Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 157

through the water column. There is alsoanimportantcontrastinthefateof
sinking POC between the margins of the ocean and the open ocean. Slowly sinking
down into the deep ocean, POC has plenty of time to be recycled back to DIC
before it reaches the seabed. As a result, relatively little particulate carbon reaches
the bed sediments, and the deep ocean is an enormous reservoir for DIC which is
locked away from the atmosphere for the time scales of ocean circulation and
overturning (decades to centuries). The proximity of the seabed of the shelf and
continental slope to the photic zone makes the margins of the oceans important
regions for longer-term particulate carbon export. Sinking POC has less time to be
recycled, and so organic carbon has more chance of reaching the sediments than in
theopenocean.Inareaswithhighsedimentationrates(e.g.canyons,towhichwe
will return in Chapter 10 ) or where bacterial processes are limited by low dissolved
oxygen, the carbon has the potential to be buried in the seabed sediments and
locked away from the atmosphere for millions of years.
Shelf seas and carbon export
The export of fixed POC is called the biological pump. We would expect the exported
carbon to be the same as the amount of carbon taken from the atmosphere, so
quantifying the biological pump is a vital part of understanding the fate of carbon
within the climate system. Shelf seas play a disproportionately large role in global
export production. The average global export ratio for carbon is estimated to be about
20% (Laws et al., 2000 ). However, export ratios in shelf regions could be larger. For
instance, the proximity of riverine sources of minerals that can either ballast POC
(making it sink faster) or protect POC from bacterial degradation as it sinks, would
raise export ratios (Dunne et al., 2007 ). Also, temperature effects on autotrophic and
heterotrophic growth rates suggest the mid to high latitude shelves can have export
ratios of over 60% (Laws et al., 2000 ). Taking a reasonable increase of the continental
shelf export ratio of a factor of 3 above the oceanic mean, POC export from the ocean
margins could account for 44% of the global total (Jahnke, 2010 ).
In the open ocean the concept of new production is tied to a supply of new
nitrogen, i.e. nitrogen as nitrate transported upward to the photic zone from below
the winter surface mixed layer. The nitrate is described as 'new' because its residence
time in these deep waters is of the order of decades to centuries. In shelf seas this
concept of new nitrate is subject to some ambiguity. Close to the shelf edge the nitrate
may well be sourced from deeper in the ocean. However, particularly in regions of
wide continental shelf, the weak residual flows in many shelf seas may mean that
nitrate mixed upward into surface waters may have been remineralised from organic
matter only a few months previously. It is then arguable whether the nitrogen is 'new'
or 'recycled'. Generally we stick with the original spirit of the Eppley-Peterson
definition and use the f-ratio to describe the proportion of primary production that
uses nitrate.
The proximity of the deep water and seabed recycling communities to the photic
zone is important in shelf seas. It means that recycled organic material is always
within reach of the sea surface, particularly in temperate and high latitude shelf