Geoscience Reference
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motility, cell size, and cell light requirements all play a role in the competition for
resources, the balance between growth and loss rates, and the competition between
different species in phytoplankton layers.
7.4
Zooplankton and larger animals at the SCM
...................................................................................
We can re-visit the Pe´ clet number and estimate the swimming speed needed to
overcome the turbulence in a tidally mixed bottom layer, and see if we might expect
larger motile organisms to be able to control their position vertically. Taking
the bottom layer of the Celtic Sea to be 80 metres thick, and the turbulent diffusivity
to be 10
1
m
2
s
1
, we can calculate a necessary sustained swimming speed to be
swimming speed to the size of an organism, a speed of
>
1mms
1
sets a lower size limit
for a migrating organism of about 0.2 mm. This is much larger than any motile
phytoplankton, but it is about the lower limit of the size range associated with
mesozooplankton. So, we can expect anything bigger than an ostracod (including all
copepods) to be able to swim vertically through a tidally energetic bottommixed layer.
The SCM is a site of relatively high concentrations of food for grazing zooplankton,
particularly since it is where most of the new production occurs in summer and contains
larger phytoplankton cells than the recycling microbial community in the surface layer.
So the ability to swim vertically and find the SCM could be a useful trait. Do we see
zooplankton migrating into the SCM? Certainly in our own work we have regularly
observed signals in the backscatter from ADCPs showing the diurnal migration of
scatterers, and often the apparent targeting of the SCM during the night. A detailed
set of temperature, optical and acoustic observations from Monterey Bay, California,
has identified night time layers of zooplankton associated with layers of phytoplankton
(Holliday et al., 2010).
Fig. 7.14
shows an example, with an acoustic signal attributable
to small copepods forming a night time layer and tracking the thermocline. Note in
particular the scatterer layer rising from the seabed at dusk, and how it tracks the
deepening thermocline just after midnight. The high vertical resolution (a few cm) of
these observations showed that the zooplankton did not always simply track layers of
high phytoplankton biomass; food quality, as well as quantity, played a role in deter-
mining where zooplankton layers formed.
The acoustic signals recorded in the time series of
Fig. 7.14
were also analysed at
very high spatial and time resolution, showing individual acoustic targets identified
as small pelagic fish also following the thermocline. This supports an observation by
the fisheries scientist Reuben Lasker, who identified the requirement of stable layers
of dinoflagellates for successful first feeding of anchovy larvae off the California
SCM and the distribution and foraging of larger predators, including seabirds and
marine mammals, has also been seen, for instance in the North Sea (
Scott et al.
,
2010)
. It is clearly tempting to draw a simple connection between trophic levels,
beginning with higher phytoplankton numbers and working through to the top
>
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