Search

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.

Increase in surface water color (browning), caused by rising dissolved organic carbon (DOC) and iron concentrations, has been widely reported and studied in the last couple of decades. This phenomenon has implications to aquatic ecosystem function and biogeochemical carbon cycling. While recovery from acidification and changes in climate‐related variables, such as precipitation and length of growing season, are recognized as drivers behind browning, land‐use change has received less attention. In this study, we include all of the above factors and aim to discern their individual and combined contribution to water color variation in an unprecedentedly long (1940–2016) and highly resolved dataset (~20 times per month), from a river in southern Sweden. Water color showed high seasonal variability and a marked long‐term increase, particularly in the latter half of the dataset (~1980). Short‐term and seasonal variations were best explained by precipitation, with temperature playing a secondary role. All explanatory variables (precipitation, temperature, S deposition, and land‐use change) contributed significantly and together predicted 75% of the long‐term variation in water color. Long‐term change was best explained by a pronounced increase in Norway spruce (Picea abies Karst) volume—a measure of land‐use change and a proxy for buildup of organic soil layers—and by change in atmospheric S deposition. When modeling water color with a combination of explanatory variables, Norway spruce showed the highest contribution to explaining long‐term variability. This study highlights the importance of considering land‐use change as a factor behind browning and combining multiple factors when making predictions in water color and DOC., The importance of considering land use (afforestation) as a factor in predicting water color (browning) and DOC concentrations in surface waters is highlighted in this study. While browning is likely caused by a combination of several factors (temperature, precipitation, S deposition, and land use), we here show the importance of considering land use, which was often not considered as an important factor in the past. When considering all factors together, 75% of variation in this long‐term water color data series can be explained, with afforestation being the most important predictor.

The Öresund (the Sound), which is a part of the Danish straits, is linking the marine North Sea and the brackish Baltic Sea. It is a transition zone where ecosystems are subjected to large gradients in terms of salinity, temperature, carbonate chemistry, and dissolved oxygen concentration. In addition to the highly variable environmental conditions, the area is responding to anthropogenic disturbances in, e.g., nutrient loading, temperature, and pH. We have reconstructed environmental changes in the Öresund during the last ca. 200 years, and especially dissolved oxygen concentration, salinity, organic matter content, and pollution levels, using benthic foraminifera and sediment geochemistry. Five zones with characteristic foraminiferal assemblages were identified, each reflecting the environmental conditions for the respective period. The largest changes occurred around 1950, when the foraminiferal assemblage shifted from a low diversity fauna dominated by the species Stainforthia fusiformis to higher diversity and abundance and dominance of the Elphidium species. Concurrently, the grain-size distribution shifted from clayey to sandier sediment. To explore the causes of the environmental changes, we used time series of reconstructed wind conditions coupled with large-scale climate variations as recorded by the North Atlantic Oscillation (NAO) index as well as the ECOSMO II model of currents in the Öresund area. The results indicate increased changes in the water circulation towards stronger currents in the area after the 1950s. The foraminiferal fauna responded quickly (

Browning of surface waters, as a result of increasing dissolved organic carbon and iron concentrations, is a widespread phenomenon with implications to the structure and function of aquatic ecosystems. In this article, we provide an overview of the consequences of browning in relation to ecosystem services, outline what the underlying drivers and mechanisms of browning are, and specifically focus on exploring potential mitigation measures to locally counteract browning. These topical concepts are discussed with a focus on Scandinavia, but are of relevance also to other regions. Browning is of environmental concern as it leads to, e.g., increasing costs and risks for drinking water production, and reduced fish production in lakes by limiting light penetration. While climate change, recovery from acidification, and land-use change are all likely factors contributing to the observed browning, managing the land use in the hydrologically connected parts of the landscape may be the most feasible way to counteract browning of natural waters.

Widespread increases in iron (Fe) concentrations are contributing to ongoing browning of northern freshwaters, but the driver/s behind the trends are not known. Fe mobilization in soils is known to be controlled by redox conditions, pH, and DOC availability for complexation. Moreover, high sulfate concentrations have been suggested to constrain Fe in transition from soil to water, and declining sulfate deposition to have the opposite effect. We studied the effect of these Fe mobilization barriers in a microcosm experiment, applying high (peak S deposition) and low (present day) sulfate treatments and oxic versus anoxic conditions to boreal (O horizon) soil slurries. We hypothesized that anoxic conditions would favor Fe release. On the contrary we expected high sulfate concentrations to suppress Fe mobility, through FeS formation or by lowering pH and thereby DOC concentrations. Anoxia had positive effects on both Fe and DOC concentrations in solution. Contrasting with our hypothesis, Fe concentrations were enhanced at high sulfate concentrations, i.e. increasing acidity in high sulfate treatments appeared to promote Fe mobilization. Establishment of the basidiomycete fungus Jaapia ochroleuca in the oxic treatments 44 days into the experiment had a major impact on Fe mobilization by increasing total Fe concentrations in solution. Thus, anoxia and acidity, along with fungi mediated mobilization, were important in controlling Fe release from soil to the aqueous phase. While Fe is often assumed to precipitate as Fe(oxy)hydroxides in the transition from anoxic to oxic water in the riparian zone, Fe from anoxic treatments remained in solution after introduction of oxygen. Our results do not support reduced atmospheric S deposition as a driver behind increasing Fe concentrations in boreal freshwaters, but confirm the importance of reducing conditions—which may be enhanced by higher soil temperature and moisture—for mobilization of Fe across the terrestrial-aquatic interphase.

The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speciation, and Fe speciation likely influences how Fe distributes between the water column and sediments. In this study, the function of a lake as an Fe sink, with focus on the role of Fe speciation, was assessed for Lake Bolmen in southern Sweden. We found that a large fraction of the Fe flowing in to the lake was efficiently lost by sedimentation in the lake basin. Fe in inflowing water was a mix of organically complexed mononuclear Fe, Fe-(oxy)hydroxides and Fe-bearing clays, while surface sediments were composed of Fe-(oxy)hydroxides, Fe-bearing clays, Fe-bearing silicates and Fe sulfides. The absence of organically complexed Fe in the surface sediments indicates that the lake is mainly a sink for minerogenic fractions. Furthermore, while lakes are considered to be sinks of Fe, it has been suggested that this function may be impaired by increasing precipitation and consequently shorter water residence time. In this study there were large within- and between-year variations in precipitation and Fe concentrations. However, rather than smaller Fe losses to the sediments during wet years, within-lake losses tended to be larger due to higher loading of Fe from the catchment. Thus, forecasted increases in precipitation may result in enhanced catchment export and Fe loading to lakes, and subsequently enhanced Fe sequestration in sediments.

Riverine colloids are important carriers of macronutrients, trace metals, and pollutants into marine waters. The aim of the current study was to extend the understanding of iron (Fe) and organic carbon (OC) colloids in boreal rivers and their fate at higher salinities. X‐ray absorbance spectroscopy (XAS) and dynamic light scattering (DLS) were combined to explore Fe speciation and colloidal characteristics such as size and surface charge and how these are affected at increasing salinity. XAS confirmed the presence of two Fe phases in the river waters—Fe‐organic matter (OM) complexes and Fe(oxy)hydroxides. From DLS measurements on filtered and unfiltered samples, three particle size distributions were identified. The smallest particles (10–40 nm) were positively charged and suggested to consist of essentially bare Fe(oxy)hydroxide nanoparticles. The largest particles (300–900 nm) were dominated by Fe(oxy)hydroxides associated with chromophoric molecular matter. An intermediate size distribution (100–200 nm) with a negative surface charge was presumably dominated by OM and containing Fe‐OM complexes. Increasing the salinity resulted in a removal of the smallest distribution. Unexpectedly, both the intermediate and largest size distributions were still detected at high salinity. The collective results suggest that Fe(oxy)hydroxides and Fe‐OM complexes are both found across the wide size range studied and that colloidal size does not necessarily reflect either Fe speciation or stability toward salinity‐induced aggregation. The findings further demonstrate that also particles beyond the typically studied, Key Points Three colloidal size distributions occurred in river water samplesPositively charged Fe(oxy)hydroxide nanoparticles were removed at increasing salinityLarger size distributions were less affected by salinity

Increasing surface water concentrations of Fe and DOC (browning), have been reported around the northern hemisphere in the last couple of decades. This increase has far-reaching ecological and societal implications, as it alters the light climate in water and decreases the quality of drinking water. One of the hypothesis behind the increase has been that afforestation and a dominance of coniferous forest have increased the availability of Fe and DOC for transport from soils into surface waters. The accumulation of organic soil layers in coniferous forests increases acidity and the amounts of organic acids in soils and may thus enhance weathering, solubility and mobilization of Fe as the forest ages. In this study we examined the effects of afforestation and growth of Norway spruce on the mobilization and potential leakage of Fe and DOC from soils to surface waters. To represent the effects of ageing forest we used plots with spruce stands of different ages (35, 61, 90 years) and unforested control plots in their immediate proximity, in Tönnersjöheden experimental forest (Sweden). Soil water collected in lysimeters (installed below the organic horizon and in the mineral soil) and analyzed for Fe, Fe speciation, using X-ray absorption spectroscopy (XAS), as well as DOC, metals, major anions and cations. Soil samples were analyzed for Fe speciation and crystallinity at different depths. Results from the soil water analysis show that more Fe was mobile in older spruce forest stands with higher DOC concentrations and lower pH. Covariation of Fe and DOC concentrations in soil waters, indicate the dependence of Fe on DOC to solubilize and stay in solution. Preliminary results from our XAS analysis also indelicate a considerable amount of Fe(II) in soil water that is likely stabilized from oxidation by organic complexation. Surprisingly Fe extracted from the organic (O) soil horizon showed the highest crystallinity and crystallinity did not vary much between soils of different stand ages. The results of this study indicate that afforestation promotes Fe and DOC availability for export into surface waters as well as strengthens the notion that the effects of afforestation are not immediate, but take time as soils develop slowly. As afforestation and dominance of coniferous forest continues in many parts of the northern hemisphere, we can expect further increase of Fe and DOC in surface waters.

Disturbances are believed to be one of the main factors influencing variations in community diversity and functioning. Here we investigated if exposure to a pH press disturbance affected the composition and functional performance of a bacterial community and its resistance, recovery and resilience to a second press disturbance (salt addition). Lake bacterial assemblages were initially exposed to reduced pH in six mesocosms whereas another six mesocosms were kept as reference. Seven days after the pH disturbance, three tanks from each treatment were exposed to a salt disturbance. Both bacterial production and enzyme activity were negatively affected by the salt treatment, regardless if the communities had been subject to a previous disturbance or not. However, cell-specific enzyme activity had a higher resistance in communities pre-exposed to the pH disturbance compared to the reference treatment. In contrast, for cell-specific bacterial production resistance was not affected, but recovery was faster in the communities that had previously been exposed to the pH disturbance. Over time, bacterial community composition diverged among treatments, in response to both pH and salinity. The difference in functional recovery, resilience and resistance may depend on differences in community composition caused by the pH disturbance, niche breadth or acquired stress resistance.

Coastal areas display natural large environmental variability such as frequent changes in salinity, pH, and carbonate chemistry. Anthropogenic impacts - especially ocean acidification - increase this variability, which may affect the living conditions of coastal species, particularly, calcifiers. We performed culture experiments on living benthic foraminifera to study the combined effects of lowered pH and salinity on the calcification abilities and survival of the coastal, calcitic species Ammonia sp. and Elphidium crispum. We found that in open ocean conditions (salinity ∼35) and lower pH than usual values for these species, the specimens displayed resistance to shell (test) dissolution for a longer time than in brackish conditions (salinity ∼5 to 20). However, the response was species specific as Ammonia sp. specimens survived longer than E. crispum specimens when placed in the same conditions of salinity and pH. Living, decalcified juveniles of Ammonia sp. were observed and we show that desalination is one cause for the decalcification. Finally, we highlight the ability of foraminifera to survive under Ωcalc < 1, and that high salinity and [Ca2+] as building blocks are crucial for the foraminiferal calcification process.

Coastal ecosystems are subjected to both large natural variability and increasing anthropogenic impact on environmental parameters such as changes in salinity, temperature, and pH. This study documents the distribution of living benthic foraminifera under the influence of multiple environmental stressors in the Skagerrak-Baltic Sea region. Sediment core tops were studied at five sites along a transect from the Skagerrak to the Baltic Sea, with strong environmental gradients, especially in terms of salinity, pH, calcium carbonate saturation and dissolved oxygen concentration in the bottom water and pore water. We found that living foraminiferal densities and species richness were higher at the Skagerrak station, where the general living conditions were relatively beneficial for Foraminifera, with higher salinity and Ωcalc in the water column and higher pH and oxygen concentration in the bottom and pore water. The most common species reported at each station reflect the differences in the environmental conditions between the stations. The dominant species were Cassidulina laevigata and Hyalinea balthica in the Skagerrak, Stainforthia fusiformis, Nonionella aff. stella and Nonionoides turgida in the Kattegat and N. aff. stella and Nonionellina labradorica in the Öresund. The most adverse conditions, such as low salinity, low Ωcalc, low dissolved oxygen concentrations and low pH, were noted at the Baltic Sea stations, where the calcareous tests of the dominant living taxa Ammonia spp. and Elphidium spp. were partially to completely dissolved, probably due to a combination of different stressors affecting the required energy for biomineralization. Even though Foraminifera are able to live in extremely varying environmental conditions, the present results suggest that the benthic coastal ecosystems in the studied region, which are apparently affected by an increase in the range of environmental variability, will probably be even more influenced by a future increase in anthropogenic impacts, including coastal ocean acidification and deoxygenation.

Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial-aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)-the messenger between terrestrial and lake ecosystems-with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change-driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice-out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.

Increases in the terrestrial export of dissolved organic carbon (C) to rivers may be associated with additional loading of organic nitrogen (N) and phosphorus (P) to the coastal zone. However, little is known about how these resources interact in the regulation of heterotrophic bacterioplankton metabolism in boreal coastal ecosystems. Here, we measured changes in bacterioplankton production (BP) and respiration (BR) in response to full-factorial (C, N, and P) enrichment experiments at two sites within the Öre estuary, northern Sweden. The BR was stimulated by single C additions and further enhanced by combined additions of C and other nutrients. Single addition of N or P had no effect on BR rates. In contrast, BP was primarily limited by P at the site close to the river mouth and did not respond to C or N additions. However, at the site further away from the near the river mouth, BP was slightly stimulated by single additions of C. Possibly, the natural inflow of riverine bioavailable dissolved organic carbon induced local P limitation of BP near the river mouth, which was then exhausted and resulted in C-limited BP further away from the river mouth. We observed positive interactions between all elements on all responses except for BP at the site close to the river mouth, where P showed an independent effect. In light of predicted increases in terrestrial P and C deliveries, we expect future increases in BP and increases of BR of terrestrially delivered C substrates at the Öre estuary and similar areas.

Recent reports of increasing iron (Fe) concentrations in freshwaters are of concern, given the fundamental role of Fe in biogeochemical processes. Still, little is known about the frequency and geographical distribution of Fe trends or about the underlying drivers. We analyzed temporal trends of Fe concentrations across 340 water bodies distributed over 10 countries in northern Europe and North America in order to gain a clearer understanding of where, to what extent, and why Fe concentrations are on the rise. We found that Fe concentrations have significantly increased in 28% of sites, and decreased in 4%, with most positive trends located in northern Europe. Regions with rising Fe concentrations tend to coincide with those with organic carbon (OC) increases. Fe and OC increases may not be directly mechanistically linked, but may nevertheless be responding to common regional-scale drivers such as declining sulfur deposition or hydrological changes. A role of hydrological factors was supported by covarying trends in Fe and dissolved silica, as these elements tend to stem from similar soil depths. A positive relationship between Fe increases and conifer cover suggests that changing land use and expanded forestry could have contributed to enhanced Fe export, although increases were also observed in nonforested areas. We conclude that the phenomenon of increasing Fe concentrations is widespread, especially in northern Europe, with potentially significant implications for wider ecosystem biogeochemistry, and for the current browning of freshwaters.

Previous studies report high and increasing iron (Fe) concentrations in boreal river mouths. This Fe has shown relatively high stability to salinity-induced aggregation in estuaries. The aim of this study was to understand how the speciation of Fe affects stability over salinity gradients. For Fe to remain in suspension interactions with organic matter (OM) are fundamental and these interactions can be divided in two dominant phases: organically complexed Fe, and colloidal Fe (oxy)hydroxides, stabilized by surface interactions with OM. The stability of these two Fe phases was tested using mixing experiments with river water and artificial seawater. Fe speciation of river waters and salinity-induced aggregates was determined by synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy. The relative contribution of the two Fe phases varied widely across the sampled rivers. Moreover, we found selective removal of Fe (oxy)hydroxides by aggregation at increasing salinity, while organically complexed Fe was less affected. However, Fe–OM complexes were also found in the aggregates, illustrating that the control of Fe stability is not explained by the prevalence of the respective Fe phases alone. Factors such as colloid size and the chemical composition of the OM may also impact the behavior of Fe species.

Rivers discharge a notable amount of Fe (1.5 x 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 riverine Fe is considered to be lost to the sediment due to aggregation during estuarine mixing. Recently however, several studies demonstrate 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 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 and confirmed a significant but variable contribution of Fe-OM in relation to Fe (oxy)hydroxides among river mouths. We further found that 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 in low-order streams in a catchment than at the river mouth. The stability of Fe to increasing salinity correlated well to the relative contribution of Fe-OM, i.e. confirming that organic complexes promote Fe transport capacity. This study suggests that boreal rivers may provide significant amounts of potentially bioavailable Fe to marine waters beyond the estuary, due to organic matter complexes.

Recent studies report trends of strongly increasing iron (Fe) concentrations in freshwaters. Since Fe is a key element with a decisive role in the biogeochemical cycling of major elements, it is important to understand the mechanisms behind these trends. We hypothesized that variations in Fe concentration are driven mainly by redox dynamics in hydraulically connected soils. Notably, Fe(III), which is the favored oxidation state except in environments where microbial activity provide strong reducing intensity, has several orders of magnitude lower water solubility than Fe(II). To test our hypothesis, seasonal variation in water chemistry, discharge, and air temperature was studied in three Swedish rivers. Methylmercury and sulfate were used as indicators of seasonal redox changes. Seasonal variability in water chemistry, discharge, and air temperature in the Eman and Lyckeby Rivers implied that the variation in Fe was primarily driven by the prevalence of reducing conditions in the catchment. In general, high Fe concentrations were observed when methylmercury was high and sulfate was low, indicative of reducing conditions. The Fe concentrations showed no or weak relationships with variations in dissolved organic matter concentration and aromaticity. The seasonal variation in Fe concentration of the Ume river was primarily dependent on timing of the snowmelt in high- versus low-altitude areas of the catchment. There were long-term trends of increasing temperature in all catchments and also trends of increasing discharge in the southern rivers, which should increase the probability for anaerobic conditions in space and time and thereby increase Fe transport to the aquatic systems.

Estuarine sediments are key sites for removal of phosphorus (P) from rivers and the open sea. Vivianite, an iron (Fe)(II)-P mineral, can act as a major sink for P in Fe-rich coastal sediments. In this study, we investigate the burial of P in the Öre Estuary in the northern Baltic Sea. We find much higher rates of P burial at our five study sites (up to ~ 0.145 mol m−2 yr−1) when compared to more southern coastal areas in the Baltic Sea with similar rates of sedimentation. Detailed study of the sediment P forms at our site with the highest rate of sedimentation reveals a major role for P associated with Fe and the presence of vivianite crystals below the sulfate methane transition zone. By applying a reactive transport model to sediment and porewater profiles for this site, we show that vivianite may account for up to 40 % of total P burial. With the model, we demonstrate that vivianite formation is promoted in sediments with a low bottom water salinity and high rates of sedimentation and Fe oxide input. While high rates of organic matter input are also required, there is an optimum rate above which vivianite formation declines. Distinct enrichments in sediment Fe and sulfur at depth in the sediment are attributed to short periods of enhanced riverine Fe and organic matter input linked to variations in rainfall on land. Most of the P associated with the Fe in the sediment is likely imported from the adjacent eutrophic Baltic Proper. Our work demonstrates that variations in land-to-sea transfer of Fe may act as a key control on burial of P in coastal sediments. Ongoing climate change is expected to lead to a decrease in bottom water salinity and contribute to continued high inputs of Fe oxides from land, further promoting P burial as vivianite in the coastal zone of the northern Baltic Sea. This may enhance the role of this oligotrophic area as a sink for P imported from eutrophic parts of the Baltic Sea.

Fluvial export of organic matter from the terrestrial catchment to the aquatic system is a large and increasing carbon flux. The successful reduction in sulfuric acid deposition since the 1980s has been shown to enhance the mobility of organic matter in the soil, with more terrestrially derived dissolved organic matter (DOM) reaching aquatic systems. Changes in soil acidity also affect the quality of the DOM. In this study we explore the consequences this may have on the reactivity and turnover of the terrestrially derived DOM as it reaches the aquatic system. DOM of different quality (estimated by absorbance, fluorescence and size exclusion chromatography) was produced through extraction of boreal forest O-horizon soils from podzol at two sulfuric acid concentrations corresponding to natural throughfall in spruce forest in Southern Sweden around 1980 and today. Extraction was done using two different methods, i.e. field leaching and laboratory extraction. The DOM extracts were used to assess if differences in acidity generate DOM of different reactivity. Three reactivity experiments were performed: photodegradation by UV exposure, biodegradation by bacteria, and biodegradation after UV exposure. Reactivity was assessed by measuring loss of dissolved organic carbon and absorbance, change in fluorescence and molecular weight, and bacterial production. DOM extracted at lower sulfuric acid concentration was more susceptible to photooxidation, and less susceptible to bacterial degradation, than DOM extracted at a higher sulfuric acid concentration. Thus the relative importance of these two turnover processes may be altered with changes in acid deposition.

pH is an important factor determining bacterial community composition in soil and water. We have directly determined the community tolerance (trait variation) to pH in communities from 22 lakes and streams ranging in pH from 4 to 9 using a growth-based method not relying on distinguishing between individual populations. The pH in the water samples was altered to up to 16 pH values, covering in situ pH ± 2.5 U, and the tolerance was assessed by measuring bacterial growth (Leu incorporation) instantaneously after pH adjustment. The resulting unimodal response curves, reflecting community tolerance to pH, were well modeled with a double logistic equation (mean R 2 = 0.97). The optimal pH for growth (pH opt ) among the bacterial communities was closely correlated with in situ pH, with a slope (0.89 ± 0.099) close to unity. The pH interval, in which growth was ≥90% of that at pH opt , was 1.1 to 3 pH units wide (mean 2.0 pH units). Tolerance response curves of communities originating from circum-neutral pH were symmetrical, whereas in high-pH (8.9) and especially in low-pH (opt ( R 2 = 0.83), allowing for easy determination of pH tolerance during rapid changes in pH.

Microbial decomposers colonising submerged leaf litter are in close spatial proximity with periphytic algae and can use carbon (C) exudates released during photosynthesis. We investigated whether labile C delivered as algal exudates could affect the microbial colonisation and decomposition of leaf litter. Using microcosms, we submerged leaf litter in pond water and monitored fungal and bacterial growth over time and tested the effect of algal photosynthetic exudates by comparing microcosms in light and dark. In order to experimentally assign the effect of algal products to labile C delivery and test for a C driven mechanism, we ran a parallel experiment with microcosms in the dark where we mimicked the delivery of algal labile C by continuously adding glucose. Labile C delivered as algal exudates or glucose resulted in a dominance of fungal decomposers over bacteria, and stimulated the acquisition of more N-rich OM fractions from litter during periods of active fungal growth. Our results highlight that labile C stimulates fungal decomposers and increases N removal from leaf litter. Since fungal necromass is more resistant to degradation than bacterial, we expect that a fungal-dominated litter degradation might contribute to more protected C pools.

Rivers have traditionally been viewed as negligible sources of iron (Fe) to marine waters, as most Fe gets lost during estuarine mixing. However, recent findings demonstrate that Fe from boreal riv ...

1. Thousands of lakes in the Northern Hemisphere are experiencing a continuous increase in water temperature and colour. While increasing temperature is an effect of climate change, several factors are suggested to drive the increasing water colour, including climate change, altered land use and reversed acidification. 2. In this mesocosm study, we study the effects on pelagic production and sedimentation of organic carbon from a 3 degrees C increase in temperature, a doubling of water colour and a combination of increased temperature and water colour. These manipulations correspond to the predicted development in water colour and temperature in northern temperate systems within the life time of the next generation. 3. The water and plankton communities in the mesocosms were taken from an oligohumic and mesotrophic lake, and the increase in water colour was achieved by the additions of water from a humic lake. The experiment ran from early March to late October. 4. In accordance with our predictions and previous cross-system analyses, net carbon production (NCP) and organic carbon sedimentation decreased in response to increasing water colour. 5. At odds with current theory, NCP and sedimentation increased in heated treatments, as not only respiration but also gross production increased in response to temperature. There were no synergistic effects between warming and browning. 6. These results imply that warming may enhance pelagic delivery of organic carbon to sediments. This could potentially offset a reduction in burial efficiency of organic carbon, which has been projected based on a positive relationship between carbon mineralisation in sediments and temperature. (Less)

Future aquatic ecosystems will be impacted synergistically by large-scale environmental changes, such as climate change and increased humic content. Now research shows that community responses are determined by food-chain length and that the top trophic level, and every second level below that, can be expected to benefit from climate change, whereas trophic levels in between are likely to suffer.

It is frequently observed that the local relative abundances of aquatic microbial taxa are correlated with their average relative abundance at the regional scale, which results in the composition of different communities being more similar than expected by chance or invariant. The degree to which communities within a region match the regional average community is variable and likely depends on several different mechanisms that control the process of microbial community assembly. Here, we show that environmental variables were associated with the community specific degree of regional invariance in 9 of 10 datasets of microbial communities in aquatic systems, being the main set of variables explaining differences in regional invariance in 5 of them. This indicates that variation in local environmental conditions across a region reduces the degree of regional invariance amongst communities. Spatial distances between communities were not related to the degrees of regional invariance, but in 7 of the datasets, regional invariance differed among different parts of the regions, particularly for phytoplankton communities. This suggests an influence of spatial or historical processes on the community specific degree of regional invariance. We conclude that both local environmental conditions and spatial/historical processes cause between-site differences in the degree of invariance between local and regional abundances in aquatic microbial metacommunities. We argue that studies of regional invariance can be an important complement to other statistical methods due to its propensity to detect variation in stochastic processes along gradients.

Browning of inland waters has been noted over large parts of the Northern hemisphere and is a phenomenon with both ecological and societal consequences. The increase in water color is generally ascribed to increasing concentrations of dissolved organic matter of terrestrial origin. However, oftentimes the increase in water color is larger than that of organic matter, implying that changes in the concentration of organic matter alone cannot explain the enhanced water color. Water color is known to be affected also by the quality of organic matter and the prevalence of iron. Here we investigated trends in water color, organic matter and iron between 1972 and 2010 in 30 rivers draining into the Swedish coast (data from the national Swedish monitoring program), and performed a laboratory iron addition experiment to natural waters, to evaluate the role of iron and organic matter in determining water color. By comparing the effect of iron additions on water color in the experiment, to variation in water color and iron concentration in the monitoring data, we show that iron can explain a significant share of the variation in water color (on average 25 %), especially in the rivers in the north of Sweden (up to 74 %). Furthermore, positive trends for iron are seen in 27 of 30 rivers (21–468 %) and the increase in iron is larger than that of organic matter, indicating that iron and organic matter concentrations are controlled by similar but not identical processes. We speculate that increasing iron concentrations can be caused by changes in redox conditions, that mean that more anoxic water with high concentrations of soluble FeII are feeding into the surface waters. More studies are needed about why iron is increasing so strongly, since both causes and consequences are partly different from those of increasing organic matter content.

1. Aquatic ecosystems in Northern Europe are expected to face increases in temperature and water colour (TB) in future. While effects of these factors have been studied separately, it is unknown whether and how a combination of them might affect phenological events and trophic interactions. 2. In a mesocosm study, we combined both factors to create conditions expected to arise during the coming century. We focused on quantifying effects on timing and magnitude of plankton spring phenological events and identifying possible mismatches between resources (phytoplankton) and consumers (zooplankton). 3. We found that the increases in TB had important effects on timing and abundance of different plankton groups. While increased temperature led to an earlier peak in phytoplankton and zooplankton and a change in the relative timing of different zooplankton groups, increased water colour reduced chlorophyll-a concentrations. 4. Increased TB together benefitted cladocerans and calanoid copepods and led to stronger top-down control of algae by zooplankton. There was no sign of a mismatch between primary producers and grazers as reported from other studies. 5. Our results point towards an earlier onset of plankton spring growth in shallow lakes in future with a stronger top-down control of phytoplankton by zooplankton grazers. (Less)

Carlos M. Duarte et al., The Arctic marine ecosystem contains multiple elements that present alternative states. The most obvious of which is an Arctic Ocean largely covered by an ice sheet in summer versus one largely devoid of such cover. Ecosystems under pressure typically shift between such alternative states in an abrupt, rather than smooth manner, with the level of forcing required for shifting this status termed threshold or tipping point. Loss of Arctic ice due to anthropogenic climate change is accelerating, with the extent of Arctic sea ice displaying increased variance at present, a leading indicator of the proximity of a possible tipping point. Reduced ice extent is expected, in turn, to trigger a number of additional tipping elements, physical, chemical, and biological, in motion, with potentially large impacts on the Arctic marine ecosystem., This is a contribution to the Arctic Tipping Points project (www.eu-atp.org) funded by FP7 of the European Union (contract #226248).

The aim of this study was to explore how acid deposition may affect the concentration and quality of dissolved organic matter (DOM) in soil-water. This was done by a small-scale acidification experiment during two years where 0.5 × 0.5 m(2) plots were artificially irrigated with water with different sulfuric acid content, and soil-water was sampled using zero-tension lysimeters under the O-horizon. The DOM was characterized using absorbance, fluorescence, and size exclusion chromatography analyses. Our results showed lower mobility of DOM in the high acid treatment. At the same time, there was a significant change in the DOM quality. Soil-water in the high acid treatment exhibited DOM that was less colored, less hydrophobic, less aromatic, and of lower molecular weight, compared to the low acid treatment. This supports the hypothesis that reduction in sulfur deposition is an important driver behind the ongoing brownification of surface waters in many regions.

We hypothesized that habitats differing in water flow regime would differ in bacterial function either because of differences in the local environment, in bacterial community composition (BCC), or in the mechanism shaping BCC (community assembly). In 20 lakes and 17 inlet streams BCC was analyzed by terminal restriction fragment length polymorphism of the gene coding for 16S ribosomal RNA, and bacterial function was estimated as bacterial production rate (BP, measured as leucine incorporation) per content of dissolved organic carbon (DOC) (BP : DOC). BCC in both lakes and streams appeared to be shaped by local environmental forces (i.e., species sorting according to metacommunity theory), but not by massive introduction of cells from the drainage area (mass effect). BP : DOC was lower in streams than in lakes, which appeared to be both because of differences in BCC and environment between lakes and streams, independent of each other. We found no support for an effect of water flow regime in itself (i.e., cell dispersal rate) causing the lower functionality of the streams. In streams, BP : DOC was correlated to both BCC and environment, independent of each other, while in lakes function could not be explained by either BCC or environment. The greater environmental variability among our streams than among our lakes may be the cause for the stronger BCC-function coupling in our streams, since smaller environmental variation among our lakes would allow a greater functional redundancy.

The aim of this study was to explore the variation and regulation of bacterial carbon processing at a coastal oligotrophic site of the Island of Majorca in the Mediterranean Sea. In situ bacterial production (BP), respiration (BR), growth efficiency, and carbon demand in relation to environmental parameters were studied over an annual cycle. In addition, the response of bacterial carbon processing to an experimental resource (phosphate) and temperature manipulations was tested. While concentrations of dissolved organic carbon (DOC) and phosphorus were fairly stable over the year, BP and BR varied 65-fold and 79-fold, respectively. Addition of phosphate stimulated both BP and BR during most of the year, suggesting that phosphorus limitation keeps a tight rein on bacterial DOC utilization. Both BP and BR responded positively to a 2 degrees C experimental increase, but at higher temperature increases BP and BR leveled off or decreased. In situ BP and BR were positively related to temperature, suggesting that elevated water temperature would yield increased BP and BR. BR responded more strongly to temperature than BP, suggesting that increased temperature may result in a decrease in bacterial growth efficiency. (Less)

Bacteria play a central role in the cycling of carbon, yet our understanding of the relationship between the taxonomic composition and the degradation of dissolved organic matter (DOM) is still poor. In this experimental study, we were able to demonstrate a direct link between community composition and ecosystem functioning in that differently structured aquatic bacterial communities differed in their degradation of terrestrially derived DOM. Although the same amount of carbon was processed, both the temporal pattern of degradation and the compounds degraded differed among communities. We, moreover, uncovered that low-molecular-weight carbon was available to all communities for utilisation, whereas the ability to degrade carbon of greater molecular weight was a trait less widely distributed. Finally, whereas the degradation of either low- or high-molecular-weight carbon was not restricted to a single phylogenetic clade, our results illustrate that bacterial taxa of similar phylogenetic classification differed substantially in their association with the degradation of DOM compounds. Applying techniques that capture the diversity and complexity of both bacterial communities and DOM, our study provides new insight into how the structure of bacterial communities may affect processes of biogeochemical significance.

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 a 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.

Four whole-lake inorganic 13C addition experiments were conducted in lakes of differing trophic status. Inorganic 13C addition enriched algal carbon in 13C and changed the $$\delta^{13}$$ C-DOC by +1.5‰ to +9.5‰, depending on the specific lake. This change in $$\delta^{13}$$ C-DOC represented a significant input of algal DOC that was not completely consumed by bacteria. We modeled the dynamics in $$\delta^{13}$$ C-DOC to estimate the fluxes of algal and terrestrial carbon to and from the DOC pool, and determine the composition of the standing stock. Two experiments in lightly stained, oligotrophic lakes indicated that algal production was the source of about 20% of the DOC pool. In the following year, the experiment was repeated in one of these lakes under conditions of nutrient enrichment, and in a third, more humic lake. Algal contributions to the DOC pool were 40% in the nutrient enriched lake and 5% in the more humic lake. Spectroscopic and elemental analyses corroborated the presence of increased algal DOC in the nutrient enriched lake. Natural abundance measurements of the $$\delta^{13}$$ C of DOC in 32 lakes also revealed the dual contributions of both terrestrial and algal carbon to DOC. From these results, we suggest an approach for inferring the contribution of algal and terrestrial DOC using easily measurable parameters.

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.

Increased anthropogenic pressures on coastal marine ecosystems in the last century are threatening their biodiversity and functioning. Global warming and increases in nutrient loadings are two major stressors affecting these systems. Global warming is expected to increase both atmospheric and water temperatures and increase precipitation and terrestrial runoff, further increasing organic matter and nutrient inputs to coastal areas. Dissolved organic nitrogen (DON) concentrations frequently exceed those of dissolved inorganic nitrogen in aquatic systems. Many components of the DON pool have been shown to supply nitrogen nutrition to phytoplankton and bacteria. Predictions of how global warming and eutrophication will affect metabolic rates and dissolved oxygen dynamics in the future are needed to elucidate their impacts on biodiversity and ecosystem functioning. Here, we experimentally determine the effects of simultaneous DON additions and warming on planktonic community metabolism in the Baltic Sea, the largest coastal area suffering from eutrophication-driven hypoxia. Both bacterioplankton community composition and metabolic rates changed in relation to temperature. DON additions from wastewater treatment plant effluents significantly increased the activation energies for community respiration and gross primary production. Activation energies for community respiration were higher than those for gross primary production. Results support the prediction that warming of the Baltic Sea will enhance planktonic respiration rates faster than it will for planktonic primary production. Higher increases in respiration rates than in production may lead to the depletion of the oxygen pool, further aggravating hypoxia in the Baltic Sea.

It has been suggested that autochthonous (internally produced) organic carbon and allochthonous (externally produced) organic carbon are utilized by phylogenetically different bacterioplankton. We examined the relationship between the source of organic matter and the structure and function of lake bacterial communities. Differences and seasonal changes in bacterial community composition in two lakes differing in their source of organic matter were followed in relation to environmental variables. We also performed batch culture experiments with amendments of various organic substrates, namely fulvic acids, leachates from algae, and birch and maple leaves. Differences in bacterial community composition between the lakes, analysed by terminal restriction fragment length polymorphism, correlated with variables related to the relative loading of autochthonous and allochthonous carbon (water colour, dissolved organic carbon, nutrients, and pH). Seasonal changes correlated with temperature, chlorophyll and dissolved organic carbon in both lakes. The substrate amendments led to differences in both structure and function, i.e. production, respiration and growth yield, of the bacterial community. In conclusion, our results suggest that the source of organic matter influences community composition both within and among lakes and that there may be a coupling between the structure and function of the bacterial community.

Organic carbon (C) in lakes originates from two distinct sources—primary production from within the lake itself (autochthonous supply) and importation of organic matter from the terrestrial watershed (allochthonous supply). By manipulating the 13C of dissolved inorganic C, thereby labeling within-lake primary production, we examined the relative importance of autochthonous and allochthonous C in supporting bacterial production. For 35 days, NaH13CO3 was added daily to two small, forested lakes. One of the lakes (Peter) was fertilized so that primary production exceeded total respiration in the epilimnion. The other lake (Tuesday), in contrast, was low in productivity and had high levels of colored dissolved organic C (DOC). To obtain bacterial C isotopes, bacteria were regrown in situ in particle-free lake water in dialysis tubes. The contribution of allochthonous C to bacterial biomass was calculated by applying a two-member mixing model. In the absence of a direct measurement, the isotopic signature of the autochthonous end-member was estimated indirectly by three different approaches. Although there was excess primary production in Peter Lake, bacterial biomass consisted of 43–46% allochthonous C. In Tuesday Lake more than 75% of bacterial growth was supported by allochthonous C. Although bacteria used autochthonous C preferentially over allochthonous C, DOC from the watershed contributed significantly to bacterial production. In combination with results from similar experiments in different lakes, our findings suggest that the contribution of allochthonous C to bacterial production can be predicted from ratios of chromophoric dissolved organic matter (a surrogate for allochthonous supply) and chlorophyll a (a surrogate for autochthonous supply).

We examined the response of bacteria and proto- and metazooplankton to photomodified dissolved organic matter (DOM). Sterile filtered water from a eutrophic and a humic lake, that was either exposed to artificial ultraviolet (UV) radiation or kept dark, was added to semicontinuous laboratory microcosms that lasted for 7 weeks. Bacterial production responded positively to photochemical modification of DOM regardless of lake type. Final heterotrophic biomass (bacteria + proto + metazooplankton) was 47 +/- 5 and 37 +/- 5 mu g carbon (C) L-1 in microcosms with UV-exposed and unaltered eutrophic water DOM and 15 +/- 4 and 11 +/- 2 mu g C L-1 in microcosms with UV-exposed and unaltered humic water DOM, respectively. For the eutrophic water, there were no significant differences in proto- or metazooplankton biomasses between microcosms receiving UV-exposed or nonexposed DOM. Differences between eutrophic water microcosms were not significant when flagellates, ciliates, cladocerans, and copepods were examined separately. In microcosms with UV-exposed humic water, biomasses of heterotrophic flagellates, rotifers, nauplii, and cladocerans were higher than in those with nonexposed DOM. Higher final metazooplankton biomass following addition of UV-exposed humic water indicates that photochemically modified DOM can be effectively transferred through the microbial loop. (Less)

Whole-lake additions of dissolved inorganic C-13 were used to measure allochthony (the terrestrial contribution of organic carbon to aquatic consumers) in two unproductive lakes (Paul and Peter Lakes in 2001), a nutrient-enriched lake (Peter Lake in 2002), and a dystrophic lake (Tuesday Lake in 2002). Three kinds of dynamic models were used to estimate allochthony: a process-rich, dual-isotope flow model based on mass balances of two carbon isotopes in 12 carbon pools; simple univariate time-series models driven by observed time courses of delta(13)CO(2); and multivariate autoregression models that combined information from time series of delta(13)C in several interacting carbon pools. All three models gave similar estimates of allochthony. In the three experiments without nutrient enrichment, flows of terrestrial carbon to dissolved and particulate organic carbon, zooplankton, Chaoborus, and fishes were substantial. For example, terrestrial sources accounted for more than half the carbon flow to juvenile and adult largemouth bass, pumpkinseed sunfish, golden shiners, brook sticklebacks, and fathead minnows in the unenriched experiments. Allochthony was highest in the dystrophic lake and lowest in the nutrient-enriched lake. Nutrient enrichment of Peter Lake decreased allochthony of zooplankton from 0.34-0.48 to 0-0.12, and of fishes from 0.51-0.80 to 0.25-0.55. These experiments show that lake ecosystem carbon cycles, including carbon flows to consumers, are heavily subsidized by organic carbon from the surrounding landscape.

During the past 20 yr, aquatic microbiologists have reported 2 strong patterns which initially appear contradictory. In pelagic systems, bacterial growth and biomass is well correlated with the growth and biomass of primary producers. However, bacterial respiration often exceeds net primary production, which suggests that bacteria are subsidized by external inputs of organic matter. We hypothesize that bacterial growth efficiency (BGE) varies systematically between autochthonous and allochthonous carbon (C) sources and that this variation resolves the above conundrum. To test these ideas, we examined the ecological regulation of bacterial secondary production (BP), bacterial respiration (BR) and BGE in a series of lakes dominated by terrestrial (allochthonous) C inputs. BP was correlated with autochthonous C sources (chlorophyll a) even though the lakes were net heterotrophic (i.e. heterotrophic respiration consistently exceeded primary production). The results were simulated by a simple steady-state model of bacterial utilization of autochthonous and allochthonous dissolved organic C. A higher preference and greater growth efficiency of bacteria on autochthonous C may explain why BP is coupled to autochthonous production also in net heterotrophic ecosystems where the use of allochthonous C by bacteria is high. These results suggest that little of the allochthonous C assimilated by bacteria is likely to reach higher consumers.

Organic substrates for pelagic bacteria are derived from dissolved organic carbon (DOC) in the water column. DOC is a heterogeneous mixture of molecules, some of which are imported from the watershed (allochthonous DOC) and others that are produced by autotrophs within the system (autochthonous DOC). We examined the importance of autochthonous versus allochthonous DOC in supporting the growth of pelagic bacteria by manipulating the 13 C content of autochthonous sources in a whole-lake experiment. NaH 13 CO3 was added daily to two small forested lakes for a period of 42 d, thereby strongly labeling autochthonous primary production. To obtain bacterial carbon isotopes, bacteria were regrown in vitro in particle-free lake water and in situ in dialysis tubes; little difference was found between the two methods. The contribution of autochthonous versus allochthonous carbon to the bacterial biomass was estimated by applying a two-member mixing model using a 13 Co f228‰ as the

Gonyostmum semen is a freshwater raphidophyte that has increased in occurrence and abundance in several countries in northern Europe since the 1980s. More recently, the species has expanded rapidly also in north-eastern Europe, and it is frequently referred to as invasive. To better understand the species history, we have explored the phylogeography of G. semen using strains from northern Europe, United States, and Japan. Three regions of the ribosomal RNA gene (small subunit [SSU], internal transcribed spacer [ITS] and large subunit [LSU]) and one mitochondrial DNA marker (cox1) were analyzed. The SSU and partial LSU sequences were identical in all strains, confirming that they belong to the same species. The ITS region differentiated the American from the other strains, but showed high intra-strain variability. In contrast, the mitochondrial marker cox1 showed distinct differences between the European, American, and Japanese strains. Interestingly, only one cox1 haplotype was detected in European strains. The overall low diversity and weak geographic structure within northern European strains supported the hypothesis of a recent invasion of new lakes by G. semen. Our data also show that the invasive northern European lineage is genetically distinct from the lineages from the other continents. Finally, we concluded that the mitochondrial cox1 was the most useful marker in determining large-scale biogeographic patterns in this species.

This study reports increasing iron concentrations in rivers draining into the Baltic Sea. Given the decisive role of iron to the structure and biogeochemical function of aquatic ecosystems, this trend is likely one with far reaching consequences to the receiving system. What those consequences may be depends on the fate of the iron in estuarine mixing. We here assess the stability of riverine iron by mixing water from seven boreal rivers with artificial sea salts. The results show a gradual loss of iron from suspension with increasing salinity. However, the capacity of the different river waters to maintain iron in suspension varied greatly, i.e. between 1 and 54% of iron was in suspension at a salinity of 30. The variability was best explained by iron:organic carbon ratios in the riverine waters – the lower the ratio the more iron remained in suspension. Water with an initially low iron:organic carbon ratio could keep even higher than ambient concentrations of Fe in suspension across the salinity gradient, as shown in experiments with iron amendments. Moreover, there was a positive relationship between the molecular size of the riverine organic matter and the amount of iron in suspension. In all, the results point towards a remarkably high transport capacity of iron from boreal rivers, suggesting that increasing concentrations of iron in river mouths may result in higher concentrations of potentially bioavailable iron in the marine system.

To examine the potential influence of terrestrially derived DOM on the Baltic Sea, a year-long study of dissolved organic matter (DOM) was performed in three river catchments in Sweden. One catchment drains into the Bothnian Sea, while two southern catchments drain into the Baltic proper. Dissolved organic carbon (DOC) concentrations were positively correlated with discharge from forested catchments over the year. While the overall concentrations of DOC were several times higher in the southern two catchments, higher discharge in the northern catchment resulted in the annual loadings of DOC being on the same order of magnitude for all three catchments. Biological oxygen demand (BOD) was used as a proxy for the lability of carbon in the system. The range of BOD values was similar for all three catchments, however, the ratio of BOD to DOC (an indication of the labile fraction) in Ume river was four times higher than in the southern two catchments. Total annual BOD loading to the Baltic Sea was twice as high in the northern catchment than in the two southern catchments. Lower winter temperatures and preservation of organic matter in the northern catchment combined with an intense spring flood help to explain the higher concentrations of labile carbon in the northern catchment. Lower lability of DOM as well as higher colour in the southern catchments suggest that wetlands (i.e. peat bogs) may be the dominant source of DOM in these catchments, particularly in periods of low flow. With climate change expected to increase precipitation events and temperatures across the region, the supply and quality of DOM delivered to the Baltic Sea can also be expected to change. Our results indicate that DOM supply to the Baltic Sea from boreal rivers will be more stable throughout the year, and potentially have a lower bioavailability.

Biological invasions often cause major perturbations in the environment and are well studied among macroorganisms. Less is known about invasion by free-living microbes. Gonyostomum semen (Raphidophyceae) is a freshwater phytoplankton species that has increased in abundance in Northern Europe since the 1980's and has expanded its habitat range. In this study, we aimed to determine the genetic population structure of G. semen in Northern Europe and to what extent it reflects the species' recent expansion. We sampled lakes from 12 locations (11 lakes) in Norway, Sweden and Finland. Multiple strains from each location were genotyped using Amplified Fragment Length Polymorphism (AFLP). We found low differentiation between locations, and low gene diversity within each location. Moreover, there was an absence of genetic isolation with distance (Mantel test, p = 0.50). According to a Bayesian clustering method all the isolates belonged to the same genetic population. Together our data suggest the presence of one metapopulation and an overall low diversity, which is coherent with a recent expansion of G. semen.

The field of genetic diversity in protists, particularly phytoplankton, is under expansion. However, little is known regarding variation in genetic diversity within populations over time. The aim of our study was to investigate intrapopulation genetic diversity and genetic differentiation in the freshwater bloom-forming microalga Gonyostomum semen (Raphidophyceae). The study covered a 2-year period including all phases of the bloom. Amplified fragment length polymorphism (AFLP) was used to determine the genetic structure and diversity of the population. Our results showed a significant differentiation between samples collected during the two blooms from consecutive years. Also, an increase of gene diversity and a loss of differentiation among sampling dates were observed over time within a single bloom. The latter observations may reflect the continuous germination of cysts from the sediment. The life cycle characteristics of G. semen, particularly reproduction and recruitment, most likely explain a high proportion of the observed variation. This study highlights the importance of the life cycle for the intraspecific genetic diversity of microbial species, which alternates between sexual and asexual reproduction., SI

Microbial ecology has focused much on causes of between-site variation in community composition. By analysing five data-sets each of aquatic bacteria and phytoplankton, we demonstrated that microbial communities show a large degree of similarity in community composition and that abundant taxa were widespread, a typical pattern for many metazoan metacommunities. The regional abundance of taxa explained on average 85 and 41% of variation in detection frequency and 58 and 31% of variation in local abundances for bacteria and phytoplankton, respectively. However, regional abundance explained less variation in local abundances with increasing environmental variation between sites within data-sets. These findings indicate that the studies of microbial assemblages need to consider similarities between communities to better understand the processes underlying the assembly of microbial communities. Finally, we propose that the degree of regional invariance can be linked to the evolution of microbes and the variation in ecosystem functions performed by microbial communities.