Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 258

(a) (b)

55 55

54 54

Muddy
seabed
Nephrops
areas

53

53

m=s -1

3

6 5 4

6 5 4 3

Longitude °W

Longitude °W

10 cm

Figure 8.19 (a) The distribution of muddy seabed within the Irish Sea, along with the strongly
correlated distribution of Nephrops. After White et al., 1988 , courtesy Oxford University Press.
(b) Amplitude of the M 2 tidal current speed (m s 1 ) within the Irish Sea (model data provided
by Sarah Wakelin, National Oceanography Centre, UK), showing the lower current speeds
associated with the muddy regions. The photograph is of an adult Nephrops. (Photo by Inigo
Martinez, Marine Scotland, Aberdeen.)

western Irish Sea. The adult Nephrops require a muddy substrate within which to
make burrows. Fig. 8.19a shows the correspondence between the Nephrops
distribution and the two main areas of muddy seabed in the Irish Sea (White et al.,
1988 ). Fig. 8.19b indicates that the muddy substrate corresponds to regions of weak
tidal currents, with maximum M 2 tidal currents less than 0.5 m s 1 ; this contrasts
markedly with the central Irish Sea where currents can reach over 0.9 m s 1 and the
seabed is predominantly sand or gravel. Nephrops larvae are free-swimming, so the
question arises as to how these isolated populations are maintained, particularly as
observations have shown that a strong southward buoyancy current along the Irish
coastline in early spring acts to transport larvae away from the adult area (White
et al., 1988 ).
The weak tidal currents that are responsible for the muddy seabed are also weak
enough to allow thermal stratification in summer. The western Irish Sea Nephrops
distribution corresponds with a thermally stratified region in the west of the generally