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lecture notes voor geochemische processen aan het aardoppervlak
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LecturesLECTURE 1 ESTUARIES
Estuary = Partly enclosed body of brackish water (diluted seawater), with input from rivers/streams
and a free connection with the sea. Place where fresh and sea water mix.
Why are estuaries important to society?
Of major economic interest:
• recreation
• fisheries
• our living environment
Sensitive to environmental change:
• Pollutants (e.g., Hg(mercury) in fish, plastics)
• Harmful algal blooms → fertilizers leak into the sea → nutrients trigger harmful algal
blooms → low oxygen in bottom waters → organic matter sinks and degrades into
deeper water layers→ oxygen supply and demand are out of balance
• Low oxygen in bottom waters
Why are estuaries important for Earth Sciences?
Trapping and processing of dissolved and particulate matter:
Examples (from the field of geochemistry):
• “Filter” for nutrients and carbon (but also pollutants)
(removal of nitrogen through denitrification; burial of phosphorus and organic
carbon)
Denitrification: NO3- → N2(g) (by organisms, nitrogen gas goes into atmosphere)
• Source of CO2 to the atmosphere(degradation of organic matter from rivers)
Degradation of organic matter: CH2O + O2 → CO2 + H2O
Types of estuaries:
• Based on geology: drowned coastal river
valley, delta, flooded glacial channels
• Water circulation in estuaries:
o Delta type systems: river flow
dominant, salt wedge estuary
below river flow, little tidal mixing
(numbers are salinity)
o River flow still dominates, but is
modified by tidal currents, highly
stratified. Fjords with deep sills,
deep estuaries.
o slightly stratified, river flow and tidal
, mixing equally important. More vertical salinity gradients. Shallow estuaries
Why does (higher)salinity move land inward in fall? salinity moves land inward in fall due to less river
runoff
o Well mixed, vertically homogenous
dominated by tidal currents, scheldt
estuary. Tidal oscillation in salinity,
outward/seaward increase in
salinity.
o Silled estuary, surface layer
flow, restricted flow at depth,
fjord with shallow sill/black
sea/Baltic Sea. Mostly stagnant
deep water → anoxic)
Residence time and flushing time:
• Residence time of constituents defined for steady state:
Black Sea: 500-1000 years
• Flushing time = volume of freshwater / river discharge
Well-mixed estuary: 1-10 days, fjord: decades
Biogeochemical processes in estuaries (sink/source)
• Dissolved components
• Suspended matter
• Chemical processes
o Adsorption/desorption
o Precipitation/dissolution
• Biological processes
o Production (of organic matter)
o Consumption/Transformation (of organic matter)
• Sediment-water exchange (influx, efflux, burial)
• Other: Inputs/removal in the estuary
(sewage, dredging)
Role of biological processes:
• Production of organic matter by diatoms and algae
• Removal of nutrients (N, P) from surface water (diatoms also Si)
• Primary production in estuaries(very high) is typically ~230g C/m2/yr in open ocean ~50g
C/m2/yr
• ‘Permanent’ removal in estuaries by sedimentation (C, N, P, Si), transformation to gaseous
forms (N, C)
C:N:P ratios and limiting nutrients in estuaries
, • Redfield ratio for organic matter: C:N:P = 106:16:1, molar ratio, phytoplankton
• N:P ratio of the water determines the limiting nutrient
• The limiting nutrient may vary from location to location depending on
o N:P in river loads
o Role of recycling (P recycled more easily, nitrogen lacks more easily)
o Denitrification (NO3- → N2(g))
o N-fixation (to store more N) → by phytoplankton for example, more N input into
estuary
Estuaries as a sink or source of elements
• How can we assess how river water is altered during transport through an estuary? Loss or
gain of solutes during mixing.
• Conservative behaviour: no loss or gain during mixing (no change in solutes)
• Non-conservative behaviour: removal or addition during mixing (change in solutes)
H4SiO4 = dissolved silica
, total = conservative, so if release of DIP from particles, then removal of P in estuary (Dissolved
Inorganic Phosphate)
Mixing models don’t always work
• Two-end member model only works if you have two well defined end members (e.g. river,
oceanic)
• The end member composition may vary with time
• Third end member (e.g. other tributary)
How humans are altering estuaries: eutrophication as an example
• Excess nutrients driving elevated production of organic material in surface waters
• Oxygen demand exceeds oxygen supply due to dead organic matter rain → low oxygen in
bottom waters
• Examples:
o Scheldt estuary in the past
o Baltic Sea at present
(denitrification removes
N, P is efficiently
recycled, N fixation by
cyanobacteria)
o Massive algal blooms
coastal zone of China
Physical processes also matter e.g.,
Suspended matter deposition/transport.
Factors include:
• Flocculation of clays, (Fe-
)aggregation at higher salinity
• Current velocity (tides, river
flow)
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