Estuaries
I. What are estuaries?
A. Definitions
1. Where freshwater
drainages meet the sea
2. Semi-enclosed coastal
bays in which fresh water derived from land drainage and sea water from
the ocean mix
3. Classifications
· salt wedge
· patterns of salt and freshwater (stratified/partially
mixed/mixed)
B. Very productive systems
1. Allochthonous from rivers
and land – detritus is very important
2. Autochthonous
a.
Phytoplankton
b.
Sea grasses and macroalgae
II. Physical Characteristics of Estuaries
A. Salinity
1. single most important
characteristic; nowhere is variation in salinity more pronounced than
in
estuaries
2. variability is
horizontal, vertical and seasonal
3. causes of variability
a.
amount of freshwater input
b.
evaporation
c.
vertical variability (density differences)
d.
tides are periodic events
B. Substrate
1. soft mud
2. terrestrial and marine
derived materials
3. gradient of sediment size
that depends on current strength
4. storms and floods can
reshape estuaries
C. Wave Action and Currents
1. currents from combination
of tidal and river flows
2. flushing time
D. Turbidity --
1. often high due to
resuspension of particles
2. lowest at mouth of
estuary and highest inland and when river flow is large
3. decreases light
penetration -- reduces primary production
E. Oxygen
1. generally high due to
mixing
2. will vary in salt and
freshwater layers (salinity; temperature, stratification...)
3. can be anoxic conditions
4. depleted in the muddy substrates
III. Interesting organisms and their adaptations in estuaries
A. Number of estuarine species is less than in
adjacent marine and freshwater habitats
B. Faunal composition
1. Marine and freshwater
biota
a. stenohaline
– very narrow salinity tolerance
b. euryhaline
– broad salinity tolerance
(1)osmotic regulation
(a) osmoconformers – osmotic concentration of their internal
fluids fluctuates with the
external environment
(b) osmoregulators –
(c) some may osmoregulate at low salinities and osmoconform at high
salinities
c.
Plankton
(1) reduced number of species
(2) composition and biomass depends on turbulence, turbidity and
flushing rate
d.
Benthos --often are lots of anoxic sediments
(1) get tolerant organisms and organisms that aerate their burrows
(2) oyster beds
(3) seagrasses in lower areas
(4) macroalgae
(5) benthic algae -- diatoms; filamentous cyanobacteria
2. brackish water/estuarine
species -- adapted to life in intermediate salinities; not in
freshwater or seawater (5-18 psu)
3. some species are
transitional – passing in and out of the estuary during some of its
lifecycle;
a.
migratory fish
b.
some shrimps
c.
breeding and feeding grounds for many birds, fishes
IV. Human impacts on estuaries
A. Have historically been treated as wastelands that
should be reclaimed for human use --
housing developments, marinas, seaports, industrial parks, cities,
garbage dumps…
1. Dikes and drainages; fill
2. 1/3 of all estuaries in
the US are completely gone
3. dredging of navigation
channels
B. Rivers input modifications
1. damming and diversion
often reduces freshwater flow from rivers and affects salinity of
estuaries
2. carry pollutants -
3. carry extra nutrients
leading to eutrophication (more later in course!)
C. Exotic species – ports and ballast water (more
later in course!)
D. Overharvesting (fish, shellfish)
E. Aquaculture
Salt Lakes
I. Distribution
A. half of the water volume in the world's lakes is
fresh and half is saline
B. global distribution
1. generally restricted to
semiarid and arid regions
2. some in other regions if
near underground salt sources or a salt mine
C. Interesting facts: saline lakes include
1. the largest (Caspian)
2. highest (Nan Tso, Tibetan
Plateau 4,718 m.a.s.l.)
3. and lowest (Dead Sea, 400
m.b.s.l.) lakes of the world
D. less studied
II. Characteristics
A. salinity
1. Definition of saline
lakes -- salinity greater than or equal to 3,000 mg/L = 3 ppt
a.
often a breakpoint for biota
b. a
point at which certain chemical equilibria change
c.
subsaline lakes >0.5 ppt (500 mg/L) but <3
d.
freshwater <0.5 ppt
2. Subcategories of saline
lakes
a. hyposaline –
3-20
ppt
b. mesosaline –
20-50 ppt (oceans are ~33 ppt)
c. hypersaline -
> 50 ppt
3. records
B. Freezing point depression
C. Sometimes high pH
D. Variety of chemical composition
E. Extremely high nutrient levels
F. High, potentially toxic levels of trace metals
G. Low oxygen
III. Unique fauna and flora
A. Almost all species in saline lakes are derived
from freshwater, not marine species
B. Decreasing numbers of species phytoplankton,
zooplankton and benthos as salinity increases
C. Species are often cosmopolitan
D. The few species present are often in high
densities and very productive
E. Some specific organisms and food webs
1. some tilapias (fish) can
live in moderately saline lakes and wetlands
2. brine shrimp, Artemia
- occurs world wide in fishless saline lakes and tolerates high
salinities (>300 ppt).
3. Lake Nakuru (Kenya) -
flamingo population
4. Lake Werowrap, Australia
– even simpler food web
IV. Human impacts on salt lakes
A. Mining of minerals – salt, uranium, lithium,
borax, potassium, nitrates, sodium carbonates
B. Harvesting of phytoplankton -- Spirulina
C. Harvesting of animals – brine shrimp; sometimes
spread to other lakes
D. Global warming
E. Case studies -- under threat from diversion of
water from rivers for irrigation of crops in the semiarid zone (like
Aral Sea)
1. Great Salt Lake
2. Dead Sea
3. Mono Lake, CA – tufa;
brine shrimp and brine fly; waterfowl;
ecosystem, lake level and water management model interact with politics
of water management in the Western U.S.