Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 257

mechanisms may contribute to the full explanation of stock development, with the
balance between them being dependent on species and environment.
At this stage we should note that there seems to be something of a paradox here.
We have clear signals of primary production and phytoplankton responding to tidal
mixing fronts, and at the other end of the food chain we have evidence of larger
marine animals targeting the fronts. The evidence for the link provided by the
zooplankton within this chain seems to be rather patchy. There are several possible
reasons why a frontal response in zooplankton numbers may either be absent or, if it
does occur, be difficult to observe. Zooplankton distributions are generally far
patchier than phytoplankton, which makes observation of cross-frontal contrasts
challenging; with limited sampling the patchiness is likely to dominate over any
consistent horizontal distribution. Also, zooplankton respond to their environment
at a far slower rate than the phytoplankton. The time between an increase in food
supply, through to egg laying, successful hatching and maturation could be several
days to a few weeks, compared to the relatively immediate response (a few days) of
phytoplankton to increases in light or nutrients. This time lag will, to varying extents,
de-couple zooplankton distributions from their physico-chemical environment
(Abraham, 1998 ). For instance, we might expect any signal in frontal copepod
production to gradually decrease from eggs, through to nauplii, through to adult
copepod stages due to physical dispersion and the developmental time between egg
and adult copepod, a pattern supported by observations off northeast Scotland
(Kiørboe and Johansen, 1986 ; Kiørboe et al., 1988 ). Other factors that need to be
considered are the limited cross-frontal transports (Perry et al., 1993 ), and the impact
of different predation pressures affecting zooplankton on the mixed and stratified
sides of the front (Meise and Oreilly, 1996 ). The examples of the basking sharks (Sims
and Quayle, 1998 ), the reef fish (Kingsford et al., 1991 ) and the seabirds (Durazo
et al., 1998 ) indicate the importance of accumulations of zooplankton at frontal
convergences; these convergences can be very localized so that single 'frontal' sites
chosen for study on the basis of the chlorophyll distribution could miss a region of
higher zooplankton abundance. Detecting any frontal signatures in zooplankton
requires many samples with fine horizontal resolution and with taxonomic resolution
that can separate species and stages of species.

8.6.4 Fronts and fisheries

We conclude our discussion of the links between the physics and biology of fronts by
looking in some detail at two examples of fisheries being supported by frontal
systems - one where the front acts as a barrier to dispersion of larvae and another
where both advection and retention associated with a tidal mixing front play roles in
the recruitment of larvae to the adult stock.

(i) Nephrops norvegicus and the western Irish Sea gyre

Nephrops norvegicus (see photograph in Fig. 8.19 ), also known locally as the Dublin
Bay prawn, is a demersal organism of considerable commercial importance in the