Chapter 6
The zoology of zooplankton
For some practititioners of biological oceanography, zooplankton are simply “Z”, the
grazers on phytoplankton in pelagic ecosystem models. This quantity Z is varied as
necessary to supply a loss term to reduce the spring bloom or balance the growth of
phytoplankton. Real zooplankton do the same things, but we are severely limited in
determining their rates of activity both individually and on average. On the other
hand, they are structurally, developmentally, and behaviorally elaborate, and we have
a wealth of detailed information about them. Some of that knowledge is sketched here
and in Chapters 7 to 10. The zoological range of the studies is wide, since marine
zooplankton include a number of protozoan groups and members of most of the phyla,
from Cnidaria and Platyhelminthes to Chordata.
(^) Zooplankton are free-swimming animals that live in oceans and lakes. Most are
small, a few centimeters or less, but some jellyfish and pyrosomes are a meter across
and several meters long. Again, the Greek word πλαγκτoς, which is the source of the
term “plankton” introduced by Viktor Hensen, means “wandering or drifting”. Thus, it
implies relative passivity. In using the term, we separate the weaker swimmers from
more active forms, nekton (νεκoν), that swim with sufficient strength to maintain their
geographical positions or to travel at will despite ocean currents. Fish, porpoises, and
squid are examples of nekton. This definition of zooplankton is not, however, an
operational definition. That is, it contains no rule for deciding whether a given animal
captured at sea is planktonic or nektonic. An operational definition is simply that
animals caught in plankton nets are zooplankton. Planktologists distinguish between
holoplankton that live in the water column through their entire life cycle, and
meroplankton that are planktonic larvae. Meroplankton either grow into swimming
capability qualifying them as nekton or settle to the seafloor becoming benthos.
Collection
(^) The basic method of capturing marine (lacustrine, too) zooplankton is necessarily
filtration (Box 6.1) (Wiebe & Benfield 2003). There are two usual approaches: nets
(cones of mesh towed through the water), and pumps that deliver water to the deck of
a ship for filtration aboard. Generally we use industrial filter-cloth woven from
monofilament nylon melted together at the crossings of warp and woof. The holes are