The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 338

bottom boundary layer driving a flux of deep ocean nitrate toward the shelf. Elevated
diapycnal mixing at the shelf edge front then mixes the nutrients upward to the photic
zone (Coachman and Walsh, 1981 ). These eddy fluxes of nutrients act down the
nutrient gradient, which in the case of the Bering Sea is particularly strong due to
iron-limitation of adjacent oceanic primary production resulting in High Nutrient
Low Chlorophyll (HNLC) conditions. Advection of bottom shelf water within the
bottom boundary layer of the along-shelf edge currents plays an important role here
by supplying much of the iron required by the shelf edge primary production
(Aguilar-Islas et al., 2007 ). The mechanism responsible for increasing the diapycnal
mixing over the shelf edge, mixing both the macronutrients and the iron upward at
the shelf edge, has not yet been identified.

10.8 Internal tides, mixing and shelf edge ecosystems: the Celtic

Sea shelf edge
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We next focus on the effects of the internal tide on shelf edge biology, using the
Celtic Sea as a case study. Studies of the internal tide in this region began with
the work of Pingree, Mardell and Holligan in the early 1980s, with the early
recognition of the role that the breaking internal waves had on the nutrient
and chlorophyll distributions (Pingree and Mardell, 1981 ; Holligan et al., 1985 ).
Subsequently our own detailed measurements of the physical and chemical envir-
onment and the phytoplankton community response have shown how the across-
shelf edge gradients in vertical turbulent mixing are pivotal in supporting a distinct
ecosystem.

10.8.1 Nutrient supply and primary production at the shelf edge

The impact of the internal tide on the shelf edge biogeochemistry has already been
anticipated in Fig. 10.16 , where we saw a marked correspondence between the region
of cool water mixed upward by the breaking internal tide and a band of increased
concentrations of sea surface chlorophyll. Figure 10.24 shows that this is a regional
phenomenon, with the band of elevated shelf edge chlorophyll reaching from north-
ern Biscay, round the west of Ireland, and up to the north of Scotland - a distance of
about 1500 km. This consistent pattern of shelf edge chlorophyll is strikingly similar
to the 'Green Belt' of the Bering Sea. The mixing of cool water upwards at the Celtic
Sea shelf edge during summer stratification is expected to be associated with a flux of
nutrients, and the band of cool water has been observed with locally increased
concentrations of surface nitrate of 0.5 mmol m 3 (Pingree and Mardell, 1981 ).

This concentration of nitrate is small compared to the concentrations we might
expect below the pycnocline in a summer stratified shelf region, but there is an
important contrast with the surface layer away from the shelf edge where nitrate is
generally undetectable using the usual analysis techniques used for shelf sea work (i.e.
<

∼

0.05 mmol m 3 ).