Week 8 - Intro to Estuaries Flashcards
Estuaries - Setting the stage
body of water where a river meets the sea and
where fresh and saltwater mix
Estuaries as Ecotone
Estuaries are transitional areas that straddle the land and the sea, as well as freshwater and saltwater ecosystems
a rare “4-way ecological intersection”
Estuaries as mixing zone
trap nutrients and sediment carried from the land by rivers and carried from the ocean by tides
constant mixing of these nutrients with the ebb and flow of the tide
supports a high biomass, diversity of plant and animal life
unique physical-chemical conditions
Estuaries habitat types
freshwater river
saltwater marsh
mudflats
eelgrass beds
other: mangroves, wetlands, swamps
Estuaries rich communities
support a rich and diverse assemblage of aquatic and terrestrial organisms and plants
used by many species for key life stages
invertebrates and microbial decomposers/recyclers
Estuary as salmon nursery
for salmonids, estuaries provide critical habitat
where they adjust physiologically from fresh to
saltwater TWICE in their life cycle
Estuary as traditional value
culturally significant for indigenous food gathering
“when the tide went out, the table was set”
Estuary - high productivity
among the most productive ecosystems on earth
tropical mangrove and tropical forest: 2.2 kg/m/yr
tropical algal beds and reefs: 2.5 kg/m/yr
temperate salts marshes: 1.7 kg/m/yr
temperate forest: 1.2 kg/m/yr
C (carbon) - is the currency of estuaries
Estuaries - Carbon
Carbon energy supply:
- River transported carbon
(autochthonous/ allochthonous OM, LWD) - Emergent marsh vegetation (sedges,
rushes…) - Submergent estuary vegetation (eelgrass)
- Carbon transported from the ocean (plankton,
fish)
Estuary as carbon storage
Like terrestrial forests, estuaries capture and store large amounts of carbon but much more efficiently (up to 20X)
“Sequestration”: capturing and storing C02 from the atmosphere, in solid or dissolved forms
“Blue Carbon”: carbon that is captured, stored, and released from coastal and marine habitats
Produce less GHG than their freshwater counterparts due to salinity
Estuaries in BC
442 estuaries in BC, make up 3% of the coastline, area of 745 km2, used by approx 80% wildlife species on the coast, only 13.5% is protected
many have been degraded by urban, industrial, and agricultural development
Most critical habitat for carbon sequestration
is eelgrass beds, followed by salt marsh
sedges
“Blue Carbon” is stored in plants and
sediments where it is stable for thousands of
years
Roughly 400 km2 of BC salt marsh and
eelgrass meadows store equivalent carbon as
BC’s area of boreal forest
Restoration - eelgrass-rich intertidal zones and BC estuaries are high priority in conservation and restoration
Estuary Restoration
Techniques include:
removal of dykes/training walls, reconnection with floodplain
relocation of log booms to dryland sorting yards
placement of large woody debris (LWD)
re-establishment of estuarine vegetation
relocation of industry where possible
exclusion of non-migratory Canada Geese
Restoration sites in BC
Campbell River, Comox, Courtney, Cowichan, Englishman, Little Qualicum, Nanaimo, Stamp-Somass, Squamish, Fraser River, small sites in Burrard Inglet
Salinity of Estuary
ocean water salinity - 35 ppt (parts per
thousand)
estuary water - salinity ~ 0.5 - 30
ppt “BRACKISH”
freshwater - salinity almost zero
highly variable -salinity of estuarine water varies from estuary to estuary, and can change from day to day depending on tides, weather, or other factors
Salinity (Salt) - dominated by Cl- and Na+, followed by SO4-2 and Mg+2,
Brakish Water
brackish water supports unique plant and animal communities
freshwater “plume” floats on top of the denser
seawater, despite the suspended sediment load
how some freshwater fish migrated to
Vancouver Island during spring freshet
Tides
daily tides are a major influence on the dynamic nature of estuaries
Gravity, sun, and the moon - caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth
Patterns - most areas experience two high and two low tides in a day
Factors - geographic location, shape of the coastline and ocean floor, depth of the water, local winds, any restriction to the water flow
Unique to each estuary - With the variety of conditions across the Earth, each estuary displays a tidal pattern unique to its location
Type of tides
Semi Diurnal Tide - two almost high tides and two low tides each day
Diurnal Tide - only one high and one low tide each day
Mixed Tide - two uneven tides a day
Why each estuary unique?
many estuaries are protected from the full
force of ocean waves, winds, and storms by reefs, barrier islands, or fingers of land,
mud, or sand that surrounds them
Characteristics of each estuary depends upon the local climate, freshwater input, tidal patterns and currents, surrounding geomorphology
No two estuaries are the same
Stratification and Circulation
in salt water, density increases progressively to
freezing point of -2 °C
salts add mass without adding volume
higher density promotes the sinking of cold sea
water, which drives global ocean circulation
water density
Temperature - freshwater declines in density below 4C, whereas saltwater continues to get more dense. Higher density promotes the sinking of cold sea water, which drives global ocean circulation
Depth - Saltwater density at the
ocean’s surface is approx 1,025 kg/m3 therefore its specific gravity is 1.025. The pycnoline is the depth gradient in salinity. Saltwater increases density with decreasing temp. Saltwater increases density with increasing depth
(to a point)
Estuary Classifications
Based on 1) Geology and 2) Stratification and Fresh-Seawater Circulation
- Coastal-plain estuaries or drowned river valleys
- Fjord-type estuaries
- Tectonically caused estuaries
- Delta estuaries
- Bar-built estuaries
- Lagoons
Downed-River Valleys (coastal plain estuaries)
- this type of estuary is common around the world
- depth increases towards the rivers mouth
- valleys are usually shallow with gentle sloping bottom
- sediment accumulates within the river valley
West Coast examples:
Fraser River
Campbell River
Columbia River
Umpqua River
Sacramento River
San Joaquin
Fjord-Type Estuaries
- fjords are valleys that have been cut deeper by moving glaciers and then invaded by the sea
- created by glacial action, characterized by the steep slope of adjacent lands and great depth
- shallow sill near their mouth that limits water exchange between the deeper waters of the
fjord and the sea - found in areas previously covered by glaciers
Local examples are: Alberni Inlet, Howe Sound, Puget Sound, and Hood Canal
Tectonic Estuaries
- movement of the earth’s plates causes large cracks or faults and folds to form in the crust
- created when the sea fills in the “hole” or basin
that was formed by the sinking land.
San Fransisco Bay is an example
Delta Estuaries
- form at the mouths of large rivers, when sediments and silt accumulate rather than being washed away by currents or ocean waves
- over time, a complex set of channels, sand barriers, and marshes form at the mouth of the river
- as sediments continue to accumulate seaward, the course of the river may even change
Bar Built-Estuaries
- rarest type of estuary on the West Coast
- formed by ocean waves and currents pushing sediments, shoreward, building up sandbars, and forming barrier islands.
Lagoon Estuaries
- formed by wave-built sand spits, which are breached during winters storms, then rebuilt in spring-fall
- found primarily along Gulf Coast and some
small lagoon estuaries along the West Coast (eg, San Dieguito) - often narrow outlet to the sea
- minimal freshwater inflow
- higher salinity levels than in coastal-plain and other estuaries
Stratification and Circulation major types
1) Salt wedge
2) Fjord
3) Slightly Stratified
4) Vertically mixed
5) Freshwater
Density and Mixing (Review)
- freshwater flowing into the estuary is less salty and less dense than water from the ocean, so it floats on top of the heavier seawater
Mixing:
Depends on:
1. the direction and speed of the wind
2. the tidal range (the difference between the
average low tide and the average high tide)
3. the estuary’s shape
4. the volume and flow rate of river water entering the estuary
1) Salt-Wedge Estuaries
- most stratified (least mixed) estuary type
- also called “highly stratified” estuaries
- occur when a rapidly flowing river discharges into the ocean where tidal currents are weak
- recall – freshwater is LESS dense so it floats
- some mixing occurs at the boundary between
the two water masses, but it is generally slight - location of the wedge varies with the weather
and tidal conditions and discharge - examples are the Columbia River (WA, OR) and
Fraser River (BC)
2) Fjord Estuaries
- found along de-glaciated coastlines
- sill at entrance restricts water circulation and dense
- seawater seldom flows up over the sill into the
estuary - very little tidal mixing; thus, the water remains
highly stratified
3) Slightly Stratified Estuaries
- also known as ‘partially mixed’ estuaries
- saltwater and freshwater mix at all depths
- the salt water is mixed upward
and fresh water is mixed downward - lower layers of water typically
remain saltier than the upper layers - very deep estuaries, such as Puget Sound (WA) and San Francisco Bay (CA)
4) Vertically mixed estuaries
- occurs when river flow is low and tidally
generated currents are moderate to strong - the salinity of water in a vertically-mixed estuaries is the same from the water surface to the bottom of the estuary due to strong tidal currents
- found in large, shallow estuaries
Freshwater Estuaries
- Exclusive to the Great Lakes
- Not seawater, but chemically-distinct
combinations of river and lake water - Not tide-driven but STORM- DRIVEN (“seiches”)
- semi-enclosed bay areas in which the lake water becomes mixed with water fafrom the river or stream
