can considerably influence the metal bioaccumulation potential in the estuarine clamC. fluminea. The relatively high Cd accumulation capacity ofC. fluminea characterized by the high AE, high dissolved influx rate and low efflux rate, suggested that this clam species can be used as an efficient biomonitor for the Cd contamination in freshwater and estuarine environments.     

Dissolved copper and copper-organic complexes in the Narragansett Bay estuary

The geochemistry of dissolved copper-organic complexes was investigated in the estuarine waters of Narragansett Bay. A transect survey was conducted in August 1980, while one mid-bay station was monitored from March through August of that year. The transect data indicated that most of the copper-organic complexes enter the bay via sewage effluent which is discharged into the Providence River at the head of the bay. Organic copper concentrations in the estuary ranged from 0.12 to 2.30 μg kg^?1 and comprised from 14 to 70% of the total dissolved copper. The concentration of copper-organic complexes was not directly related to the amount of dissolved organic matter; and recently generated organic material from phytoplankton production within the bay had a negligible influence on the fraction of dissolved copper which was organically bound.The major source of total copper to the bay is anthropogenic inputs from sewage effluents. Particulate and dissolved copper concentrations ranged from 0.06 to 2.42 and 0.23 to 16.4 μg kg^?1, respectively, giving average values of about 40% particulate and 60% dissolved copper. Particulate copper concentrations decreased rapidly from the upper to the lower bay as a result of both removal and dilution. About 75% of the dissolved copper entering the bay is rapidly removed in the Providence River and upper bay, and the remaining portion (which is largely organic copper) follows conservative mixing in the mid to lower bay. The data suggest that copper binding by dissolved organic matter may be an important control on the riverine flux of dissolved copper through estuaries into coastal and oceanic waters.     

Seasonal variations of dissolved and particulate copper species in estuarine waters

This one-year survey conducted in the macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) was aimed at examining the variations of the various dissolved and particulate copper species. Ten field stations along the whole freshwater–seawater mixing zone were sampled each month. Different biogeochemical parameters (SPM, chl-a, pH and DOC) were also measured. The levels in total dissolved and total particulate copper ranged from 1.8 to 9.5 nM and from 5 to 98 μg g⁻¹, respectively; such amounts are indicative of a pollution-free system. Extractable C₁₈ copper (non-polar hydrophobic organic copper species), in winter and spring, accounted for 30–40% of the total dissolved copper. In summer, this contribution rapidly rose to 60% in the salinity range 20–30; over the same period of time, total particulate copper decreased. The change in dissolved copper speciation and the lowering of particulate copper concentrations were attributed to the release of strong organic ligands by phytoplankton. Our field data evidenced a highly variable behaviour for the various copper species over the seasonal cycle, and then led us to identify the following mechanisms: (i) metal desorption from organic river-flown particles (winter and spring), (ii) metal desorption from resuspended sediment in the upstream section (summer), (iii) competition between particles, non-extractable C₁₈ organic ligands and phytoplankton-released extractable C₁₈ organic ligands to complex copper in the downstream section (summer), and (iv) removal of non-extractable C₁₈ organic copper by adsorption (autumn). Dissolved copper species fluxes were assessed: most of metal inputs to the estuary (60–74%) corresponded to non-extractable C₁₈ organic copper. Winter and spring metal output fluxes were mainly constituted of non-extractable C₁₈ organic complexes; on the other hand, extractable C₁₈ organic complexes were predominant in summer and autumn output fluxes.     

Organic complexation and its control of the dissolved concentrations of copper and zinc in the Scheldt estuary

Cathodic stripping voltammetry (CSV) is used to determine total (after UV-irradiation) and labile dissolved metal concentrations as well as complexing ligand concentrations in samples from the river Scheldt estuary. It was found that even at high added concentrations of catechol (1 m for copper and 0·4 m for iron) and of APDC (1 m for zinc) only part of the dissolved metal was labile (5–58% for copper, 34–69% for zinc, 10–38% for iron); this discrepancy could be explained by the low solubility of iron which is largely present as colloidal material, and by competition for dissolved copper and zinc by organic complexing ligands. Ligand concentrations varied between 28 and 206 n for copper and between 22 and 220 n for zinc; part of the copper complexing ligands could be sub-divided into strong complexing sites with concentrations between 23 and 121 n and weaker sites with concentrations between 44 and 131 n . Values for conditional stability constants varied between (logK′ values) 13·0 and 14·8 for strong and between 11·5 and 12·1 for weaker copper complexing ligands, whereas for zinc the values were between 8·6 and 10·6. The average products of ligand concentrations and conditional stability constants (a-coefficients) were 6 × 10² for zinc and 6 × 10⁶ for copper.The dissolved zinc concentration was found to co-vary with the zinc complexing ligand concentration throughout the estuary. It is argued that the zinc concentration is regulated, in this estuary at least, by interactions with dissolved organic complexing ligands. A similar relationship was apparent between the dissolved copper and the strong copper complexing ligand concentration. The total copper complexing ligand concentrations were much greater than the dissolved copper concentrations, suggesting that only strongly complexed copper is kept in solution.These results provide evidence for the first time that interactions of copper and zinc with dissolved organic complexing ligands determine the geochemical pathway of these metals.     

An electrochemical study of the speciation of copper,zinc and iron in two estuaries in England

The organic speciation of copper, iron and zinc in estuarine waters is studied using electrochemical techniques. Complexing capacities for copper and zinc were determined by cathodic stripping voltammetry (CSV) of their complexes with respectively catechol and amino pyrrolidine dithiocarbamate (APDC). Iron speciation was studied by CSV measurements of dissolved iron before (‘free iron’) and after acidification and UV-irradiation (‘total iron’) of the filtered samples. Complexing capacities of copper were found to vary between 1·4 and 5 × 10⁻⁷m with conditional stability constants, logK′_CuL, between 9·2 and 10·3 in the Tamar estuary. Complexing capacities of zinc were less at between 0·4 and 1·6 × 10⁻⁷m with values for logK′_ZnL between 8·1 and 9·4. Copper complexing capacities generally decreased with increasing salinity, and variations in the results were related to high concentrations of suspended material. Similar variations in the dissolved vanadium concentrations suggested that part of this element was associated with colloidal material. The total dissolved iron concentration in samples from the River Ribble decreased from 10⁻⁶m at low salinity to 10⁻⁷m at high salinity, but the free iron concentration was found to decrease from 8 to 3 × 10⁻⁸m over the same salinity range, which may be compared with the calculated solubility of iron in seawater of 2 × 10⁻⁸m. Comparative experiments showed that on average about 24% of the non-labile iron fraction was stabilized by organic material, the rest being composed of inorganic colloidal material.     

Collection and detection of natural iron-binding ligands from seawater

Iron (Fe) is an essential element for the biochemical and physiological functioning of terrestrial and oceanic organisms, including phytoplankton, which are responsible for the primary productivity in the world's oceans. However, due to the low solubility of Fe in seawater, phytoplankton are often limited by their inability to incorporate enough Fe to allow for optimal growth rates in regions with dissolved Fe concentrations below 1 nM. It has been postulated that certain phytoplankton may produce compounds to facilitate the uptake of Fe from seawater to overcome this limitation. Dissolved Fe in the oceans is overwhelmingly complexed (>99%) by strong organic ligands that may control the uptake of Fe by microbiota; however, the identity, origin, and chemical characteristics of these organic chelates are largely unknown. Although it has been implied that some components of natural Fe-binding ligands are siderophores, no direct analyses of such compounds from natural seawater have been conducted. Here, we present a simple solid-phase extraction technique employing Biobeads SM-2 and Amberlite XAD-16 resins for concentrating naturally occurring dissolved iron-binding compounds from large volumes (>200 l) of seawater. Additionally, we report on the first successful determination of molecular weight size classes and preliminary iron-binding functional group characterization within those size classes for isolates collected from the surface and below the photic zone (150 m) in the central California coastal upwelling system. Electrochemical analyses using competitive ligand equilibration/adsorptive cathodic stripping voltammetry (CLE-ACSV) showed that isolated compounds had conditional Fe-binding affinities (with respect to inorganic iron—Fe′) of K_FeL,Fe′^cond=10^11.5–10^11.9 M⁻¹, similar to purified marine siderophores produced in laboratory cultures and to the ambient Fe-binding ligands observed in seawater. In addition, 63% of the extracted compounds from surface-collected samples fall within the defined size range of siderophores (300–1000 Da). Hydroxamate or catecholate Fe-binding functional groups were present in each compound for which Fe binding was detected. These results illustrate that the functional groups previously shown to be present in marine and terrestrial siderophores extracted and purified from laboratory cultures are also present in the natural marine environment. These data provide evidence that a significant fraction of the organic Fe-binding compounds we collected contain Fe-binding functional groups consistent with biologically produced siderophores. These results provide further insight into characteristics of the Fe-binding ligands that are thought to be important in controlling the biological availability of Fe in the oceans.     

Contrasting complexing capacity of dissolved organic matter produced during the onset,development and decay of a simulated bloom of the marine diatom Skeletonema costatum

The capacity of natural dissolved organic matter (DOM) produced during the onset, development and decay of a simulated bloom of the marine diatom Skeletonema costatum to complex free copper has been followed for a 2 week period. Copper binding capacity of the culture was measured by anodic stripping voltammetry (ASV) with a hanging mercury drop electrode (HMDE). The concentration of dissolved organic carbon (DOC) and two fluorophores, M (humic-like, Ex/Em: 320 nm/410 nm) and T (protein-like, Ex/Em: 280 nm/350 nm), were followed during the course of the incubation. Models using DOC concentrations alone could not accurately predict the complexing capacity of the culture, especially at the end of the bloom, and better predictions were obtained when fluorescence measurements were considered. They were helpful in characterising two types of copper ligands produced in the culture. The first type, traced by the fluorescence of peak T, was related to labile DOC directly exuded by phytoplankton. The second type, traced by the fluorescence of peak M, was the refractory humic-like material presumably produced in situ as a by-product of the bacterial degradation of phytogenic materials. During the onset and development of the bloom (days 0 to 7), the fluorescence of peak T explains 60–80% of the total complexing capacity of the culture, suggesting that exuded “protein-like” compounds among other exuded complexing agents efficiently complexed free copper. On the contrary, during the decay (days 8 to 13), these ligands were replaced by humic substances as the complexing agent for copper.     

Determination of copper complexation with natural organic ligands in seawater by equilibration with MnO2 II. Experimental procedures and application to surface seawater

The MnO₂ adsorption method combined with voltammetry is proposed for the direct determination of metal complexation in seawater of various salinities as a more satisfactory alternative to direct voltammetric measurements and bioassay methods. A small quantity of MnO₂ is equilibrated with copper ions in filtered seawater. Natural organic ligands in the seawater compete for copper with the MnO₂. Total dissolved copper is measured by differential pulse anodic stripping voltammetry after filtration and acidification of the sample. Preconcentration of natural water samples is unnecessary and measurement is performed at the natural equilibrium pH of the aerated sample. The analytical limit of detection of the method depends on contamination from the filtration step, and for copper complexation a ligand concentration of 5 × 10^?8 M was obtained. The sensitivity can be increased by use of radioisotopes as tracers. The method is very versatile in that complexation of various metals may be determined by any analytical method that measures total dissolved metal concentrations. Neither organic ligands nor their complexes with copper adsorb on the MnO₂ at pH8, but at pH 1.8 MnO₂ is an efficient scavenger for electroactive organic material.Samples of surface water from the Irish Sea and the Atlantic Ocean were found to contain ligand concentrations of 1.7 × 10^?7 and 1.1 × 10^?7 M, with conditional stability constants (log values) of 9.84 ± 0.13 and 9.86 ± 0.23, respectively, at pH 8.0.     

Chemical and microbiological studies of sea-surface films in the Southern Gulf of California and off the West Coast of Baja California

Sea-surface films and the corresponding 10-cm subsurface waters were sampled on three cruises to the eutrophic and oligotrophic waters in the Gulf of California and off the west coast of Baja California. The following constituents and properties were measured: NH₄⁺, NO₂⁻, NO₃⁻, PO₄³⁻, SiO₃²⁻, urea, ATP and Chl-a; dissolved and particulate organic carbon and nitrogen; lipid, protein and carbohydrate; total viable and nitrifying bacteria; simulated in situ bacterial heterotrophy; microplankton and simulated in situ primary productivity; surface potential and film pressures; ultraviolet absorption; and film-formation rates using surface potential and chemical methods.Mean enrichment factors (film concentration/10-cm concentration) for the three cruises were: 1.1–2.4 for the soluble inorganic nutrients, dissolved organic carbon, nitrogen, urea, carbohydrate, and lipid; 1.3–2.0 for ATP, Chl-a, microplankton and bacteria; and 1.1–3.7 for particulate carbon and nitrogen and both dissolved and particulate protein. Particulate and dissolved carbon and nitrogen were the only constituents never depleted in the films relative to the subsurface waters. Systematic, significant correlations between the various chemical and biological parameters measured were few, reflecting the complexity of processes which form and maintain surface films.Protein, carbohydrate and lipid carbon accounted for 15–114% (mean = 50%) of the total particulate organic carbon and 14–42% (mean = 28%) of the total dissolved organic carbon in both the films and 10-cm waters. Lipid was not the major identified constituent of films, averaging 18% of the particulate organic carbon and 2.5% of the dissolved organic carbon. There was more protein relative to carbohydrate in film samples compared to 10-cm water; there was also more protein relative to carbohydrate in particulate compared to dissolved mater. Microplankton plus bacterial carbon averaged 16% of the particulate organic carbon in films and 19% in 10-cm waters.     

The Interacting Effect of Hydrodynamics and Organic Matter on Colonization: a Soft-Sediment Example

This study examined whether buried organic matter in the form of seagrass influenced colonization by the corophiid amphipod Paracorophium excavatum in an intertidal sandflat. Laboratory manipulations were compared to results from field studies to determine the role of hydrodynamics in mediating the effects of the organic matter. At levels of organic loading greater than 1% of sediment dry weight, size frequency effects and decreases in numbers of colonizers occurred in still water conditions. In the field, however, at levels of organics up to 2% of sediment dry weight, only size-frequency effects were observed. A change in hydrodynamic conditions and the influence of this on sediment geochemistry is suggested as the most reasonable explanation for the disparity in colonization both between sites in the field and between laboratory and field manipulations. Two effects of organics upon corophiid amphipod colonization are proposed: a lower level of organic loading that can lead to a greater proportion of juveniles in the colonizing population and a higher level of organics that can lead to a decrease in the numbers of amphipods colonizing. The threshold at which these effects take place is modulated by the hydrodynamics of the environment. The same organic addition can therefore have different effects upon colonization across a tidal flat, which is dependent upon the variation in hydrodynamics     

Organic copper and chromium complexes in the interstitial waters of Narragansett Bay sediments

Dissolved organic copper and chromium complexes were measured in both overlying and interstitial waters of Narragansett Bay and mesocosm sediments using C₁₈ reverse-phase liquid chromatography and atomic absorption spectroscopy. In the interstitial and overlying waters, the isolation procedure recovered 22–67% of the total dissolved copper, 23–55% of the total dissolved chromium and 14–40% of the dissolved organic carbon. The distribution of both total and organic copper decreased with depth in the cores and exhibited a subsurface maximum near the zero Eh level (z = 2–4 cm). Below that depth, both forms of copper continued to decrease until an apparent equilibrium with sulfide minerals was established (7–8 cm). Dissolved chromium exhibited a different geochemistry, with both total and organic chromium increasing in concentration with depth in the cores, possibly due to remobilization from some mineral phase such as chromic hydroxide or chromite.     

Determination of conditional stability constants of organic copper and zinc complexes dissolved in seawater using ligand exchange method with EDTA

To clarify the nature of organic metal complexes dissolved in seawater, a ligand exchange reaction between ligands of natural origin and an aminopolycarboxylic acid (EDTA) was used to determine the conditional stability constants of organic metal complexes. The results indicate that more than two organic molecules complexed with copper and zinc exist in surface seawater. It is found that the conditional stability constants of these naturally-occurring organic metal complexes are 1–3 orders of magnitude higher than those of EDTA-Cu and EDTA-Zn complexes. These estimates of the conditional stability constants for the dominant species of organic copper and zinc complexes are 10^11.8 and 10^9.3, respectively, at pH 8.1. The results indicate that these naturally-occurring organic metal complexes are stable species and not easily dissociated or displaced with others in the marine environment.     

Enhanced preservation of organic matter in sediments deposited within the oxygen minimum zone in the northeastern Arabian Sea

The presence of a strongly developed oxygen minimum zone (OMZ; [O₂]<2 μM) in the northeastern Arabian Sea affords the opportunity to investigate whether oxygen deficiency in bottom waters enhances the preservation of organic matter in the underlying sediments. We explored if the observed patterns of organic matter accumulation could be explained by differences in productivity, sedimentation rate, water depth, and mineral texture. The differences in the burial rates of organic matter in sediments deposited within or below the OMZ could not be explained on the basis of these factors. All collected evidence points to a coupling of low oxygen concentrations and enhanced organic matter preservation. Under more oxygenated conditions bioturbation as well as the presence of labile manganese and iron oxides are probably important factors for a more efficient microbially mediated degradation of organic matter. Pore water profiles of dissolved Mn²⁺ and Fe²⁺ show that reduction of manganese and iron oxides plays a minor role in sediments lying within the OMZ and a larger role in sediments lying below the OMZ.     

Impact of dynamic feedbacks between sedimentation,sea-level rise,and biomass production on near-surface marsh stratigraphy and carbon accumulation

Salt marshes accrete both organic and inorganic sediments. Here we present analytical and numerical models of salt marsh sedimentation that, in addition to capturing inorganic processes, explicitly account for above- and belowground organic processes including root growth and decay of organic carbon. The analytical model is used to examine the bias introduced by organic processes into proxy records of sedimentation, namely ¹³⁷Cs and ²¹⁰Pb. We find that accretion rates estimated using ²¹⁰Pb will be less than accretion rates estimated using the ¹³⁷Cs peak in steadily accreting marshes if (1) carbon decay is significant and (2) data for ²¹⁰Pb extend below the ¹³⁷Cs peak. The numerical model expands upon the analytical model by including belowground processes such as compaction and root growth, and by explicitly tracking the evolution of aboveground biomass and its effect on sedimentation rates. Using the numerical model we explore how marsh stratigraphy responds to sediment supply and the rate of sea-level rise. It is calibrated and tested using an extensive data set of both marsh stratigraphy and measurements of vegetation dynamics in a Spartina alterniflora marsh in South Carolina, USA. We find that carbon accumulation in marshes is nonlinearly related to both the supply of inorganic sediment and the rate of sea-level rise; carbon accumulation increases with sea-level rise until sea-level rise reaches a critical rate that drowns the marsh vegetation and halts carbon accumulation. The model predicts that changes in carbon storage resulting from changing sediment supply or sea-level rise are strongly dependent on the background sediment supply: if inorganic sediment supply is reduced in an already sediment poor marsh the storage of organic carbon will increase to a far greater extent than in a sediment-rich marsh, provided that the rate of sea-level rise does not exceed a threshold. These results imply that altering sediment supply to estuaries (e.g., by damming upstream rivers or altering littoral sediment transport) could lead to significant changes in the carbon budgets of coastal salt marshes.     

Organic and inorganic speciation of copper in the Irish Sea

The metal complexing ability of surface water of the Irish Sea has been measured by the MnO₂ adsorption method. In all samples strong copper-chelating compounds are present at concentrations of 60–150 nM, with conditional stability constants (log values) of 10.0–10.4. The concentrations of Cu, Pb and Cd in the samples are 16–39 nM, 1–7 nM and 0.1–2 nM, respectively; much less than the ligand concentrations. The organic compounds form complexes with 94–98% of dissolved copper, and therefore constitute the major form of copper in surface water of the Irish Sea. Recalculation of speciation of the inorganic fraction of copper in seawater reveals that the major complex ion is that of CuCO₃⁰ (60%), followed by CuOH⁺ (16%) and Cu(OH)₂⁰ (16%). Complexes with borate ions form a small and rather insignificant fraction of 1%.     

Rates of chitin degradation in an estuarine environment

In vitro chitin degradation rates in pure cultures and in mixed natural cultures have been determined and compared with those of other workers.In situ studies in the salt marsh shrimp nursery grounds along the southeastern Louisiana coast showed that chitin degradation was most rapid (118 mg d^–1 g^–1 chitin) when water temperature averaged 30C. Maximum degradation rates were noted at the water-sediment interface and when substrate particles were reduced in size (0.25 cm²). Of the several types of chitin tested, including treated and untreated, native chitin was most rapidly solubilized. Microbial populations on this substrate developed more rapidly, suggesting that chitin degradationin situ is a function of initial colonization. This is further supported by the observation that degradation rates were most rapid when total bacteria, chitinoclastic bacteria, and the ratio of chitinoclasts to total bacterial biomass (15.6 %) was greatest.