The Oceans 221utilization in proteins. When phytoplankton die they decompose, releasing the
nitrogen as ammonium (NH 4 +) hence N is still in its –3 oxidation state. Similarly,
when phytoplankton are eaten by zooplankton, the consumers excrete nitrogen
primarily as NH 4 +. This NH 4 +is then available for reuse by phytoplankton: NH 4 +
is the preferred form of available nitrogen since there is no energy requirement
in its uptake and utilization. Alternatively, the ammonium is oxidized via nitrite
(NO 2 - ) to NO 3 - , the thermodynamically favoured stable species. These rapid recy-
cling processes maintain euphotic zone NH 4 +at low concentrations. In the deep
ocean, the only NH 4 +source is from the breakdown of organic matter sinking
from the surface waters. The amount of NH 4 +released from this source is small
and rapidly oxidized, maintaining very low NH 4 +concentrations.
Elements showing nutrient-like distribution often have long oceanic residence
times, although shorter than conservative elements. The residence times of NO 3 -
silicon and DIP have been estimated to be 57 000, 20 000 and 69 000 years respec-
tively (Table 6.9). The vast reservoirs of nutrients in the deep ocean mean that
increases in the concentrations of NO 3 - in riverwaters due to human activity (seein the spring—and larger—moving
polewards.
The duration of the spring bloom is
limited by nutrient availability and/or grazing
by zooplankton. Phytoplankton growth and
abundance then decline to lower levels,
which are maintained throughout the
summer by nutrient recycling within the
euphotic zone. In some locations, limited
mixing in autumn can stimulate another
small bloom, before deep winter mixing
returns the system to its winter condition.
In tropical waters, vertical stratification
persists throughout the year and production
is permanently limited by nutrient supply
rates, which are controlled by internal
recycling and slow upward diffusion from
deep water. Under these conditions,
productivity is low throughout the year.
These seasonal cycles of productivity
are shown schematically in Fig. 1, along with
a map of current estimates of primary
production rates. In the north Pacific
zooplankton population growth supresses
the spring bloom.
Since production rates vary with time and
place, the data on the figure are uncertain,
but are consistent with satellite-derived maps
of chlorophyll concentrations (see Plate 6.3,
facing p. 138). These maps show that, on an
annual basis, the short, high-production
seasons of temperate and polar areas fix
more carbon in organic tissue than organisms
in tropical waters. There are a few exceptions
to this, in so-called upwelling areas, for
example the Peruvian, Californian, Namibian
and North African coasts and along the line
of the equator (Plate 6.3, facing p. 138 )&
Fig. 1). In upwelling areas, ocean currents
bring deep water to the surface, providing a
large supply of nutrients in an area with
abundant light. Very high rates of primary
production ensue and the phytoplankton are
the basis of a food chain that supports
commercially important fisheries.
The spring bloom and upwelling areas
are not simply times and regions of higher
productivity; changes in the structure of the
whole ecosystem result. For example, the
phytoplankton community in areas of higher
production is usually rich in diatoms,
organisms which, upon death, efficiently
export carbon and nutrients to deep waters.
This contrasts with the tropics, where the
phytoplankton community has adapted to
the low-nutrient waters by recycling nutrients
very efficiently, with little export to deep
waters. Phytoplankton are able to live in
low-nutrient tropical waters because they are
typically smaller, with larger surface area to
volume ratios that increase their efficiency
in diffusing nutrients across the cell wall.