So far, we've learned a little about the different kinds of producers
that live in an estuary: emergent plants, submerged plants, drifting algae
and finally plankton. All of these are powered by the sun.
But last time I left you hanging with an unanswered question; I claimed
there were two completely different power sources in an estuary, and asked
you to figure out what the second one is. The first, of course, is the
But what's the second one? Would you believe the moon?
Good gosh! How does the moon count as a source of power for an estuary?
It surely can't reflect sunlight to power much in the way of
photosynthesis, so how does the moon provide a second source of power to
Simple. Tides and tidal currents.
Think about all the critters living in the estuary that depend on the
tidal currents to bring food to them. The list includes lots of mollusks
(clams, oysters, scallops, mussels and more), barnacles, anemones, worms
of many kinds, even some shrimps and crabs, and lots more. These animals
could not live without the tidal currents to sweep food by so they can
filter it out of the water. In general, these critters are called
What kind of food are they eating? Obviously, it's not pieces of leaf
or stem -- they are too big to handle. In fact, most of their food is
Do any other estuarine animals depend on the tides? Find a commercial
fisherman and ask him or her. Or ask one of those other fishermen who
always seem to come home with fish.
Many of the fish we want to catch (predators like striped bass, snook,
weakfish and more) often lay up in grass beds and wait for the tide to
bring food to them. They are ambush feeders. When the tide is dropping,
they lay in channels and wait for prey to come to them from the shallows.
When the tide is rising, they often move up into the shallows to chase
food. I take advantage of this: fishing the little channels on a falling
tide, moving up onto shallow flats on the rising tide.
Another factor to consider: most predators are very energy conscious.
Since food for them is not guaranteed, they will spend as little energy as
possible while they wait for food to come to them. Tidal currents eddy
around obstacles, just like rivers eddy around rocks, and these eddies
behind the obstacle are places of quiet water where a predator can wait
for the current to bring food while using as little energy as possible.
Most estuaries have lots of docks and pilings, and these produce eddies
downcurrent where predators lie in wait.
We've seen that tidal currents can transport food. Can they also
transport other things?
Of course! Seeds from emergent plants float around with the currents,
occasionally becoming lucky enough to germinate in a suitable location and
start another stand of grasses or rushes or mangroves. Many of these
plants depend entirely on the tidal currents to propagate themselves. And
the submerged grasses depend on the currents to do what bees do on land:
carry their pollen to female flowers. These grasses don't have to attract
pollinating insects, so their "flowers" are reduced in size,
hard to find (no pretty colors) and have little or no colorful petals.
How about an oyster? It can't get up and move over to a suitable mate,
so how do they do it? Again, tidal currents. Oysters, and many other
organisms which can't move around in the estuary, depend on the tidal
currents to carry their sperm to eggs to start the next generation. Which,
when you think about it, implies an exquisite sense of timing, to make
sure the eggs are afloat just as the sperm reach them. For oysters, it
seems the trigger for mating - the release into the water column of eggs
and sperm - depends on temperature.
Another effect of the moon involves its phases. You know from basic
science that when the moon is directly opposite the sun, rising just as
the sun sets, we call this a "full moon." Then, two weeks later,
the moon is lined up between earth and sun so that we cannot see it, and
we call this the "new moon".
These two periods, every two weeks, lead to more gravitational
attraction (sun and moon combined) which makes high tides higher and low
tides lower. This in turn means that more water moves in and out than
normal, creating much stronger tidal currents. We call these extra strong
tides "spring tides".
A rule of thumb for fishermen is that night fishing is better on a full
moon (especially if you have a clear night) because the fish can see prey
better, and that fishing during the day - especially at dawn and dusk - is
better on a new moon, when the fish haven't had enough light through the
night to be full of food by daybreak.
A general rule of thumb for tidal amplitude - that is, the difference
in height between low and high tide - is that it is least in the tropics,
and gradually becomes greater as you approach the poles. Where I live, on
the east coast of Florida, the tidal amplitude is generally only 1 1/2 to
2 feet, while further north it may be 6 or 8 feet. The shape of the body
of water can also affect tidal amplitude; the Bay of Fundy in Nova Scotia
is shaped like a funnel, and the range between high and low tides up at
the narrow end of the bay can be as much as 50 feet on a good spring tide.
And, of course, those same tidal currents which make life possible for
many of the estuary's inhabitants are also capable of carrying pollutants
of all kinds to the far corners of the estuary. How many of you have seen
trash floating around? I hope you pick it out of the water and put it into
a dumpster! And, while we're thinking about pollutants, consider that most
of the most harmful ones are invisible: dissolved in the very water
Next time, we'll look into more of the magic found in estuaries, and
consider what we can do to make them even more productive than they are
Later this year, we'll examine ponds, lakes, reservoirs and streams
Bob Bergen, Professor of Science, Retired. Teen Angler Mentor
2008[National Teen Anglers]. All rights reserved.
Revised: February 10, 2010
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