The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 339

60°N

North
Sea

55°N

UK

>5

4

Celtic
Sea

3

50°N

2

1

mg Chl m -3

0

15°W 10°W 5°W 0° 5°E 10°E

Figure 10.24 See colour plates version . Satellite image of sea surface chlorophyll over the
NW European shelf seas, taken May 30, 2004. Isobaths are contoured in white, with the 200-
metre isobath marking the edge of the continental shelf. Image data courtesy NEODAAS,
Plymouth Marine Laboratory UK. Bathymetry data from GEBCO.

Direct measurements of the vertical turbulent transport of nitrate from the
deeper water towards the surface at the Celtic Sea shelf edge have indicated a
strong dependence on the contrasts in mixing by the internal tide over the spring-
neap cycle (Sharples et al., 2007 ). By combining turbulent dissipation measure-
ments and observations of the vertical nitrate gradient, neap tide nitrate trans-
port was found to be about 1.3 mmol m 2 d 1 , similar to the flux measured into
the base of the shelf sea thermocline away from the shelf edge (Sharples et al.,
2001b ; Rippeth et al., 2009 ). At spring tides, however, the nitrate flux was found
to increase to

9mmolm 2 d 1 , attributable to packets of high-frequency
internal waves (e.g. such as those shown in Fig. 10.13b ). Moreover, while at neap
tides the surface nitrate concentration was below detection limits, at spring
tides surface waters were found to have

∼

0.5 mmol m 3 of nitrate, suggesting
that the flux at spring tides exceeded the phytoplankton community's capacity to
assimilate it.

∼