The estuarine ecosystem Flashcards
What are the main features of an estuary?
Freshwater, sea water mixing (semi enclosed system)
Sediments being deposited from river and sea
Tidal activity
Industrial activity
Estuaries form and disappear over a geographical time line.
Ephemeral features (short lived) current estuaries have only been around for 1000 years.
How are estuaries dynamic?
Dynamic – ever changing
Varies minute by minute basis
Low tide- just river water, low salinity
High tide – salinity increases
Less mixing during neap tides and more mixing on spring tide (happens on a full moon or new moon, occur every two weeks)
Varies in seasons
What is progradation and transgression?
Progradation – high levels of sediment. Sediment dries as tide drops so estuary disappears
Transgression – low levels of sediment and sea levels like after the last ice-age so estuaries form.
What are the key features of a highly stratified- salt wedge estuary in terms of mixing and circulation.
Freshwater overlies seawater which sits on bottom as a wedge
At interface get slight mixing and seawater becomes ‘entrained’ into freshwater flow
Little seawater flows in to replace entrained water
Very sharp halocline
Salinity of surface water constant, until get near mouth
Sudden meeting with seawater
– often shown as a “front”
What is a salt wedge estuary and why is it not good for the industrial industries?
salt wedge estuary – wedge of high salinity water that sits at the bottom of the estuary due to higher density (highly stratified – layered) 2 layers of fresh and salt water. Entrainment mixing as river water moves over saline water. Turn over of sea water is slow. Halocline (salt gradient)
Not good for industries that need waste to be removed because very low levels of mixing and would cause environmental problems.
What are the key features of a highly stratified- fjord in terms of mixing and circulation.
Similar to salt wedge but seawater forms deep thick layer
Thin layer of freshwater on top
Water flow over sill can be strong but doesn’t mix deep layers (shallow sill)
Deep water renewed slowly, monthly / annually
bottom waters can become anoxic
e.g. Norwegian fjords, sea lochs
Horizontal layer of salt water. Slight mixing.
Problems with deoxygenation because of build up of materials that aren’t being turned over and aerated by turbid waters and becomes anoxic.
What are the key features of a partially mixed estuary in terms of mixing and circulation.
Tidal influence is increasing, river flow decreasing
Oscillating tides produces turbulence and mixing
Bottom velocities move in opposite directions
Where meet = ‘null point’ no current
Surface salinity gradually increases as you go out to sea
Vertical salinity gradient more gentle
Gradual change with depth
e.g. Thames, Forth
More common in temperate regions.
Reasonable amount of mixing.
As you move through the estuary the salinity of surface waters will gradually increase as you move to the mouth of the estuary closer to the sea.
As you move to closer to the sea, each iso-haline will drop in salinity concentration. Gradual increase of salinity as you go vertically down to the sea bed. More blending.
Changes in salinity is difficult to cope with for organisms.
What are the key features of a well mixed (homogenous) estuary in terms of mixing and circulation.
Very large tidal range
Whole of water column gets mixed
Estuary is vertically homogenous
Macrotidal / Hypertidal estuaries
If estuary is very wide can get lateral ‘inhomogenous’ conditions
e.g. Solway Firth
Vertical iso-halines. Salinity concentration almost same between top and bottom.
Strong currents so organisms must be adapted to living under those conditions.
What is the corolis force?
Corolis force deflects currents to the right in northern hemisphere
Wide estuaries are deflected to either side which causes different levels of salinity from one bank to the other (Lateral differences)
