Regarding zooplankton

Debbie MacKenzie debimack at
Wed Feb 20 11:43:10 EST 2002

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:

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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