Hello marine biology students.
In this video we're going to talk about where
rivers meet the ocean.
We're going to talk about estuaries.
[Intro Music]
An estuary is a semi-enclosed area where fresh
water and sea water meet and mix.
Estuaries can experience wide changes in salinity,
temperature, and exposure to air.
In estuaries there will normally be soft sediments
and typically there will be low species diversity,
meaning the number of different types of species,
especially in adults or permanent residents
of estuaries, but typically high biomass,
the total mass of living organisms, and this
includes the meiofauna in addition to actually
many plants that are right on the border,
the salt marshes, of the interface between
the water and the land.
There are several different types of estuaries,
but again, they normally involve freshwater
and saltwater mixing in a sheltered environment.
One type of estuary, in fact the most common,
are called drowned river valleys or also called
ria's.
They're the most common type of estuary and
they are formed by a rising of the sea level
after the last ice age around a lowland for
a river or the mouth of a river.
So they are formed by the drowning of lowland
around the mouths of rivers when sea levels
rose after the last ice age.
The Chesapeake Bay is an example of this,
here we can also see a drowned river valley
estuary from Australia in the picture.
Another type of estuary are bar-built estuaries.
In this case, it's a sand bar or a bar of
sediments that end up sheltering or enclosing
an area where rivers are entering or meeting
the sea.
These are built up by the accumulation of
sediments
into sandbars or barrier islands and so the
coast protected by Cape Hatteras barrier islands
are an example of a bar built estuary.
There are also tectonic estuaries, where land
bordering the oceans begins to subside due
to the movements of the Earth's crust 
at faults.
San Francisco Bay is an example of a tectonic
estuary.
And the last category of estuaries are fjords.
These are 
deep valleys cut on the coast as a result
of retreating glaciers.
These can be found in Alaska, Norway, New
Zealand, and Chile.
The best and most developed estuaries are
going to be in areas where the coastal plain
is flat and the continental shelf is wide.
These are the passive margins.
Whereas active margins usually have a very
narrow continental shelf and they have a more
abrupt transition from land to sea, and so
on an active margin you aren't going to have
an abundance of estuaries.
A defining feature of the estuaries is freshwater
and saltwater coming in contact with each
other.
Now, the density of freshwater is significantly
less than saltwater and so the seawater is
going to form what's called a salt wedge as
tide comes in or as the interface of these
two water sources mix.
Another way of thinking about this is that
the freshwater will attempt to form a thin
surface layer Salinity in an estuary varies
from 5 to 30 parts per thousand.
It varies according to the distance from the
sea water or the tidal input and the quantity
of the freshwater or the river input.
The depth also contributes to the salinity
profile.
Again, because saltier water is more dense,
it will be found at deeper levels.
A defining characteristic of estuaries is
that the salinity just is not uniform.
The salt water is heavier and sinks below
the freshwater.
As water is brought in by the tides, a salt
wedge is formed.
Organisms in estuaries are normally euryhaline,
in that they tolerate a wide range of salinities.
Some are osmoregulators 
and have mechanisms for keeping their internal
solute concentrations stable, others are osmoconformers
and have internal concentrations that vary
with their surroundings.
In this diagram we see the typical salinity
tolerances of organisms, whether they're stenohaline
marine organisms, euryhaline marine organisms,
brackish water species, or freshwater species.
This chart shows us the difference between
osmoregulators and osmoconformers, with an
osmoregulator typically having a flat line,
whereas osmoconformers would have a line at
a set slope.
We see these crabs are able to somewhat regulate
their internal concentration at least for
certain ranges of salinity.
Estuaries aren't just about the organisms
living in the water, but also those living
adjacent to the water.
There are many flowering plants in estuaries
and they often contribute a large amount to
the productivity of these environments.
Now some of these have special mechanisms
to deal with the salty water that they're
around.
They can expel excess solutes, such as the
salt glands in the cordgrass Spartina and
also many mangroves.
The photo we see here is of a mangrove which
secretes a solute-rich solution onto its leaves
and as the water evaporates away, you can
see the salt crystals left behind.
Some of these flowering plants actually concentrate
the solutes in certain glands, such as the
pickleweed Salicornia.
The substrate in estuaries is almost always
soft sediment.
It's usually going to be mud or sand, and
they often because they are sheltered, have
low wave activity, which is why this sediment
hasn't been washed away or taken as sediment
down to the seafloor.
So, normally they will be continental sediments.
Mud is difficult for many animals to move
through.
The mud and sand also shift and are unstable,
making it a difficult environment for organisms
that need a hard substrate to grow on.
Particle sizes of the sediments are also usually
small enough that most of the sediment areas
are actually anoxic, as denoted by the dark
black color to the soil where anaerobic bacteria
are breaking down organic matter in the absence
of oxygen.
That black sediment would likely smell very
strongly of sulfur and be quite unpleasant.
Temperature is another factor that varies
in estuaries.
As with tide pools, there can be wide variation
in temperature, especially at low tide.
This can be stressful on marine organisms
when the difference in temperatures are the
greatest, especially in summer.
These physical challenges can be limiting
to the diversity of species which can survive
there permanently.
A very important role of estuaries is actually
that of a larval development ground.
They are the nurseries for many marine species.
When we look at contributions from the open
water into an estuary, the dominant organisms
include plankton 
both in the form of phytoplankton and zooplankton
and also fishes.
It turns out that many marine species spend
at least a portion of their lives in estuaries,
mostly as larvae.
This also speaks to estuaries almost hidden
importance to the survival of other marine
communities as well.
Another difficulty of estuaries has to do
with water transparency.
They are usually so full of sediments from
terrestrial regions that the ability of light
to penetrate the waters is greatly reduced.
So, normally water transparency is very poor
due to suspended sediments and particles from
rivers in the water.
This reduced clarity 
makes it more difficult for photosynthesis
in autotrophs in the water.
Most of the primary production of estuaries
is the result of flowering plants on land
as opposed to sea weeds or phytoplankton.
This reduced visibility also can make it difficult
for certain visual predatory fish to find
their prey, however sharks have been known
to swim in estuaries and up rivers seeking
prey and their ability to detect electrical
impulses using their Ampullae of Lorenzini
allowed them to find prey even when the water
is murky.
Salt marshes are a common type of estuary,
with cordgrass
making up a dominant species in these salt
marshes.
They have high primary productivity and serve
as a nursery for the young of many species.
These salt marshes often contribute detritus,
both to the estuary and also to some of the
neighboring intertidal regions outside of
the estuary as well.
Most estuaries will have a soft bottom and
so many burrowing organisms.
Even though the sediments themselves are anoxic,
they will have mechanisms to connect them
to the oxygenated water above, either along
siphons or mechanisms for making burrows and
actively pumping water through those burrows
to maintain an oxygenated living area.
The fat innkeeper worm, which is an echiurine,
a type of annelid, has a burrow that will
often be shared with many other species and
so their name of being a fat innkeeper worm
is because so many other species are dependent
on their burrow in addition to themselves.
When we look at food webs within estuaries,
we see a lot of the primary productivity is
going to come from the salt marshes and that
contributes to a detritus pool.
There will be phytoplankton and some benthic
diatoms and seaweeds and those can be preyed
on directly by herbivores, but we see that
filter feeders and deposit feeders are more
common in the estuary environment and the
carnivores are going to come in the form of
fish and birds.
As I had mentioned, sharks have been known
to frequent river mouths and also estuary
environments where they can use their special
abilities to detect prey even if the water
is not clear.
And this completes our introduction to estuaries.
Now before our next video, I want you to think
about “What is your preferred way to use
coastline?”
We'll talk about that in the next video.
