PARTICULATE ORGANIC MATTER I N SEA WATER
27
mechanism which concentrates phytoplankton as well as non-living
particles into convective downdrafts, and moreover since zooplankton
appears to congregate in these areas, the real environment of zooplankton with respect to its food supply is very different from the impression
that we get with our ordinary crude sampling devices.
We can expect that the usual zooplankton tow will cut across a
number of lines of convergence and will provide a realistic estimate of
average zooplankton abundance, although it tells us nothing about
microdistribution. Phytoplankton samples are commonly taken from
a string of water bottles, They are almost never taken in sufficient
abundance to give a true picture of micro-distribution, and by the law
of averages most of the samples will be taken in the broad areas between
convergences rather than in the convergences themselves. Neither type
of observation is realistic with respect to phytoplankton-zooplankton
relations as they exist in nature, but one suspects that the true abundance of zooplankton food materials has been seriously underrated.
This should give some comfort to the investigators, too numerous
to cite by reference, who have examined zooplankton grazing rates and
food requirements in the laboratory and found that the estimated
requirements often exceeded the amount of food that appeared to be
available in nature. One can hardly deny nature, and there seemed to
be some flaw in the experiments. Now it seems likely that the flaw
may be in our observations of nature-that food may be more abundant
than we realized in the micro-habitats where it is being consumed.
What has been said about horizontal micro-distribution in the surface layer applies equally to horizontal layering in underlying thermoclines. There is increasing evidence (Stommel and Federov, 1967 ;
Lovett, 1968; Tait and Howe, 1968) that thermoclines are not smooth
gradients but rather are laminar structures in which thin, almost
homogeneous layers alternate with transitional zones of steep temperature gradient. Theoretically, sinking particles will accumulate in any
gradient strong enough to cause a significant reduction in sinking speed.
Many writers have described accumulations of phytoplankton in
thermoclines, although seldom in sufficient detail to reveal fine structure. Biggs and Wetzel (1968) found concentrations of particulate
carbohydrate, presumably mostly non-living particles, in discontinuity
layers. Limbaugh and Rechnitzer (1955) reported that this layering
can be sufficiently dense to be detected visually when diving. Moreover, Harder (1 968) demonstrated experimentally that copepods and
some other marine plankton organisms tend to congregate at discontinuity layers.
Thus there is abundant qualitative evidence that physical processes
27
mechanism which concentrates phytoplankton as well as non-living
particles into convective downdrafts, and moreover since zooplankton
appears to congregate in these areas, the real environment of zooplankton with respect to its food supply is very different from the impression
that we get with our ordinary crude sampling devices.
We can expect that the usual zooplankton tow will cut across a
number of lines of convergence and will provide a realistic estimate of
average zooplankton abundance, although it tells us nothing about
microdistribution. Phytoplankton samples are commonly taken from
a string of water bottles, They are almost never taken in sufficient
abundance to give a true picture of micro-distribution, and by the law
of averages most of the samples will be taken in the broad areas between
convergences rather than in the convergences themselves. Neither type
of observation is realistic with respect to phytoplankton-zooplankton
relations as they exist in nature, but one suspects that the true abundance of zooplankton food materials has been seriously underrated.
This should give some comfort to the investigators, too numerous
to cite by reference, who have examined zooplankton grazing rates and
food requirements in the laboratory and found that the estimated
requirements often exceeded the amount of food that appeared to be
available in nature. One can hardly deny nature, and there seemed to
be some flaw in the experiments. Now it seems likely that the flaw
may be in our observations of nature-that food may be more abundant
than we realized in the micro-habitats where it is being consumed.
What has been said about horizontal micro-distribution in the surface layer applies equally to horizontal layering in underlying thermoclines. There is increasing evidence (Stommel and Federov, 1967 ;
Lovett, 1968; Tait and Howe, 1968) that thermoclines are not smooth
gradients but rather are laminar structures in which thin, almost
homogeneous layers alternate with transitional zones of steep temperature gradient. Theoretically, sinking particles will accumulate in any
gradient strong enough to cause a significant reduction in sinking speed.
Many writers have described accumulations of phytoplankton in
thermoclines, although seldom in sufficient detail to reveal fine structure. Biggs and Wetzel (1968) found concentrations of particulate
carbohydrate, presumably mostly non-living particles, in discontinuity
layers. Limbaugh and Rechnitzer (1955) reported that this layering
can be sufficiently dense to be detected visually when diving. Moreover, Harder (1 968) demonstrated experimentally that copepods and
some other marine plankton organisms tend to congregate at discontinuity layers.
Thus there is abundant qualitative evidence that physical processes