Estuaries and Intertidal Communities

Chapters 11 and 12

Intertidal Communities

• The effects of the tides and the nature of the community depends on the substrate.

• Two major types of intertidal communities:

• Rocky shore communities

• Soft bottom tidal communities

Rocky Shore Communities

• Occur mostly on active margins.

• Most rocky shore inhabitants live on the rock surface- called epifauna.

• Low tide- epifauna are exposed to the air.

• Emersion time- time spent out of the water.

• Emersion time is longer the higher up the intertidal you go.

Challenges to epifauna

• Water loss- they will dessicate if exposed too long.

• Two mechanisms for coping-

• Run and hide, or ‘clam up’

• Run and hide- escape to tide pools or hide in crevices

• Clam up- protective shell to hold in water.

Some Shelled Residents

Whelks

Cross-Section of Whelk

Temperature and salinity

• Exposure to the air creates greater changes in temperature and salinity.

• Air has greater daily temp. range, and is flooded with fresh water during rains.

• Tide pools can experience very large temperature and salinity variations.

Feeding strategies

• Filter feeders cannot feed at low tide.• must be submerged to filter• often clam up during low tide• Deposit feeders are rare in the rocky

intertidal zone – few sediments to “hold” nutrients

• Animals higher in the intertidal have less time to feed, often grow more slowly than animals lower in the intertidal.

Wave Energy

• Incoming waves refract along the shore.

• This causes waves to come in parallel to the shore.

• Headlands receive the most wave impact.

• Bays receive less wave energy.

Wave Refraction

Wave energy on a coastline

Coping with wave energy

• Organisms that live on exposed shores Organisms that live on exposed shores must deal with waves.must deal with waves.

• Strategies:Strategies:• HoldfastsHoldfasts• Strong attachmentsStrong attachments• Thick shellsThick shells• Low ProfilesLow Profiles• FlexibilityFlexibility• Finding ShelterFinding Shelter

Limiting Resources

• In rocky intertidal communities, food is abundant.

• Space is the greatest limiting factor- all sessile organisms will die if not strongly attached.

• Competition for space is intense.• Rocky intertidal zones exhibit vertical

zonation- distinct bands at different heights.

Vertical Zonation on Rocky Shores

• Upper intertidal- lichens, cyanobacteria, and periwinkles.

• Middle Intertidal- Barnacles dominate in upper area. Lower areas compete with mussels and predators.

• Mussels are the dominant space competitors. They are limited by air exposure and predation by sea stars.

• Lower intertidal- red, green and brown seaweeds.

Vertical Zonation

Vertical Zonation

Soft bottom intertidal communities

• Substrate is sediment- sand, clays, mud.

• Fine sediments are often found in calm areas.

• Coarse sediments on coasts that experience wave action.

• Many animals burrow into the sediments, they are called infauna.

Sediment sizes

Life in the sediment• Less dessication- sediments hold water.• Many deposit feeders- most feed on detritus.

Fine sediments have more detritus, sand has less.

• Oxygen- decaying organic matter uses up oxygen- therefore interstitial water often is lacking in oxygen.

• Anaerobic bacteria thrive here- produce H2S.

• Other infauna must pump O2- rich water from above the sediment, or have extra hemoglobin and slow metabolism.

Life in the sediment- cont’d

• Some worms help to oxygenate the sediment by mixing up the sediment.

• Mobility- many soft bottom animals burrow in the sediment.

• clams use their muscular foot.

• Many tiny species live in between the sand grains, they are called meiofauna.

Clam Locomotion

Feeding strategies

• Most soft bottom species feed on detritus.• Some plankton is also in their diet.• Deposit feeders- sea cucumbers and

worms eat sediment, and digest the organic matter.

• Sand dollars- use tube feet to pick up organic particles.

• Clams use long siphons to reach nutrient rich surface sediment.

Predation

• Moon snails burrow in the sediments looking for clams.

• use their radula to drill through the clam shell and eat it.

• During low tides, birds are important predators.

Zonation in Soft Bottom Intertidal Communities• Less obvious than on rocky shores.

• Upper beach- Beach hoppers (sand fleas), ghost crabs and fiddler crabs.

• Lower beach- Polychaete worms, clams, moon snails.

• Just below tide line- sand dollars, blue crabs, sea cucumbers

• On muddy shores- very little zonation.

Estuaries

• Estuaries are semi-enclosed areas where fresh and sea water meet.

• Low in biodiversity, but high in productivity.• Often among the most affected by human

activity.• Large cities are often built in estuary areas

or along the rivers that feed them.• Dredging or filling in of these areas, as

well as pollution can be disastrous.

Types of Estuaries

• Drowned river valleys- formed after the last Ice age - like the Chesapeake Bay

• caused by sea level rise.

• Bar Built estuaries-Sand bars and barrier islands are common.

• Formed by deposition that isolates a body of water from the open ocean- like the Outer Banks.

Cape Hatteras is a bar-built estuary.

Types of Estuaries

• Tectonic estuaries- formed when the land subsided (sank) due to plate movements.

• Ex- San Francisco Bay

• Glacial estuaries- also called fjords.

• Formed by retreating glaciers carving out valleys along the coast.

• Common in Alaska and Norway.

A fjord in New Zealand

Salinity and Estuaries

• Dramatic fluctuations in salinity are common throughout the day.

• Salinity drops as you travel upstream.

• Salinity often increases with depth.

• As salty tidal water come in, it forms a ‘salt wedge’- fresher, less dense water flows on the surface.

Salinity varies in an Estuary

During High tide

During Low Tide

Coping with Salinity changes

• How far up and estuary an animal can live depends upon how well it can tolerate low salinities.

• Animals that can tolerate a wide range are Euryhaline.

• Stenohaline organisms can only survive a narrow range- they are less successful in estuaries.

Dealing with Osmosis

• Estuarine species must be good osmoregulators or osmocomformers.

• Many species do both, during extreme salinity fluctuations.

• Many invertebrates osmoregulate at low salinities, and osmoconform in high salinities.

Other Factors in Estuaries

• Substrate- like soft bottom intertidal communities.

• Often less primary production by plankton due to poor water clarity.

• Anoxic conditions are common. In these areas, anaerobic bacteria thrive.

Types of Estuarine Communities

• Open water- Limited primary production.• -rich in shellfish- 90% of

commercially important fish and shellfish in the Gulf of Mexico rely on estuaries.

• Often called the oceans nurseries because many juvenile fishes seek shelter in estuaries.

• Migration routes for catadromous and anadromous fishes.

Types of Estuarine Communities

• Mudflats- found mostly where there are gentle slopes.

• Mud exposed at low tide- community must deal with dessication as well as salinity changes.

• Only a few producers like sea lettuce• Bacteria and burrowing infauna are common.• Worms, clams, snails, ghost shrimp, fiddler

crabs are common• Important feeding grounds for shorebirds

Birds feeding on a mudflat at low tide

Mudflats on Prince Edward Island

Resource Partitioning- the different shapes and lengths of these birds beaks allow them to forage for different foods

• Fiddler crabs hide in the sand and come out at low tide to feed.

• Feed on detritus in the sand.• Male fiddlers have a one large claw to

attract mates.• They attract mates by waving their claws

at females, and by fighting each other.• Different species of fiddler crabs have

different waving patterns to attract the ‘right’ mate

Sandpipers use their bills as probes in the sand

Plovers use visual cues to feed

Salt Marsh Communities

• Extensive grassy areas that are flooded at high tide.

• Develop in areas sheltered from strong wave action.

• Lower area is dominated by cordgrasses.

• Upper areas give way to pickle weeds and rushes.

Salt Marsh near Atlantic Beach, NC

Animals of the Salt marsh

• Marsh plants provide shelter for many animals.• Many burrowing animals live in the substrate.• Air breathing snails live in the upper marsh, such

as coffee bean snails and periwinkles• Small fish live in the tide pools and move into

marsh grasses during high tide to avoid predators.

• Fiddler crabs live at the boundary of the salt marsh and mudflat

• Land birds and mammals forage in the salt marsh during low tide

The Atlantic Horse mussel

can breathe air during low

tide

Salt Marsh

Wading birds feed in the Salt marsh at high tide

Mangroves

• Often considered the tropical version of salt marshes.

• Mangroves are trees and shrubs that are adapted to live in the intertidal.

• 60-70% of sheltered tropical shores are mangrove forests- not all are estuarine.

• Some need freshwater to live, others can excrete excess salts through glands on the leaves.

Animals of the Mangrove

• Both land and marine animals live in mangrove forests.

• Mudskippers- fish that can spend hours out of the water feeding, getting oxygen to their gills form the air.

• Sponges are abundant, attached to mangrove roots- a mutualistic relationship

• Many shrimp, lobsters and fish mature in the protection of the mangrove roots.

• Many birds and reptiles live in the canopy

Mudskippers feed on insects up on the exposed roots of mangroves

Distribution of salt marshes and Mangroves

Mangroves

Mangroves in Belize

The roots of mangroves provide shelter for marine life

Oyster Reef in Beaufort, NC