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  • 1. Adam, Birgit
    et al.
    Klawonn, Isabell
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Svedén, Jenny B.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Bergkvist, Johanna
    Nahar, Nurun
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Littmann, Sten
    Whitehouse, Martin J.
    Lavik, Gaute
    Kuypers, Marcel M. M.
    Ploug, Helle
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. University of Gothenburg, Sweden.
    N2-fixation, ammonium release and N-transfer to the microbial and classical food web within a plankton community2016In: The ISME Journal, ISSN 1751-7362, E-ISSN 1751-7370, Vol. 10, no 2, p. 450-459Article in journal (Refereed)
    Abstract [en]

    We investigated the role of N2-fixation by the colony-forming cyanobacterium, Aphanizomenon spp., for the plankton community and N-budget of the N-limited Baltic Sea during summer by using stable isotope tracers combined with novel secondary ion mass spectrometry, conventional mass spectrometry and nutrient analysis. When incubated with 15N2Aphanizomenon spp. showed a strong 15N-enrichment implying substantial 15N2-fixation. Intriguingly, Aphanizomenon did not assimilate tracers of 15NH4+ from the surrounding water. These findings are in line with model calculations that confirmed a negligible N-source by diffusion-limited NH4+ fluxes to Aphanizomenon colonies at low bulk concentrations (<250 nm) as compared with N2-fixation within colonies. No N2-fixation was detected in autotrophic microorganisms <5 μm, which relied on NH4+uptake from the surrounding water. Aphanizomenon released about 50% of its newly fixed N2 as NH4+. However, NH4+ did not accumulate in the water but was transferred to heterotrophic and autotrophic microorganisms as well as to diatoms (Chaetoceros sp.) and copepods with a turnover time of ~5 h. We provide direct quantitative evidence that colony-formingAphanizomenon releases about half of its recently fixed N2 as NH4+, which is transferred to the prokaryotic and eukaryotic plankton forming the basis of the food web in the plankton community. Transfer of newly fixed nitrogen to diatoms and copepods furthermore implies a fast export to shallow sediments via fast-sinking fecal pellets and aggregates. Hence, N2-fixing colony-forming cyanobacteria can have profound impact on ecosystem productivity and biogeochemical processes at shorter time scales (hours to days) than previously thought.

  • 2. Conley, Daniel J.
    et al.
    Carstensen, Jacob
    Aigars, Juris
    Axe, Philip
    Bonsdorff, Erik
    Eremina, Tatjana
    Haahti, Britt-Marie
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Stockholm University, Stockholm Resilience Centre, Baltic Nest Institute.
    Jonsson, Per
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Kotta, Jonne
    Lannegren, Christer
    Larsson, Ulf
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Maximov, Alexey
    Medina, Miguel Rodriguez
    Stockholm University, Stockholm Resilience Centre, Baltic Nest Institute.
    Lysiak-Pastuszak, Elzbieta
    Remeikaite-Nikiene, Nijole
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Wilhelms, Sunhild
    Zillen, Lovisa
    Hypoxia is increasing in the coastal zone of the Baltic Sea2011In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 45, no 16, p. 6777-6783Article in journal (Refereed)
    Abstract [en]

    Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955-2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.

  • 3.
    Ekeroth, Nils
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Kononets, Mikhail
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Blomqvist, Sven
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Hall, Per O. J.
    Effects of oxygen on recycling of biogenic elements from sediments of a stratified coastal Baltic Sea basin2016In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 154, p. 206-219Article in journal (Refereed)
    Abstract [en]

    Benthic nutrient dynamics in the coastal basin Kanholmsfjarden, NW Baltic proper, were studied by in situ flux measurements and sediment samplings in 2010-2013. The benthic release of NH4 and DIP from anoxic sediments in Kanholmsfjarden were calculated to renew the standing stock inventories of DIN and DIP in the overlying water in roughly 1 year. Starting in summer 2012, mixing of oxygen-rich water into the deep part of the basin temporarily improved the oxygen conditions in the deep water. During the 1 year oxygenated period, the total phosphorus inventory in the surficial sediment increased by 0.4 g P m(-2) or 65%. This was most likely due to stimulated bacterial P assimilation under oxygenated conditions. By July 2013, the bottom water had again turned anoxic, and DIP and DSi fluxes were even higher than earlier in the study period. These high fluxes are attributed to degradation of sedimentary pools of P and Si that had accumulated during the bottom water oxygenation in 2012. The strong correlation between DIP and DSi fluxes and the similar dynamics of DIP and DSi in the sediment pore water and near bottom water, suggest a similar redox dependency of benthic-pelagic exchange for these nutrients.

  • 4.
    Ekeroth, Nils
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Kononets, Mikhail
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Hall, Per
    Recycling of biogenic elements in sediments of a stratified coastal Baltic Sea basin: Effects of bottom water oxygen oscillationManuscript (preprint) (Other academic)
  • 5. Franzen, Frida
    et al.
    Kinell, Gerda
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Elmgren, Ragnar
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Soderqvist, Tore
    Participatory Social-Ecological Modeling in Eutrophication Management: the Case of Himmerfjarden, Sweden2011In: Ecology & society, ISSN 1708-3087, E-ISSN 1708-3087, Vol. 16, no 4, p. 27-Article in journal (Refereed)
    Abstract [en]

    Stakeholder participation is increasingly seen as central in natural resource management. It is also required by the European Union Water Framework Directive, which identifies three levels of participation; information, consultation, and active involvement. In this paper we discuss the active involvement of stakeholders, using our experience from a case study in the Himmerfjarden region, which is a coastal area southwest of Stockholm, Sweden. Our study used the systems approach proposed by the European Union research project called Science and Policy Integration for Coastal System Assessment (SPICOSA), in which local stakeholders and a study site team constructed an integrated simulation model of a crucial coastal management issue. In this case the issue was nitrogen enrichment. We showed how stakeholder participation in the modeling process helped identify interesting and currently relevant management scenarios, and how the modeling process facilitated communication of the likely ecological, economic, and social effects of these scenarios to the stakeholders. In addition, stakeholders also reported social gains in terms of network building. We managed to actively involve local stakeholders in water issues, and the research process clearly strengthened the social capital in the Himmerfjarden region, and created a basis for future collaboration regarding water management. Our experience indicates that the approach we tried is a useful tool for promoting active stakeholder involvement in water management projects. Also, the results of our science and policy integration approach indicated that the study site team assumed a leadership role, which is a commonly recognized factor in successful natural resource management.

  • 6.
    Hansson, Sture
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Larsson, Ulf
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Site fidelity in perch (Perca fluviatilis) and roach (Rutilus rutilus) in two Baltic Sea coastal areas2019In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 226, article id UNSP 106261Article in journal (Refereed)
    Abstract [en]

    Perch (Perca fluviatilis) and roach (Rutilus rutilus) are among the more common coastal fish species in the Baltic Sea. They are often targeted in environment monitoring programs as well as in ecological research, in which knowledge of their basic biology, including migration and feeding ranges, are needed in the sampling design and for interpretation of data. Body condition (length-mass relationship) differences between stations separated by at most a few kilometres show that both species are reasonably sedentary even in areas without obvious migration barriers. Collecting representative samples, even from a reasonably small water body, may thus require careful planning.

  • 7.
    Harvey, Therese
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Kratzer, Susanne
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Karlson, Bengt
    Andersson, Agneta
    The effect of optical properties on Secchi depth and implications for eutrophication managementManuscript (preprint) (Other academic)
    Abstract [en]

    Successful management of coastal environments requires reliable monitoring methods and indicators. Secchi depth and chlorophyll-a concentration (Chl-a) are used as indicators for the assessment of eutrophication, both within the European Commission’s Water Framework and Marine Strategy Directives and the Helsinki commission. Chl-a is a used as a proxy for phytoplankton biomass and Secchi depth is used as a measure of changes in Chl-a. However, Secchi depth is more closely correlated with the light climate, affecting for example benthic vegetation. The public strongly link Secchi depth to the perceived water quality. Due to its simple measurement method Secchi depth is included in many monitoring programmes, often with the longest available time-series. In optically complex waters, Secchi depth is influenced by other factors than Chl-a, such as coloured dissolved organic matter (CDOM) and suspended particulate matter (SPM). In this study we evaluate how much Chl-a, CDOM and inorganic SPM each contribute to the variations in Secchi depth. We collected in situ data from different Swedish coastal gradients in three regions, Bothnian Sea, Baltic proper and Skagerrak during 2010-2014. Two linear multiple regression models for each region, with Chl-a, CDOM and inorganic SPM as predictors, explained the Secchi depth well (R2adj=0.54/0.8 for the Bothnian Sea, R2adj=0.81/0.81 for the Baltic proper and R2adj=0.53/0.64 for the Skagerrak). The slope for inorganic SPM was not significant in all models, but still included in the models, as significant correlations were found, both with Secchi depth and between parameters. The follow-up analysis of the multiple regressions by commonality analyses showed differences between the regions in the unique and common effects of the variables to the variance of the R2adj for Secchi depth. In the Bothnian Sea the unique effects for Chl-a were relatively low, 6% and 20%. The highest unique effect were from CDOM (~46% in summer and 20% in spring), whereas inorganic SPM had no unique contribution in summer but in spring with ~6%. The common effects from CDOM and inorganic SPM were large (71% in spring and 42% in summer). In the Baltic proper the optical variables had a different effect on the Secchi depth, with the largest part from the common effects of all three parameters, explaining up to 42-45% of the variations. The largest unique effects were from inorganic SPM (24%) or from Chl-a (15%). The models in the Skagerrak showed another pattern with CDOM having a very high unique effect, 71% for one model and the almost equally to Chl-a in the other 26% (Chl-a 28%). The common effects between CDOM and Chl-a were also pronounced, ~21% and the inorganic SPM had the lowest effect. The models were used for applying the levels for the reference value and the threshold for good/moderate status for Chl-a within the EU directives. The results showed, that in optically complex waters, Secchi depth is not a sufficient indicator for eutrophication, or as a response to Chl-a changes. Differences in natural processes have an indirect effect on the optical components determining the Secchi depth. For example land and river run-off, resuspension, bottom substrate, hydrography and salinity may explain the differences seen between the regions. The natural coastal gradients in Secchi depth will influence the determination of reference conditions for other eutrophication indicators, such as the depth distribution of macro algae. Hence, setting targets for Secchi depth based on reducing Chl-a might in some cases have no or only limited effect.

  • 8.
    Kari, Elina
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Merkouriadi, Ioanna
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Leppäranta, Matti
    Kratzer, Susanne
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Development of under-ice stratification in Himmerfjärden bay, north-western Baltic proper, and their effect on the phytoplankton spring bloom2018In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 186, p. 85-95Article in journal (Refereed)
    Abstract [en]

    Seasonal sea ice cover reduces wind-driven mixing and allows for under-ice stratification to develop. These under-ice plumes are a common phenomenon in the seasonal sea ice zone. They stabilize stratification and concentrate terrestrial runoff in the top layer, transporting it further offshore than during ice-free seasons. In this study, the effect of sea ice on spring stratification is investigated in Himmerfjärden bay in the NW Baltic Sea. Distinct under-ice plumes were detected during long ice seasons. The preconditions for the development of the under-ice plumes are described as well as the typical spatial and temporal dimensions of the resulting stratification patterns. Furthermore, the effect of the under-ice plume on the timing of the onset and the maximum of the phytoplankton spring bloom were investigated, in terms of chlorophyll-a (Chl-a) concentrations. At the head of the bay, bloom onset was delayed on average by 18 days in the event of an under-ice plume. However, neither the maximum concentration of Chl-a nor the timing of the Chl-a maximum were affected, implying that the growth period was shorter with a higher daily productivity. During this period from spring bloom onset to maximum Chl-a, the diatom biomass was higher and Mesodinium rubrum biomass was lower in years with under-ice plumes compared to years without under-ice plumes. Our results thus suggest that the projected shorter ice seasons in the future will reduce the probability of under-ice plume development, creating more dynamic spring bloom conditions. These dynamic conditions and the earlier onset of the spring bloom seem to favor the M. rubrum rather than diatoms.

  • 9.
    Klawonn, Isabell
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Nahar, N.
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Andersson, B.
    Olofsson, M.
    Barthel Svedén, Jennie
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Littmann, S.
    Whitehouse, M. J.
    Kuypers, M. M. M.
    Ploug, Helle
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Cell-specific nitrogen- and carbon-fixation of cyanobacteria in a temperate marine system (Baltic Sea)2016In: Environmental Microbiology, ISSN 1462-2912, E-ISSN 1462-2920, Vol. 18, no 12, p. 4596-4609Article in journal (Refereed)
    Abstract [en]

    We analysed N-2- and carbon (C) fixation in individual cells of Baltic Sea cyanobacteria by combining stable isotope incubations with secondary ion mass spectrometry (SIMS). Specific growth rates based on N-2- and C-fixation were higher for cells of Dolichospermum spp. than for Aphanizomenon sp. and Nodularia spumigena. The cyanobacterial biomass, however, was dominated by Aphanizomenon sp., which contributed most to total N-2-fixation in surface waters of the Northern Baltic Proper. N-2-fixation by Pseudanabaena sp. and colonial picocyanobacteria was not detectable. N-2-fixation by Aphanizomenon sp., Dolichospermum spp. and N. spumigena populations summed up to total N-2-fixation, thus these genera appeared as sole diazotrophs within the Baltic Sea's euphotic zone, while their mean contribution to total C-fixation was 21%. Intriguingly, cell-specific N-2-fixation was eightfold higher at a coastal station compared to an offshore station, revealing coastal zones as habitats with substantial N-2-fixation. At the coastal station, the cell-specific C-to N-2-fixation ratio was below the cellular C: N ratio, i.e. N-2 was assimilated in excess to C-fixation, whereas the C-to N-2-fixation ratio exceeded the C: N ratio in offshore sampled diazotrophs. Our findings highlight SIMS as a powerful tool not only for qualitative but also for quantitative N-2-fixation assays in aquatic environments.

  • 10.
    Klawonn, Isabell
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Nahar, N.
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Andersson, B.
    Olofsson, M.
    Littmann, S.
    Whitehouse, M.
    Ploug, Helle
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Cell-specific N2- and carbon fixation of cyanobacteria in a temperate marine system (Baltic Sea)Manuscript (preprint) (Other academic)
  • 11.
    Larsson, Ulf
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Nyberg, Svante
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Zakrisson, Anna
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Hajdu, Susanna
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Elmgren, Ragnar
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Rolff, Carl
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Baltic Sea phytoplankton: Long-term variability of major groups and primary production in spring and summer related to nutrients and temperatureManuscript (preprint) (Other academic)
  • 12.
    Svedén, Jennie B.
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Adam, Birgit
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Nahar, Nurun
    Musat, Niculina
    Lavik, Gaute
    Whitehouse, Martin J.
    Kuypers, Marcel M. M.
    Ploug, Helle
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. University of Gothenburg, Sweden.
    High cell-specific rates of nitrogen and carbon fixation by the cyanobacterium Aphanizomenon sp at low temperatures in the Baltic Sea2015In: FEMS Microbiology Ecology, ISSN 0168-6496, E-ISSN 1574-6941, Vol. 91, no 12Article in journal (Refereed)
    Abstract [en]

    Aphanizomenon is a widespread genus of nitrogen (N-2)-fixing cyanobacteria in lakes and estuaries, accounting for a large fraction of the summer N-2-fixation in the Baltic Sea. However, information about its cell-specific carbon (C)- and N-2-fixation rates in the early growth season has not previously been reported. We combined various methods to study N-2-fixation, photosynthesis and respiration in field-sampled Baltic Sea Aphanizomenon sp. during early summer at 10 degrees C. Stable isotope incubations at in situ light intensities during 24 h combined with cell-specific secondary ion mass spectrometry showed an average net N-2-fixation rate of 55 fmol N cell(-1) day(-1). Dark net N-2-fixation rates over a course of 12 h were 20% of those measured in light. C-fixation, but not N-2-fixation, was inhibited by high ambient light intensities during daytime. Consequently, the C: N fixation ratio varied substantially over the diel cycle. C-and N-2-fixation rates were comparable to those reported for Aphanizomenon sp. in August at 19 degrees C, using the same methods. High respiration rates (23% of gross photosynthesis) were measured with C-14-incubations and O-2-microsensors, and presumably reflect the energy needed for high N-2-fixation rates. Hence, Aphanizomenon sp. is an important contributor to N-2-fixation at low in situ temperatures in the early growth season.

  • 13.
    Svedén, Jennie B.
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Production and nitrogen fixation by Baltic Sea Aphanizomenon sp.  — estimates from a growth modelManuscript (preprint) (Other academic)
  • 14.
    Svedén, Jennie B.
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Walve, Jakob
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Elmgren, Ragnar
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Larsson, Ulf
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    The bloom of nitrogen-fixing cyanobacteria in the northern Baltic Proper stimulates summer production2016In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 163, p. 102-112Article in journal (Refereed)
    Abstract [en]

    In the northern Baltic Sea Proper, total nitrogen (TN) increases during the summer bloom of filamentous heterocystous cyanobacteria. To follow the fate of the nitrogen they fix, we studied several N fractions during the bloom. We measured cyanobacterial biomass, TN, particulate organic N (PON, two size fractions), dissolved organic N (DON), and PON sedimentation in two areas in 2011. TN increased mainly due to increasing PON, but also to DON. Cyanobacteria contributed about 20% of the PON increase and ~ 10% of the TN increase. About half the PON changes (increase, then decrease) could be explained by the sum of cyanobacteria, other autotrophs (> 2 μm) and zooplankton, indicating that the bloom stimulates primary and secondary production. TN decreased after the bloom mainly due to declining PON > 10 μm, but sedimentation rates did not increase and could explain little of the post-bloom N-loss. There was little settling of undecomposed cyanobacteria.

    The seasonal development of Aphanizomenon sp. and N pools was similar among stations and areas. For Nodularia spumigena between-station variability increased once patchy surface accumulations developed. A brief Dolichospermum spp. bloom indicated that sampling frequency may be more important than spatial resolution for capturing dynamics of this bloom.

  • 15.
    Walve, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Gelting, Johan
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Ingri, Johan
    Trace metals and nutrients in Baltic Sea cyanobacteria: Internal and external fractions and potential use in nitrogen fixation2014In: Marine Chemistry, ISSN 0304-4203, E-ISSN 1872-7581, Vol. 158, p. 27-38Article in journal (Refereed)
    Abstract [en]

    Even though the availability of trace metals influences nitrogen fixation and growth of cyanobacteria, field data on their cellular metal composition are scarce. In this study, contents of Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, and the major elements C, N, P and Si were studied in filamentous, nitrogen-fixing cyanobacteria sampled over the growth season March-October at two locations in the Baltic proper (years 2004 and 2007) and one location in the Bothnian Sea (2006). The Al and Ti contents indicated that lithogenic Fe was an important Fe fraction associated with Nodularia spumigena, but not with Aphanizomenon sp. Treatment with an oxalate-EDTA solution indicated that less than 5% of total Fe was adsorbed as oxides, but relatively high adsorbed fractions were found for Mn and Cu. Despite the large variation in biomass and dissolved Fe concentrations, the Fe:C ratio of Aphanizomenon was highly consistent within years and across sampling stations (76 +/- 13 mu mol mol(-1) C. average +/- 1SD), indicating growth controls other than Fe. Species-mixed samples corrected for lithogenic metals indicate similar Fe content in Nodularia as in Aphanizomenon. Calculations based on the use efficiency of Mo for N-2 fixation indicate that most Mo in Nodularia and at least a third of the Mo in Aphanizomenon are used in nitrogenase, corresponding to 5-24% of the Fe content. The high Ni content suggests excess storage or extensive use in enzymes such as Ni superoxide dismutase or in Fe-dependent Ni-hydrogenases. The trace metal composition of the investigated Baltic cyanobacteria was similar to that reported for the oceanic genus Trichodesmium, suggesting common physiological requirements of these filamentous nitrogen-fixing cyanobacteria.

  • 16.
    Walve, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Systems Ecology. marin ekologi.
    Larsson, Ulf
    Stockholm University, Faculty of Science, Department of Systems Ecology. marin ekologi.
    Blooms of Baltic Sea Aphanizomenon sp. (Cyanobacteria) collapse after internal phosphorus depletion2007In: Aquatic Microbial Ecology, Vol. 49, p. 57-69Article in journal (Refereed)
  • 17.
    Walve, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Larsson, Ulf
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Seasonal changes in Baltic Sea seston stoichiometry: the influence of diazotrophic cyanobacteria2010In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 407, p. 13-25Article in journal (Refereed)
    Abstract [en]

    We studied nutrient deficiency of Baltic Sea phytoplankton, as indicated by C:N:P ratios in filamentous N-2-fixing (diazotrophic) cyanobacteria and size-fractions of seston. Samples were collected during an annual cycle in the NW Baltic proper (1998, Landsort Deep) and in 2 summers in the central Baltic proper (1997 and 1998, Gotland Basin). Generally, seston C:N:P ratios in the top 20 m were close to Redfield values. There was a transient increase in the C:N ratio at the end of the phytoplankton spring bloom (to similar to 9 mol:mol), concomitant with the depletion of dissolved inorganic N (DIN), but not dissolved inorganic phosphorus (DIP), indicating that phytoplankton became N-limited at the end of the spring bloom. In early summer (May-June), seston C:N:P ratios indicated weak N limitation of the community of small phytoplankton. From June through summer, seston size-fractions < 10 mu m accumulated mainly C and N, resulting in a September peak in seston C:P and N:P ratios (201 and 23 mol:mol, respectively) in the surface water. This change occurred in parallel with an increase in C:P and N:P ratios of diazotrophic filamentous cyanobacteria to levels indicative of severe P limitation of cyanobacterial growth. This suggests that the cyanobacterial P demand and new N from their N-2 fixation caused a weak P deficiency also in non-diazotrophs. However, the small increase in seston N:P, as well as a seston C:N above the Redfield ratio, indicate that N and P are nearly co-limiting for non-diazotrophs at the culmination of the cyanobacterial bloom in late summer.

  • 18.
    Walve, Jakob
    et al.
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Sandberg, Maria
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Larsson, Ulf
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Lännergren, Christer
    A Baltic Sea estuary as a phosphorus source and sink after drastic load reduction: seasonal and long-term mass balances for the Stockholm inner archipelago for 1968-20152018In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 15, no 9, p. 3003-3025Article in journal (Refereed)
    Abstract [en]

    Internal phosphorus (P) loading from sediments, controlled by hypoxia, is often assumed to hamper the recovery of lakes and coastal areas from eutrophication. In the early 1970s, the external P load to the inner archipelago of Stockholm, Sweden (Baltic Sea), was drastically reduced by improved sewage treatment, but the internal P loading and its controlling factors have been poorly quantified. We use two slightly different four-layer box models to calculate the area's seasonal and annual P balance (input-export) and the internal P exchange with sediments in 1968-2015. For 1020 years after the main P load reduction, there was a negative P balance, small in comparison to the external load, and probably due to release from legacy sediment P storage. Later, the stabilized, near-neutral P balance indicates no remaining internal loading from legacy P, but P retention is low, despite improved oxygen conditions. Seasonally, sediments are a P sink in spring and a P source in summer and autumn. Most of the deep-water P release from sediments in summer-autumn appears to be derived from the settled spring bloom and is exported to outer areas during winter. Oxygen consumption and P release in the deep water are generally tightly coupled, indicating limited iron control of P release. However, enhanced P release in years of deep-water hypoxia suggests some contribution from redox-sensitive P pools. Increasing deep-water temperatures that stimulate oxygen consumption rates in early summer have counteracted the effect of lowered organic matter sedimentation on oxygen concentrations. Since the P turnover time is short and legacy P small, measures to bind P in Stockholm inner archipelago sediments would primarily accumulate recent P inputs, imported from the Baltic Sea and from Lake Mälaren.

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