lsm3254_lecture 8 the intertidal

8/3/2019 LSM3254_Lecture 8 the Intertidal

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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.

lsm3254_lecture 8 the intertidal

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