8.3 Primary production and vertical export |

Primary production consists of *new production*
(P_{N}) that is based on allochthonous, i.e. externally supplied
nutrients, and *regenerated production* (P_{R}), which is based on autochthonous, i.e.
internally recycled nutrients [139]. Hence total primary
production (P_{T}) is the sum of P_{N} and P_{R}. The amount
of carbon that enters the aphotic zone is entitled *export
production* (P_{E})
(Figure 2).

Figure: New and regenerated production are based on (a) the
supply of the limiting (allochthonous) nutrients from the
aphotic zone, by advection, run-off or from the atmosphere
(straight arrows) and (b) the recycled (autochthonous)
nutrients in the euphotic zone (circular arrows), respectively.
New and regenerated production comprise total primary
production. Export production is the amount of sinking organic
carbon at the bottom of the euphotic zone.

The concept of new production is of utmost importance for understanding
natural and eutrophicated ecosystems because the fraction f =
P_{N}/P_{T} represents the upper limit of organic matter and energy
which can be removed or extracted from the surface waters of the system
without destroying the long-term integrity of pelagic systems
[483][244][285]. P_{N} represents thus the biomass that
has to be handled by an eutrophicated recipient (e.g. mineralization,
accumulation, harvestable biomass and export of biomass to adjacent
recipients). Given the importance of P_{N} for the over-all cycling
of organic matter, considerable emphasis has recently been given to
estimating P_{N} in coastal (e.g. Wassmann, [487]) as
well as oceanic environments (e.g. Knauer et al. [267]). New
production represents the *carrying capacity* of a marine
ecosystem. New production represents the *maximum production*
capacity of an ecosystem or the *harvestable production*. New
production is a *critical component* of marine primary production
that limits the supply of food to the benthos, zooplankton, fish, and
extensive aquaculture as well as the removal rate of atmospheric
CO_{2} by the marine biota. New production estimates are of great
interest for understanding eutrophication. Increased new production
results in additional biomass that the ecosystem has to deal with in
terms of grazing, vertical export to the bottom and pelagic and benthic
degradation.

Given the practical difficulties in estimating P_{N} for lengthy
periods of time, sediment traps can be used to estimate P_{N}.
P_{E} estimates as measured by sediment traps come close to
P_{N}, but are always smaller because it comprises only the
particulate fraction and some transformation takes place from ammonium
to nitrate even in the upper layers. Calculations of the productivity
index f by applying P_{E} give, therefore, rise to underestimates
[489]. As a consequence, the term e = P_{E}/P_{T} can be applied and used as an approximation of f. In boreal,
coastal areas where steady state, if at all, cannot be assumed for
intervals of less than the length of the productive period, e is
meaningful as a base for estimating f for lengthy periods only (e.g.
>6 months). Therefore, the term <e>, representing e
for lengthy periods of time will be applied.

8.3 Primary production and vertical export |