Transcript 2006 Thomson-Brooks Cole Chapter 14 Estuaries © 2006 Thomson

Chapter 14
Estuaries
© 2006 Thomson-Brooks Cole
Key Concepts
• Estuaries form in embayments where
freshwater from rivers and streams
mixes with seawater.
• The salinity of water in estuaries varies
both vertically and horizontally.
• Mixing of nutrients from saltwater and
fresh water, combined with plentiful
sunlight and relatively shallow water,
makes estuaries very productive
ecosystems.
© 2006 Thomson-Brooks Cole
Key Concepts
• Animals and plants that live in
estuaries must be able to adapt to
changing salinity.
• The physical characteristics of
estuaries tend to favor benthic
organisms.
• Many commercially valuable fishes and
shellfishes spend a portion of their life
cycle in estuaries.
© 2006 Thomson-Brooks Cole
Key Concepts
• Estuarine communities include oyster
reefs, mud flats, seagrass meadows,
salt marshes, and mangrove forests
(mangals).
© 2006 Thomson-Brooks Cole
Physical Characteristics of
Estuaries
• Formation of an estuary
– embayments—coastal areas where
portions of the ocean are partially cut off
from the rest of the sea
– rivers and streams carry freshwater runoff
from land into some embayments
– estuary forms where fresh and salt water
are mixed
– all estuaries are partially isolated from the
sea by land, and diluted by fresh water
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Types of Estuaries
• Coastal plain estuary—forms between
glacial periods when melting glaciers
raise the sea level and flood coastal
plains
– found along the Gulf of Mexico and
eastern Atlantic coasts
• Drowned river valley estuary—forms
when melting glaciers raise the sea
level and flood low-lying rivers
– e.g. Chesapeake Bay, Long Island Sound
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Types of Estuaries
• Tectonic estuary—forms when an
earthquake causes the land to sink,
allowing seawater to cover it
– e.g. San Francisco Bay
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Types of Estuaries
• Fjord—estuary formed when a deep
valley cut into the coast by retreating
glaciers fills with water
– found in Alaska and Scandinavia
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Types of Estuaries
• Tidal flats—deltas formed in the upper
part of a river mouth by accumulated
sediments, which divide and shorten
an estuary
• Bar-built estuary—estuary in which
deposited sediments form a barrier
between the fresh water from rivers
and salt water from the ocean
– e.g. Cape Hatteras region of North
Carolina, Texas/Florida
Gulf Coasts, etc.
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Salinity varies horizontally
– salinity increases from the mouth of the
river toward the sea
• Salinity varies vertically
– uniform salinity results when currents are
strong enough to thoroughly mix salt and
fresh water from top to bottom
– layered salinity may occur, with the layers
moving at different rates
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Mixing patterns
– tidal overmixing—seawater at the surface
moves upstream more quickly; denser
seawater at the surface sinks as lighter
freshwater beneath it rises, creating a
mixing action
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Water circulation patterns
– positive estuary
• influx of fresh water from the river more than
replaces the amount of water lost to
evaporation
• surface water is less dense and flows out to
sea
• denser salt water from the ocean flows into
the estuary along the bottom
• most estuaries are positive estuaries
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Water circulation patterns (continued)
– negative estuary
• occur in hot, arid regions
• lose more water through evaporation than the
river is able to replace
• surface water flows toward the river; its
salinity is increased by evaporation
• water along the bottom moves out to sea
• usually low in productivity
• e.g. Laguna Madre estuary in Texas
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Salt-wedge estuary
– occur in the mouths of rivers that are flowing into
seawater
– freshwater flows rapidly out to sea at the surface
– denser saltwater flows upstream along the river
bottom
– rapid flow of the river prevents saltwater from
entering and produces an angled boundary
between the freshwater moving downstream and
the seawater moving upstream called a salt
wedge
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Well-mixed estuary
– river flow is low and tidal currents play a
major role in water circulation
– seaward flow of water and uniform salinity
at all depths
– lines of constant salinity move back and
forth with the tides
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Partially-mixed estuary
– strong surface flow of freshwater and a
strong influx of seawater
– tidal currents force seawater upward to
mix with surface water
– rapid exchange of surface water between
the estuary and ocean
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Salinity and Mixing Patterns
• Other mixing patterns
– e.g. Galveston Bay (Texas), fjords
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Temperature
• Shallowness of estuaries allows
temperatures to fluctuate dramatically
• Warmth comes from solar energy and
warm tidal currents
• In some estuaries, winter turnover
results when cooler surface water sinks
and warmer deep water rises
– circulates nutrients vertically between
water and bottom sediments
© 2006 Thomson-Brooks Cole
Estuarine Productivity
• Nutrients in fresh and saltwater complement
one another
– freshwater contains nitrogen, phosphorus and
silica
– surface seawater has less nitrogen and silica but
more phosphorus
• Silt and clay dumped by rivers hold, then
release excess nutrients
• Filter feeders consume more plankton than
they can absorb, producing pseudofeces
which provide food for bottom feeders
© 2006 Thomson-Brooks Cole
Life in an Estuary
• Many are species are generalists, and
can feed on a variety of foods
depending on what is available
• Species that tolerate temperature and
salinity changes can exploit estuaries
and grow large populations
• So, estuaries contain abundant
individuals from relatively few species
© 2006 Thomson-Brooks Cole
Life in an Estuary
• Maintaining osmotic balance
– osmoconformers—animals with tissues
and cells that tolerate dilution
• e.g. tunicates, jellyfishes, sea anemones
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Life in an Estuary
• Maintaining osmotic balance
– osmoregulators—animals that maintain an
optimal salt concentration in their tissues,
regardless of the salt content of the
environment
• concentrate or excrete salts, or shield
themselves from their environment
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Life in an Estuary
• Remaining stationary in a changing
environment
– natural selection favors benthic organisms
because of the difficulty in staying still to
feed in constantly-moving water
– non-benthic animals (e.g. crustaceans,
fishes) maintain position by actively
swimming or by moving back and forth
with the movement of the tides
© 2006 Thomson-Brooks Cole
Life in an Estuary
• Estuaries as nurseries
– high level of nutrients + few predators
makes a great habitat for juveniles
– juveniles live in the estuary until they
grow large enough to be successful in the
open sea
– e.g. striped bass, shad, bluefish, blue
crabs, white shrimp
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Many hardy organisms are euryhaline
—species that can tolerate a broad
range of salinity
• Oyster reefs
– reefs form from numerous oysters
growing on the shells of dead oysters
– provide a habitat for many organisms,
which may depend on oysters for food,
protection, and a surface for attachment
– oyster drill snails prey on oysters
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Mud flats
– contain rich deposits of organic material +
small inorganic sediment grains
– bacteria and other microbes thrive in the
mud, producing sulfur-containing gases
– mud provides mechanical support for
organisms
– cohesiveness permits construction of a
permanent burrow
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Mud flats (continued)
– mud flat food webs
• main energy base = organic matter consisting
of decaying remains and material deposited
during high tides
• bacterial decomposition channels organic
matter to other organisms, and recycles
nitrogen and phosphate back to the sea floor
• deposit feeders prey on bacteria
• larger organisms eat secondary consumers of
bacteria, and so forth
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Mud flats (continued)
– animals of the mud flats
• most are burrowers living just below surface
• closely-packed silt prevents good water
circulation, so many animals have a “snorkel”
• soft-shelled clams use a siphon to filter feed
and obtain oxygenated water, then metabolize
anaerobically during low tide
• lugworms are common mud flat residents
• innkeeper worms house many other organisms
in their burrows, as do ghost shrimp
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Seagrass meadows
– seagrass productivity
• depends on the ability of seagrasses to extract
nutrients from the sediments
• depends on activity of symbiotic, nitrogenfixing bacteria
• also depends on productivity of algae that
grow on and among seagrasses
• nutrients from drawn from sediments are
released into the water by seagrasses, for use
by algae
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Seagrass meadows (continued)
– seagrass food webs
• seagrasses are tough, and seldom consumed
directly by herbivores
• seagrasses are a food source to many animals
as detritus, when their dead leaves are eaten
by bacteria, crabs, sea stars, worms, etc.
• organisms from other communities feed in
seagrass meadows during high tide, exporting
nutrients to other communities
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Seagrass meadows (continued)
– seagrass meadows as habitat
• epiphytes and epifauna attach to seagrasses
• filter feeders live in the sand among blades
• rhizoids and root complexes provide more
permanent attachment sites, and protect
inhabitants from predators
• larvae and juveniles of many species live here,
protected from predators by changing salinity,
plentiful hiding places, and shallow water
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Salt marsh communities
– distribution of salt marsh plants
• low marsh—region covered by tidal water
much of the day and typically flushed twice
each day by the tides
• high marsh—region covered briefly by
saltwater each day and only flushed by the
spring tides
• cordgrass dominates the low marsh
• short, fine grasses dominate the high marsh
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Salt marsh communities (continued)
– salt marsh productivity
• tides bring in replenishing supplies of nutrients
• most primary production supports detrital food
chains
• bacteria eat decaying plant material
• deposit feeders eat bacteria
• some salt marshes export large amounts of
detritus to nearby communities; in others,
resident organisms consume most of the
detritus
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Salt marsh communities (continued)
– animals of the salt marsh
• permanent residents include periwinkles, tidal
marsh snails, ribbed mussels, purple marsh
crabs, fiddler crabs, amphipods, grass shrimp
• burrowing animals play an important role in
bringing nutrient-rich mud from deeper down
to the surface, while oxygenating deeper
sediments
• tidal visitors that come to the salt marsh to
feed include predatory birds, herbivorous
animals from land, fishes and blue crabs
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Salt marsh communities (continued)
– succession in salt marshes
• salt marshes can be the first stage in a
succession process that produces more land
• roots of marsh plants trap sediments until the
area becomes built up with sand/silt that
combine with organic material to make mud
• mud islands appear and merge, and high tide
covers less and less of them
• tall cordgrass is replaced by short cordgrass,
which is replaced by rushes and then land
plants
© 2006 Thomson-Brooks Cole
Estuarine Communities - Mangals
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Mangrove communities
– distribution of mangrove plants
• red mangroves are usually pioneering species,
and grow close to the water where the amount
of tidal flooding is greatest
• black mangroves occupy areas that experience
only shallow flooding during high tide
• white mangroves and buttonwoods (not true
mangroves) live closest to land, but can
tolerate flooding during high tide and saline
soil
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Mangrove communities (continued)
– mangrove root systems
• shallow, widely spread root systems anchor
the plants and provide oxygen for parts buried
in the mud
• red mangroves have prop roots, and black
mangroves have many pneumatophores
• prop roots and pneumatophores slow water
movement, causing suspended materials to
sink to the bottom
• eventually, this sediment build-up can
transform the estuary into a terrestrial habitat
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Mangrove communities (continued)
– mangal productivity
• primary producers (mangroves, algae and
diatoms) support a productive detrital food
web; burrowing/climbing crabs eat the leaves
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Estuarine Communities
• Mangrove communities (continued)
– mangroves as habitat
• many animals live on prop roots and
pneumatophores, such as bivalves and snails
• roots provide habitat for many organisms
found in salt marshes and mud flats
• sheltered waters provide a nursery as well
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole