Estuaries Flashcards
What is an estuary?
A semi-enclosed coastal
body of water, which has free connection to the open
seas, and within which sea water is measurably diluted
with freshwater derived from land drainage - Pritchard (1967)
Where are the limits to an estuary?
The upper limit of an estuary is referred to as its head, while the lower limit is called the mouth of the estuary. Between the freshwater head and the saline mouth of the estuary lie several zones corresponding to intermediate salinity ranges (Fig. 1).
What are the types of estuary?
Coastal plain estuaries. These estuaries have formed where pre-existing valleys were flooded at the end of the last glaciation. They are usually less than 30 m deep, with a large width-to-depth ratio. This is the main sub-type of estuary, by area, in the UK.
Bar-built estuaries. These characteristically have a sediment bar across their mouths and are partially drowned river valleys that have subsequently been inundated. Bar-built estuaries tend to be small but are widespread around the UK coast.
Complex estuaries. These have been formed by a variety of physical influences, which include glaciation, river erosion, sea-level change and geological constraints from hard rock outcrops. There are few examples of this sub-type of estuary in the UK.
Ria estuaries. Rias are drowned river valleys, characteristically found in south-west Britain. The estuarine part of these systems is usually restricted to the upper reaches.
What are the environmental parameters that cause stressful living conditions in estuaries? Describe why they are stressful
- Tides - leads to air exposure which causes desiccation
- Salinity - temporal variation due to tidal movement
- Current - river is unidirectional and tidal movement both up and down, current greatest in mid-channel (friction)
- Sediment - nutritionally very rich, Soft and mobile, difficult for colonisation, locomotion, respiration and feeding.
- Dissolved oxygen - bacteria in mud depletes oxygen and accumulates hydrogen sulphide (rotten egg smell) making less oxygen available to organisms. Also in water column where there is high organic input (e.g. sewage)
- Human activity - sewage disposal leads to organic enrichment, - bank reclamation changes current and sediment patterns
- Temperature - natural seasonal variation increases microbial action (summer), freezing rates vary river-sea, influence of cooling water effluent.
What is the maximum salinity at the head of an estuary?
5 PSU
What is the salinity range in the middle reaches of an estuary?
18-25 PSU
What is the salinity range in the lower reaches of an estuary?
25-30 PSU
What are the main patterns of
organism distribution?
need pic of remane diagram
What is an oligohaline organism?
An organism tolerant of only a moderate range of salinities (<5 PSU), eg Erpobdella testacea leech, Radix balthica wandering snail
What are true estuarine organisms?
Brackish water species found ONLY found in estuaries (2-25PSU), eg Corophium volutator small crustacean, Osmerus eperlanus smelt (fish)
What are euryhaline marine organisms?
Marine species that extend up estuaries, often commonest species, exploit food, a few reach a salinity of 5. Examples include Hediste diversicolor
(ragworm), Platichthys flesus (flounder), Tubificoides benedii (oligochaete)
What are Stenohaline marine organisms
Fully marine species that are confined to high salinity areas e.g. Ostrea edulis (oyster), Lanice conchilega (sandmason worm).
Describe what are considered migrants in an estuary.
Migrants temporarily occur in estuaries, en route from sea-freshwater (or vice versa). E.g. Salmo salar (salmon), Eriocheir sinensis (Chinese mitten crab)
