lsm3254_lecture 8 the intertidal
TRANSCRIPT
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LSM3254 Ecology of Aquatic Environments
and soft sediments
Peter Todd
Dept of Biological Sciences
By the end of this lecture you should be able to discuss variousaspects of the intertidal, including:
Learning outcomes:
e t es
Coastal geomorphology and wave exposure.
Physical factors, e.g. temperature, desiccation,
salinity, and wave exposure (the vertical emersion gradient).
Physical and biological interactions - vertical zonation.
The particle size gradient.
Representative fauna.
Feeding strategies.
SUPRALITTORAL LITTORAL
(intertidal)SUBLITTORAL
LAND SEA
High tide level
SEA
Battle of the Origin of Bulges
What causes the opposite bulge?
Most textbooks explain by the differences in centripetal
and centrifugal forces across the Earth
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Battle of the Origin of Bulges
What causes the opposite bulge?
Alternatively:
Presence of the o osite bul e can be accounted for
by the vector subtraction of the gravitational pull by the
moon.
Bulge is formed due to the tangential component of the
resultant force from the Moons attraction.
Not because water particles are pulled directly towards
SunNP
7 days7 days
SunNP
7 days 7 days
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Intertidal (or littoral) life along the coast is controlled
by 2 very important gradients:
The horizontal wave-action gradient(this determines what kind of substrate is available)
The vertical emersion gradient(this determines a large range of environmental parameters)
The horizontal wave-action gradient is a complex combination of
geomorphological and climatic/meteorological factors that determine
The horizontal wave-action gradient
whether an area of shore is sheltered or exposed (or in between)!
In general, headlands tend to be exposed and rocky whereas bays are
often sheltered and sandy/muddy.
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Horizontal wave action gradient
Wave get bent, or refracted, by features such as headlands and outcrops
McGraw Hill
Hitting a cliff can mean
a wave gets reflected.
Exposure to wave action
Prevailingwind directionand fetch areveryimportant
Wave-action gets focused on headlands and outcrops anddiffused in bays
Maps of two indented coastlines to show variations in
exposure to wave action
Maximum wave fetch less than 10 km; usually areas ofProtected:P
Maximum wave fetch less than one kilometre; usually the
location of all-weather anchorages, marinas and harbours.
Very Protected :VP
British Columbia Estuary Mapping System
Maximum wave fetch distances between 50 and 500 km.
Swells, generated in areas distant from the shore unit create
relatively high wave conditions. During storms, extremely
lar e waves createhi h waveex osures
Semi-exposed :SE
Maximum wave fetch distances in the range of 10 to 50 km.
Waves are low most of the time except during high winds.
Semi-protected:SP
provisional anchorages and low wave exposure except in
extreme winds.
Maximum wave fetch distances greater than 500 km. High
ambient wave conditions usually prevail within this exposure
category, which is typical of open-Pacific type conditions
Exposed:E
Fetch not everything sea bottom also important
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Measure how fast some erodible substance (e.g. balls of plaster ofParis clod cards) wears away.
Measuring exposure to wave action
Measure some aspect of wave velocity (Helmuth and Denny, 2003.)
Wiffle golf balls!
How exposed is
Singapore?
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e roc y s ore
The intertidal is the most familiar of the marinesystems because it is the most accessible.
Rock shores are articularl well studied due to
The rocky shore
extensive epifauna (as opposed to soft sediments).
Little specialist equipment
is needed and it is(usually!) easy to return tothe same spot for research.
Features of the rocky shore
Have few sediments as they are washed away.
Organisms cannot easily burrow only a few rock-boring bivalveslike piddocks (in wood, chalk and even sandstone).
Toothed shell that doesnt
fully shut. Muscles are
attached in a way that allows
for a rotational grinding
movement.
Most organisms live on the top of things = epifauna. However, thismakes them vulnerable to the effects of exposure.
Rocky shores can be uplifted/ing rock that has had little time erode.
Hawaii a good example of young islands, with new land beingcreated on a regular basis.
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Hawaii: volcanic activity not created by spreading or subduction but by a hot spot. The Pacific Plate has moved northwest acrossthe hot spot creating the island chain the oldest being Kaui andthe youngest and most active being Hawaii or the Big island
5,000,000 yrs old
The vertical emersion gradient is where the
The vertical emersion gradient
, .
The physical conditions along the gradient from lower to the uppershore are extremely variable especially compared to the sea itself,which is a relatively constant environment
It is a highly stressful environment for
the time spent emersed, i.e. exposedto the air (as opposed to immersed).
The high degree ofvariability makesacclimatization difficult
The only stresses that may increase at lower shore levelsare predation and light availability for plants/algae
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Low temp: Under typical New England winter conditions, as much as 50-70% of the water content of invertebrates and algae freezes.
Intertidal organisms have two main strategies dealingwith these stresses: tolerate (sessile) or avoid (motile).
Desiccation: Some barnacles can survive 28 days out of water and therough periwinkle Littorina saxatilis, can survive over 42 days out of water.
Salinity: Carcinus maenascontrols osmotic pressure of internal organs,regardless of external conditions.
Low O2: In the periwinkles Littorina neritoides(supralittoral fringe) the
. .
Wave actionWaves:
Dislodge things
Encourage scour by sand ors ng e
Smother things
Cause continuous rapid
movement (impedes foraging,larval settlement, etc.)
But waves also:
Renew oxygen
Deliver food and nutrients
Disperse gametes
Remove waste
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Predation and feeding
Not much sediment so few deposit feeders.
Lots of filter/suspension feeders but time available for
feeding decreases higher up the shore.
Many grazers scraping algae and bacteria etc.
Also man redators but all tend to seek shelter whenthe tide is out.
Terrestrial predators too e.g. birds and rats.
Predation and competition are superimposed upon the
physical characteristics. INTERACTIONS
The effects oflimpet grazing
limpets = 100% coverof the green seaweedEnteromorphaintestinalis
Pisaster ochraceusMytilus sp.
Interactions and vertical zonation
Rocky shore is limited by space not limited by food, especially forsessile organisms
Two main strategies - get there first (good dispersal and colonisingabilities) or take over:
Taking over may involve buldozing neighbours (barnacles are good atthis), blocking out light (various seaweeds), smother by growing over
(e.g. musselbeds).
The biological response to the environmental gradient between landand sea is vertical zonation
The vertical distribution of any one species is controlled by a complexinteraction between physical and biological factors.
Different levels/ zones on the rocky shore are occupied by differentassemblages of algae and animals, each with a main abundance withina particular zone where conditions are favourable.
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McGraw Hill
Intertidal vertical zonation is found throughout the world although species
will vary. Easy to spot due to distinct bands.
As a general rule the upper limit is determined by physical
factors (emersion) whereas the lower limit is controlled by
predation and competition.
The main h sical factors controll in zonation are the tidal ran e andfrequency, and how exposed the shore is.
Greater tidal ranges result in more extensive intertidal zones. However,even in the absence of tides, a zone exists in which the sea laps against
the shore or waves break and splash (the splash zone).
The (rocky) intertidal can be splitup into 3 to 4 principle zones
Often linked
McGraw Hill
Tidal range
Usually based on a combination of physical and biological gradientsMcGraw Hill
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Soft sediments
McGraw Hill
Remember the horizontal wave-action gradient? In general, headlands tend tobe exposed and rocky whereas bays are often sheltered and sandy/muddy.
The type of community encountered is closely linked to the substrate/s present.
In this lecture we will focus on soft sediments.
Soft sediment = soft bottom = anything that isnt rock orvery hard = can burrow into it easily!
Shore types: Eroding (usually rock) or Depositing (usually occur in bays, inlets and estuaries)
The geological history of an area determines the availability of
sediment types (e.g. pebbles, sand and mud).
Most of these sediments are deposited by
longshore currents.
Coral reef associated sand has a different history!
Longshore current result in
longshore transport or beach
drift
Notethe zi za effect
McGraw Hill
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The particle size gradient
Pebbles and coarse sand on exposed
Muddy sand and mud as shelterincreases (and thus water movementdecreases)
most sheltered conditions
Sediment composition (the relative amount of pebbles, sand, silt andclay) is directly related to water motion.
Think of what happens if you shake a jar full of different
sand, pebbles and mud (silt and clay).
McGraw Hill
Sediments are defined by their grain size
Wentworth Classification and the Phi Scale
Grade name Particle size Phi units
range (mm)
Feel for
yourself!
Boulder >256 beyond -8.0
Cobble 25664 -8.0-6.0
Pebble 644 -6.0-2.0
Granule 42 -2.0-1.0Very coarse sand 21 -1.00
Coarse sand 10.5 01.0
e um san . . . .
Fine sand 0.250.125 2.03.0
Very fine sand 0.1250.0625 3.04.0
Silt 0.06250.0039 4.08.0
Clay
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Life in soft sediments
Muddy bottoms tend not to shift at all and therefore host a
shifting (in fact, some move
offshore altogether in the
winter!).
. .tact in mud, whereas they disintegrate relatively quickly in sand.)
Pebble beaches really are lifeless as the churning of the pebblesby waves basically grinds everything to death. This also happenson coarse sand beaches - albeit to a lesser extent. Particle size gradient (mix of particles also important)
Particle size gradient and organisms
Surface dwelling species are present at both ends of the
particle size gradient
Infauna (organisms live in the substrate) are restricted to
smaller particle sizes
(Non-transient) macrobiota are absent from the middle sectionof the gradient but surface may have a few microscopic
species.
Not a comfortable
living space!
Exposed, sandy beaches
Sandy vs muddy shores
rone o g empera ures an es c ca on
Little fluctuation in salinity
Well oxygenated
Reduced organic matter and hence limited bacterial activity
Sheltered mudflats
Rarely (if ever) dries out (thus not so hot)
Greater fluctuation in salinity
Poorly oxygenated
Increased organic matter and thus greater bacterial activity
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Temperatures
The midday heat of a summer sun may raise the surfacetemperature of the sand much higher than the returning sea.
u a s are no a ec e so muc as ey are u ere ystanding water.
The marine-freshwater gradient of salinity
Coarse sandy beaches - sufficientlyrapid drainage of water - little
evaporate or be diluted by rainwater
On mudflats - often a great deal ofstanding water thus heavy rainfall orevaporation at low tide mayconsiderably change its salinity.
Oxygen availability
No sunlight below a few mm and therefore no photosynthesis.
What oxygen there is gets used up by the animal respiration.
Sand contains little organic matter (it is washed away) and thereforefeels clean. Mud is the opposite!
Muddy bottoms especially bad as they have more organic material butless oxygen getting in. Deeper down water conditions become anoxic(no oxygen at all) and thus anaerobic respiration found. Sandy
beaches can also have this layer just deeper down (1m +).
Chemocline.
Other organisms also have to adapt to low levels of oxygen:
Some pump (oxygen-rich) water through their borrows
Some use siphons to suck water from the surface Others have properly adjusted to low levels of oxygen (rather than just
avoiding them) through special hemoglobins and reduced metabolism.
A few have symbiotic bacteria
Sandy vs muddy shores
Most organisms on soft sediments are either too small to beseen with the naked eye or they are buried so they dontattract the same amount of attention as rocky shores, for
The fauna of mud and sand
.
Many animals borrow in the sediment to keep from beingwashed away, i.e. infauna.
Interstitial fauna are microscopic organisms that live among thetiny spaces between grains and are represented by. Most phylaare represen e .
Note: interstitial and meiobenthos and meiofauna are often usedinterchangeably (Some say the latter are technically those organisms between0.5mm and 62m, however other sources say meiofauna is 100-1000m, 2-1000m, etc.)
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Tardigrades
Copepods
Cnidarians
a es
Gastrotrichs Molluscs
Polychaetes
Macrofauna (infauna and epifauna)
Burrowing
Bivalves
Use their muscular foot
All burrowing
organisms are
bioturbators
one direction
Worms
Elongated body Use penetration and terminal anchors
Shrimps/crustaceans
urrow ea rs w e r appen ages
Urchin
Heart urchin has spatulate spines for burrowing
McGraw Hill
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Much the same basic idea for both molluscs and worms Types of feeding
Predators, e.g. Snails (Polinicesmoon snail), crabs, worms,fishes and birds.
Suspension feeders, e.g. bivalves, worms, etc.
Deposit feeders, e.g. amphipods, sea cucumbers, worms andsnails.
suspension feeder deposit feeder Deposit feeding
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Hydrobia
Deposit feeding
Gorbushin A.M. 1997 Field evidence for trematode induced gigantism inHydrobiaspp. (Gastropoda: Prosobranchia). J. Mar. Biol. Ass. U.K., 77 , 785-800.
McGraw Hill
In summary:
Tide are a result of the sun and moons gravity.
Rocky shores are found on more exposed areas of coast.Soft sediments
s ores are oun on s e ere areas o coas .
Along the shoreline life is only found ON rock or IN relatively fine
sediments
Organisms living in the intertidal have to survive a wide rage of stresses.Intertidal organisms have various adaptations to cope with this harsh
environment.
Generally, the upper limit of marine life on the rocky shore is determined
by physical factors (emersion) whereas the lower distribution iscontrolled by predation and competition.
Organisms living soft sediments must adapt to this shifting environment.
Most soft sediment organisms have the ability to burrow and deposit and
suspension feeding is common.