Estuaries

What is an estuary?

An inlet of the ocean that extends into a river valley as far as the upper limit of the tide, characterized by mixing of saltwater and freshwater

What are 4 types of estuaries?

• Coastal plain

• Tectonic estuary

• Fjord

• Lagoon

What is a common characteristic of all estuaries?

• Tend to be young in geological terms

• Usually less than 10,000 years old (last glaciation)

What are coastal plains/drowned river valleys?

Estuaries that form between glacial periods, when water from melting glaciers raises the sea level and floods coastal plains and low-lying rivers

What are 3 examples of coastal plains/drowned river valleys?

• Saint-Lawrence River estuary in Quebec

• Chesapeake Bay in the eastern US

• Mississippi River estuary in the eastern US

What is one of the largest estuaries in the world?

Saint-Lawrence River estuary

What are tectonic estuaries?

Estuaries that form as a result of earthquakes that cause land to sink or subside, creating a connection between the sea and wetlands behind the broken barrier

What is an example of a tectonic estuary?

San Francisco Bay

What are fjords or fjards?

• Estuaries that are formed by glaciers that cut valleys into coasts

• After the glaciers retreated, water filled those valleys

What are characteristics of fjords?

Shallow sill or a series of shallow sills and deep basins at their mouth that restricts the exchange of the deep fjord water and the sea

What are characteristics of fjards?

• Less defined sills

• Lower relief

• Gently undulating topography

• Irregular shape

• Tend to include many islands

What are 4 examples of fjords?

• Many in Norway, New Zealand, and Labrador

• Indian Arm and Burrard Inlet pass between Vancouver and North Vancouver

What are lagoons?

Formed when sandbars build up parallel to the coast to form barrier islands that partially cut off the waters behind from the sea

Where are lagoons common?

• Off the coast of North Carolina

• Gulf of Florida

• Netherlands

• Parts of Australia

Where do positive estuaries occur?

Temperate areas

Where do negative estuaries occur?

Arid areas

What is a positive estuary?

Sufficient influx of freshwater, leading to mixing and increased salinity towards the mouth of the estuary

What is a negative estuary?

High evaporation rate at the surface causes the surface water to become hypersaline, sinks to the bottom, and moves out of the estuary as a bottom current, increased salinity towards the head

What are 4 types of mixing patterns?

• Salt wedge

• Partially mixed

• Well mixed

• Fjord

What is a salt wedge?

• Freshwater flows out at the surface while seawater flows in at the bottom, creating an angled wedge

• Salinity break

What is the main influence of a salt wedge?

River flow

What is a partially mixed pattern?

• Strong surface flow of freshwater and influx of seawater

• Changing salinity with depth

What are the main influences of a partially mixed pattern?

• River

• Tides

What is a well mixed pattern?

• Low river flow

• Uniform salinity with depth

What is the main influence of a well mixed pattern?

Tidal currents

What is a fjord mixing pattern?

• Freshwater remains at the surface

• Little mixing

• Horizontal salinity stratification

What are key environmental features of estuaries?

• Salinity

• Mixing

• Temperature

• Sediment

• Turbidity

• Dissolved oxygen

What are freshwater species?

Cannot tolerate salinity higher than 5 ppt

Where do freshwater species live in an estuary?

Closer to the head of the estuary

What are euryhaline marine species?

Tolerate varying amounts of reductions in salinity

What are stenohaline marine species?

Barely able or unable to tolerate changes in salinity

Where do stenohaline marine species live in an estuary?

Closer to the mouth of the estuary

What are 2 examples of transitional species?

• Salmon

• Eels

What are 2 challenges of life in the estuary?

• Dealing with low or changing salinity, maintaining osmotic balance

• Remaining stationary in a tidal system

What are osmoconformers?

• Maintain an internal environment which is isotonic to their external environment

• Osmotic pressure in cells = osmotic pressure in environment

What are osmoregulators?

Maintain constant salt concentration in body

What are physiological adaptations to deal with low or changing salinity and maintain osmotic balance in the estuary?

• Absorb ions through their gills

• Concentrate ions in their bodies when salinity is low

• Excretion when salinity is too high

How do freshwater fish maintain osmotic balance in the estuary?

• Do not drink

• Actively take up ions through gills

• Absorb water through skin

• Excrete dilute urine

• Body fluids are saltier

How do marine fish maintain osmotic balance in the estuary?

• Drink a lot

• Excrete ions through gills

• Lose water through skin

• Excrete concentrated urine

• Body fluids are hypoosmotic, lower solute concentration

What are morphological adaptations to maintain osmotic balance in the estuary?

Body surface that reduces permeability (exoskeleton or mucous glands on body surface)

What is a behavioural adaptation to maintain osmotic balance in the estuary?

Seek habitats that are less affected by environmental change (mud)

How do organisms remain stationary in a tidal system?

• Take advantage of the currents, move in the water column

• Live on the bottom

• Live in the sediment

What are 2 ways that organisms can live on the bottom of the estuary?

• Permanently attached

• Wandering epifauna

What are epifauna?

Organisms that live on the sediment

What are infauna?

Organisms that live in the sediment

What are benefits of living in the sediment?

• Less variation in salinity and temperature

• Protection

Why is there less variation in salinity and temperature in the sediment?

Sediment pore water exchanges slowly with overlying water

Why are estuaries important nursery habitats?

• Nutrient-rich

• Relatively few predators

What is the base of estuarine food webs?

• Primarily detritus-based

• From different sources but not produced by the estuary itself

How many trophic levels are in estuarine food webs?

3

What is an important trophic level of estuarine food webs not seen in other food webs?

Microbial/bacterial detritivores

What are important predators in estuarine food webs?

Birds

What dictates how deep a prey different species of birds can catch?

• Bill length

• Bill curvature

What are ecosystem services?

Functions that are useful to humans and are provided for free by nature

What are ecosystem services of estuaries?

• Important nursery habitat for many commercially important species

• Coastal protection

What are threats to estuaries?

• Conversion: draining, filling, damming, dredging

• Pollution: agricultural and industrial runoff