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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