Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 254

Stratified Front Mixed

Zooplankton > 200

m

μ

< 200 μ m

Protozoa

Bacteria

Surface

Phytoplankton

> 200 μ m
< 200 μ m

Zooplankton

Protozoa

Bacteria

Deep

Phytoplankton

0 50 100

% Plankton carbon

Figure 8.17 The distribution of organic carbon amongst plankton groups for surface (0-24 m)
and deep (24-60 metres) waters in vertically mixed, frontal and stratified waters of the
western English Channel. Adapted from Holligan et al., 1984 , courtesy of Inter Research
Journals.

Calculations for typical fronts suggest that increased vertical mixing and the
spring-neap adjustment could together supply 80% of the phytoplankton nitrate
requirements, with the remaining 20% coming from eddy transfers and the transverse
circulation (Loder and Platt, 1985 ) . However, the details are likely to vary for
different fronts. For instance, the front on Georges Bank may be fuelled by signifi-
cant amounts of nitrate supplied by horizontal advection (Horne et al., 1996 ).
Modelling studies for Georges Bank indicate that much of the nutrient supply to
the surface frontal waters is driven by short but intense mixing events during each
semi-diurnal tidal cycle (Franks and Chen, 1996 ), with the steep slopes of the bank
generating on-bank residual flows that will transport bottom layer nutrients to the
front (Chen and Beardsley, 1998 ).

8.6.2 Zooplankton and tidal mixing fronts

Given such a clear increase in phytoplankton production and biomass within the
surface waters of a front, we might reasonably ask if there is evidence of further
impacts on the frontal ecosystem at higher trophic levels. Fig. 8.17 shows the average
plankton distributions at a tidal mixing front in the western English Channel,
indicating that the increase in phytoplankton carbon within the front emerges as
the clearest signal (Holligan et al., 1984 ). Phytoplankton represent 90% of total
plankton carbon in the surface layer of the frontal zone, compared to 70% in the
mixed water and 25% in the surface stratified water. There does not, however, appear
to be any corresponding frontal response in the zooplankton biomass in Fig. 8.17 .