Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 163

plankton are actively avoiding visual detection by predators in the well-lit surface
waters during the day. Migration is also seen in the opposite sense: upwards to the
surface during the day, and downward during the night. This seems like a dubious
strategy for avoiding predators. However, zooplankton undergoing this 'reverse
migration' have been seen to switch their migration behaviour to downward during
daylight in response to an increase in predators (Frost and Bollens, 1992 ). Other
reasons for a migration strategy have been proposed (e.g. see the summary in
Lampert, 1989 ), including taking advantage of different metabolic rates in warm
surface water and deep colder water. Thinking of the physical environment within
which migrations take place, we might also suggest that the vertical shear between
surface and deeper currents would lead to the zooplankton sampling a different body
of surface water at each upward migration. As zooplankton horizontal distributions
tend to be patchier than those of the phytoplankton (Abraham, 1998 ) this would
increase the food supply to the zooplankton by giving them access to phytoplankton
communities that have potentially received lower recent grazing.
An issue to which we will return to in Chapters 6 and 9 is that many large
marine organisms begin their life in larval stages that cannot control their hori-
zontal position in the ocean and so are at the mercy of ocean currents. However,
the interaction between vertical position and shear in mean flows can be utilised to
achieve net movement. Vertical movement of meroplankton can be ontogenic,
meaning that their position within the water column depends on their age or
stage. We will see some examples later where this ontogenic shift in position
within depth-varying mean flows can be a vital component in the survival of
populations.
We have already described the importance of new primary production in the
carbon cycle. All mesozooplankton play a very important role in the biological
pump of carbon from surface waters to the deep ocean and to shelf sediments
(e.g. Wassmann, 1998 ). The mesozooplankton eat relatively small food particles,
but they concentrate the waste products into faecal pellets that have much faster
sinking speeds than the original prey items. Rapid sinking helps particles reach the
seabed and be buried (sequestered) before microbial activity can recycle the organic
material. Faecal pellets are also an important food source for heterotrophic bacteria,
whose main foraging strategy involves waiting in the water column to be collected by
a passing pellet.
The other role for the mesozooplankton, especially important in shelf seas, is
the support of fish, including commercial fish stocks. As an example, consider
the analysis of the diet of juvenile mackerel found at the shelf edge of the Celtic
Sea (Conway et al., 1999 ), shown in Fig. 5.16 . The gut contents show a progres-
sion from feeding on phytoplankton for larvae of lengths

5 mm, to ingestion of
copepod eggs for larval lengths between 4 and 7 mm, and consistent ingestion of
copepod nauplii for larval lengths

<

4 mm. The availability of suitable phyto-
plankton for the early larval stages, followed by a population of reproducing
copepods, is required to sustain the larvae towards recruitment to the adult stock.
The semi-digested remains of the phytoplankton inside the larvae were not

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