Three-dimensional fluorescence as a tool for investigating the dynamics of dissolved organic matter in the Lake Biwa watershed

Quantitative and qualitative characterizations of dissolved organic matter (DOM) were carried out at the watershed level in central Japan by measuring dissolved organic carbon (DOC) concentration and the three-dimensional excitation–emission matrix (3-D EEM). DOC concentration was low (mean 37 ± 19 µM C) in the upstream waters, whereas, in general, it increased toward the downstream areas (mean 92 ± 47 µM C). Significant variations in DOC concentration were detected among rivers and channels. DOC concentration in the epilimnion of Lake Biwa increased during the summer period and decreased during the winter period. The lake hypolimnion has lower DOC concentration (mean 87 ± 7 µM C) compared with the epilimnion (107 ± 15 µM C). Fulvic acid (FA)-like substances in the DOM were directly characterized by 3-D EEM. The fluorescence peak for upstream DOM was found in regions with longer wavelengths (excitation/emission 386 ± 6/476 ± 5 nm) compared with downstream and lake DOM (351 ± 12/446 ± 15 nm and 341 ± 6/434 ± 6 nm, respectively). The DOC concentration is correlated with fluorescence peak intensity of FA-like substances in DOM in river waters. Such a relationship was not found in lake DOM. A blueshift of the fluorescence peak from upstream to lake DOM was observed. A decrease in fluorescence intensities was also detected during the summer period. These results may suggest that the degradation of FA-like substances in DOM occurs from natural solar irradiation. Protein-like fluorescence was significantly detected in the lake epilimnion during the summer period. A linear relationship between DOC concentration and protein-like fluorescence indicated that an autochthonous input of DOM gave rise to the increase in DOC concentration in the lake epilimnion during the summer. These results may suggest that the 3-D EEM can be used as a tool for the investigation of DOM dynamics at the watershed level with concurrent measurement of DOC concentration and the fluorescence properties of fulvic acid-like and protein-like substances.     

Fluorescence and amino acid characteristics of molecular size fractions of DOM in the waters of Lake Biwa

Dissolved organic matter (DOM) in the waters from Lake Biwa, Japan was fractionated using tangential flow ultrafiltration, and subsequently characterized by fluorescence properties and amino acids. While major dissolved organic carbon (DOC), UV absorbance (Abs), humic-like fluorescence (Flu) and total hydrolyzed amino acids (THAA) occurred in the less than 5 kDa molecular size fraction, they were not evenly distributed among various molecular size fractions. Flu/Abs ratios increased, and THAA/DOC ratios decreased with decreasing molecular size. Humic-like fluorescence occurred in all molecular size fractions, but protein-like fluorescence only occurred in the 0.1 m-GF/F fraction. Subtle differences in amino acid compositions (both individuals and functional groups) were observed between various molecular size fractions, this may indicate the occurrence of DOM degradation from higher to lower molecular weight. The results reported here have significance for further understanding the sources and nature of DOM in aquatic environments.     

Chemical Characteristics and Origin of Dissolved Organic Matter in the Yukon River

Monthly (or bi-weekly) water samples were collected from the Yukon River, one of the largest rivers in North America, at a station near the US Geological Survey Stevens Village hydrological station, Alaska from May to September 2002, to examine the quantity and quality of dissolved organic matter (DOM) and its seasonal variations. DOM was further size fractionated into high molecular weight (HMW or colloidal, 1 kDa–0.45 μm) and low molecular weight (LMW, <1 kDa) fractions. Dissolved organic carbon (DOC), colored dissolved organic matter (C-DOM) and total dissolved carbohydrate (TCHO) species were measured in the size fractionated DOM samples. Concentrations of DOC were as high as 2830 μmol-C l⁻¹ during the spring breakup in May and decreased significantly to 508–558 μmol-C l⁻¹ during open-water season (June–September). Within the DOC pool, up to 85% was in the colloidal fraction (1 kDa–0.45 μm) in early May. As DOC concentration decreased, this colloidal portion remained high (70–85% of the bulk DOC) throughout the sampling season. Concentrations of TCHO, including monosaccharides (MCHO) and polysaccharides (PCHO), varied from 722 μmol-C l⁻¹ in May to 129 μmol-C l⁻¹ in September, which comprised a fairly constant portion of bulk DOC (24±2%). Within the TCHO pool, the MCHO/TCHO ratio consistently increased from May to September. The C-DOM/DOM ratio and the size fractionated DOM increased from May to September, indicating that DOM draining into the Yukon River contained increased amounts of humified materials, likely related to a greater soil leaching efficiency in summer. The average composition of DOM was 76% pedogenic humic matter and 24% aquagenic CHO. Characteristics of soil-derived humic substances and low chlorophyll-a concentrations support a dominance of terrestrial DOM in Yukon River waters.     

Distribution of dissolved organic carbon and dissolved fulvic acid in mesotrophic Lake Biwa, Japan

The dissolved organic carbon (DOC) concentrations in mesotrophic Lake Biwa were determined by a total organic carbon (TOC) analyzer, and DOC molecular size distributions were determined by size exclusion chromatography (SEC) using a fluorescence detector at excitation/emission (Ex/Em) levels of 300/425 nm with the eluent at pH 9.7. The fluorescence wavelengths for detection were chosen from the result of excitation–emission matrix spectrometry (EEM) analysis for dissolved fulvic acid (DFA) extracted from Ado River (peak A, Ex/Em = 260–270/430–440 nm; peak B, Ex/Em = 300–310/420–430 nm). Ado River DFA was eluted with a retention time (RT) of 7.4–8.9 min and the apparent molecular weight was estimated at 22–87 kDa based on the elution curve for the spherical protein molecular weight standard. A DFA peak eluted at the same retention time as Ado River DFA also appeared in all the samples of Lake Biwa water. From the linear relationship between the peak areas with an RT of 7.4–8.9 min by SEC analysis and DOC values of DFA by TOC analysis of a series of DFA samples (r² = 0.9995), the concentrations of DFA in the lake water were roughly calculated. DFA was distributed within the range 0.25–0.43 mg C l⁻¹ and accounted for 15%–41% of DOC, with the highest ratios observed at a depth of 70 m in August and the lowest at 2.5 m in May.     

Effects of different molecular weight fractions of dissolved organic matter on the growth of bacteria,algae and protozoa from a highly humic lake

Effects of different molecular size fractions (< 1000 MW, < 10 000 MW, < 100 000 MW and <0.1 μm) of dissolved organic matter (DOM) on the growth of bacteria, algae and protozoa from a highly humic lake were investigated. DOM from catchment drainage water as well as from the lake consisted mostly (59–63%) of high molecular weight (HMW) compounds (> 10 000 MW). With excess inorganic nutrients, the growth rate and yield of bacteria were almost identical in all size fractions. However, in < 1000 MW fractions and with glucose added, a longer lag phase occurred. Without added nutrients both the growth rates and biomasses of bacteria decreased towards the smaller size fractions and the percentage of dissolved organic carbon (DOC) used during the experiment and the growth efficiency of bacteria were lower than with excess nutrients. The growth efficiency of bacteria was estimated to vary between 3–66% in different MW fractions, largely depending on the nutrient concentrations, but the highest growth efficiencies were observed in HMW fractions and with glucose. The growth of algae was clearly lowest in the < 1000 MW fraction. In dim light no net growth of algae could be found. In contrast, added nutrients substantially enhanced algal growth and in deionized water with glucose, algae achieved almost the same growth rate and biomass as in higher MW fractions of DOM. The results suggested that bacteria and some algae were favoured by DOM, but protozoans seemed to benefit only indirectly, through bacterial grazing. The utilization of DOM by bacteria and algae was strongly affected by the availability of phosphorus and nitrogen.     

Fluorescence spectroscopic characterization of dissolved organic matter in the waters of Lake Fuxian and adjacent rivers in Yunnan,China

The fluorescence properties of dissolved organic matter (DOM) in the water of Lake Fuxian and its adjacent rivers on the Yunnan Plateau, southwestern China, were studied to specify the characterization of DOM in the lake and river waters. The fluorescence properties with the excitation–emission matrix in the water of Lake Fuxian are different from those in the river water. The differences in these properties between the lake and river water could arise not only from their sources but also from the reactivity of the photobleaching of DOM. In the lake, the supplying of allochthonous fluorescent materials from inflowing rivers to the fluorescent DOM is less significant than the photobleaching of fluorescent substances.     

Spectroscopic characterization and molecular weight distribution of dissolved organic matter in sediment porewaters from Lake Erhai, Southwest China

Dissolved organic matter (DOM) in sediment porewaters from Lake Erhai, Southwest China was investigated using dissolved organic carbon (DOC) concentration, UV absorbance, fluorescence and molecular weight distribution. DOC exhibited a high concentration at the sediment–water interface with a rapid decrease to the oxic–anoxic interface at approximately 7 cm, and then increased with depth. Similar trends were also found for the UV absorption coefficients at 254 and 280 nm in the porewaters. DNA in the sediment was also measured, which confirmed the high abundance of aerobic bacteria in the upper layer of the sediment. Both humic-like (peaks A and C) and protein-like (peaks B and D) fluorescence were observed in the porewater DOM, and their fluorescence intensities exhibited a similar porewater profile as DOC concentration. A strong correlation was found between the peak fluorescence intensity ratio r(A, C) and r(D, B). Both the fluorescence index and UV absorption coefficient at 254 nm suggested a dramatic increase in aromaticity of porewater DOM across the oxic–anoxic interface. Porewater DOM exhibited a multimodal distribution of molecular weight with a relatively low polydispersity. The results of this study offer significant insight into the nature and properties of DOM in freshwater ecosystems.     

Characterization of Nanming River (southwestern China) sewerage-impacted pollution using an excitation-emission matrix and PARAFAC

Nanming River, the largest urban river in Guizhou Province, southwestern China plateau, has been severely polluted for decades. This study characterizes the organic materials and their sources in the upstream and downstream waters by dissolved organic carbon (DOC), excitation emission matrix (EEM) spectroscopy, parallel factor (PARAFAC) analysis and photo-microbial experiments. DOC concentrations were low (47–120 μM C) upstream and relatively high (146–462 μM C) downstream. The PARAFAC studies on the sample EEM spectra demonstrated that the upstream dissolved organic matter (DOM) was mostly composed of one component that had a fulvic acid-like substance; downstream DOM was composed of two components with mixtures of tryptophan-like and fulvic acid-like substances. From the results of the sewerage drainage samples collected along the bank of the river, it is evident that both household detergent-like and protein-like or tryptophan-like substances are predominantly present, indicating that untreated sewerage effluents are the major sources of organic matter pollution in Nanming River. The degradation experiments conducted on river, sewerage drainage and commercial detergent samples demonstrated that the detergent-like and tryptophan-like substances are both photochemically and microbiologically more decomposable than fulvic acid-like materials under sunlight and dark incubations. These results suggest that the input of the untreated sewerage effluents along the streams is the major pollution source in Nanming River, and the fluorescent DOM was efficiently affected by both photochemical and microbial processes.     

Temporal and spatial distributions of dissolved organic carbon and nitrogen in two small lakes on the Southwestern China Plateau

Temporal and spatial distributions of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), chlorophyll-a and inorganic nitrogen were investigated in two small mountainous lakes (Lake Hongfeng and Baihua), on the Southwestern China Plateau, based on almost 2 years’ field observation. DOC concentrations ranged from 163 μM to 248 μM in Lake Hongfeng and from 143 μM to 308 μM in Lake Baihua, respectively, during the study period. DON concentrations ranged from 7 μM to 26 μM in Lake Hongfeng and from 14 μM to 47 μM in Lake Baihua. DOC showed vertical heterogeneity with higher concentrations in the epilimnion than in the hypolimnion during the stratification period. The DON concentration profiles appeared to be more variable than the DOC profiles. Apparent DON maxima occurred in the upper layer of water. In Lake Hongfeng, DOC concentration in the surface water was highest at the end of spring and early summer. DON concentration was 2–5 μM higher in May 2003 and in June 2004 than in adjacent months. DOC and chlorophyll-a concentrations were significantly correlated (r = 0.79, P < 0.05). The period of highest concentrations of DOC in Lake Hongfeng was also the season of concentrated rainfall. Algae activity and allochthonous input might result in an increase of DOC and DON concentrations together. In Lake Baihua, the maximum concentrations of DOC and DON in the surface water occurred simultaneously in May 2003 and February 2004. DOC concentrations were significantly correlated with DON (r = 0.90, P < 0.01), indicating the common sources. Allochthonous input, biological processes, stratification and mixing were the most important factors controlling the distributions and cycling of dissolved organic matter (DOM) and inorganic nitrogen in these two lakes. Inference from the corresponding vertical distributions of DOM and inorganic nitrogen indicated that DOM played potential roles in the internal loading of nitrogen and metabolism in the water body in these small lakes. The carbon/nitrogen (C/N) ratio showed a potential significance for tracing the source and biogeochemical processes of DOM in the lakes. These results are of significance in the further understanding of biogeochemical cycling and environmental effects of DOM and nitrogen in lake ecosystems.     

Fluorescence characteristics and biodegradability of dissolved organic matter in forest and wetland soils from coastal temperate watersheds in southeast Alaska

Understanding how the concentration and chemical quality of dissolved organic matter (DOM) varies in soils is critical because DOM influences an array of biological, chemical, and physical processes. We used PARAFAC modeling of excitation–emission fluorescence spectroscopy, specific UV absorbance (SUVA₂₅₄) and biodegradable dissolved organic carbon (BDOC) incubations to investigate the chemical quality of DOM in soil water collected from 25 cm piezometers in four different wetland and forest soils: bog, forested wetland, fen and upland forest. There were significant differences in soil solution concentrations of dissolved organic C, N, and P, DOC:DON ratios, SUVA₂₅₄ and BDOC among the four soil types. Throughout the sampling period, average DOC concentrations in the four soil types ranged from 9–32 mg C l⁻¹ and between 23–42% of the DOC was biodegradable. Seasonal patterns in dissolved nutrient concentrations and BDOC were observed in the three wetland types suggesting strong biotic controls over DOM concentrations in wetland soils. PARAFAC modeling of excitation–emission fluorescence spectroscopy showed that protein-like fluorescence was positively correlated (r ² = 0.82; P < 0.001) with BDOC for all soil types taken together. This finding indicates that PARAFAC modeling may substantially improve the ability to predict BDOC in natural environments. Coincident measurements of DOM concentrations, BDOC and PARAFAC modeling confirmed that the four soil types contain DOM with distinct chemical properties and have unique fluorescent fingerprints. DOM inputs to streams from the four soil types therefore have the potential to alter stream biogeochemical processes differently by influencing temporal patterns in stream heterotrophic productivity.     

Biomass and potential feeding, respiration and growth of zooplankton in the Bransfield Strait (Antarctic Peninsula) during austral summer

Biomass (as dry weight and protein content), gut fluorescence, electron transfer system (ETS) and aspartate transcarbamylase (ATC) activities were studied in different size fractions (200–500, 500–1000 μm and 1–14 mm) in the Bransfield Strait (Antarctic Peninsula) during January 1993. Very low values of zooplankton biomass were observed in all the size classes studied. About 56% of total biomass was due to the large size fraction (1–14 mm) while the smallest one (200–500 μm) accounted for about 26%. Gut fluorescence values increased in relation to the size class considered, as expected, being the differences from the smaller to the highest size fractions of orders of magnitude. Calculated ingestion rates showed that about 60–80% of total zooplankton ingestion (<14 mm) was due to the smaller organisms. Higher average values and higher variability of specific ETS activity was observed in the smaller size fraction while no differences between size classes were observed for the specific ATC activity. Biomass, gut fluorescence, ETS and ATC activities were not significantly different between the Bellingshausen and Weddell waters, although higher standard deviation was normally found at the former area. With the restrictions of using the above indices to estimate physiological rates, potential grazing of mesozooplankton (<14 mm) accounted for a rather low portion (<10%) of the primary production. The index of growth showed high values, suggesting no food limitation of mesozooplankton. Therefore, other processes such as predation should account for the very low biomass found and for the fate of a large portion of primary production. Accepted: 26 March 2000     

Abundance and composition of the summer phytoplankton community along a transect from the Barguzin River to the central basin of Lake Baikal

The abundance and composition of phytoplankton were investigated at six stations along a transect from the Barguzin River inflow to the central basin of Lake Baikal in August 2002 to clarify the effect of the river inflow on the phytoplankton community in the lake. The water temperature in the epilimnion was high near the shore at Station 1 (17.3°C), probably due to the higher temperature of the river water, and gradually decreased offshore at Station 6 (14.5°C). Thermal stratification developed at Stations 2–6, and a thermocline was observed at a 17–22-m depth at Stations 2–4 and an 8–12-m depth at Stations 5 and 6. The concentrations of nitrogen and phosphorus nutrients in the epilimnion at all stations were <1.0 μmol N l⁻¹ and <0.16 μmol P l⁻¹, respectively. Relatively high concentrations of nutrients (0.56–7.38 μmol N l⁻¹ and 0.03–0.28 μmol P l⁻¹) were detected in the deeper parts of the euphotic zone. Silicate was not exhausted at all stations (>20 μmol Si l⁻¹). The chlorophyll a (chl. a) concentration was high (>10 μg l⁻¹) near the shore at Station 1 and low (<3 μg l⁻¹) at five other stations. The <2 μm fraction of chl. a in Stations 2–6 ranged between 0.80 and 1.85 μg l⁻¹, and its contribution to total chl. a was high (>60%). In this fraction, picocyanobacteria were abundant at all stations and ranged between 5 × 10⁴ and 5 × 10⁵ cells ml⁻¹. In contrast, chl. a in the >2 μm fraction varied significantly (0.14–11.17 μg l⁻¹), and the highest value was observed at Station 1. In this fraction, the dominant phytoplankton was Aulacoseira and centric diatoms at Station 1 and Cryptomonas, Ankistrodesmus, Asterionella, and Nitzschia at Stations 2–6. The present study demonstrated the dominance of picophytoplankton in the pelagic zone, while higher abundance of phytoplankton dominated by diatoms was observed in the shallower littoral zone. These larger phytoplankters in the littoral zone probably depend on nutrients from the Barguzin River.     

Fate and Transport of Organic Nitrogen in Minimally Disturbed Montane Streams of Colorado,USA

In two montane watersheds that receive minimal deposition of atmospheric nitrogen, 15–71% of dissolved organic nitrogen (DON) was bioavailable in stream water over a 2-year period. Discharge-weighted concentrations of bulk DON were between 102 and 135 μg/l, and the C:N ratio differed substantially between humic and non-humic fractions of DON. Approximately 70% of DON export occurred during snowmelt, and 40% of that DON was biologically available to microbes in stream sediments. Concentrations of bioavailable DON in stream water were 2–16 times greater than dissolved inorganic nitrogen (DIN) during the growing season, and bioavailable DON was depleted within 2–14 days during experimental incubations. Uptake of DON was influenced by the concentration of inorganic N in stream water, the concentration of non-humic DON in stream water, and the C:N ratio of the non-humic fraction of dissolved organic matter (DOM). Uptake of DON declined logarithmically as the concentration of inorganic N in stream water increased. Experimental additions of inorganic N also caused a decline in uptake of DON and net production of DON when the C:N ratio of non-humic DOM was high. This study indicates that the relative and absolute amount of bioavailable DON can vary greatly within and across years due to interactions between the availability of inorganic nutrients and composition of DOM. DOM has the potential to be used biotically at a high rate in nitrogen-poor streams, and it may be generated by heterotrophic microbes when DIN and labile DOM with low relative nitrogen content become abundant.     

Urea decomposing activity of fractionated brackish phytoplankton in Lake Nakaumi

The influence of brackish phytoplankton cell classes upon the response of urea decomposition was investigated in Lake Nakaumi. The urea decomposition rate was 5 to 350 μmol urea m⁻³ h⁻¹ in the light and 3 to 137 μmol urea m⁻³ h⁻¹ in the dark. The urea decomposition rates in the light were obviously higher than in the dark. An extremely high rate (350 μmol urea m⁻³ h⁻¹) was observed in Yonago Bay. The rate in the smaller fraction (<5 μm) exceeded that in the middle (5–25 μm) and larger fractions (>25 μm). The chlorophyll- and photosynthesis-specific rates for urea decomposition in the light were 0.5 to 3.9 μmol urea mg chl.a ⁻¹ h⁻¹ and 0.3 to 1.3 μmol urea mg photo.C⁻¹. The specific urea decomposing activities were higher in the smaller fraction than in the other two fractions. The present results suggest that in brackish waters urea decomposition occurred with coupling to the standing crop and photosynthetic activity of phytoplankton. Received: May 22, 1999 / Accepted: August 15, 1999     

Stream dissolved organic matter bioavailability and composition in watersheds underlain with discontinuous permafrost

We examined the impact of permafrost on dissolved organic matter (DOM) composition in Caribou-Poker Creeks Research Watershed (CPCRW), a watershed underlain with discontinuous permafrost, in interior Alaska. We analyzed long term data from watersheds underlain with varying degrees of permafrost, sampled springs and thermokarsts, used fluorescence spectroscopy, and measured the bioavailabity of dissolved organic carbon (DOC). Permafrost driven patterns in hydrology and vegetation influenced DOM patterns in streams, with the stream draining the high permafrost watershed having higher DOC and dissolved organic nitrogen (DON) concentrations, higher DOC:DON and greater specific ultraviolet absorbance (SUVA) than the streams draining the low and medium permafrost watersheds. Streams, springs and thermokarsts exhibited a wide range of DOC and DON concentrations (1.5–37.5 mgC/L and 0.14–1.26 mgN/L, respectively), DOC:DON (7.1–42.8) and SUVA (1.5–4.7 L mgC⁻¹ m⁻¹). All sites had a high proportion of humic components, a low proportion of protein components, and a low fluorescence index value (1.3–1.4), generally consistent with terrestrially derived DOM. Principal component analysis revealed distinct groups in our fluorescence data determined by diagenetic processing and DOM source. The proportion of bioavailable DOC ranged from 2 to 35%, with the proportion of tyrosine- and tryptophan-like fluorophores in the DOM being a major predictor of DOC loss (p < 0.05, R ² = 0.99). Our results indicate that the degradation of permafrost in CPCRW will result in a decrease in DOC and DON concentrations, a decline in DOC:DON, and a reduction in SUVA, possibly accompanied by a change in the proportion of bioavailable DOC.     

Effects of Watershed History on Dissolved Organic Matter Characteristics in Headwater Streams

Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humic-like components in chromophoric DOM. In addition, excitation–emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.     

Association of Chernobyl-derived 239+240Pu, 241Am, 90Sr and 137Cs with different molecular size fractions of organic matter in the soil solution of two grassland soils

Radiocesium is normally bound only rather weakly and unspecifically by humic substances, in contrast to the actinides Pu and Am. Recently, however, it was observed that fallout ¹³⁷Cs in the soil solution from an Of-horizon of a podzol forest soil (slightly decomposed plant material) was associated essentially only with one single size fraction of the humic substances. In deeper soil layers with well humified material (AOh-horizon), radiocesium was associated with all size fractions of the dissolved organic matter (DOM). To examine whether this unexpected behaviour is also observable for DOM isolated from other soils, we determined the association of fallout ¹³⁷Cs,⁹⁰Sr,²³⁸Pu, ^239+240Pu and ²⁴¹Am with various size fractions of DOM from in situ soil solutions isolated from two layers (0–2 cm and 2–5 cm) of two grassland soils (a soddy podzolic soil and a peat soil) within the 10 km zone of the nuclear reactor at Chernobyl (Ukraine). The four size fractions of DOM as obtained by gel filtration of the soil solution were (mean nominal molecular weight in daltons): fraction I: ≥2000, fraction II: 1300; fraction III: 560, fraction IV: inorganic compounds. The results for the well humified DOM (humus accumulation horizon of podzol, deeper layer of peat soil) showed that Pu and Am are essentially associated with the high molecular weight fractions, while Sr is present only in the `inorganic' fraction. Radiocesium is found in all the size fractions separated. A quite similar pattern was also found for Pu, Am, and Sr in the soil solution from only slightly decomposed plant material (0–2 cm of peat soil), but not for radiocesium. This radionuclide was again essentially only observable in one single low molecular weight fraction of DOM. The above results thus support our recent observations in the different horizons of a forest podzol mentioned above, even though no reason for the different binding of radiocesium by well humified soil organic matter and by only slightly decomposed plant material can be given at present. The data demonstrate, however, that information on only the total amount of a radionuclide in the soil solution will not be sufficient to interpret or predict its fate adequately in the soil. Received: 13 February 1998 / Accepted in revised form: 14 July 1998     

Speciation of iodine in high iodine groundwater in china associated with goitre and hypothyroidism

Iodine intake affects the occurrence of disease in a population. Excessive iodine intake may be caused by a high iodine content of drinking water. Tap water in few locations in Europe contains up to 139 μg/L mostly bound to humic substances, probably leaching from marine sediments in the aquifers. Even higher iodine contents have been found in Chinese waters, previously shown to associate with goitre and hypothyroidism. The aims were to elucidate speciation of high iodine groundwater from deep wells in China and to compare with high iodine waters from Europe. Water was sampled from eight wells in five villages along Bohai Bay, China. Macro-molecules and low molecular weight (MW) substances were separated by size exclusion chromatography (high performance liquid chromatography, Superose 12 HR 10/30, buffer 0.1 M Tris, pH 7.0). Organic material was evaluated by A280 and iodine in fractions measured by the Ce/As method after alkaline incineration. Iodine content of well water varied from 135 to 880 μg/L (median 287 μg/L). The amount of organic material in water was low with A280, <1–5 mAU. The chromatographic traces were similar between samples: One peak of iodine eluted around K _AV 0.65 corresponding to MW 5 kDa (humic substances) and one peak at V _total (iodide/low MW substances). The fraction of iodine in macro-molecules, suggested to be humic substances, varied from 8% to 70% (median 27%). Iodine and peak absorbance were associated (p = 0.006). In conclusion, iodine in iodine-rich deep well water in northern China may have marine origin and may associate with humic substances, comparable to shallow well iodine-rich water in Europe. High iodine intake from iodine-rich water suggests the cause of endemic goitre and hypothyroidism in some areas in China being iodine.     

Degradation efficiency and molecular size alteration during the aerobic microbial treatment of lignite pyrolysis deposit water

 Investigations into aerobic biological degrada-tion were carried out as part of an extensive programme designed to facilitate the cheap remediation of a pyrolysis waste-water deposit. Attention was focused on the processes of carbon conversion by different populations. The susceptibility of a body of lignite-processing deposit water to microbiological degradation was examined in batch investigations in a Sapromat system and in continuous bench-scale fermenter cultivations, with respect to nutrient supply, inoculation culture and molecular size distribution. It was found that degradation best occurs with an adapted mixed culture. The autochthonous culture removes 30% less dissolved organic carbon (DOC) and has a 40% higher specific oxygen demand. A shortage of phosphorus, investigated with a view to avoiding additional eutrophication problems in the open water in the case of in situ remediation, causes reduced DOC degrada-tion and significantly higher specific oxygen demand. The biological process is overlapped by abiotic oxidation. During aerobic treatment, a concentration of colour-giving aromatic substances of between 0.5 kDa and 5 kDa was observed. This phenomenon is caused by the oxidation of low- and high-molecular-mass compounds. The removal of DOC is limited to 65% and mainly occurs in the range below 0.5 kDa (30%) and in the 0.5–1 kDa range (12%); the removal is negligible in the ranges 1–3 kDa (0.8%) and 3–5 kDa (2%) and a little higher in the ranges 5 kDa–0.3 μm (5%) and above 0.3 μm (6%). In the investigations it was discovered that DOC removal causes in the ranges below 0.5 kDa, 0.5–1 kDa and 5 kDa–0.3 μm mainly as a result of degradation, but the range above 0.3 μm is chiefly caused by bioadsorption. Aerobic microbiological treatment is able to remove most low-molecular-mass substances. In order to remove the macromolecular and colour-giving part of the deposit water, an additional treatment stage, e.g. flocculation, is required. Received: 10 October 1995/Received revision: 9 January 1996/Accepted: 15 January 1996     

Seasonal variability of inorganic and organic nitrogen in the North Sea

This study considers the cycling of nitrogen in the waters of the North Sea, particularly focussing on organic nitrogen. Dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and particulate organic nitrogen (PON) were measured in the North Sea over a one-year period (autumn 2004–summer 2005). The surface water concentrations of nitrate, ammonium, DON and PON during the present study ranged from <0.1–7.2 μM, <0.1–2.0 μM, 1.9–11.2 μM and 0.3–5.6 μM, respectively, with DON the dominant fraction of total nitrogen at all times. These nutrients concentrations were significantly lower compared to previous studies in the southern North Sea. The seasonal variations showed high mean surface concentrations of nitrate (4.7 ± 0.6 μM) and DON (8.9 ± 0.9 μM), low ammonium (<0.1 μM) and PON (0.8 ± 0.1 μM) in winter, shifting to low nitrate (0.3 ± 0.3 μM) and DON (4.2 ± 1.2 μM) in summer, with high ammonium (0.8 ± 0.4 μM) in autumn and PON (2.5 ± 1.2 μM) in spring. Highest mean surface DON concentration was measured in winter and may be due to resuspension of the organic matter from the bottom sediments. For autumn and spring, phytoplankton DON release was likely to be the most significant source of DON as shown by high concentrations of low molecular weight (LMW) DON and its positive correlation to chlorophyll a. Low total and LMW DON concentrations during summer were likely to be due to the uptake of the LMW DON fraction by phytoplankton and bacteria and the stratification of the water column. DON is therefore shown to be a potentially important source of nitrogen in shelf seas especially after the spring bloom has depleted nitrate to limiting concentrations. Handling editor: L. Naselli-Flores