Chemical processes in estuaries 2

Question 1

What are the properties of fluorine in estuaries?

Answer 1

Conservative

Higher conc in seawater

Question 2

What are the properties of iron in estuaries?

Answer 2

Non conservative
Loss of Fe from solution in upper estuary
Due to coagulation of colloidal Fe(III) as river water mixes with seawater
Concentration of Fe(II) decreases with increasing salinity

Question 3

How does Fe speciation show the non-conservative behaviour of Fe?

Answer 3

• Most Fe is lost as Fe(III)
• Conc of Fe(II) can be high; stabilised by ligands?
• Concentration of Fe(II) decreases with increasing salinity; loss of organic ligands via flocculation and/or effects of increased ionic strength

Question 4

What is the behaviour of silicon in estuaries?

Answer 4

• dissolved silicon can show both conservative and non conservative behaviour
• some examples of extreme removals in some estuaries

Question 5

Why is estuarine residence time important for silicon?

Answer 5

If residence time is short (high riverwater flux) then any dissolved silicon used by diatoms is rapidly replenished.

Question 6

How does Narragansett bay, rhode island support further diatom growth?

Answer 6

Salt marshes are a source of dissolved silicon in the spring due to remineralisation of biogenic silicon

Over course of the year, marsh is a net SINK of both dissolved and biogenic silicon

Question 7

What is another SOURCE of dissolved silicon in estuaries?

Answer 7

desorption from resuspended sediments/dissolution of riverbourne particulate silicon

Question 8

How can silicon isotopes be used to study nutrient utilisation & remineralisation of biogenic silicon?

Answer 8

diatoms preferentially utilise LIGHT Si isotopes to form frustules, so the remaining dissolved silicon is enriched with heavier isotopes

Question 9

Behaviour of dissolved silicon may change over the course of a year. What does this reflect?

Answer 9

• rate of biological uptake/remineralisation (if rate is low compared to rate of re-supply of dissolved silicon via rivers then may not show up as non-conservative behaviour)
• residence time of water within the estuary (depends on river flow rate)
• other factors like turbidity (particle loading may affect light penetration; river biological production is inhibited by turbidity)

Question 10

Why does manganese undergo intensive cycling in many estuaries?

Answer 10

Its chemistry is linked to changes in redox conditions.

Many estuarine sediments are reducing, due to high rates of burial of organic carbon.

In anoxic sediments, Mn(IV) is reduced to Mn(II) which is soluble.

Mn(II) may diffuse out of sediment, and flux is increased by resuspension of sediments.

Question 11

At what scale of salinity is the maximum concentration of dissolved Mn found at?

Answer 11

Intermediate salinities.

Question 12

When can high concentrations of Mn be found in the water column?

Answer 12

If circulation is restricted, e.g. a fjord.

Question 13

How does fluorine behave during estuarine mixing?

Answer 13

Conservatively

Question 14

How does iron behave in estuaries?

Answer 14

Non-conservatively; it it lost from solution largely due to coagulation/flocculation of colloidal Fe(III).

Question 15

What happens to dissolved Si levels when biological productivity is high?

Answer 15

It falls due to uptake by diatoms.

Question 16

How can addition of biogenic Si occur?

Answer 16

By remineralisation of biogenic Si, or via resuspension of bottom sediments.

Question 17

Why is manganese often added at intermediate salinities?

Answer 17

Due to inputs of Mn(II) from reducing sediments.