question nitrates phosphates

Debbie MacKenzie debimack at
Tue Jan 28 18:20:45 EST 2003

Hello Dianne, coral-list,

At 06:42 PM 25/01/2003 -0400, you wrote:
>Anita at Noaa suggested a post this for consideration.
>I live on a small island in the Grenadines in the Caribbean.  We are =
>seeing long black stringy algae covering reefs and choking it out.  =
>Hoping the answer was in water testing we bought LaMotte phosphate and =
>nitrate test kits....our results are not as expected

I'm curious: what did you expect to find (and why?) and what did you
actually find? Were you expecting to find elevated nutrient levels?
is my first guess, since it is routinely thought to be an important
of increased algae growth these days.)

I can't help you in the specifics of discussing exactly what is growing
your reefs, but am interested in your observations.

The increasing dominance of marine algae, especially fine or filamentous
forms, is a very broad pattern that seems to be occurring in marine
environments virtually everywhere now. I am not convinced (although many
are) that this change is basically a reflection of an increasing
availability of plant nutrients in coastal waters. (Besides increasing
nutrient availability, declining populations of herbivores are also
sometimes suspected, and doubtless this plays a role...but it is the
of changing nutrient dynamics on the algae that interests me the most.)

The pattern of declining populations of marine invertebrates and their
replacement by algal growths is also strongly evident in temperate
In my area, Atlantic Canada, we have no shallow water coral reefs but
has been a marked decline in coastal sessile invertebrates (barnacles,
mussels...roughly analogous to your corals). Unpolluted areas that were
once dominated by barnacles and mussels now predominantly support
(e.g. see )  And within the
established seaweed communities there is a marked shift away from the
long-dominant, heavier perennial species toward shorter lived algae with
much finer structures. The automatic interpretation of this changing
appearance of seaweed seems to be that this new, elaborate algal growth
must have resulted from an increasing availability of plant nutrients
pollution). To some, it seems cut and dried.

But the signals from the long-established, older, heavier seaweeds as
decline strongly suggest that these plants are experiencing a lowered,
rather than an increased, availability of nutrients. Stunted growth,
lowered levels of pigmentation, lowered resistance to environmental
stressors such as light, heat and dessication...these patterns are
in all of the typical large seaweed groups in my area. And the increased
growth of filamentous algae is dramatic, indeed it also gives the
impression that it might be "choking out" the others. (I have a
of photos on my website, several galleries are linked from this page:  )

I have been puzzled at the quick conclusion that commonly seems to be
that the increase in filamentous algae has been stimulated by an
in nutrient availability. The filamentous growth style, with its
high surface area:volume ratio, gives these organisms a natural
over the thicker-fleshed algae under conditions where nutrient
is lowered.

Maybe at some level the interpretive difficulty is related to our
(subconscious?) comparison of seaweeds to terrestrial plants. It is
commonly known that applying fertilizer to a vegetable garden results in
more lush, elaborate growth of plants. So, when we witness an
elaborate growth of seaweed, is this partly why we assume that it must
the result of increased fertilization?

But when we view the vegetable garden, we are generally only seeing the
"tops" of the plants (stalks, leaves, fruits, etc, responsible for
photosynthesis and energy storage) as the "roots" (oft-filamentous
responsible for uptake of dissolved nutrients) are invisible. In
terrestrial plants, "tops" and "roots" do not respond to fertilizer in
same way. Agricultural research has shown that, when well fertilized,
root:top ratio of plants (for example, corn) declines. Relatively more
elaborate root development occurs in the fertilizer-poor plot, and roots
then make up a greater fraction of the total mass of the plant. (And
type of plasticity is not at all surprising.)

At first glance, seaweeds may well remind us of the "tops" of
plants, but what is waving in the water column is in fact analogous to
the "tops" and the "roots" of the cornfield. Viewed in this light,
proliferation of filamentous marine algae (more "root"-like types) and
decline in heavier in heavier, fleshy species (more "top"-like) should
raise the suspicion that one fundamental change that is occurring in the
oceans is a decline in the availability of plant nutrients. (One may
imagine the corals as being more essentially like "tops" than

Dianne, it may seem as if I have strayed rather far from your initial
question, but the matter of discovering the true relationship between
trends in fertilizer availability and growth changes in marine algae is
hugely important one. Thanks for raising it.

Debbie MacKenzie

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