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Fate of riverine iron over estuarine salinity gradients

Herzog, Simon LU (2018)
Abstract (Swedish)
Floder och åar är viktiga källor för järn och andra näringsämnen till Östersjön. De senaste decennierna har järnkoncentrationen i svenska och finska åar ökat kraftigt, vilket skulle kunna leda till en ökad transport av järn till Östersjön. Vilken effekt detta kan ha för Östersjön beror på vad som händer med järnet när det möter det salta vattnet i Östersjön. Järn transporteras huvudsakligen i form av små partiklar - kolloider. Dessa kolloider består antingen av järnoxider eller organiska järnkomplex. Kolloidernas kemiska sammansättning är av betydelse då det påverkar vad som händer med järnet i saltgradienten. Om de järnbärande kolloiderna stannar kvar i vattenfasen kan järnet nå de öppna vattnen i Östersjön.
Järn är ett essentiellt... (More)
Floder och åar är viktiga källor för järn och andra näringsämnen till Östersjön. De senaste decennierna har järnkoncentrationen i svenska och finska åar ökat kraftigt, vilket skulle kunna leda till en ökad transport av järn till Östersjön. Vilken effekt detta kan ha för Östersjön beror på vad som händer med järnet när det möter det salta vattnet i Östersjön. Järn transporteras huvudsakligen i form av små partiklar - kolloider. Dessa kolloider består antingen av järnoxider eller organiska järnkomplex. Kolloidernas kemiska sammansättning är av betydelse då det påverkar vad som händer med järnet i saltgradienten. Om de järnbärande kolloiderna stannar kvar i vattenfasen kan järnet nå de öppna vattnen i Östersjön.
Järn är ett essentiellt näringsämne växtplankton, eftersom järn är inblandat i nyckelprocesser såsom fotosyntes och respiration. Under kraftiga algblomningar kan järn begränsa tillväxten, speciellt för de potentiellt giftiga blågrönalgerna som har särskilt stort järnbehov. Förekomsten av blågrönalgblomningar i Östersjön har ökat de senaste decennierna, vilket är ett hot mot ekosystemet med tanke på att blågrönalger kan vara giftiga för djur och människor. Dessutom kan blågrönalger skapa en positiv feedback för övergödning, genom att tillföra kväve till systemet genom kvävefixering, och genom att ge upphov till syrebrist när algbiomassan bryts ner. Bristen på syre kan i sin tur leda till att fosfor frigörs från sedimenten, och en förlust av biodiversitet och levnadsmiljöer, samt förändringar i födoväven.
Den gängse bilden är emellertid att merparten av järnet från åar och floder aggregerar när det möter det salta vattnet och sjunker till sedimentet. I sedimentet kan järnet binda in fosfor och hålla det borta från vattnet. På så vis kan järnet i stället motverka övergödning, genom att verka som en fosforsänka.
För att bättre förstå transporten av järn från åar till öppet hav ville jag undersöka vad som händer med järnet när sötvatten från vattendragen blandas med marint saltvatten. I det syftet studerades tio åar från norra till södra Sverige. Järnets sammansättning – huruvida det var järnoxider eller organiska järnkomplex – analyserades med röntgenabsorptionsspektroskopi vid synkrotronljus-anläggningar.
Resultaten visade att alla åmynningar hade en blandning av organiska järnkomplex och järnoxider. Det relativa bidraget av de två järnformerna varierade, men överlag var andelen organiska järnkomplex oväntat stor, och den var större under höga flöden på våren än låga flöden på hösten. Ett centralt resultat var att de organiska järnkomplexen var mycket mer stabila över salinitetsgradienter än järnoxider som i stor utsträckning aggregerade. Detta, tillsammans med den stora andelen organiska järnkomplex, indikerar att de ökande järnkoncentrationerna i många åar kan leda till att mer järn når till öppna vatten i Östersjön där de kan gynna algblomningar. Även om andelen organiska järnkomplex var oväntat hög, så var merparten av järnet i form av järnoxider, och kan följaktligen öka inbindningen av fosfor i sedimenten, vilket kan motverka övergödning. Fler studier som belyser vilken roll järn spelar för viktiga näringscykler i Östersjön behövs.
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Abstract
Rivers have traditionally not been considered important sources of bioavailable iron (Fe) to the marine waters, since most Fe is lost by salinity-induced aggregation and sedimentation during estuarine mixing. However, recent studies from boreal rivers found a remarkably high Fe stability, probably due to the interaction with organic matter. Recent studies have suggested that Fe speciation is a key factor, i.e. that Fe (oxy)hydroxides are effectively removed by aggregation processes, but that organic Fe complexes (Fe-OM) are less affected by increasing salinity. This hypothesis has been supported by indirect assessments of Fe speciation (based on molecular size and Fe:C ratios). One aim of the current thesis was to directly assess... (More)
Rivers have traditionally not been considered important sources of bioavailable iron (Fe) to the marine waters, since most Fe is lost by salinity-induced aggregation and sedimentation during estuarine mixing. However, recent studies from boreal rivers found a remarkably high Fe stability, probably due to the interaction with organic matter. Recent studies have suggested that Fe speciation is a key factor, i.e. that Fe (oxy)hydroxides are effectively removed by aggregation processes, but that organic Fe complexes (Fe-OM) are less affected by increasing salinity. This hypothesis has been supported by indirect assessments of Fe speciation (based on molecular size and Fe:C ratios). One aim of the current thesis was to directly assess variability in Fe speciation in boreal river waters, by X-ray absorbance spectroscopy (XAS), and to examine the link to Fe stability, which was assessed by artificial seawater mixing experiments. Another aim was to explore what factors may control temporal and spatial variability in Fe speciation within and across rivers. It has further been proposed that variation in Fe isotopic composition reflects Fe speciation, which was tested in this thesis by subjecting a set of samples to both analyses. Finally, to test how colloidal size distributions in river waters vary with regards to surface charge and Fe speciation, XAS analysis was combined with dynamic light scattering and zeta potential. The combination of these methods provided the potential for a comprehensive picture of the Fe phases present in river water and how they react to increasing salinity. The overarching aim of this thesis was to gain a better understanding of what factors determine the fate of Fe from boreal rivers across estuarine salinity gradients. For this purpose, 10 rivers, from the north to the south of Sweden, with different catchment characteristics were considered. Among the river mouths a significant, but variable contribution of Fe-OM 
in relation to Fe (oxy)hydroxides was detected. That Fe (oxy)hydroxides are more affected by increasing salinity than Fe-OM was confirmed by selective removal of Fe (oxy)hydroxides due to salinity-induced aggregation. Moreover, the relative contribution of Fe-OM correlated well with Fe transport capacity (FeTC), i.e. supporting that organic complexation of Fe favors Fe export to open waters. However, Fe-OM complexes were also found in the aggregated fraction, illustrating that the control of Fe stability is not simply explained by the prevalence of the respective Fe phases alone.
The Fe-OM contribution was more dominant further upstream in a catchment than at the river mouth, and also more prevalent during high-flow than at low-flow conditions. Consequently, the higher contribution of Fe-OM during spring, when much of the annual Fe discharge is taking place, resulted in a higher FeTC. Separation of salinity-induced aggregates from the fraction remaining in suspension revealed that Fe (oxy)hydroxide displayed lower δ56Fe values, and Fe-OM displayed higher δ56Fe values than the in situ Fe. This points to the possibility of inferring Fe speciation of the fraction that survives the estuarine mixing zone from isotope analysis. The combination of XAS and DLS analysis demonstrated the existence of three size distributions; Fe (oxy)hydroxide were observed both as nanoparticles (10-40 nm) with positive surface charge, and larger aggregates with OM interactions (300-900 nm). An intermediate (100-200 nm) and negatively charged distribution was inferred to contain Fe-OM. After increasing salinity, the smallest Fe (oxy)hydroxide nanoparticles were no longer detected. Interestingly, both the larger size distributions were still detected at high salinity.
In all, the results from this thesis support that boreal rivers may provide significant amounts of bioavailable Fe to marine 
waters beyond the estuary, due to significant contributions of Fe-OM complexes. Moreover, the results illustrates that a division between small Fe-OM complexes that “survive” estuarine salinity gradients and large Fe (oxy)hydroxides that are aggregated and lost to the sediment, is too simplistic, since both phases can be found in a wide size range. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Ass. Research Professor Schroth, Andrew, University of Vermont, USA
organization
publishing date
type
Thesis
publication status
published
subject
keywords
iron (Fe), Organic matter, boreal rivers, XAS, DLS, Fe isotope
pages
166 pages
publisher
Lund University
defense location
Blå hallen, Ekologihuset, Sölvegatan 37, Lund
defense date
2019-01-11 09:00
ISBN
978-91-7753-936-0
978-91-7753-937-7
language
English
LU publication?
yes
id
ff0b3ebd-3796-40da-a4d0-13cdecb68142
date added to LUP
2018-12-17 13:05:23
date last changed
2018-12-17 14:20:31
@phdthesis{ff0b3ebd-3796-40da-a4d0-13cdecb68142,
  abstract     = {Rivers have traditionally not been considered important sources of bioavailable iron (Fe) to the marine waters, since most Fe is lost by salinity-induced aggregation and sedimentation during estuarine mixing. However, recent studies from boreal rivers found a remarkably high Fe stability, probably due to the interaction with organic matter. Recent studies have suggested that Fe speciation is a key factor, <i>i.e.</i> that Fe (oxy)hydroxides are effectively removed by aggregation processes, but that organic Fe complexes (Fe-OM) are less affected by increasing salinity. This hypothesis has been supported by indirect assessments of Fe speciation (based on molecular size and Fe:C ratios). One aim of the current thesis was to directly assess variability in Fe speciation in boreal river waters, by X-ray absorbance spectroscopy (XAS), and to examine the link to Fe stability, which was assessed by artificial seawater mixing experiments. Another aim was to explore what factors may control temporal and spatial variability in Fe speciation within and across rivers. It has further been proposed that variation in Fe isotopic composition reflects Fe speciation, which was tested in this thesis by subjecting a set of samples to both analyses. Finally, to test how colloidal size distributions in river waters vary with regards to surface charge and Fe speciation, XAS analysis was combined with dynamic light scattering and zeta potential. The combination of these methods provided the potential for a comprehensive picture of the Fe phases present in river water and how they react to increasing salinity. The overarching aim of this thesis was to gain a better understanding of what factors determine the fate of Fe from boreal rivers across estuarine salinity gradients. For this purpose, 10 rivers, from the north to the south of Sweden, with different catchment characteristics were considered. Among the river mouths a significant, but variable contribution of Fe-OM 
in relation to Fe (oxy)hydroxides was detected. That Fe (oxy)hydroxides are more affected by increasing salinity than Fe-OM was confirmed by selective removal of Fe (oxy)hydroxides due to salinity-induced aggregation. Moreover, the relative contribution of Fe-OM correlated well with Fe transport capacity (FeTC), <i>i.e.</i> supporting that organic complexation of Fe favors Fe export to open waters. However, Fe-OM complexes were also found in the aggregated fraction, illustrating that the control of Fe stability is not simply explained by the prevalence of the respective Fe phases alone. <br/>The Fe-OM contribution was more dominant further upstream in a catchment than at the river mouth, and also more prevalent during high-flow than at low-flow conditions. Consequently, the higher contribution of Fe-OM during spring, when much of the annual Fe discharge is taking place, resulted in a higher FeTC. Separation of salinity-induced aggregates from the fraction remaining in suspension revealed that Fe (oxy)hydroxide displayed lower δ<sup>56</sup>Fe values, and Fe-OM displayed higher δ<sup>56</sup>Fe values than the in situ Fe. This points to the possibility of inferring Fe speciation of the fraction that survives the estuarine mixing zone from isotope analysis. The combination of XAS and DLS analysis demonstrated the existence of three size distributions; Fe (oxy)hydroxide were observed both as nanoparticles (10-40 nm) with positive surface charge, and larger aggregates with OM interactions (300-900 nm). An intermediate (100-200 nm) and negatively charged distribution was inferred to contain Fe-OM. After increasing salinity, the smallest Fe (oxy)hydroxide nanoparticles were no longer detected. Interestingly, both the larger size distributions were still detected at high salinity.<br/> In all, the results from this thesis support that boreal rivers may provide significant amounts of bioavailable Fe to marine 
waters beyond the estuary, due to significant contributions of Fe-OM complexes. Moreover, the results illustrates that a division between small Fe-OM complexes that “survive” estuarine salinity gradients and large Fe (oxy)hydroxides that are aggregated and lost to the sediment, is too simplistic, since both phases can be found in a wide size range. },
  author       = {Herzog, Simon},
  isbn         = {978-91-7753-936-0},
  keyword      = {iron (Fe),Organic matter,boreal rivers,XAS,DLS,Fe isotope},
  language     = {eng},
  pages        = {166},
  publisher    = {Lund University},
  school       = {Lund University},
  title        = {Fate of riverine iron over estuarine salinity gradients},
  year         = {2018},
}