Jens Würgler Hansen

The anaerobic degradation of organic matter in Danish coastal sediments: Iron reduction, manganese reduction, and sulfate reduction

We used a combination of porewater and solid phase analysis, as well as a series of sediment incubations, to quantify organic carbon oxidation by dissimilatory Fe reduction, Mn reduction, and sulfate reduction, in sediments from the Skagerrak (located off the northeast coast of Jutland, Denmark). In the deep portion of the basin, surface Mn enrichments reached 3.5 wt%, and Mn reduction was the only important anaerobic carbon oxidation process in the upper 10 cm of the sediment. In the less Mn-rich sediments from intermediate depths in the basin, Fe reduction ranged from somewhat less, to far more important than sulfate reduction. Most of the Mn reduction in these sediments may have been coupled to the oxidation of acid volatile sulfides (AVS), rather than to dissimilatory reduction. High rates of metal oxide reduction at all sites were driven by active recycling of both Fe and Mn, encouraged by bioturbation. Recycling was so rapid that the residence time of Fe and Mn oxides, with respect to reduction, ranged from 70-250 days. These results require that, on average, an atom of Fe or Mn is oxidized and reduced between 100-300 times before ultimate burial into the sediment. We observed that dissolved Mn2+ was completely removed onto fully oxidized Mn oxides until the oxidation level of the oxides was reduced to about 3.8, presumably reflecting the saturation by Mn2+ of highly reactive surface adsorption sites. Fully oxidized Mn oxides in sediments, then, may act as a cap preventing Mn2+ escape. We speculate that in shallow sediments of the Skagerrak, surface Mn oxides are present in a somewhat reduced oxidation level (< 3.8) allowing Mn2+ to escape, and perhaps providing the Mn2+ which enriches sediments of the deep basin.

Pathways of organic carbon oxidation in three continental margin sediments

We have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of Denmark and Norway. Rates of NH4+ and Sigma CO2 liberation sediment incubations were used with O2 penetration depths to conclude that O2 respiration accounted for only between 3.6-17.4% of the total organic carbon oxidation. Dentrification was limited to a narrow zone just below the depth of O2 penetration, and was not a major carbon oxidation pathway. The processes of Fe reduction, Mn reduction and sulfate reduction dominated organic carbon mineralization, but their relative significance varied depending on the sediment. Where high concentrations of Mn-oxide were found (3-4 wt% Mn), only Mn reduction occurred. With lower Mn oxide concentrations more typical of coastal sediments, Fe reduction and sulfate reduction were most important and of a similar magnitude. Overall, most of the measured O2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon. We suspect that the importance of O2 respiration in many coastal sediments has been overestimated, whereas metal oxide reduction (both Fe and Mn reduction) has probably been well underestimated.

Manganese oxidation and in situ manganese fluxes from a coastal sediment

Abstract Manganese fluxes from the seafloor were measured in situ in Aarhus Bay, Denmark, with a free operating benthic flux-chamber lander (ELINOR). Constant effluxes were observed during 3 h incubations. The benthic Mn flux resulted in bottom water concentrations of dissolved Mn up to 0.6 μM, whereas concentrations above the pycnocline were about 0.1 μM. Similar fluxes were observed from sediment cores incubated in the laboratory under in situ conditions. A large variation between cores was attributed to the small area covered by each core. Manganese reduction in the upper 0–1 cm of the sediment supported steep porewater gradients of Mn towards the surface. However, calculated diffusive Mn fluxes towards the sediment surface were 3–16 times higher than the benthic effluxes. This demonstrated high rates of Mn oxidation in the 1–2 mm thin oxic surface layer with turnover times of 2 h or less. Model calculations including measured microdistributions of O2 and pH yielded rate constants more than 1000 times higher than those reported for abiotic Mn oxidation implying that Mn oxidation in the sediment was microbially mediated. The rapid oxidation was, as an internal source of oxidized Mn, essential to the intense redox cycling of Mn in the surface sediment.

Regime shift in a coastal marine ecosystem

We demonstrate changes in ecosystem stable states in a coastal lagoon that are consistent with what a regime shift would hypothesize. In the nutrient-stressed Ringkøbing Fjord, Denmark, a small change in one variable (salinity) facilitated by a change in sluice management, caused a sudden regime shift from a bottom-up controlled turbid state, into a top-down controlled clear-water state. The change in dominating pathway of organic matter production from pelagic turnover to benthic-pelagic coupling was facilitated by new recruitment and growth of existing suspension-feeding clams, Mya arenaria. With the invasion of clams, benthic grazing became the key feature of the biological structure. Phytoplankton composition and zooplankton abundance were also affected by the change in biological structure. The small, but sudden, increase in salinity caused by the change in sluice management led to a dramatic reduction in biomass and coverage of benthic vegetation and thus affected herbivorous waterbird populations. In recent years, plant coverage has been increasing, as can be expected with increased water transparency. The regime shift has some major implications for coastal water management and revealed some conflicts between different types of nature and environmental protection legislation.

Thiosulfate and sulfite distributions in porewater of marine sediments related to manganese, iron, and sulfur geochemistry

Depth distributions of thiosulfate (S[sub 2]O[sub 3][sup 2[minus]]) and sulfite (SO[sub 3][sup 2[minus]]) were measured in the porewaters of a Danish salt marsh and subtidal marine sediments by HPLC analysis after derivatization with DTNP [2,2[prime]-dithiobis(5-nitropyridine)]. The distributions were compared to the redox zonation as indicated by Eh and Mn[sup 2+], Fe[sup 2+], and H[sub 2]S distributions. Concentrations of S[sub 2]O[sub 3][sup 2[minus]] varied from below detection (<50 nM) to 600 nM while SO[sub 3][sup 2[minus]] concentrations generally were 2-3 times higher, 100-1500 nM. Depth distributions of the two species were roughly similar. Lowest concentrations were found in the oxidized zone, including both the oxic surface layer and the suboxic and into the reduced, sulfidic zone. The similarity of SO[sub 3][sup 2[minus]] and S[sub 2]O[sub 3][sup 2[minus]] profiles suggested a close coupling of the cycling of the two species. Rates of consumption were suggested as the main factor governing their distribution. Rapid turnover times for S[sub 2]O[sub 3][sup 2[minus]] and H[sub 2]S of 4 and 1.1 h, respectively, were estimated for the upper 0-1 cm of a subtidal sediment.

Marine litter in Nordic waters

This report provides an overview of the currently available data from studies on marine litter in the Nordic countries. This covers various field studies on amount, distribution, characteristics an ...

Nitrogen Inputs from Agriculture: Towards Better Assessments of Eutrophication Status in Marine Waters

Nutrient enrichment of coastal marine waters caused by losses of nitrate (NO3−) from agriculture is an increasing global problem. In the European Union, the Nitrates Directive (ND) of 1991 was meant to be a cornerstone in reducing eutrophication effects in coastal waters downstream from intensively farmed catchments. Although reductions in losses of nitrate have been attained, very few Member States have yet been able to reduce eutrophication effects caused by inputs of NO3− from agriculture. We report trends in concentrations of NO3− and chlorophyll-a (Chl-a) in Danish coastal and open marine waters during the period from 1996 to 2011 together with an assessment of eutrophication status based on multiple indicators (e.g. nutrient concentrations, Chl-a, submerged aquatic vegetation and benthic macroinvertebrates). Despite decreasing concentrations of both NO3−and Chl-a, Danish coastal waters are not yet to be classified as ‘unaffected by eutrophication’. In order to improve future pan-European evaluations of the effectiveness of the ND, we argue for the added value of including indicators and assessment principles from other European Directives, i.e. the Water Framework Directive and the Marine Strategy Framework Directive.