The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 329

10.5 The biogeochemical and ecological importance

of the shelf edge
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We have seen that the relatively steep seabed slope at the shelf edge is fundamental to
limiting the transfers of water and its constituents between the shelf seas and the open
ocean. Along-slope flows are far stronger than cross-slope transfers. This has meant
that, to some extent, it has been possible to treat the shelf and the deep ocean
separately. However, while the cross-slope physical flows are weak, the cross-slope
biogeochemical gradients can be very strong so that biogeochemical fluxes between
the open ocean and the shelf seas can be substantial. The high rates of primary
production in shelf seas require adequate supplies of the macronutrients nitrate,
phosphate and (for the diatoms) silicate (see Section 5.1.6 ). We saw in Chapter 9
that agricultural and wastewater runoff into rivers can supply large quantities of
nutrients, particularly nitrate, but their effects are seen predominantly close to the
coast. Taking the shelf seas as a whole, biogeochemical budgets suggest that 80-90%
of the nitrogen and 50-60% of the phosphate that is required by shelf sea export
production are supplied to the shelf seas from the deep ocean (Liu et al., 2010 ). Silica,
in contrast to nitrogen and phosphorus, is mainly supplied through the weathering of
rocks on land and subsequent transfer down rivers into the shelf seas (Treguer et al.,
1995 ); processes at the shelf edge, therefore, control the flux of silicate from the shelf
seas to the open ocean (Liu et al., 2010 ). As well as being the gateway controlling the
fluxes of nutrients between the shelf seas and the open ocean, the shelf edge is often a
site of increased biological production which is implicated in supporting distinct
ecosystems that contrast with both the open ocean and the shelf seas.

10.6 Upwelling, nutrient supply and enhanced biological production

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The upwelling of deep oceanic water by persistent alongshore winds (see Sections
10.3.1 and 3.5.3 ), and the resulting supply of nutrients and enhancement of coastal
primary production is probably the most studied aspect of the biological impacts of
physical processes along the edges of the continental shelves. Along the narrow
shelves of Peru, northwest North America, northwest and southwest Africa, this
upwelling supplies nutrients across the continental shelf edge into the shallow coastal
water, leading to enhanced primary production. These archetypal, wind-driven
upwelling regions are well represented in the literature (see our suggestions
for further reading at the end of this chapter). Here we will briefly address the
biological consequences of wind-driven upwelling, focusing on the time scales of
responses of different components of the ecosystem to changes in the alongshore
winds. We will also consider how nutrients can be supplied by upwelling which is
driven through the bottom Ekman layer of along-slope flows (see Section 10.3.2 ),
particularly in regions where a western boundary current is forced to flow along a
continental slope.