Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 158

seas where winter convective mixing results in efficient connection between the
seabed and the sea surface. This provides one of the important contrasts with
the open ocean, and a reason for the relatively high production rates that we see
on the shelf.
In broad shelf seas, away from the edge of the continental shelf, carbon export
to the shelf sediments may only remove the carbon from access to the atmosphere
for seasonal time scales. In temperate shelf seas particularly, the connection
between the sediments and the atmosphere in winter allow DIC in the deep shelf
water or released from the sediments to achieve equilibrium with the CO 2 in the
atmosphere. Carbon arriving in the bottom layer of a shelf sea water column, or
settling on the seabed, thus may be exported only for a few months, until the next
winter period of convective mixing. Long-term export of carbon requires the
carbon to reach the deeper waters of the open ocean below the winter mixed layer
depth, or the sediments of the continental rise and slope. The physical processes
that might lead to such export of shelf primary production are the subject of
Chapter 10 .

5.2.2 Food for the heterotrophs

Heterotrophic organisms need to consume organic material in order to survive
and grow, and so they are ultimately reliant on the ability of the autotrophs to fix
carbon. Heterotrophs encompass an enormous size range, with the consumption
of organic matter being carried out by organisms ranging from bacteria to whales
(and humans). It is difficult to root our discussion in some common metabolic
process more specific than the basic need for organic fuel. However, the relative
sizes of heterotroph grazers compared to their autotrophic food particle provides
a basis for understanding the fate of the carbon fixed by the autotrophs. One
useful ecological rule is that on average a planktonic predator tends to be about
ten times larger than its prey (e.g. Sheldon et al., 1977 ). This predator:prey size
ratio does vary between heterotroph groups (Hansen et al., 1994 ), but the gener-
ally consistent larger predator size has important implications for structuring the
phytoplankton community that we will examine in more detail later. This size-
dependence of who eats who also has implications for the size of phytoplankton
cells that are more efficient at transferring organic matter upward to higher
trophic levels. The 5 m cell size that we earlier noted as important in determining

the potential for carbon export via cell sinking is also viewed as the lower size
limit for phytoplankton organic material to be transferred rapidly up to larger
heterotrophs (Legendre and Rivkin, 2002 ). As we will see later in this chapter, the
same size is viewed as setting the lower limit to particles likely to be grazed by fish
larvae. The different size of heterotrophs will consume different sizes of the
phytoplankton, and so will play different roles in the recycling, export and trophic
transfer of organic material.
As with the phytoplankton, the heterotrophs cover a range of sizes, from bacteria

m

( < 1 m) upwards. We will introduce heterotrophs from the bacteria up to the

m