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Aggregation of iron in estuaries and the effect of increasing iron concentration in riverine waters

Bedmar Villanueva, Ana (2013) BIOM35 20121
Degree Projects in Biology
Abstract
Abstract

The significance of terrestrially derived iron (Fe) for marine ecosystems and its implications on other biogeochemical cycles is well recognised. It has been recently highlighted that the concentration of Fe in several Swedish rivers has increased strongly over the last 40 years. Four rivers draining into the Baltic Sea were used in the present study in order to assess the aggregation process of dissolved Fe and organic carbon along a salinity gradient. Experimental additions of Fe were conducted in a parallel set of experiments in order to assess the potential consequences of higher riverine Fe concentrations reaching estuaries. Data from the last 49 years was used to extrapolate a rough prediction of the Fe concentration of... (More)
Abstract

The significance of terrestrially derived iron (Fe) for marine ecosystems and its implications on other biogeochemical cycles is well recognised. It has been recently highlighted that the concentration of Fe in several Swedish rivers has increased strongly over the last 40 years. Four rivers draining into the Baltic Sea were used in the present study in order to assess the aggregation process of dissolved Fe and organic carbon along a salinity gradient. Experimental additions of Fe were conducted in a parallel set of experiments in order to assess the potential consequences of higher riverine Fe concentrations reaching estuaries. Data from the last 49 years was used to extrapolate a rough prediction of the Fe concentration of the studied rivers in 72 years. The results obtained here suggest that the character of the riverine dissolved organic matter play a significant role in the Fe transport capacity of the river waters. The Fe amendments caused an enhancement of the Fe aggregation in two of the studied rivers. In contrast, an increase of the concentration of Fe kept in solution by another river was recorded. There is evidence that increasing Fe in freshwater systems might be a widespread phenomenon, however, it is not known what are the potential consequences of an increasing build up of Fe and associated substances in the sediments. In contrast, a potential natural Fe fertilization of the oceans might have implications at the global scale. Thus the findings highlighted here open a broad field for further research.

Popular science summary:

Iron is the main factor limiting important processes such as photosynthesis and nitrogen fixation in vast areas of the ocean. In oxygenated and saline waters iron has very low solubility, and that is why it is present only in very low concentrations. An important source of Fe to oceans is riverine export, where Fe is mainly present in association with dissolved organic matter. Recent research highlights a strong trend of increasing Fe concentration in rivers throughout Sweden, ranging from 21 to 468 % over the last 40 years. Such a strong trend may have important implications.
The present study aimed to assess to what extent riverine iron may be maintained in solution in the marine environment, where it could e.g. relieve cyanobacteria from iron limitation, and to what extent it may aggregate and sediment.

Methods
Water from four Swedish rivers were used. Helgeån, Lyckebyån and Emån drain into the Baltic Proper. Umeälven drains into the Bothnian Sea. The rivers were chosen to differ with respect to Fe concentration and the quality of the organic matter. An artificial salt mixture was added to filtered river water to create a salinity gradient ranging from 0 to 30 ‰. In a parallel experiment, different concentrations of Fe were added to the river waters to assess the effect of increasing Fe concentrations on the aggregation process. After the salt addition samples were mixed and left to settle in a constant temperature room for 24 hours, and then centrifuged for 8 hours. During centrifugation the aggregates concentrated at the bottom of the tubes, while the iron and organic matter that remained in solution was still in the water phase. The water phase was sampled to estimate how much iron and organic matter remained in solution and to assess the quality of the organic matter.

Results
The aggregation of Fe increased in all waters as salinity increased strongly from 0 to 15 ‰, so that only a fraction of the initial iron remained in solution (~5 %). However, in the water from Emån much more Fe remained in solution in the salinity gradient (~35 %) . When I added even more iron to the water samples, the aggregation and loss of Fe occurred even faster in Helgeån and Lyckebyån, while the water from Emån maintained high Fe concentrations. The results suggest that increasing Fe concentrations reaching estuaries may lead to an increase of Fe and associated substances sinking to the sediments at low salinities. On the other hand, the results for Emån point to the potential of some rivers to keep relatively high concentrations of Fe in solution at high salinities. This is of relevance because negative effects on biota caused by aggregated Fe remaining in solution have been reported in some estuaries. Moreover, increasing dissolved Fe concentrations in estuaries may enhance the productivity of the marine systems and especially favor cyanobacteria which have a very high iron demand.

Advisors: Emma Kritzberg and Heather Reader
Master´s Degree Project 30 credits in Aquatic Ecology 2013.
Department of Biology, Lund University. (Less)
Please use this url to cite or link to this publication:
author
Bedmar Villanueva, Ana
supervisor
organization
course
BIOM35 20121
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
3731563
date added to LUP
2013-04-26 12:08:40
date last changed
2013-04-26 12:08:40
@misc{3731563,
  abstract     = {Abstract

The significance of terrestrially derived iron (Fe) for marine ecosystems and its implications on other biogeochemical cycles is well recognised. It has been recently highlighted that the concentration of Fe in several Swedish rivers has increased strongly over the last 40 years. Four rivers draining into the Baltic Sea were used in the present study in order to assess the aggregation process of dissolved Fe and organic carbon along a salinity gradient. Experimental additions of Fe were conducted in a parallel set of experiments in order to assess the potential consequences of higher riverine Fe concentrations reaching estuaries. Data from the last 49 years was used to extrapolate a rough prediction of the Fe concentration of the studied rivers in 72 years. The results obtained here suggest that the character of the riverine dissolved organic matter play a significant role in the Fe transport capacity of the river waters. The Fe amendments caused an enhancement of the Fe aggregation in two of the studied rivers. In contrast, an increase of the concentration of Fe kept in solution by another river was recorded. There is evidence that increasing Fe in freshwater systems might be a widespread phenomenon, however, it is not known what are the potential consequences of an increasing build up of Fe and associated substances in the sediments. In contrast, a potential natural Fe fertilization of the oceans might have implications at the global scale. Thus the findings highlighted here open a broad field for further research.

Popular science summary:

Iron is the main factor limiting important processes such as photosynthesis and nitrogen fixation in vast areas of the ocean. In oxygenated and saline waters iron has very low solubility, and that is why it is present only in very low concentrations. An important source of Fe to oceans is riverine export, where Fe is mainly present in association with dissolved organic matter. Recent research highlights a strong trend of increasing Fe concentration in rivers throughout Sweden, ranging from 21 to 468 % over the last 40 years. Such a strong trend may have important implications.
The present study aimed to assess to what extent riverine iron may be maintained in solution in the marine environment, where it could e.g. relieve cyanobacteria from iron limitation, and to what extent it may aggregate and sediment. 

Methods
Water from four Swedish rivers were used. Helgeån, Lyckebyån and Emån drain into the Baltic Proper. Umeälven drains into the Bothnian Sea. The rivers were chosen to differ with respect to Fe concentration and the quality of the organic matter. An artificial salt mixture was added to filtered river water to create a salinity gradient ranging from 0 to 30 ‰. In a parallel experiment, different concentrations of Fe were added to the river waters to assess the effect of increasing Fe concentrations on the aggregation process. After the salt addition samples were mixed and left to settle in a constant temperature room for 24 hours, and then centrifuged for 8 hours. During centrifugation the aggregates concentrated at the bottom of the tubes, while the iron and organic matter that remained in solution was still in the water phase. The water phase was sampled to estimate how much iron and organic matter remained in solution and to assess the quality of the organic matter.

Results
The aggregation of Fe increased in all waters as salinity increased strongly from 0 to 15 ‰, so that only a fraction of the initial iron remained in solution (~5 %). However, in the water from Emån much more Fe remained in solution in the salinity gradient (~35 %) . When I added even more iron to the water samples, the aggregation and loss of Fe occurred even faster in Helgeån and Lyckebyån, while the water from Emån maintained high Fe concentrations. The results suggest that increasing Fe concentrations reaching estuaries may lead to an increase of Fe and associated substances sinking to the sediments at low salinities. On the other hand, the results for Emån point to the potential of some rivers to keep relatively high concentrations of Fe in solution at high salinities. This is of relevance because negative effects on biota caused by aggregated Fe remaining in solution have been reported in some estuaries. Moreover, increasing dissolved Fe concentrations in estuaries may enhance the productivity of the marine systems and especially favor cyanobacteria which have a very high iron demand. 

Advisors: Emma Kritzberg and Heather Reader
Master´s Degree Project 30 credits in Aquatic Ecology 2013.
Department of Biology, Lund University.},
  author       = {Bedmar Villanueva, Ana},
  language     = {eng},
  note         = {Student Paper},
  title        = {Aggregation of iron in estuaries and the effect of increasing iron concentration in riverine waters},
  year         = {2013},
}