Organic iron complexes enhance iron transport capacity along estuarine salinity gradients of Baltic estuaries

Rivers discharge a notable amount of dissolved Fe (1:5×109 mol yr-1) to coastal waters but are still not considered important sources of bioavailable Fe to open marine waters. The reason is that the vast majority of particular and dissolved riverine Fe is considered to be lost to the sediment due to aggregation during estuarine mixing. Recently, however, several studies demonstrated relatively high stability of riverine Fe to salinity-induced aggregation, and it has been proposed that organically complexed Fe (Fe-OM) can "survive" the salinity gradient, while Fe (oxy)hydroxides are more prone to aggregation and selectively removed. In this study, we directly identified, by X-ray absorption spectroscopy, the occurrence of these two Fe phases across eight boreal rivers draining into the Baltic Sea and confirmed a significant but variable contribution of Fe-OM in relation to Fe (oxy)hydroxides among river mouths. We further found that Fe-OM was more prevalent at high flow conditions in spring than at low flow conditions during autumn and that Fe-OM was more dominant upstream in a catchment than at the river mouth. The stability of Fe to increasing salinity, as assessed by artificial mixing experiments, correlated well to the relative contribution of Fe-OM, confirming that organic complexes promote Fe transport capacity. This study suggests that boreal rivers may provide significant amounts of potentially bioavailable Fe beyond the estuary, due to organic matter complexes

Spruce forest afforestation leading to increased Fe mobilization from soils

Increasing exports of Fe and DOC from soils, causing browning of freshwaters, have been reported in recent decades in many regions of the northern hemisphere. Afforestation, and in particular an increase of Norway spruce forest in certain regions, is suggested as a driver behind these trends in water chemistry. In this study, we tested the hypothesis that the gradual accumulation of organic soil layers in spruce forests, and subsequent increase in organic acid concentrations and acidity enhances mobilization of Fe. First generation Norway spruce stands of different ages (35, 61, 90 years) and adjacent arable control plots were selected to represent the effects of aging forest. Soil solutions were sampled from suction lysimeters at two depths (below organic soil layer and in mineral soil) during two years, and analyzed for Fe concentration, Fe speciation (XAS analysis), DOC, metals, major anions and cations. Solution Fe concentrations were significantly higher in shallow soils under older spruce stands (by 5- and 6-fold) than in control plots and the youngest forest. Variation in Fe concentration was best explained by variation in DOC concentration and pH. Moreover, Fe in all soil solutions was present as mononuclear Fe(III)-OM complexes, showing that this phase is dominating Fe translocation. Fe speciation in the soil was also analyzed, and found to be dominated by Fe oxides with minor differences between plots. These results confirmed that Fe mobilization, by Fe(III)-OM complexes, was higher from mature spruce stands, which supports that afforestation with spruce may contribute to rising concentrations of Fe in surface waters

Mass and UV-visible spectral fingerprints of dissolved organic matter: sources and reactivity

Advanced analytical techniques have revealed a high degree of complexity in the chemical makeup of dissolved organic matter (DOM). This has opened the door for a deeper understanding of the role of DOM in the aquatic environment. However, the expense, analytical cost, and challenges related to interpretation of the large datasets generated by these methods limit their widespread application. Optical methods, such as absorption and fluorescence spectroscopy are relatively inexpensive and easy to implement, but lack the detailed information available in more advanced methods. We were able to directly link the analysis of absorption spectra to the mass spectra of DOM using an in-line detector system coupled to multivariate data analysis. Monthly samples were taken from three river mouths in Sweden for one year. One subset of samples was exposed to photochemical degradation and another subset was exposed to long-term (4 months) biological degradation. A principle component analysis was performed on the coupled absorption-mass spectra data. Loading spectra for each principle component show distinct fingerprints for both reactivity (i.e. photochemical, biological degradation) and source (i.e. catchment land cover, temperature, hydrology). The fingerprints reveal mass-to-charge values that contribute to optical signals and characteristics seen in past studies, and emphasise the difficulties in interpreting changes in bulk CDOM characteristics resulting from multiple catchment processes. The approach provides a potential simple method for using optical indicators as tracers for more complex chemical processes both with regards to source material for DOM and the past reactive processing of DOM

Effects of wastewater treatment plant effluent inputs on planktonic metabolic rates and microbial community composition in the Baltic Sea

The Baltic Sea is the world's largest area suffering from eutrophication-driven hypoxia. Low oxygen levels are threatening its biodiversity and ecosystem functioning. The main causes for eutrophication-driven hypoxia are high nutrient loadings and global warming. Wastewater treatment plants (WWTP) contribute to eutrophication as they are important sources of nitrogen to coastal areas. Here, we evaluated the effects of wastewater treatment plant effluent inputs on Baltic Sea planktonic communities in four experiments. We tested for effects of effluent inputs on chlorophyll <i>a</i> content, bacterial community composition, and metabolic rates: gross primary production (GPP), net community production (NCP), community respiration (CR) and bacterial production (BP). Nitrogen-rich dissolved organic matter (DOM) inputs from effluents increased bacterial production and decreased primary production and community respiration. Nutrient amendments and seasonally variable environmental conditions lead to lower alpha-diversity and shifts in bacterial community composition (e.g. increased abundance of a few cyanobacterial populations in the summer experiment), concomitant with changes in metabolic rates. An increase in BP and decrease in CR could be caused by high lability of the DOM that can support secondary bacterial production, without an increase in respiration. Increases in bacterial production and simultaneous decreases of primary production lead to more carbon being consumed in the microbial loop, and may shift the ecosystem towards heterotrophy

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Forskare har ett särskilt ansvar i klimatfrågan, menar Emma Kritzberg vid Lunds universitet. Hon är en av dem som verkat för vegetariskt som standardval vid biologiska institutionen

Long-term water color in 18 lakes

Lake names, years and water color in mg Pt/L for 18 lakes

Allochthonous and autochthonous carbon sources of lake bacterioplankton

Organic substrates for pelagic bacteria are derived from dissolved organic carbon (DOC) in the water column that originates either from primary production from within the lake itself (autochthonous), or import of organic matter from the terrestrial watershed (allochthonous production). This thesis addresses the utilization of allochthonous versus autochthonous carbon (C) sources by lake bacterioplankton. In addition, I examine the connection between the source of the organic matter and the structure and function of the bacterial community. In spite of low allochthonous C inputs, high respiration:primary production and bacterial:primary production suggested that five oligotrophic clear-water lakes on the Faroe Islands were net heterotrophic (respiration exceeded primary production) and that the heterotrophic plankton were subsidized by allochthonous organic C. While the occurence of net heterotrophy does not reveal anything about the fate of the allochthonous C, the importance of autochthonous versus allochthonous DOC in supporting bacterial production was examined by whole-lake additions of 13C (Forest lakes in Northern Wisconsin), and tracing its subsequent uptake by phytoplankton and on to bacteria. Bacterial production in lakes ranging from eutrophic and low-humic to oligotrophic and humic, consisted of 40 ? 80 % allochthonous C, confirming the often stated hypothesis that autochthonous carbon alone does not support bacterial production. Yet, autochthonous DOC was preferentially utilized relative to terrestrial DOC, although the preference for autochthonous C was not complete. Furthermore, based on a correlative study of ten lakes in the same area, bacteria seemed to grow more efficiently on autochthonous C, suggesting that this C is more effectively transferred to higher organisms. The results of the lake survey and the whole-lake isotope additions were simulated by a simple steady state model of bacterial utilization of autochthonous and allochthonous dissolved organic C. This showed that a higher preference and greater growth efficiency of bacteria on autochthonous C could explain why bacterial production was coupled to autochthonous production also in net heterotrophic ecosystems where the use of allochthonous C by bacteria was high. Finally, the source of the organic substrate (autochthonous and allochthonous) appeared to influence the bacterial community composition as well as general bacterial functions (production, respiration, growth yield and substrate utilization). Hence, bacterial sub-populations growing at different rates on substrates of differing origin, might explain why we observe a selective utilization of autochthonous C by the total bacterial community

Centennial-long trends of lake browning show major effect of afforestation

Observations of increasing water color and organic carbon concentrations in lakes are widespread across the Northern Hemisphere. The drivers of these trends are debated. Declining atmospheric sulfur deposition has been put forward as an important underlying factor, since recovery from acidification enhances mobility of organic matter from surrounding soils. This would suggest that the current browning represents a return to a more natural state. This study explores historical lake data from Sweden—1935 to 2015—providing a unique opportunity to see how and why water color has varied during almost a century. The data shows that sulfur deposition has not been the primary driver of water color trends over this period. I propose that the observed browning is to a large extent driven by a major transition from agriculture to forestry

Land-use and climate variables

Spruce coverage in percent, based on sediment pollen records from Lake Fiolen. The modeling of vegetation coverage is assumed to be representative for an area with a 500-kilometer radius from the lake. 10-running means of growing degree days, i.e. number of days each year where average temperature was above 5°C, in Jönköping. 10-year running means of discharge from Lake Allgunnen