Exam revision chapter 5

what is the behaviour of fluorine

conservative, no addition or removal higher concentration in seawater

what is the behaviour of iron

non conservative, reduction/loss of Fe from solution in the upper part of the estuary

what causes reduction of iron

more likely due to coagulation of colloidal Fe(III) as river water mixes with seawater (when Fe(II) hits sea water it precipitates) - most lost before salinity of 5 so most of removal in upper estuary most iron lost is Fe(III) unlikely due to oxidation of Fe(II) (soluble) -> Fe(III) (insoluble) as river waters contain O2 and the rate of oxidation is fast

how do concentrations of Fe(II) differ

concentration of Fe(II) can be high, possibly stabilised by ligands concentration of Fe(II) decreases with increases in salinity - loss of organic ligands via flocculation and effects of increased ionic strength (when seawater is at 1)

what is the behaviour of silicon

dissolved silicon can show both conservative and non-conservative behaviour

examples of silicon behaviour in estuaries

Elbe estuary: removal of dissolved silicon in early autumn - uptake by diatoms for frustules Danshuei estuary: shows conservative behaviour in march charlotte harbour florida: high levels of phosphate are delivered by the Peace River (hosts phosphate industries) promoting a diatom bloom close to the river mouth

removal of silicon in relation with chlorophyll a

removal of silicon also had addition of chlorophyll this is associated with a decrease in silicate and is therefore evidence for biological activity of removing silicate

what is the relationship of residence times and behaviour

conservative behaviour = short residence time = high flux = constantly used and remineralised (e.g. diatoms rapidly replenished) non-conservative behaviour = long residence time

how can dissolved silicon be used to trace river plumes

large river plumes can be transported out to sea and past the estuary if a removal of silicon is seen can trace the plume (since silicate comes from weathered rocks)

example: Narragansett Bay, Rhode Island

salt marshes here are a source of dissolved silicon in the spring (remineralisation of biogenic silicon) supporting further diatom growth over the year, the marsh is a net sink of dissolved and biogenic silicon H4SiO4 (aq) -> SiO2-nH2O (solid) -> is biological utilisation <- is dissolution

what are other sources of dissolved silicon

desorption from resuspended sediments anthropogenic inputs aren't usually a significant source

e.g. Rappahannock Estuary

removal of dissolved silicon in river by diatoms input of dissolved silicon due to desorption/dissolution of riverbourne particulate Si

relationship of dissolved silicon, chlorophyll and salinity with concentration and distance upstream

dissolved silicon low at 0 concentration end member which is unusual as rivers are a source of Si chlorophyll is high so therefore biological activity

how can Si isotopes be used

they are used to study nutrient utilisation and remineralisation of biogenic silicon dissolved silicon shows conservative behaviour in the Chaiiang estuary but diatoms preferentially utilise the lighter Si isotope (easier to take up) to form frustules so the remaining dissolved Si is enriched with heavier isotopes

what does isotope analysis provide

isotope analyses provide evidence for cryptic removal of light Si at salinities >20, especially in July heavier isotopes are left in the solution and so there is therefore a definite removal due to biological processes

what does the behaviour of dissolved silicon changing over a year reflect

the rate of biological uptake/remineralisation if the rate is low compared to the rate of re-supply of dissolved silicon via rivers (or remin of biogenic Si), then may not show up as non-conservative as enough is constantly put in and remineralised

what does residence time of water in the estuary depend on

river flow rate

what other factors affect dissolved silicon - turbidity

particle loading (particles in the water) may affect light penetration in some estuaries where river biological production is inhibited by turbidity

why does manganese undergo intensive cycling

its chemistry is linked to changes in redox conditions

oxidising vs reducing conditions for manganese

oxidising: free O2, Mn(IV) is thermodynamically stable, MnO2 has low solubility so Mn(IV) forms particles or coats them reducing: oxygen deficient, Mn(II) is thermodynamically stable, Mn2+ is soluble and can diffuse and advect in solution

Mn released from sediments in estuaries

river: suboxic/anoxic sediment resuspends and mobilises sediment and therefore Mn, it releases Mn2+ into the tidal river estuary: once in the estuary it's under reducing conditions and is given back to the sediment -> constant supply and removal

Mn behaviour in estuaries

maximum at low salinities addition in the middle of the estuary many estuarine sediments are reducing due to high rates of burial of organic carbon

Mn in anoxic sediments

Mn (IV) is reduced to Mn(II) which is soluble this may diffuse out of the sediments and flux is increased by resuspension of sediments

what happens when Mn oxidation state changes

a phase change Mn(II) is transported into an oxidising environment and is converted to MnO2 which is insoluble and precipitates kinetics of Mn(II) oxidation are slow so it can persist at concentrations higher than predicted by thermodynamics

summary of redox Mn reactions

particulate Mn(IV) sinks from oxidising water column to reducing sediments particulate Mn(IV) in sediment, reduced to dissolved Mn(II) this diffuses out the reducing sediments to the oxidising water column to become dissolved Mn(II) this oxidising to particulate Mn(IV)

Mn2+ behaviour over a year

no Mn at surface, all at depth since low O2 at depth very low in summer

O2 behaviour over a year

in spring, huge flux of O2 fjord turned over = no mn in water column since oxygenated when O2 decreases, see Mn in water column again

when can high concentrations of Mn(II) be found in the water column

if circulation is restricted e.g. fjord - mixing event and increase of O2 removes Mn high Mn(II) in bottom waters with low O2 overturning event July/Aug replenishes bottom water O2 and concentrations of Mn(II) fall