Marine Biology, May 8, 2006
Lecture 12: Estuaries
I.
II.
III.
IV.
V.
Introduuction
A.
What is an estuary?
1.
It is a partially enclosed embayment where freshwater and seawater meet.
B.
Biological importance of estuaries?
C.
Level of human impact?
Types of estuaries
A.
Drowned river mouth/valley: increase in sea level inundated a river mouth.
1.
Several in Oregon are of this type
2.
Differences among these in how far seawater penetrates upriver:
a)
Ocean-dominated (Coos Bay, Yaquina Bay) vs. river-dominated
(outlet of Columbia and Rogue Rivers)
B.
Tectonic: instead of sea-level rise, the area subsided, lowering itself to sea level.
1.
Example: San Francisco Bay
C.
Bar-built: A sand spit creates a separate estuarine environment which receives
very little freshwater inflow.
1.
Sand Lake and Netarts Bay, are "bar-built”
a)
Sand Lake has a watershed of only 14 square miles.
2.
Blind: Small estuaries where low river flow in summer results in a sand bar
completely closing off the mouth of the estuary
a)
Elk River and Sixes River in Curry County,
D.
Fjords: Glacier-cut (Norway, Alaska, British Columbia, Chile, New Zealand)
Parts of an estuarine system
A.
Marine
B.
Bay
C.
Slough
D.
Riverine
Salinity
A.
Classification of estuaries based on salinity gradients
1.
Positive (or “salt wedge”) estuary [Fig. 12.3]
a)
Text calls this a “typical” estuary.
b)
Understand why the salinity gradients from top to bottom and river to
sea are the way they are!
2.
Negative (“evaporite”) estuary [Fig. 8.2b, Nybakken text]
a)
Blind estuaries: Very little freshwater input, coupled with high
evaporation at surface leads to higher salinity at the surface.
b)
Can actually have the reverse salinity gradient than the positive
estuary.
B.
Effects of tides on salinity [Fig. 12.4]
C.
Coriolis effect and salinity
D.
Effects of season on salinity
Other physical characteristics
A.
Substrate
1.
Main type of substrate?
VI.
a)
Why is this the dominant type of substrate?
2.
What effect might strong currents have on the substrate composition?
3.
How does the change in salinity within the substrate throughout a day
compare to that in the water column? [Fig. 8.5, Nybakken text]
B.
Temperature
1.
More variable than in coastal waters
a)
Why? (Several reasons)
b)
Is the temperature more variable in the substrate or in the water
column of the estuary?
C.
Wave action and currents
1.
More protected than adjacent coast against wave action
2.
Tidal currents can be intense
a)
Anecdote: Diving the Newport jetties
b)
Tidal currents can cause erosion within more narrow channels
3.
Flushing time: time for the entire mass of fresh water contained in the
estuary to travel through it.
a)
How does biological productivity relate to flushing time and why?
D.
Turbidity
1.
Lower flow leads to _________ turbidity
2.
What does phytoplankton growth do to turbidity? What does this, in turn, do
to phytoplankton growth?
E.
Oxygen
1.
Shallowness and mixing increase oxygen concentration, while higher
temperatures and salinity decrease it
a)
Can be stratified: As in open ocean, can be highest at surface
2.
Low within sediments, especially as you go deeper Why?
The biota of estuaries
A.
Basics
1.
Fewer species, but can achieve high biomass
2.
Most have origins in the marine ecosystem
3.
Distribute themselves based on their ability to deal with different saline
conditions [Fig. 12.5]
a)
Stenohaline
b)
Euryhaline
c)
Brackish
d)
Freshwater
4.
Osmoconformers vs. osmoregulators [Fig. 12.6]
a)
Understand how change in salinity affects the movement of water into
(or out of) the organism, and what osmoregulators have to do at
different places along the estuary.
5.
Adaptations to muddy substrate
a)
Infauna: burrowing, tube building

How can buried organisms deal with low oxygen levels found
in deeper mud?
b)
Epifauna: stay on surface

Examples…
B.
Estuarine food webs [Fig. 12.23]
1.
Main “primary” sources of energy are:
a)
Organic matter

Some is created within the estuary (examples…)

Also comes from external sources, such as…?
b)
Phytoplankton
c)
Algal mats
d)
Plants
e)
NOTE: Much debate over where what the “key” sources of energy
are, but probably very different for different estuaries.
2.
Primary consumers (not strictly…)
a)
Deposit feeders (several worm species)

Feed on the detritus or on bacteria that break down the detritus

Some clams, such as Macoma, are detritus feeders that use
their incurrent siphons like vacuum cleaners
b)
Suspension feeders and filter feeders (i.e. clams)

Feed on phytoplankton and other suspended organic molecules
in the water column

Difference between suspension feeders and filter feeders?

Disadvantage for filter-feeders in an estuary?
3.
Predators
a)
Bird predation

Niche partitioning based on beak length and prey depth [Fig.
12.12]

Differences in prey-location tactics

sandpipers are tactile while plovers are visual [Fig.
12.13]
b)
Variety of predatory fish

Anadromous (salmon) and catadromous (freshwater eels)

Estuaries as nurseries for larval and juvenile fish.
C.
Some common estuarine animals in Oregon
1.
Callianassa californiensis
2.
Crangon spp.
3.
Clams: a variety of species
4.
A variety of worms
a)
Nereis
b)
Abarenicola and similar worms
5.
Various juvenile fish species
a)
Find tubesnouts and pipefish near Hatfield
VII. Salt marshes and mangroves
A.
Why are there more on the east coast of North America than on the West Coast?
B.
Salt marshes give way to mangroves in subtropical and tropical regions
C.
Plants distributed in both these habitats based on how well they deal with salt stress
1.
Three different ways that plants may adapt to salt water… (Know them!)
D.
Prop roots of mangroves are especially important nursery grounds for tropical reef
fish and invertebrates [Fig. 12.21]