The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 348

Africa which supports large fish catches based mainly on plankton-eating fish.
Upwelling and enhanced primary production can also be induced by along-slope
flow, though the evidence here is far more limited. Perhaps rather surprisingly,
downwelling may, in some cases, be favourable to plankton growth. Both
upwelling and downwelling can result in a transport of fixed carbon from the
shelf edge region out into the deeper waters of the open ocean, and it is possible
that the generation of organic material in upwelling regions acts as a source of
organic nutrients to the nutrient-impoverished oligotrophic gyres. We also saw
that the thermocline in the outer shelf waters often plays a role in the export of
carbon, by trapping DIC remineralised from sinking organic material. The rela-
tive timing between the processes driving cross-shelf edge transports of the deeper
shelf water and the seasonal breakdown of shelf stratification is important, as
winter mixing on the shelf will re-establish contact between the high DIC water
and the atmosphere.
As the internal tide and shorter internal waves propagate towards and onto
the shelf, they induce vertical mixing which broadens the thermocline, cools the
surface water and promotes the nutrient supply to support enhanced plankton
growth over the slope and the adjacent shelf. While there is often a focus on
this increased production at the shelf edge, the case study of the Celtic Sea
illustrates how significant contrasts in the species structure of the phytoplankton
community are set up across the shelf edge and correlate with the horizontal
differences in vertical supply of nitrate. The existence of large-celled phyto-
plankton species at the shelf edge is not a response solely to the observed
enhanced supply of nutrients, but is also conjectured to require decoupling
between the phytoplankton and their grazers. The very distinct contrasts across
the shelf edge may make the region an ideal study site for assessing the links
leading to phytoplankton community biodiversity. The importance of the shelf
edge for commercial fisheries may be linked to this shift in community structure,
as might the often highly diverse benthic ecosystems found on the continental
slope.
The water at the outer edges of temperate and high latitude shelf regions can be
prone to substantial cooling in winter, increasing its potential to absorb atmospheric
CO 2 and also making it sensitive to density-driven cascades down the continental
slope. Fluxes of inorganic and organic carbon in such flows represent an important
export of carbon, and high sedimentation rates within canyons make them sites of
long-term burial of POC.
It is perhaps remarkable how a narrow band around the edges of the ocean
basins acts as such a fundamental control on the productivity of the shelf seas and
on the export of carbon to the deep sea, This narrow band is also a region of
marked horizontal gradients in ecosystem structures. Understanding, and cor-
rectly simulating, the physics of the shelf edge is vital because of the region's role
in controlling crucial biogeochemical fluxes and supporting large commercial
fisheries.