Regarding zooplankton

[Debbie MacKenzie]

Dear coral-list,

Further to our discussion last year on the possibility of food shortage
as
a cause of mass coral bleaching and coral epidemics, I remain very
concerned that the declining abundance of oceanic zooplankton is an
important variable in the picture. I raised this question earlier - "is
tropical zooplankton declining also?" - and did not get much feedback
beyond a few offlist comments to the effect that the declines noted
elsewhere (e.g. off California) have been the result of decreased
vertical
mixing in ocean areas where mixing is more pronounced and so has a
greater
effect on productivity. Therefore, if zooplankton productivity is felt
to
be directly related to mixing, then I suppose a decline in the
not-very-mixed tropics would not be anticipated(?) However, zooplankton
is
declining markedly in the seemingly-normally-mixed North Atlantic - in
fact
there's lots of phytoplankton to indicate that mixing is occurring, but
zooplankton is declining nevertheless. I suspect that the decline is
related to the loss of fish, and fish spawn, which contributes
considerable
amounts of energy and nutrients to the plankton. What do you think of
this
idea? I find it intriguing, for instance, that the giant bluefin tuna
produces some of the smallest fish eggs in the sea, and that this fish
fattens itself in the temperate seas and then migrates to the tropics to
spawn millions of little food packages small enough to be consumed by a
coral polyp...

And of course, the reduction in the spawning stock of bluefin tuna has
been
considerable. I would appreciate any of your comments on this article:
http://www.fisherycrisis.com/fisheggs.html

sincerely,
Debbie MacKenzie

Abstract of article:

Declining numbers of oceanic zooplankton cannot be entirely accounted
for
by the climate-warming-induced slacking of upwelling currents. For
example,
the North Atlantic ocean offers a picture today of unusually low
zooplankton numbers in the presence of unusually high phytoplankton
numbers, a finding which runs directly contrary to expectations. The
life
histories of common zooplankton organisms (e.g. copepods) reveal that
while
juvenile stages are herbivorous, adult stages are not. They are
frequently
omnivores or carnivores. The reproduction of herbivorous zooplankton
therefore is not solely related to the availability of plant food. The
abundance and quality of carnivorous food available to the adult stages
also impacts directly on the successful reproduction of the herbivores.

Pelagic spawn released by marine fish and invertebrates is accessible
food
to these carnivorous zooplankton. The massive reduction in "spawning
stocks" of multiple marine species that has been induced by human
fishing
is therefore plausibly implicated in the current declining numbers of
zooplankton. Spawn production in the ocean has obviously declined with
the
elimination of spawning fish, and this gradual withdrawl of a rich
carnivorous food supply to the zooplankton may have negatively affected
their ability to reproduce. It also follows that total new organic
production (and carbon fixation) in the marine system will have been
reduced. The widespread declining trend in age and size at maturity in
marine fish and invertebrate species possibly represents a systemic
reaction to the falling density of zooplankton, in the form of an
increased
effort to produce spawn in compensation.

If fishing has "undercut" the zooplankton in this manner, and it seems
entirely plausible that it has, then one "unintended, unanticipated
negative consequence" of continued fish removals will be the collapse of
virtually the entire marine ecosystem. Continuing to focus our
explanatory
efforts solely on temperature changes and "global warming," despite
remarkable inconsistencies between predicted and observed effects,
merely
continues a long-ingrained human thought pattern of denial...denial of
the
full impact of our ruthless destruction of marine life...and perhaps
denial
that many of our earlier theories about how the ecosystem functions are
now
proving to be, to a rather significant extent, incorrect. For example,
the
concept of energy flowing in only a single direction in marine systems,
upwards through successively higher trophic levels, is challenged by
consideration of the significant quantity of energy that is actually
channelled directly downwards in the form of marine spawn. Although it
inevitably dissipates, energy can be seen to follow many circular routes
within the ecosystem, cycling along with nutrients. In this way, animate
marine life in total acts as a catalyst for the continuation of high
rates
or primary production...a greater abundance fish in the sea, therefore,
ultimately results in a higher rate of marine carbon fixation.
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