Red Tide: the whys, whens, and wherefores - loooong ;-), the first place to look for a dive instructor

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Posted by AADIVER on September 16, 2003 at 17:02:05:

Hi Folks,

I've been neglecting my red-tide-y duties lately, and have not kept you
informed about the goings-on in our vasty deep. Okay, our nearshore,
relatively shallow waters.

As you might have noticed, the waters are a dark brownish purple. This is
caused by dense accumulations near the surface of dinoflagellates (think:
tiny cellulose-covered balls with two little whips for propulsion). As luck
would have it, today's bloom (as opposed to the one last month) is dominated
by our friend Lingulodinium polyedrum. This dinoflagellate is photosynthetic
(makes sugars from light and carbon dioxide), and bioluminescent (makes its
own light). Each cell is about 30-40 microns across (there are 1000 microns
in a millimeter, which is about the thickness of a dime). The cells have
tiny sacs of enzymes that react when the cell is jostled (for example by the
breaking surf). When the enzymes react, they give off a bluish flash of
light. So our waves (and your footsteps on the beach; your hands and feet
when you swim) will be adorned with gorgeous flashes of light at night.

The bioluminescence of these cells is on a circadian rhythm - they don't
bother making light during the day 'cause no one could see it. So here's
what you do to amaze your friends, astound your children, and be the toast
of your neighborhood:
Get a clear jar or bottle, and fill it with some water from the surfzone
(ask a friendly neighborhood surfer to fill it for you in deeper water, to
get less sand in it). Take your bottle home, and wait until after the sun
has gone down. Then take your bottle into a dark room (bathroom or closet
with no windows). Wait for your eyes to adjust to the darkness (a minute or
so), and then give your bottle a swirl. You should see a really amazing
light show. For extra added excitement, add some vinegar to the bottle.
You'll get a particularly bright flash (but then all the cells will die, so
it only works once). The acid of the vinegar makes the enzymes react inside
the cell, even without stirring.

Some frequently asked questions (that I mostly cannot answer):
Why is there a red tide?
I don't know. Red tides are natural occurrences; the plankton community
becomes dominated by one or a few species at extraordinarily high
concentrations. The only thing I can say for certain is that the net growth
rate of the cells (including accumulation by swimming) is higher than their
net loss rate (due to grazing, etc.). It's certainly possible that the
growth of the cells is fueled by nutrients brought to the surface by
upwelling (the water temperature dropped suddenly back in August).

Why are red tides so dense?
The organisms that make red tides (at least around here) tend to be able to
swim. They might be swimming upward to photosynthesize, and downward to take
up nutrients. If they are all doing the same thing, then they can form dense
layers that are visible from the surface (like today). It's possible that
the total amount of nutrients in the red-tide organisms is higher than the
nutrients that were in the water before the bloom. This means that the cells
must have moved relative to the water, in order to accumulate nutrients.

Why are the cells bioluminescent?
Two thoughts on this (Mike Latz is the expert...): One is that the organisms
eating the red-tide cells don't like lights flashing in their face, and so
don't graze on the flashing cells. The other thought is that the cell's
flashing acts like a burglar alarm: an organism eating a red-tide cell
causes it to flash, attracting the notice of a visual predator (the police)
who then eats the grazer (burglar). Swift justice.

Why don't you so-called experts know anything about where the red tides come
It's embarrassing, isn't it? To really know why we have a red tide off the
Scripps Pier right now, we would have had to do dense and continuous
sampling all up and down the Southern California Bight for several months
prior to the bloom. We would have had to measure the growth rates of the
red-tide organisms, their swimming behaviors, and all their sources of
mortality. Furthermore (and this is the easy part), we would have had to
measure all the water velocities, since the water motions move the blooms
around. It's quite likely that our present bloom is the same one that was
off Huntington beach a couple of weeks ago. On the upside, there are plans
afoot to put observation networks in place all along the US coast, that
would help generate data that might give us some clues about how and why
these blooms form. Get back to me in about 15-20 years. From a scientific
point of view, these red tides are a wonderful examples of
physical-biological interactions leading to a massive perturbation of the
marine ecosystem. I think that if we could understand why they occur, we'd
learn a lot about how the planktonic ecosystem works the rest of the time.

And what the heck was that foam on the beach?
Probably degradation products and exudates from a different phytoplankton
bloom (maybe the "green tide" of August). According to Lihini Aluwihare, the
foam was mostly protein (a lot like egg whites). I'll defer all foam
questions to her.

Is the water toxic?
No. You'll die from salt water before any phytoplankton toxins kill you.

Then why do I get skin rashes/ear/nose/throat infections/and-or eye
Don't know. Try bathing more.

Al the best,

Your red tide guy,
Peter Franks

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