Estuaries Flashcards
What is an estuary?
It is a semi-enclosed coastal body of water with free connection to open sea
Boundaries mark the end of the influences of sea and river
Seawater is diluted by freshwater from land draining
There are fluctuations in water temperature and salinity due to tidal cycle
Organisms have to adapt to both freshwater and saltwater
What are the two types of estuaries and what are they classified by?
Classified by salinity variation
Salt wedge: Gentle slop and high river flow= Looks like there is a wedge (hill) which the freshwater goes on top
OR Vertically mixed: Steep slope and high tidal flow= No wedge, travel straight through, it gets increasingly saline
(There are pictures if it helps…..)
However most estuaries lie somewhere between these 2 extremes= Partially mixed
What are intermittent estuaries?
Connect with sea only during periods of high river flow, other times they are sealed from the sea by a sand bar
How does the ionic composition change along the estuary?
River water= Higher in Ca2+, bicarbonate, silicate
Sea water= Higher in Na+, Cl-, sulphate
What are the 2 ways in which estuarine sediments arise?
Sediments: Deposited from further in the shore
1) River delta= Deposition of materials brought down by river when they encounter higher salinities
2) Offshore sources= Transport of mud or sand into the river mouth from offshore sources
What effect does having lots of fine suspended sediment have on phytoplankton growth?
Poor light penetration= Less photosynthesis= Less phytoplankton growth
What are the levels of productivity like in estuaries?
High productivity due to:
Detritus: Dead organic matter from river and salt-marches leading to high levels of organic matter as they can be carried down with the sediments = Detritus feeding animals such as Hydrobin snails and Corophium mud shrimps
What is the species diversity like?
Detritus feeding animals are abundant
Main vertebrate feeding predators= Aquatic species such as Flounder and Plaice
Terrestrial species: Feed on invertebrates at low tide but usually only for short periods of year= Lots of birds at this time
However: Biodiversity form the middle of an estuary tends to be lower than for either the river or the marine environment= Distribution of organisms is controlled by abiotic rather than biotic factors
What are some abiotic gradients which will affect surface dwelling and free swimming animals?
1) Temperature: More variable than in in marine environment
2) Salinity: Minimum salinity at low tide and maximum river flow= Restricts upper limit of marine species
Maximum salinity at high tide and minimum river flow= Restricts lower limit of freshwater species
3) Oxygen: Saturation at lower concentrations in the sea than in freshwater
What largely affects the condition of benthic species living in mud/sand?
Estuaries are divided into 3 tidal zones: supratidal, intertidal and subtidal zones, where it increasingly gets deeper
Benthic= Bottom dwelling
Largely governed by the distribution of sediments within an estuary
Tidal currents distribute sediments= Subtidal channels (always covered by water)=Coarsest sediments
Particle diameter decreases with depth= The bigger the depth, the smaller the particle diameter
Intertidal zone: Relationship is complicated by wave action
How is the oxygen transported to subtidal zone?
Diffusion via interstitial water= water in between mud grains
This only oxygenates the top few mm= Below this, oxygen is removed by bacteria to give a black anoxic layer
Intertidally: Oxygen is transported through water movement between grains
Where is organic matter mostly deposited?
In areas with little water movement e.g. mud
Negative correlation between median grain size and %organic matter + N content = As the grain size gets bigger, the smaller the amount of organic matter
Finer sediment grains have larger surface area= More bacteria to break down organic matter= More adsorbed onto them= Higher Nitrogen content from bacterial proteins
What is the different properties which differ between Mud and sand?
1) Oxygen penetration
Mud: Poor
Sand: Better
2) Temperature change within sediment
Mud: Small
Sand: Larger
3) Salinity change within sediment
Mud: Small
Sand: Larger
4) Food availability (detritus, bacteria)
Mud: High
Sand: Low
Type of sediment also important
What is standing crop? What is the productivity like in benthic organisms in estuaries?
Organisms present at an one moment
Densities of benthic invertebrates (e.g. crustaceans) can reach very high levels
Because many benthic species have short lifespans= High turnover of individuals= Annual productivity is higher than standing crop
What do tidal power plants do? What are the pros and cons?
They make use of the difference in potential energy between high and low tide by storing water in dams with sluice= Reservoir
Sluices close when tide changes from high to low tide= Water released through rotating turbines= Energy
Pros: Reduced carbon emissions, Renewable
Cons: Environmental impacts Reduced mixing= Reduced salinity and build up of contaminants and eutrophication Loss of habitats such as mudflats Change in benthic habitats Affect feeding birds Damage to migratory fish and mammals
