Estuaries are places where fresh
water from the land mixes and mingles with salty water from the
ocean. This not-quite-fresh - but not so salty either -
water is often called "brackish."
It makes for a stressful
environment. Critters that live in fresh water - like
largemouth black bass - can't handle the saltiness, and critters
that live in the ocean can't tolerate the fresher water.
We'll look at some other stressors a little later.
But estuaries are biologically
extremely productive; that is, they are home to lots of living
things of different kinds and
they produce more food per acre per year than almost any other
ecosystem on earth. There are only two other ecosystems
that are more productive than estuaries, and you can probably
name them: tropical rain forests and coral reefs.
In the end, though, we humans, even
with all our modern technology, can't grow as much food on our
farms as a healthy estuary does.
So what makes estuaries so
And that's where the magic gets
explained. You know there is no such thing as true
"magic;" it's misdirection and illusion, easy to
understand once someone shows and explains it to you.
Same thing in the estuary.
They aren't really magic; it's just that we are just now
learning how they really work, and why they are so productive.
Let's compare an estuary with a
forest, or a prairie, or even a corn field. A first glance
shows that an estuary is really a mix of different plant
communities, and since we know that plants are the base of any
food chain, the more different kinds of plants the better.
A forest, on the other hand, is made up mostly of one kind of
plant: trees. And a prairie is made up of small
shrubs, wildflowers, grasses and other smaller plants. A
corn field is tended by the farmer to prevent any other kinds of
plants from growing; they are called weeds.
Since plants are the only critters
on the planet that can make food from sunlight, we call them
producers. Any estuary worth its salt has at least three
different kinds of producer communities: submerged plants,
emergent plants, and floating, microscopic plants we call
plankton. The forest, the prairie, the corn field only
have one kind of plant: roots in the soil, leaves up in
the air. No plankton floating around.
And, since we call the plants
producers, then animals become the consumers.
These three different kinds of
estuarine plant communities provide shelter and food for many
kinds of animals. We'll explore some of these.
communities: rooted in soil under water, with stems and
leaves out of the water. Think mangrove trees in the
tropics, cordgrass and black rush further
north. Mangroves drop leaves year-round, while the grasses
and rushes die back each winter. In both of these, the
dead leaves and plants are the base of the food
chain. They are fed upon by bacteria and fungi, which
become food for grazers, which become food for crustaceans,
which become food for fish.
seagrasses, which provide shelter from sunlight and predators,
and which provide solid surfaces for things to colonize:
tiny sponges, barnacles,
larvae of many kinds, algae and many others. These in turn
become food for animals like the green sea turtle, which grazes
on the grass and gets all the encrusting
animals as extra protein.
float on the currents of the estuary and are the basic food of
many filter-feeders like oysters and barnacles, and many kinds
of fish such as
herrings, sardines and anchovies.
No other ecosystem on earth has all
three of these producer communities. The coral reef has no
emergent plants, the tropical rain forest has no plankton, the
prairie has no submerged plants - and the corn field is a
killing ground for anything other than corn.
A basic rule in ecology is that
species diversity leads to stability. A healthy estuary
certainly has that diversity.
Let's explore this diversity and its
role in the estuary a little further.
The emergent community is a part of
the littoral zone, where the bottom of the estuary gradually
shallows until it reaches the edge of the land. If you
were in the Navy, the littoral is where the latest generation of
submarines operates - in water less than 600 feet deep. If
you are an estuarine ecologist, the littoral is the zone
affected by the tides at the edge, where low tide finds mud
flats exposed to the air and high tide finds them under several
feet of water. In estuaries, the tides control what grows
where in the littoral zone. North of Florida, cordgrass
grows in the deepest water, where the soil is wet even at low
tide, while black rush grows on higher ground wet only at
high tide. In the tropics, different species of mangrove
trees occupy different elevations in the littoral zone.
The animals that live in the
littoral zone must adapt to periods of dryness followed by
flooding tides on a daily basis. Where do fiddler crabs go
when the tide comes in?
Little fish (little predators,
really) move through the marsh or swamp following the water to
find different kinds of food. Worms of many species
retreat into their burrows at high tide, coming out and feeding
only when they are again covered with water on the incoming
tide. Oysters close their shells, barnacles close up, sea
anemones retreat into little round clumps of soft stuff, all
waiting for the tide to cover them again and bring food to them.
We are still learning about these animals.
Another important role of the
littoral zone is so common-sensical it is often overlooked or
ignored. Shallow water is an exceptionally good hiding
place for an inch-long minnow; what one pound predator can swim
in water only an inch deep? But there's a price to pay if
you are that minnow: wading birds like herons and egrets
which wade those same shallows looking for you and your brothers
Principle: a healthy estuary
produces such a surplus of baby critters that the birds and the
larger fishes cannot eat them all.
The seagrass meadows live in water
deep enough to (nearly) always be flooded. These grassy
meadows - and they are real grasses, only living underwater
rather than on your front lawn - don't tolerate drying out, so
they live in shallow water just below the low tide line.
There are often several species of grasses, with different
species of algae living with them, attached to the grass blades
or just floating and drifting through the grasses.
Now grasses of all kinds like direct
sunlight, the more the better. So a typical estuary, with
water muddied up by runoff from the land, only has grass beds
where the sun can reach. Often that means their grass beds
can only exist to a depth of three or four or five feet.
Deeper than that, and not enough sunlight gets through the murk
to support photosynthesis. If you have a big tree in your
yard, grass won't grow in its shade. An exception is the
Florida Keys, where I have dived on seagrass beds in 30 feet of
incredibly clear water.
These grass beds provide food for
many predators, who hide in the grasses and ambush prey.
Other predators graze on the grasses themselves, and the
communities of organisms which live on the grass blades.
Many of these grazers and predators are camouflaged to resemble
the grasses; pipefish and some shrimps are good examples.
Finally, we come to the plankton
The plankton community is a world in
itself. Microscopic algae like the many species of diatoms
are fed upon by microscopic animals like copepods, and all of
these tiny critters are then fed upon by predators like clams
and oysters and herrings and sardines. But these tiny,
microscopic producers and consumers cannot swim far enough to
move around an estuary. So how do they get to the
barnacles and worms that eat them?
Although we've talked a little about
the magic of estuaries, we have not yet talked about the power
of estuaries. And that's where we'll start next time.
And, yes, it has to do with how plankton move through the
Can you figure it out before next