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Figure 8.18 Distributions of the
major zooplankton taxa across
the tidal mixing front in the
western Irish Sea, based on
data in (Scrope-Howe and
Jones, 1985 ). (a) The three
most abundant taxa,
illustrating a common trend of
highest numbers in the
stratified water, decreasing
across the front into the
mixed water. (b) The only
three of the reported taxa that
demonstrated an enhancement
at the front. Note the
different scales on the y-axes.
(a)
8
Pseudocalanus elongatus
Acartia clausi
Calanus finmarchicus
6
4
2
0
0
Stratified
Front
Mixed
(b)
3
Polychaete larvae
Echinoderm larvae
Gastropod larvae
2
1
0
Stratified
Front
Mixed
The lack of apparent zooplankton enhancement at a tidal mixing front is perhaps a
little surprising; with such a strong response of the primary production and primary
biomass to the front, we would expect the zooplankton to take advantage of an
enhanced food supply. Is this lack of response a general property or does it apply to
some zooplankton and not to others? To answer this question we can look at regions
where the distributions of zooplankton species have been assessed in more detail.
Fig. 8.18 summarises some of the data from a study of the seasonal changes in
zooplankton across the western Irish Sea front (Scrope-Howe and Jones, 1985 ).
There it was found that there was almost no consistent localised response of the
zooplankton. In Fig. 8.18a you can see that concentrations at the front for most
species were intermediate between those in stratified and mixed water. However, this
well-resolved taxonomic study did show a small number of species to have marked
increases in abundance at the front, for instance polychaete larvae, gastropod larvae,
and the free-swimming stage of echinoderms ( Fig. 8.18b ). It is perhaps noteworthy
that these are all larvae of benthic organisms. Similarly, observations have shown
that some taxa of zooplankton exhibit peaks in abundance at the front on the north
flank of Georges Bank (Wishner et al., 2006 ), while for most others the frontal jet acts
simply as a boundary between taxa (Butman et al., 1987 ; Meise and Oreilly, 1996 ).
There is a good deal of evidence supporting a link between chlorophyll concen-
tration (as a food supply) and the rate of copepod egg production (Uye and Shibuno,
1992 ; Runge et al., 2006 ), so despite the lack of a clear signal in zooplankton biomass,
we might expect tidal fronts to be sites of higher zooplankton production.
For copepods, such a frontal increase in egg production has been observed in the
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