Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 162

0.03

0.02

0.01

0.0

-0.01

-0.02

196 197 198 199 200 201

Yearday 1998

Figure 5.15 Vertical swimming velocities of zooplankton population measured using an
upward-looking ADCP on the Malin shelf west of Scotland. The dashed boxes indicate the
hours of darkness. From Rippeth and Simpson, 1998 , with permission from ASLO.

could out-compete the plankton that rely solely on autotrophy or heterotrophy
(Katechakis and Stibor, 2006 ).

Mesozooplankton (size ∼ 100 m m - a few mm)
The mesozooplankton are multicellular animals that feed on the microzooplankton
and on phytoplankton. Mesozooplankton can be either holoplanktonic, meaning
that their entire life is spent in the plankton, or meroplanktonic, which means
spending only the early part of their life as plankton. Holoplankton include the
copepods ( Fig. 5.13d ), krill, ostracods (a small crustacean), chaetognaths (predatory
worms), tunicates (or sea squirts, including the salps) and amphipods ( Fig. 5.13e ).
The meroplankton are the larvae of larger marine organisms, particularly fish but
also crustaceans and other benthic organisms. We could also include the young of the
larger gelatinous plankton (e.g. jellyfish and larger tunicates) which often target the
same food supply as the mesozooplankton.
Many of the mesozooplankton are capable of maintaining a sustained swimming
speed that is able to compete successfully with turbulent velocities in shelf seas. For
instance, a copepod might be able to sustain speed of 10 m hr 1 , which if used in
Equation (5.13) suggests that the copepod should be able to maintain directed
swimming in turbulence as strong as K z

10 2 m 2 s 1 ; mixing at this rate is
generated close to the seabed during strong tidal flows, but is rarely seen in the
interior of the shelf seawater column. A common feature of mesozooplankton
swimming behaviour is vertical migration with a daily periodicity. The most common
migration is that of nightly excursions into the surface waters, with rapid downward
migration to deeper waters as dawn approaches. An example is shown in Fig. 5.15
based on ADCP measurements of vertical velocity on the Malin shelf west of
Scotland. This vertical migration is often viewed as an anti-predation strategy: the

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