Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 155

have a larger proportion of dinoflagellates and diatoms compared to the subtropical
oligotrophic gyres. Conversely, in the temperate subpolar gyres, turbulence is
frequently injected into the surface ocean by extra-tropical storms and so phyto-
plankton community structure can be similar to that on the shelf.

5.2 The fate of organic matter: recycling, carbon export

or food for heterotrophs
......................................................................................................................
Broadly, there are three possible routes for organic material to take once it has been
fixed by the phytoplankton. First, it could be recycled by the heterotrophic bacteria,
releasing inorganic compounds back into the water column. Over some time scale
dependent largely on where in the shelf seawater column the recycling takes place,
e.g. the photic zone, the deep water or the sediments, the inorganic carbon and
nitrogen will again become accessible to the autotrophs. Second, if the organic
material can reach the waters below the winter thermocline of the open ocean, or
be buried rapidly in the sediments of the open ocean or the seafloor of the continental
slope, then it has been exported away from the phytoplankton and from contact with
the atmosphere for some considerable time. Quantifying such long-term export of
organic carbon is a key part of determining the role of the oceans in absorbing and
removing carbon from the atmosphere. Third, the organic material could be
consumed by larger heterotrophs, supporting a marine food chain that reaches up
to large predatory fish, marine mammals and commercial fisheries.
We will first describe the export of carbon, and point to the role that the shelf seas
are thought to play globally. Then we will look at how organic material can support
the different components of the heterotrophic community.

5.2.1 Carbon export

When quantifying how much carbon is removed, or exported, from the surface
ocean, we need to understand how the primary production that fixed the carbon
was supported by nutrients. The bacterial recycling of organic material that generates
ammonium will also be remineralising organic carbon. If this recycling is taking place
in the photic zone, then both the ammonium and the recycled carbon can be utilised
by the phytoplankton and the primary production will not require a source of carbon
from elsewhere. Importantly this means that the growth will not require any flux of
CO 2 from the atmosphere to the ocean. This primary production is referred to as
regenerated production. If, however, there is a source of nitrate to the phytoplankton
community, say from below the seasonal thermocline, then additional carbon may be
fixed. Primary production using such a new supply of nitrogen is called new produc-
tion and is important because it has the potential to result in a flux of CO 2 from the
atmosphere to the ocean as the phytoplankton utilise the dissolved inorganic carbon
(DIC) in the surface water. The ratio of new production to total production is the
f-ratio, first defined by Richard Eppley and Bruce Peterson in an investigation of the