Dissolved organic matter of a low-coloured stream

SUMMARY. The dissolved organic matter (DOM) of White Clay Creek (Pennsylvania, U.S.A.), a low-coloured stream, was characterized by gel permeation chromatography, humic acid determination, compound classification analysis and gas chromatography.
Large polymers (approximate mol.wt above 5000) were virtually absent. The majority of the DOM consisted of fulvic acid-like material, of probable mol.wt less than 3000. Highly coloured material represented only a small portion of the total. Substances classifiable as phenols, carbohydrates, lipids, amino acids or proteins made up only a small fraction of the DOM. Sixteen low mol. wt compounds (carboxylic acids, amino acids, carbohydrates and an amide) were tentatively identified.
DOM concentration had a mean value of 6.4 mgC/l and showed annual fluctuations, with maxima in autumn and in late winter.     

Origin of carbon in organic matter in the Neva estuary

Analysis of the isotopic composition of tissues of representatives of zoobenthos and organic matter of seston have shown that the major part of carbon participating in the biological turnover in the ecosystem of the upper part of the Neva estuary is of terrestrial origin. Obviously, it is discharged from the lake drainage area to the Lake Ladoga–Neva–Neva estuary system. The results have revealed an important role in aquatic ecosystems of the humid zone of allochthonous humic substances creating a supplementary stock of nutrients and enhancing the productivity of ecosystems. Detailed investigations of the role of various forms of allochthonous organic compounds of terrestrial origin are necessary for elaboration of efficient measures for alleviation of eutrophication of the Neva estuary.     

Dissolved organic matter in Cumbrian lakes and streams

SUMMARY. 1. A survey of dissolved organic carbon concentration [DOC] and optical absorbance at 340 nm (A340) was carried out for thirteen lakes and twenty-three streams in Cumbria over the period May 1986 to April 1987.
2. Values of [DOC] were in the range 0-6.2 mg 1-1-1, no water having a mean [DOC] greater than 3.6mgI-1. Streamwaters showed no gross seasonal dependence in [DOC], but there was a peak in [DOC] associated with the first major rain event of autumn 1986. Summer values of [DOC] in lakes were greater than winter values, probably because of the production of DOC by phytoplankton.
3. In streams, [DOC] was weakly positively correlated (r2 <0.3) with discharge (as estimated from rainfall); A340 was more strongly correlated (0.4 < r2 < 0.6). Linear regression of mean A340 against mean [DOC] for streams gave a slope consistent with the absorptivities of aquatic humic substances, and a positive intercept on the abscissa that suggests the presence of, on average 0.6 mg 1-1 of DOC that does not absorb at 340 nm. Lake samples taken in winter gave a similar A.340/[DOC] relationship to that for streams, but in summer A340/[DOC] ratios were lower, suggesting that phytoplankton-derived [DOC] absorbs weakly or not at all at 340nm.     

Dissolved organic matter utilization and oxygen uptake in algal-bacterial microcosms.

Under closed laboratory conditions, at non-limiting nutrient levels, the biomass of Anabaena variabilis, Anacystis nidulans, Chlorella pyrenoidosa, and Selanastrum capricornutum increased with increasing levels of dissolved organic matter (DOM) as a result of bacterially produced carbon dioxide (CO2) and (or) cofactors. Oxygen (O2) produced as a result of algal photosynthesis was sufficient to supply the najority of O2 required by the bacterial community. The percentage of DOM utilized by bacteria which was subsequently incorporated into algal biomass varied with individual species indicating that the association between individual algal species and the bacterial microbiota varied. Under natural conditions bacteria could provide CO2 and (or) cofactors for algal photosynthesis which in turn supplies O2 for bacterial respiration. This mutualistic association in aquatic environments could result in an increase in planktonic and epiphytic algal biomass if other nutrients are available.     

Dissolved organic matter under native Cerrado and Pinus caribaea plantations in the Brazilian savanna

The transformation of native Cerrado into Pinus caribaea Morelet plantations changes the DOM dynamics including changed rates of mineralisation, denitrification, and C export to the groundwater. To examine the differences in quantity, temporal dynamics, and quality of DOM between Cerrado and Pine plantations we collected rainfall, throughfall, stemflow, litter leachate (under pine only) and soil solution at 15, 80, and 200 cm depth in weekly intervals during the rainy seasons 1997/98 and 1998/99. We determined total dissolved organic carbon (DOC) concentrations and assessed DOM quality by separating hydrophilic and hydrophobic fractions and by NMR analysis of organic layer extracts. The rainfall had a mean DOC concentration of 2.6 mg L^–1. The mean concentrations of DOC in the throughfall of the pine plantations (5.0–10.5 mg L^–1) were significantly above those of Cerrado (2.6–4.9 mg L^–1). During the first part of the rainy seasons (October–December), the concentrations of DOC in the soil solution (15–200 cm depth) under Cerrado and pine did not differ significantly. During the second part of both rainy seasons (January–April), the concentrations of DOC in the soil solution under Cerrado (4.4–5.1 mg L^–1) exceeded those under PI (1.4–2.7 mg L^–1). Possible explanations of the latter include higher DOM input into the Cerrado soil and a stronger retention and/or faster mineralisation of the pine DOM than of the Cerrado DOM in the mineral soil. As the structural composition of DOM extracted from the organic layer under Cerrado and pine did not differ significantly, faster mineralisation was the most likely explanation for partly lower DOC concentrations in the soil solution under pine than under Cerrado. This assumption was supported by increasing contributions of hydrophobic DOM to total DOM with increasing depth under pine while, under Cerrado, the DOM composition did not change with depth. The reason for DOM mineralisation under pine was probably the higher N availability because total N concentrations were 11–23 times higher under pine than under Cerrado.     

Suspended matter in the Scheldt estuary

The Scheldt estuary is characterised by a specific energy pattern resulting from the interaction of wave energy, tidal energy and river energy. It divides the estuary into three parts and governs suspended matter transport and distribution pattern. Observation of suspended matter transport shows the existence of three estuarine turbidity maxima (ETM), a marine-dominated ETM in the lower estuary at the river mouth, a river-dominated ETM in the upper estuary with suspended matter concentration reaching up to 300 mg/l, and the most important tide-dominated ETM in the middle estuary with suspended matter concentrations from several hundred milligrams per litre up to a few grams per litre. Resuspension is the dominant phenomenon in this last ETM due to the tidal related bottom scour, which is initiated when a critical erosion velocity of 0.56 m/s is exceeded. An assessment of residual current along the axis of the estuary shows distinctive pattern between the surface water flow and the near bottom water flow. Also the local morphology of the river, natural or man-made, has a prominent effect on the orientation and strength of the residual currents flowing along either side of the river or river bend. Evaluation of suspended matter concentration in relation to the current flow shows no systematic correlation either because of phenomena as scour lag and settling lag mainly in the middle estuary, or because of the current independency character of uniform-suspension mainly in the upper and lower estuary. Quantification of suspended matter load exhibits a net downstream transport from the upper estuary, a near-equilibrium sustainable status in the middle estuary and a net upstream transport of suspended matter from the lower estuary. The characteristic of suspended matter is induced by and is a function of e.g. tidal phase, spring-neap tide, longitudinal and vertical distribution mechanisms, seasons, short and long terms of anthropogenic influence and/or estuarine maintenance. Suspended matter is dominated by complex and cohesive organo-mineral aggregates. It consists of a variable amount of an inorganic fraction (average of 89%) and an organic fraction and occurs largely as flocs, the size of which is remarkably larger in the upper estuary and smallest within the ETM in the middle estuary. Independent time series measurements (1990–2000) of suspended matter property show an increasing sand fraction, a decreasing organic matter content, a rise in ¹³C as well as a decrease in water transparency. These independent measurements exhibit coherent consequences of estuarine maintenance operations. Maintenance dredging of the shipping channel and harbours and dumping operation in the Scheldt strengthen marine influence further landward, resulting in a sustained tidal range increment and upstream flow and transport of suspended matter.     

Organic matter in the Elbe estuary

In the low salinity region of the Elbe estuary in March–April 1992 the turbidity zone was characterized by high loads of suspended matter, 7% of which was organic material (750 μM C) at the surface. Particulate nitrogen, phosphorus and carbohydrates concentrations reached 55 μM N, 10 μM P and more than 15 μM glc. eq., corresponding to 13% of total C, at the surface and increasing threefold near the bottom. In spite of the peaking of particulate organic material levels in the maximum turbidity zone, there were only consistent qualitative changes in total particulate C, N, P, and carbohydrates along the Elbe estuary. Downstream, both the percentage of particulate organic material and the turbidity: organic material ratio decreased, indicating decomposition in the upper estuary and dilution with inorganic suspended matter from the lower estuary. Diatoms, the dominant phytoplankton group, decreased from the upper reaches towards the turbidity zone by 0.3 (surface) and 1.5 mg C l⁻¹ (bottom). This corresponded to 12 and 60% of the decrease in total particulate carbon. Estimated local input of organic carbon by primary production (21 μg Cl⁻¹d⁻¹) was almost compensated by calculated minimum grazing (14 μg C l⁻¹d⁻¹). Considering net primary production and grazing, the dissimilation by zooplankton (5 μg C l⁻¹d⁻¹) and heterotrophic bacterial decomposition (48 μg C l⁻¹d⁻¹), when summed over the estimated flushing time (12 days) represented a loss of suspended organic matter of 0.6 mg Cl⁻¹. Since this was only 20% of the observed decrease in particulate carbon, significant dilution processes must be assumed. Dissolved organic nitrogen decreased from 35 to 10 μM N and dissolvd organic phosphorus from 0.6 to 0.1 μM P towards the sea, mainly due to dilution. The distribution of phosphate, with highest loads in the turbidity maximum of 2.4 μM, suggested an interaction with the accumulated load of particulate material.     

Bio-seismic and sequence stratigraphy of the Neogene of the Northwest Damietta Concession,Nile Delta,Egypt

The stratigraphic framework of the Neogene sequence drilled by two offshore wells located in the north-eastern shore of the Nile Delta (the wells Sekhmet-1 and Sekhmet-2) has been established. The lithostratigraphic units with their sequences, from older to younger, are as follows: the Sidi Salim Formation (including Sr1 SB, Sr2 SB, Sr2 MFS, Sr3 SB and Sr3 MFS), a sequence representing the uppermost part of the Sidi Salim and most of the lower part of the Qawasim Formations (including Tor 1.1 SB, Tor 1.2 SB, Tor 1.3 SB, Tor 1.4 SB and ?Tor 2 SB), a sequence representing the uppermost part of Qawasim and the lower part of the Abu Madi Formations (including ?Me1 SB, Me2 SB and Me2 MFS), the Kafr El Sheikh Formation (including alternatively Za 1 and 2 SB and MFS and Ge 1 SB and MFS), the El Wastani Formation (including Ge 2 SB and MFS) and a Quaternary sequence represented by the topmost part of El Wastani and Mit Ghamr/Bilqas Formation (including alternatively ?Cala 1 and 2 SB and MFS and ?Io 2 SB). The lower part of the Qawasim in well Sekhmet-2 includes two LST: Tor 2 LST and Me 1 LST.     

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

The partitioning of organic matter (OM) between dissolved and particulate phases is an important factor in determining the fate of organic carbon in the ocean. Dissolved organic matter (DOM) release by phytoplankton is a ubiquitous process, resulting in 2–50% of the carbon fixed by photosynthesis leaving the cell. This loss can be divided into two components: passive leakage by diffusion across the cell membrane and the active exudation of DOM into the surrounding environment. At present there is no method to distinguish whether DOM is released via leakage or exudation. Most explanations for exudation remain hypothetical; as while DOM release has been measured extensively, there has been relatively little work to determine why DOM is released. Further research is needed to determine the composition of the DOM released by phytoplankton and to link composition to phytoplankton physiological status and environmental conditions. For example, the causes and physiology of phytoplankton cell death are poorly understood, though cell death increases membrane permeability and presumably DOM release. Recent work has shown that phytoplankton interactions with bacteria are important in determining both the amount and composition of the DOM released. In response to increasing CO₂ in the atmosphere, climate change is creating increasingly stressful conditions for phytoplankton in the surface ocean, including relatively warm water, low pH, low nutrient supply and high light. As ocean physics and chemistry change, it is hypothesized that a greater proportion of primary production will be released directly by phytoplankton into the water as DOM. Changes in the partitioning of primary production between the dissolved and particulate phases will have bottom-up effects on ecosystem structure and function. There is a need for research to determine how these changes affect the fate of organic matter in the ocean, particularly the efficiency of the biological carbon pump.     

青藏高原源头河流水体溶解性有机物组分及其关键调控因素

水体溶解性有机物(Dissolved Organic Matter, DOM)是河流生态系统的重要组成部分,其含量及组分的变化与生态系统功能密切相关。以青藏高原东缘龙苍沟流域27条Strahler 1级源头河流为研究对象,采用DOM荧光特性表示组分特征,同时调查各河流地理特征、气候特征和水化学特征,探究源头河流DOM含量和组分的关键调控因素。研究结果表明:溶解性有机碳(DOC)浓度在0.35-1.50 mg/L之间,平均值为0.85 mg/L。荧光指数(Fluorescence index, FI)的均值分别为0.91和1.11,类色氨酸与类酪氨酸比值(Trypto/Tyro)的均值为0.76,新鲜度指数β/α均值为0.61,表明蛋白质的生物可利用性较差、微生物活性较低。随着海拔的降低,龙苍沟流域河流DOC浓度降低,DOM组分外源性降低,而微生物生物活性升高(P ＜ 0.05)。DOM组分受地形、流域面积和气候因素影响不显著(P ＞ 0.05)。荧光指数 FI、新鲜度指数 β/α与Ca²⁺/Mg²⁺和NO₃^-浓度显著正相关(P ＜ 0.05)。逐步回归结果显示,海拔、坡降、温度、pH、氧化还原电位、DOC浓度、Ca²⁺/Mg²⁺和NO₃^-浓度都对DOM组分起到了一定作用(P ＜ 0.05)。结构方程模型结果显示,在Strahler 1级源头河流中Ca²⁺/Mg²⁺和 NO₃^-浓度是驱动DOM组分海拔变异的主要环境因素。综合以上分析,随着海拔降低,岩石风化加剧,同时人类活动的增加引起NO₃^-浓度的增加改变了水化学环境,导致DOM组分品质改善。对源头河流DOM组分空间变异和调控因素进行了研究,加深对源头河流有机物代谢过程的认识。     

Dissolved organic matter in small streams along a gradient from discontinuous to continuous permafrost

The Yenisei river passes every type of permafrost regime, from south to north, being characterized by increasing continuity of the permafrost and by decreasing thickness of the active layer. We used that situation to test the hypothesis that amounts and properties of dissolved organic matter (DOM) in small streams draining forested catchments respond to different permafrost regimes. Water samples were taken from eight tributaries along the Yenisei between 67°30′N and 65°49′N latitude. The samples were analysed for dissolved organic carbon (DOC) and nitrogen (DON) and DOM was characterized by its chemical composition (XAD‐8 fractionation, sugars, lignin phenols, amino acids, protein, UV and fluorescence spectroscopy), and its biodegradability. Most properties of the tributary waters varied depending on latitude. The higher the latitude, the higher were DOC, DON and the proportion of the hydrophobic fraction of DOC. The contribution of hexoses and pentoses to DOC were higher in southern tributaries; on the other hand, phenolic compounds were more abundant in northern tributaries. Mineralizable DOC ranged between 4% and 28% of total DOC. DOM in northern tributaries was significantly (P<0.05) less biodegradable than that in southern tributaries reflecting the differences in the chemical properties of DOM. Our results suggest that the differences in DOM properties are mainly attributed to differences of permafrost regime, affecting depth of active layer, soil organic matter accumulation and vegetation. Soil organic matter and vegetation determine the amount and composition of DOM produced in the catchments while the depth of the active layer likely controls the quantity and quality of DOM exported to streams. Sorptive interactions of DOM with the soil mineral phase typically increase with depth. The results imply that a northern shift of discontinuous permafrost likely will change in the long term the input of DOM into the Yenisei and thus probably into the Kara Sea.     

Dissolved organic matter quantity and quality from freshwater and saltwater lakes in east-central Alberta

Concentrations of dissolved organic matter (DOM) in surface waters of sub-humid to semi-arid lakes in east-central Alberta increase with increasing salinity and water residence time from about 20 to 330 mg L^–1 as dissolved organic carbon (DOC). This pattern is opposite to that observed among freshwater lakes spanning a gradient in water residence times, and is probably caused by evaporative concentration of refractory DOM. The proportion of total DOC, operationally defined as humic substances using XAD-8 resin, was high, though similar to surface waters typically referred to as "humic", and independent of salinity. Very long water residence times (hundreds of years) in saline lakes favors evapoconcentration of low-color, low molecular weight DOM, with N-content characteristic of allochthonous DOM.     

Fertilizer applications and the organic matter status in the agege experimental field in Nigeria

Summary The influence of 14 years application of mineral fertilizers on the organic carbon content of the Agege sandy soil was appraised. The fertilizer treatments in comparison to unfertilized plots led to higher as well as lower organic carbon. The sole applications of N. P, and K were compared, while N alone increased the organic carbon content of the soil, P alone tended to decrease it, but K alone especially decreased the soil organic carbon. Increased P application, however, could compensate this decrease, whereas increased K-accelerated the decrease. NP and NK combinations showed similar trends to decrease the organic carbon but this decrease was more with NK than NP. But PK fertilizer combinations in contrast to the effect of each of these nutrients alone influenced the organic carbon positively, whereby higher P contents showed again favourable effects than higher K. The fertilizer combinations of the three nutrients (NPK) showed in comparison to unfertilized variants not much higher carbon contents. The complete fertilizer decreased the organic carbon as against unfertilized variants. It was therefore concluded that not always and everywhere can harmonious fertilizer lead to higher organic carbon in the soil, this depends on the type of soil and kinds of cations available in the soil concerned.Ammonium sulphate and urea influenced the content of organic carbon in the soil differently, owing to their different influences on P and K in the soil. Ammonium sulphate in contrast to urea led to mobility of organic carbon in the soil, therefore for better results split application of ammonium sulphate will minimize this effect. For better results of the organic carbon in the soil, mineral fertilizer should not be applied alone but in combination with organic manure. It is also very essential not only to assess the quantity but also qualitative aspect of organic matter as influenced by 14 years of applications.     

Dissolved and particulate organic matter in contrasting Zostera marina (eelgrass) sediments

The influence of seagrass Zostera marina on sediment characteristics was examined in two contrasting sediments, one organic-rich and one organic-poor. The presence of plants leads to reduced sediment redox potential in both sediment types compared to bare sediment with the largest effects in the organic-poor sediment. Z. marina stimulated the sulfate reduction rates in organic-poor sediment with ∼50% and higher pools of dissolved organic carbon (DOC) were found. In contrast, sulfate reduction rates were lower in vegetated compared to bare sites in the organic-rich sediment. Despite a low contribution of dissolved carbohydrate (DCHO) to the DOC pool (<5%), the seagrass vegetation was responsible for an increase of ∼50% in DCHO pools with a peak in the root zone suggesting that Z. marina supplied DCHO to the pore waters. The Z. marina meadows also enhanced the contribution of particulate carbohydrate (PCHO) to sedimentary particulate organic carbon (POC) pools by 6-14% compared to bare sediment. Although the PCHO pools were higher in organic-rich than organic-poor sediments, the analyses of carbohydrate composition revealed that three groups of neutral sugars including glucose, galactose and mannose+xylose were the major compounds of PCHO and contributed with >60% to sedimentary carbohydrate pools at both sites. Only glucose showed depletion with depth in the vegetated sediments, whereas the percentage of ribose and rhamnose increased indicating a selective degradation of labile carbohydrates in the meadows. Galactose and mannose+xylose appeared to represent a refractory part of carbohydrate that remained after degradation of the more labile components. The sugar content was rather constant with depth at the bare organic-rich sediment indicating that only recalcitrant carbohydrate pools were buried. There was less difference in the PCHO composition profiles between vegetated and bare organic-poor sediments.     

Dissolved organic matter (DOM) concentration and quality in a forested mid-Atlantic watershed, USA

Understanding the quantity and quality of dissolved organic matter (DOM) in potential watershed sources is critical for explaining and quantifying the exports of DOM in stream runoff. Here, we examined the concentration and quality of DOM for ten watershed sources in a 12?ha forested catchment over a two-year period. DOM composition was evaluated for: throughfall, litter leachate, soil water (zero and tension), shallow and deep groundwater, stream water, hyporheic zone, and groundwater seeps. DOM quality was measured using a suite of optical indices including UV–visible absorbance and PARAFAC modeling of fluorescence excitation-emission matrices (EEMs). DOM concentrations and quality displayed a pronounced trend across watershed sources. Surficial watershed sources had higher DOM concentrations and more humic-like DOM with higher molecular weight whereas deeper groundwater sources were rich in % protein-like fluorescence. The greater % contribution of protein-like fluorescence in groundwater suggested that a larger fraction of groundwater DOM may be bioavailable. DOM for wetland groundwater was more aromatic and humic-like than that at the well-drained riparian location. Principal component analyses (PCA) revealed that the differences in surficial watershed compartments were dictated by humic-like components while groundwater sources separated out by % protein-like fluorescence. Observations from optical indices did not provide any conclusive evidence for preferential association of dissolved organic carbon (DOC) or dissolved organic nitrogen (DON) with any particular DOM quality pools.     

Dissolved organic matter and nutrient dynamics of a coastal freshwater forested wetland in Winyah Bay, South Carolina

Seasonally flooded, freshwater cypress-tupelo wetlands, dominated by baldcypress (Taxodium distictum), water tupelo (Nyssa aquatica), and swamp tupelo (Nyssa sylvatica var. biflora) are commonly found in coastal regions of the southeastern United States. These wetlands are threatened due to climate change, sea level rise, and coastal urban development. Understanding the natural biogeochemical cycles of nutrients in these forested wetlands as ecosystems services such as carbon sequestration and nitrogen processing can provide important benchmarks to guide conservation plans and restoration goals. In this study, surface water and soil pore water samples were collected weekly from a cypress-tupelo wetland near Winyah Bay, South Carolina and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), inorganic nitrogen, and phosphate during its flooding period between October 2010 and May 2011. DOC was further characterized by specific ultra-violet absorbance at 254 nm, spectral slope ratio (S_R) (ratio of two spectral slopes between 275–295 nm and 350–400 nm), E2/E3 ratio (ratio between A254 and A365), and fluorescence excitation-emission matrix. In addition, litterfall was collected on a monthly basis for a year while the biomass of the detritus layer (i.e., decomposed duff lying on the wetland floor) was determined before and after the flooding period. Results of the field study showed that concentrations of DOC, DON, NH₄ ⁺–N, and (NO₂ ^? + NO₃ ^?)–N in the surface water were generally higher during the fall, or peak litterfall season (October to December), than in the spring season (March to May). Highest concentrations of 54.8, 1.48, 0.270, and 0.0205 mg L^?1, for DOC, DON, NH₄ ⁺–N, and (NO₂ ^? + NO₃ ^?)–N respectively, in surface waters were recorded during October. Lower SUVA, but higher S_R and E2/E3 ratios of DOC, were observed at the end of the flooding season comparing to the initial flooding, suggesting the wetland system converts high aromatic and large DOC molecules into smaller and hydrophilic fractions possibly through photochemical oxidation. A similar trend was observed in soil pore water, but the pore water generally had greater and relatively stable concentrations of dissolved nutrients than surface water. No obvious temporal trend in phosphate concentration and total nitrogen to total phosphorus ratio (N:P) were found. Results of the laboratory extraction and mass balance calculation suggested fresh litter was a major source of DOC whereas decomposed duff was the source of dissolved nitrogen in surface water. In summary, the biogeochemistry of this isolated cypress-tupelo wetland is not only driven by the vegetation within the wetland system but also by hydrology and weather conditions such as groundwater table position, precipitation, and temperature.     

Rapid organic matter mineralization coupled to iron cycling in intertidal mud flats of the Han River estuary,Yellow Sea

Organic matter oxidation represents a transfer of elements to inorganic nutrients that support biological productivity and food web processes. Therefore, quantification of the controls of organic matter mineralization is crucial to understanding the carbon cycle and biogeochemical dynamics in coastal marine environments. We investigated the rates and pathways of anaerobic carbon (C) oxidation in an unvegetated mud flat (UMF) and a vegetated mud flat (VMF) of the Ganghwa intertidal zone of the macro-tidal Han River estuary, Yellow Sea. Analyses of geochemical constituents revealed relatively oxidized conditions and high reactive Fe(III) concentrations (40–100 μmol cm⁻³) in the sediments. A pronounced depth stratification in Fe(III) was observed at the VMF site likely due to the lower number of infaunal burrows along with dense root formation by the macrophytes, Suaeda japonica. Depth-integrated rates of anaerobic C mineralization as well as sulfate- and Fe(III) reduction at the VMF were consistently higher than those at the UMF, likely driven by the dense vegetation that supplied organic C substrates and electron acceptors to the rhizosphere. Sediment inventories revealed that solid Fe(III) was up to 17 times more abundant than pore water sulfate, and direct rate measurements showed that microbial Fe(III) reduction comprised an equal or larger percentage of C oxidation (36–66 %) in comparison to sulfate reduction (36–40 %) at both sites studied. Time-course experiments indicated that sulfate reduction rates were likely underestimated, especially in the VMF rhizosphere, due to the reoxidation of reduced S in the presence of high Fe(III). The high rates of C mineralization suggest that the Ganghwa intertidal mud flats are a significant sink against the external loading of organic compounds, and organic matter mineralization is enhanced by chemical exchange regulated by extreme tidal flushing and macro-microorganisms interactions.     

Dissolved organic matter in Arctic multi-year sea ice during winter: major components and relationship to ice characteristics

Ice cores were collected between 10.03.93 and 15.03.93 along a 200 m profile on a large ice floe in Fram Strait. The ice was typical of Arctic multi-year ice, having a mean thickness along the profile of 2.56 ±0.53 m. It consisted mostly of columnar ice (83%) grown through congelation of seawater at the ice bottom, and the salinity profiles were characterized by a linear increase from 0 psu at the top to values ranging between 3 and 5 psu at depth. Distributions of dissolved organic carbon (DOC) and nitrogen (DON) and major nutrients were compared with ice texture, salinity and chlorophyll a. DOC, DON, dissolved inorganic nitrogen (DIN), NH₄ ⁺ and NO₂ ^– were present in concentrations in excess of that predicted by dilution curves derived from Arctic surface water values. Only NO₃ ^– was depleted, although not exhausted. High DOC and DON values in conjunction with high NH₄ ⁺ levels indicated that a significant proportion of the dissolved organic matter (DOM) was a result of decomposition/grazing of ice algae and/or detritus. The combination of high NH₄ ⁺ and NO₂ ^– points to regeneration of nitrogen compounds. There was no significant correlation between DOC and Chl a in contrast to DON, which had a positively significant correlation with both salinity and Chl a, and the distribution of DOM in the cores might best be described as a combination of both physical and biological processes. There was no correlation between DOC and DON suggesting an uncoupling of DOC and DON dynamics in multi year ice.