Week 8 - Intro to Estuaries

Question 1

Estuaries - Setting the stage

Answer 1

body of water where a river meets the sea and
where fresh and saltwater mix

Question 2

Estuaries as Ecotone

Answer 2

Estuaries are transitional areas that straddle the land and the sea, as well as freshwater and saltwater ecosystems

a rare “4-way ecological intersection”

Question 3

Estuaries as mixing zone

Answer 3

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

Question 4

Estuaries habitat types

Answer 4

freshwater river
saltwater marsh
mudflats
eelgrass beds

other: mangroves, wetlands, swamps

Question 5

Estuaries rich communities

Answer 5

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

Question 6

Estuary as salmon nursery

Answer 6

for salmonids, estuaries provide critical habitat
where they adjust physiologically from fresh to
saltwater TWICE in their life cycle

Question 7

Estuary as traditional value

Answer 7

culturally significant for indigenous food gathering

“when the tide went out, the table was set”

Question 8

Estuary - high productivity

Answer 8

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

Question 9

Estuaries - Carbon

Answer 9

Carbon energy supply:

1. River transported carbon
   (autochthonous/ allochthonous OM, LWD)
2. Emergent marsh vegetation (sedges,
   rushes…)
3. Submergent estuary vegetation (eelgrass)
4. Carbon transported from the ocean (plankton,
   fish)

Question 10

Estuary as carbon storage

Answer 10

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

Question 11

Estuaries in BC

Answer 11

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

Question 12

Estuary Restoration

Answer 12

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

Question 13

Salinity of Estuary

Answer 13

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,

Question 14

Brakish Water

Answer 14

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

Question 15

Tides

Answer 15

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

Question 16

Type of tides

Answer 16

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

Question 17

Why each estuary unique?

Answer 17

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

Question 18

Stratification and Circulation

Answer 18

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

Question 19

water density

Answer 19

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)

Question 20

Estuary Classifications

Answer 20

Based on 1) Geology and 2) Stratification and Fresh-Seawater Circulation

1. Coastal-plain estuaries or drowned river valleys
2. Fjord-type estuaries
3. Tectonically caused estuaries
4. Delta estuaries
5. Bar-built estuaries
6. Lagoons

Question 21

Downed-River Valleys (coastal plain estuaries)

Answer 21

• 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

Question 22

Fjord-Type Estuaries

Answer 22

• 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

Question 23

Tectonic Estuaries

Answer 23

• 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

Question 24

Delta Estuaries

Answer 24

• 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

Question 25

Bar Built-Estuaries

Answer 25

• rarest type of estuary on the West Coast
• formed by ocean waves and currents pushing sediments, shoreward, building up sandbars, and forming barrier islands.

Question 26

Lagoon Estuaries

Answer 26

• 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

Question 27

Stratification and Circulation major types

Answer 27

1) Salt wedge
2) Fjord
3) Slightly Stratified
4) Vertically mixed
5) Freshwater

Question 28

Density and Mixing (Review)

Answer 28

• 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

Question 29

1) Salt-Wedge Estuaries

Answer 29

• 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)

Question 30

2) Fjord Estuaries

Answer 30

• 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

Question 31

3) Slightly Stratified Estuaries

Answer 31

• 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)

Question 32

4) Vertically mixed estuaries

Answer 32

• 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

Question 33

Freshwater Estuaries

Answer 33

• 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