Annelie Skoog

Dissolved organic matter and nutrients in the Lena River, Siberian Arctic : Characteristics and distribution

Dissolved organic carbon (DOC) and nitrogen (DON), amino acids, carbohydrates and inorganic nutrients were measured on samples taken in July 1994 at 18 stations between Yakutsk and the Lena delta, East Siberia. There were no obvious gradients or features along the river, except in the tributaries, the Aldan and Vilyuy rivers, where significantly higher concentrations of several parameters were measured. Concentrations of DOC varied between 300 and 1000 μM C, with most values varying between 500 and 700 μM C (mean 570 μM C). DON concentrations ranged between 9 and 28 μM N (mean 13 μM N). The C/N ratios of bulk dissolved organic matter (DOM) varied from 30 to 58, with 75% of the values being between 45 and 55 (mean 48). Total dissolved amino acids (TDAA) ranged between 1.6 and 5.4 μM, averaged about 3.5 μM, mostly in the combined form, and represented about 28% of the DON. Free amino acids were only about 2% of TDAA. Glycine, aspartic acid and glutamic acid predominated, accounting for about 41% of TDAA. Total dissolved carbohydrates ranged from 190 to 470 μg glucose equivalents l−1 and averaged 299 μg l−1, forming only 1.2 to 2.5% of the DOC pool. The following ranges of inorganic nutrients were measured: nitrate, 0.01 to 1.4 μM N (mean 0.6 μM N); nitrite, 0.03 to 0.1 μM N (mean 0.07 μM N); ammonium, 0.01 to 0.3 μM N (mean 0.13 μM N); phosphate, 0.2 to 1 μM P (mean 0.5 μM P); silicate, 59 to 87 μM Si (mean 66 μM Si). Carbon isotope data of the suspended organic material suggest that the low inorganic nitrogen values are not due to algal uptake, but rather an inherent characteristic of the river and the catchment area. This, together with positive correlations between silicate, DOC and DON and high C/N values, suggests that the composition of DOM in the Lena River is mainly determined by the input of soil-derived, recalcitrant material and not by autochthonous sources.

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.

Non‐Redfield carbon and nitrogen cycling in the Arctic: Effects of ecosystem structure and dynamics

The C:N ratio is a critical parameter used in both global ocean carbon models and field studies to understand carbon and nutrient cycling as well as to estimate exported carbon from the euphotic zone. The so-called Redfield ratio (C:N = 6.6 by atoms) [Redfield et al., 1963] is widely used for such calculations. Here we present data from the NE Greenland continental shelf that show that most of the C:N ratios for particulate (autotrophic and heterotrophic) and dissolved pools and rates of transformation among them exceed Redfield proportions from June to August, owing to species composition, size, and biological interactions. The ecosystem components that likely comprised sinking particles and had relatively high C:N ratios (geometric means) included (1) the particulate organic matter (C:N = 8.9) dominated by nutrient-deficient diatoms, resulting from low initial nitrate concentrations (approximately 4 μM) in Arctic surface waters; (2) the dominant zooplankton, herbivorous copepods (C:N = 9.6), having lipid storage typical of Arctic copepods; and (3) copepod fecal pellets (C:N = 33.2). Relatively high dissolved organic carbon concentrations (median 105 μM) were approximately 25 to 45 μM higher than reported for other systems and may be broadly characteristic of Arctic waters. A carbon-rich dissolved organic carbon pool also was generated during summer. Since the magnitude of carbon and nitrogen uncoupling in the surface mixed layer appeared to be greater than in other regions and occurred throughout the productive season, the C:N ratio of particulate organic matter may be a better conversion factor than the Redfield ratio to estimate carbon export for broad application in northern high-latitude systems.

Methodological investigations on DOC determinations by the HTCO method

Abstract Results are presented on the function and use of the catalyst in the commonly employed high-temperature catalytic oxidation (HTCO) method for determination of organic carbon concentrations. The instrument used was a Shimadzu TOC 5000 analyzer with a 0.5% platinum-coated aluminum oxide catalyst. Some basic considerations on gas flow rates and volumes of the catalyst kiln, together with experiments using carrier gas with and without oxygen, led to the conclusion that O2 is unlikely as the source of reacted oxygen in the HTCO oxidation of organic carbon. Results from injections of 14C-labelled organic material showed that no residual carbon remains on the catalyst surface after a large number of injections. This indicates that the oxidation of organic carbon to gaseous compounds is complete and the catalyst is not a source of carry over signal between injections. The blank signal from injection of water probably originates from carbonaceous compounds contained in the catalyst particles. A series of salt water injections (salinity 34.4) showed that > 2500 injections can be made without deterioration of catalyst performance.

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

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

Multi-element speciation of trace metals in fresh water adapted to plasma source mass spectrometry

A method for speciation of trace metals at natural concentration levels has been developed. The speciation is carried out in a cleanroom laboratory and the determination is performed with an ICP-MS. The method is based on fractionation of metal species on three adsorbents. Chelex-100, Sep-Pak C-18 and Fractogel DEAE. The method distinguishes between labile complexes, non-polar organic adsorbable matter and ion exchangeable substances. The filtered water samples are pumped through columns with the adsorbents. Following a prefraction, samples for determination of metal concentrations are taken. The adsorbents are not eluted. After extensive cleaning, the used adsorbents have blank values that are low enough for trace metal analysis in fresh water. Concentration and speciation data for Al, Cd, Co, Cu, Fe, Pb, Mn, Mo, Ni and Zn are obtained.